---
abstract: 'The near-infrared spectral region is becoming a very useful wavelength range to detect and quantify the stellar population of galaxies. Models are developing to predict the contribution of TP-AGB stars, that should dominate the NIR spectra of populations 0.3 to 2 Gyr old. When present in a given stellar population, these stars leave unique signatures that can be used to detect them unambiguously. However, these models have to be tested in a homogeneous database of star-forming galaxies, to check if the results are consistent with what is found from different wavelength ranges. In this work we performed stellar population synthesis on the nuclear and extended regions of 23 star-forming galaxies to understand how the star-formation tracers in the near-infrared can be used in practice. The stellar population synthesis shows that for the galaxies with strong emission in the NIR, there is an important fraction of young/intermediate population contributing to the spectra, which is probably the ionisation source in these galaxies. Galaxies that had no emission lines measured in the NIR were found to have older average ages and less contribution of young populations. Although the stellar population synthesis method proved to be very effective to find the young ionising population in these galaxies, no clear correlation between these results and the NIR spectral indexes were found. Thus, we believe that, in practice, the use of these indexes is still very limited due to observational limitations.'
author:
- |
    Lucimara P. Martins$^{1}$[^1], Alberto Rodríguez-Ardila$^2$, Suzi Diniz$^{1,3}$,Rogério Riffel$^{3}$ and Ronaldo de Souza$^{4}$\
    $^{1}$NAT - Universidade Cruzeiro do Sul, Rua Galvao Bueno, 868, São Paulo, SP, Brazil\
    $^{2}$Laboratório Nacional de Astrofísica/MCT, Rua dos Estados Unidos 154, CEP 37501-064. Itajubá, MG, Brazil\
    $^{3}$Universidade Federal do Rio Grande do Sul - IF, Departamento de Astronomia, CP 15051, 91501-970, Porto Alegre, RS, Brasil\
    $^{4}$Instituto Astronômico e Geofísico - USP, Rua do Matão, 1226, São Paulo, SP
date: 'Accepted ? December ? Received ? December ?; in original form ? October ?'
title: 'Spectral Synthesis of Star-forming Galaxies in the Near-Infrared'
---

\[firstpage\]

Stars: AGB and post-AGB, Galaxies: starburst, Galaxies: stellar content, Infrared: galaxies

Introduction
============

The integrated spectrum of galaxies is sensitive to the mass, age, metallicity, dust and star formation history of their dominant stellar populations. Disentangling these stellar populations is important to the understanding of their formation and evolution and the enhancement of star formation in the universe. Star formation tracers in the optical region are nowadays considerably well known and studied, and have been a fundamental tool to identify star-formation in galaxies [@kennicutt88; @kennicutt92; @worthey+97; @balogh+97; @gu+06]. However, the use of this knowledge is not always possible in the case of very dusty galaxies or due to the presence of a luminous AGN. Because of these setbacks, tracers in other wavelength regions have been searched.

In this sense, the near-infrared region (NIR hereafter) offers an alternative to tackle this problem. It conveys specific information that adds important constrains in stellar population studies. Except for extreme cases such as ultraluminous IRAS galaxies (Goldader et al. 1995, Lançon et al. 1996), the dominant continuum source is still stellar. The $K$-band light of stellar populations with ages between 0.3 and 2 Gyr is dominated by one single component, namely, the thermally pulsating stars on the asymptotic giants branch (TP-AGB) [@maraston05; @marigo+08]. For populations with age larger than 3 Gyr the NIR light is dominated by stars on the red giant branch (RGB) [@origlia+93]. Their contribution stays approximately constant over large time scales [@maraston05]. By isolating the signature of these stellar evolutionary phases, one expects to gain a better understanding of the properties of the integrated stellar populations. This knowledge is of paramount importance, for example, in the study of high redshift galaxies, when the major star-formation has occurred.

Population synthesis models are beginning to account for these stars in a fully consistent way. As a result, they predict prominent molecular bandheads in the NIR. The spectral features of highest relevance to extragalactic studies are the ones located redward of 1 $\mu$m, which should be detectable in the integrated spectra of populations few times 10$^8$ years old. The detection of these bandheads would be a safe indication of the presence of the TP-AGB stars. The most massive of these TP-AGB stars can be very luminous in the NIR, exceeding the luminosity of the tip of the red giant branch by several magnitudes (Melbourne et al. 2012). Models that neglect TP-AGB stars have been shown to over-estimate the masses of distant galaxies by factors of two or more in comparison to models that include them [@ilbert+10].

Models from @maraston05 show that the combination of metallic indexes using these bands can quantify age, metallicity and even separate populations with single bursts from the ones with continuous star formation. However, for nearby objects, some of these bands are located in, or very close to, strong telluric absorption bands, rendering their predictions useless or strongly dependent on the S/N or observing conditions of the spectra.

As soon as good quality single stellar population (SSP) models in the NIR became available, the stellar population synthesis emerged as a powerful tool to study galaxies of many different types.

For example, by fitting combinations of stellar population models of various ages and metallicities @riffel+08 studied the inner few hundred parsecs stellar populations of starburst galaxies in the NIR.They observed spectra were best explained by stellar populations containing a sizable amount (20-56% by mass) of stars $\sim$1Gyr-old in the thermally pulsing asymptotic giant branch. @riffel+09 applied spectral synthesis to study the differences between the stellar populations of Seyfert 1 (Sy1) and 2 (Sy2) galaxies in the NIR. They found that the central few hundred parsecs of the studied galaxies contain a substantial fraction of intermediate-age SPs with a mean metallicity near solar. They also found that the contribution of the featureless continuum and young components tends to be higher in Sy 1 than in Sy 2.

@martins+10 also used stellar population synthesis to investigate the NIR extended spectra of NGC 1068. They found an important contribution of a young stellar population at $\sim$ 100 pc south of the nucleus, which might be associated with regions where the jet encounters dense clouds, possibly inducing star formation.

However, if on one hand we have now sophisticated models that predict the spectra of integrated populations in the NIR, on the other hand, few attempts have been made to fit them to observations in a consistent way, in order to calibrate or test these predictions. Much of the work in the NIR has focused on unusual objects with either active galactic nuclei [@larkin+98; @alonso-herrero+00; @ivanov+2000; @reunanen+02; @reunanen+03; @riffel+09; @martins+10; @riffel+10; @riffel+11; @storchi-bergmann+12] or very strong star formation [@goldader+97; @burston+01; @dannerbauer+05; @engelbracht+98; @vanzi+97; @coziol+01; @reunanen+07; @riffel+08]. @kotilainen+12 recently published NIR long-slit spectroscopy for a sample of nearby inactive spiral galaxies to study the composition of their NIR stellar populations. With these galaxies they created NIR HK-band templates spectra for low redshift spiral galaxies along the Hubble sequence. They found a dependency between the strength of the absorption lines and the luminosity and/or temperature of the stars, implying that NIR spectral indices can be used to trace the stellar population of galaxies. Moreover, evolved red stars completely dominate the NIR spectra of their sample, meaning that the contribution from hot young stars is insignificant in this spectral region, although such ages play an important role in other spectral regions [@riffelRogerio+11].

However, to identify and quantify tracers of star formation in the NIR, we need galaxies known to have a significant fraction of star formation. With this in mind, we used the NIR spectral sample of star-forming galaxies of @martins+13, which are known to have star formation from their optical observations. Our objective is to test the predictions of stellar population models in this wavelength range and verify the diagnostics that can, in practice, be used as proxies in stellar population studies. In order to do this we fit the underlying continuum between 0.8 and 2.4 $\micron$ with the stellar population synthesis technique, using the same method described in @riffel+09 and @martins+10. In §2 we present the details of our observations and reduction process; in §3 we briefly describe the stellar population synthesis method; in §4 we present our results and discussions and in §5 we show our conclusions.

The Data
========

The sample used here was presented in @martins+13 and is a subset of the one presented in the magnitude-limited optical spectroscopic survey of nearby bright galaxies of Ho et al. (1995, hereafter HO95). These galaxies are sources defined by Ho et al. (1997, hereafter HO97) as those composed of “nuclei dominated by emission lines from regions of active star formation (H[ii]{} or starburst nuclei)". In addition, five galaxies classified as non-star-forming in the optical, dominated by old-stellar population and with no detected emission lines, were included as a control sample.

All spectra were obtained at the NASA 3m Infrared Telescope Facility (IRTF) in two observing runs (2007 and 2008) - the same data from @martins+13. The SpeX spectrograph [@rayner+03] was used in the short cross-dispersed mode (SXD, 0.8-2.4 $\mu$m). The detector consists of a 1024x1024 ALADDIN 3 InSb array with a spatial scale of 0.15“/pixel. A 0.8”x 15" slit oriented in the north-south direction was used, providing a spectral resolution of 360 kms$^{-1}$. This value was determined both from the arc lamp and the sky line spectra and was found to be constant with wavelength along the observed spectra. The seeing varied from night to night but on average, most objects were observed under a 1$\arcsec$ seeing conditions.

The spectral reduction, extraction and wavelength calibration procedures were performed using SPEXTOOL, the in-house software developed and provided by the SpeX team for the IRTF community [@cushing+04]. Telluric features removal and flux calibration were done using XTELLCOR [@vacca+03], another software available by the SpeX team. The spectra were corrected for Galactic extinction using the @cardelli+89 law and @schlafly+11 extinction map.

The final sample is composed of 28 galaxies, from which 23 are classified as starbursts or star-forming and 5 are non-starforming galaxies for comparison. A different number of apertures were extracted for each galaxy, depending on the size of the extended emission across the slit. The average exctraction size is 1". More details about the observations and reduction process can be found in @martins+13. Additional information about these galaxies are presented in Table \[sample\]. The G band measurements and the H$_\alpha$ flux are presented because they can be used to characterize the old and young stellar populations, respectively, in the optical.

The wavelength and flux-calibrated spectra were de-redshifted using the $z$ values listed in column 2 of Table \[sample\]. Then, they were normalised to unity at 1.233$\mu$m (as defined in @riffel+08). This position was set because no emission or absorption features are located in or around it.

  ---------- --------- ----------- ------- ------------------- ----------------------- -- -- -- -- --
    Galaxy       z      Apertures   Class   W[(G band)]{}$^*$   log F(H$_\alpha$)$^*$              
                           (a)       (b)           (c)                   (d)                       
   NGC 0221   -0.0007    nuc + 3      N           4.54                    -                        
   NGC 0278   0.0021     nuc + 3      H           0.50                 -13.45                      
   NGC 0514   0.0082     nuc + 2      H           3.86                 -14.64                      
   NGC 0674   0.0104     nuc + 2      H           4.38                 -14.31                      
   NGC 0783   0.0173     nuc + 2      H           1.98                 -13.54                      
   NGC 0864   0.0052     nuc + 2      H           1.03                 -12.88                      
   NGC 1174   0.0091     nuc + 2      H           3.04                 -13.07                      
   NGC 1232   0.0053       nuc        N             -                     -                        
   NGC 1482   0.0064     nuc +2       H             -                     -                        
   NGC 2339   0.0074     nuc + 2      H           0.00                 -13.07                      
   NGC 2342   0.0176       nuc        H           0.67                 -12.83                      
   NGC 2903   0.0019     nuc + 7      H           0.57                 -12.63                      
   NGC 2950   0.0045     nuc + 4      N           4.98                    -                        
   NGC 2964   0.0044     nuc + 2      H           1.03                 -12.75                      
   NGC 3184   0.0020     nuc + 2      H           1.37                 -13.12                      
   NGC 4102   0.0028     nuc + 2      H           1.94                 -12.52                      
   NGC 4179   0.0042     nuc + 2      N           4.89                 -14.30                      
   NGC 4303   0.0052     nuc + 6      H           3.60                 -12.84                      
   NGC 4461   0.0064     nuc + 2      N           4.67                    -                        
   NGC 4845   0.0041     nuc + 6      H           4.18                 -13.61                      
   NGC 5457   0.0008     nuc + 2      H           1.03                 -13.33                      
   NGC 5905   0.0113     nuc + 2      H           2.15                 -13.13                      
   NGC 6181   0.0079     nuc + 2      H           4.43                 -13.57                      
   NGC 6946   0.0195       nuc        H           0.32                 -13.01                      
   NGC 7080   0.0161     nuc + 2      H             -                  -13.49                      
   NGC 7448   0.0073     nuc + 2      H           0.42                 -14.01                      
   NGC 7798   0.0080     nuc + 2      H           0.04                 -13.16                      
   NGC 7817   0.0077     nuc + 2      H           3.57                 -13.25                      
  ---------- --------- ----------- ------- ------------------- ----------------------- -- -- -- -- --

  :  Sample details

\
References: (1) @ho+95 (2) @kennicutt88 (3) @coziol+98\
(\*)Adopted from @ho+97.// Redshifts obtained from NED (http://ned.ipac.caltech.edu/) (a) Number of apertures extracted from each galaxy. (b) H means star-forming and N means non-star-forming galaxy. (c) Measured G band. (d) Log of the measured H$\alpha$ flux (in ergs/cm$^2$/s) \[sample\]

Spectral Synthesis Base Set and Method
======================================

The spectral synthesis is done using the code STARLIGHT [^2] [@cid+04; @cid+05a; @mateus+06; @asari+07; @cid+09], which mixes computational techniques originally developed for semi-empirical population synthesis with ingredients from evolutionary synthesis models. The code fits an observed spectrum O$_\lambda$ with a combination, in different proportions, of a number SSPs, to obtain the final model that fits the spectrum, M$_\lambda$. Due to the fact that the @maraston05 models include the effect of the TP-AGB phase, we use these SSPs models as the base set for STARLIGHT. The SSPs used in this work cover 14 ages, t = 0.001, 0.005, 0.01, 0.03, 0.05, 0.1, 0.2, 0.5, 0.7, 1, 2, 5, 9 and 13 Gyr, and 4 metalicities, Z = 0.02, 0.5, 1 and 2 z$_{\sun}$, summing up 56 SSPs.

It is important to mention that the spectral resolution of the models in this spectral region ranges from R $\sim$100 (K-band) to R $\sim$ 250 (z-band) while that of the observed data is significantly higher R $\sim$750 [@rayner+03]. The observations were then degraded to the model’s resolution. For this reason, STARLIGHT’s fit will depend more strongly on the continuum shape rather than on the absorption features. This comes directly from the way the synthesis works: the best models are chosen based on the $\chi^2$ values. This means that the synthesis does a point by point subtraction between models and observations. When the resolution is very low, the “distance” between models and observations becomes smaller in the absorption bands, and therefore they become “less important”. Recently, @maraston+11 published a new set of NIR models with R $\sim$500 (K-band). However, they are only available for solar metallicity. In order to test possible differences introduced in the results due to the spectral resolution, these latter models were used to construct a base using the same ages as for the low resolution base.

Besides that, we include in our spectral base 8 Planck distributions (black-body-BB), with T ranging from 700 to 1400 K, in steps of 100 K, to account for a possible contribution of hot dust to the continuum (for more details see @riffel+09). Although dust at such high temperatures is not common in starburst galaxies, its existence would indicate the presence of a hidden active galactic nucleus, for instance.

Extinction is modelled by STARLIGHT as due to foreground dust, and parametrised by the V-band extinction A$_V$. We use the @calzetti+00 extinction law to this purpose because it is more appropriated for star-forming galaxies [@Fishera+03].

Velocity dispersion is also a free parameter for STARLIGHT. This means that the code broadens the SSPs in order to better fit the absorption lines in the observed spectra. In our case the velocity dispersion results lack of significance because of the low resolution of the models compared to that of the observations.

Emission lines were masked, since STARLIGHT only fits the stellar population continuum. Lines masked in this work were \[SIII\]$\lambda$ 0.907 $\micron$, \[SIII\]$\lambda$ 0.953 $\micron$, HeI 1.083 $\micron$, Pa$\gamma$ $\lambda$ 1.094 $\micron$, \[FeII\]$\lambda$ 1.257 $\micron$, Pa$\beta$ $\lambda$ 1.282, \[FeII\]$\lambda$ 1.320 $\micron$, \[FeII\]$\lambda$ 1.644 $\micron$, Pa$\alpha$ $\lambda$ 1.875 $\micron$ and HI $\lambda$ 1.945 $\micron$.Spurious data (like bad teluric correction regions) were also masked out.

Results
=======

The results of the spectral synthesis fitting procedure for the nuclear apertures are presented in Figures \[specfit1\] to \[specfit7\]. For each galaxy, the top panel shows the observed and modeled spectra normalised to unity at 1.233 $\mu$m. The middle panel shows the observed spectrum after subtraction of the stellar contribution (O$_\lambda$ - M$_\lambda$). This residual can be understood as the gas emission component. The bottom panel shows the contribution of each SSP to the continuum found by the synthesis. This last panel can be understood in terms of the star formation history of the galaxy.

![image](starlighfit_1.ps){width="180mm"}

![image](starlighfit_2.ps){width="180mm"}

![image](starlighfit_3.ps){width="180mm"}

![image](starlighfit_4.ps){width="180mm"}

![image](starlighfit_5.ps){width="180mm"}

![image](starlighfit_6.ps){width="180mm"}

![image](starlighfit_7.ps){width="180mm"}

Figures \[specfit1\] to \[specfit7\] show that overall, the the spectral synthesis reproduces well the continuum shape and the most conspicuous absorption signatures, like the CaT and CO. Examples of good results are NGC 0864 and NGC 4102. However in many galaxies some strong absorption signatures are not being reproduced by the models. Clear examples are the features that appear around 0.94 $\mu$m (TiO) and 1.1 $\mu$m (CN) in many galaxies. Worst cases seem to be NGC 2950, NGC 4179 and NGC 6181. It is important to mention that we tested many different weights for different regions in the synthesis process, but the features were still not reproduced. Many things might be playing a role here, from telluric contamination around these regions to lack of precision from the method due to the low resolution of the models, or even the strength of these signatures in the models.

The quality of the fits are measured by the reduced $\chi^2$ and the [*adev*]{} parameter, which is the percentage mean deviation over all fitted pixels, $|$O$_\lambda$ - M$_\lambda$$|$/O$_\lambda$. Following @cid+05b, we present our results using a condensed population vector to take into account noise effects that dump small differences between similar spectral components. This is obtained by binning the population vector x into young (x$_Y$: t $\leq$ 5 $\times$ 10$^7$ yr), intermediate-age (x$_I$: 1 $\times$ 10$^8$ $\leq$ t $\leq$ 2 $\times$ 10$^9$ yr) and old (x$_O$: t $>$ 2 $\times$ 10$^9$ yr) components, using the flux distributions. Results for the condensed vectors for each galaxy (nuclear and off-nuclear apertures) are presented in Table \[synthesis\]. Additional results from the synthesis, namely the extinction value A$_v$, the mean age $<$log(t$_{\textrm{av}}$)$>$ and mean metallicity $<$z$_{\textrm{av}}$$>$ of the stellar population, weighted by the light fraction are also shown. These quantities are defined as @cid+05b:

$$\langle {\rm log} t_{ av} \rangle_{L} = \displaystyle \sum^{N_{\star}}_{j=1} x_j {\rm log}t_j,$$

and

$$\langle Z_{ av} \rangle_{L} = \displaystyle \sum^{N_{\star}}_{j=1} x_j Z_j,$$

where N$^*$ is the total number of SSPs used in the base. Table \[synthesis\] also presents the current mass of the galaxy which is in the form of stars (M\*), the mass that was processed in stars during the galaxy lifetime (M$_{ini}$) and the star formation rate in the last 100 Myrs (SFR$_{100}$). We also present the reduced $\chi^2$ and the [*adev*]{} values for each fit. The last column of this table shows the distance of the aperture (in arcseconds) to the nucleus. More information about these quantities can be obtained in the STARLIGHT manual.

@cid+05b tested the effects of the S/N in the optical on the stellar population synthesis results, concluding that for observed spectra with S/N $\ge$ 10, in the continuum the results are robust. For that reason we only applied the synthesis method for apertures obeying this criteria.

Average metallicites tend to be between 1 and 2 times the solar value. No hot dust contribution to the continuum was found from the synthesis for any galaxy. This result rules out the presence of a hidden AGN in our sample and agrees with previous observations in other wavelengths bands, where no mention to the possible presence of an AGN, even of low-luminosity, is made for any of the targets studied. Moreover, we found a significant contribution of young stellar population (x$_Y$ $\geq$ 10%) in 17 out of the 23 galaxies classified as starforming galaxies. In contrast, four of the five galaxies classified as non-star-forming have zero contribution of young stars, being dominated by an old stellar population whose contribution is larger than 75%. This agrees with the work of @kotilainen+12 who also found negligible contribution of young stars in their sample of non-active galaxies by means of $H-$ and $K$-band spectroscopy. NGC 4461 the only non-star-forming galaxy with anomalous result, has a young population fraction of x$_Y$ $\sim$ 48% in the nuclear region. This is surprising, as no emission lines were detected in that object. Moreover, its NIR spectrum is almost featureless, with no apparent CaII or CO 2.3$\mu$m absorption features, leading us to consider this result with caution. Indeed, the remaining non-star-forming galaxies of our sample, as well as those of @kotilainen+12, display these two systems of absorption lines/band. We therefore believe that our fits failed because no constraint could be set either from the continuum or from absorption features.

Ideally one would like to compare these results with stellar populations synthesis results from the optical. However, as shown by @martins+13, the apertures from the optical observations and from the NIR are very diffent, which leads to the sampling of different stellar populations (the optical spectra were obtained through a slit with 5 times the area of the NIR slit). Despite this, we believe our results can be trusted as consistent with the optical emission line tracers for the stellar population, which is further detailed in section 4.3.

Figure \[gradpop\] shows the gradient of the stellar populations for galaxies with five or more apertures. In this plot, negative distances represent the north direction and positive distances the south. One of these galaxies is a non-star-forming galaxy (NGC 2950, top right plot), and no variation of the stellar population is seen along the galaxy. For the three remaining galaxies (NGC 2903, NGC 4303 and NGC 4845), the fraction of younger populations are clearly rising in the outer apertures, coinciding with where the emission lines are stronger.

![image](starlight_grad_1.ps){width="180mm"}

[cccccccccccccc]{}

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NGC0221 & nuc & 0 & 30& 70 & 0.476 & 9.57 & 0.026 & 2.10E+07 & 3.27E+07 & 0.00E+00 & 2.21 & 1.71 & 0.00\
& S1 & 0 & 24& 76 &-0.421 & 9.60 & 0.027 & 6.58E+06 & 1.03E+07 & 0.00E+00 & 2.25 & 2.48 & 2.00\
& S2 & 0 & 5& 95 &-0.428 & 9.76 & 0.023 & 2.32E+06 & 3.70E+06 & 0.00E+00 & 2.31 & 2.24 & 4.00\
& N1 & 0 & 25& 75 & 0.174 & 9.65 & 0.027 & 8.15E+06 & 1.29E+07 & 0.00E+00 & 2.80 & 1.22 & -2.00\
NGC0278 & nuc & 0 & 33& 67 & 0.066 & 9.56 & 0.036 & 7.26E+07 & 1.14E+08 & 0.00E+00 & 1.64 & 2.78 & 0.00\
& S1 & 0 & 33& 67 &-0.180 & 9.56 & 0.030 & 3.24E+07 & 5.08E+07 & 0.00E+00 & 1.30 & 5.06 & 2.00\
& S2 & 9 & 25& 65 & 0.210 & 9.56 & 0.032 & 1.50E+07 & 2.33E+07 & 3.37E-03 & 1.17 & 12.31 & 4.00\
& N1 & 0 & 20& 80 & 0.369 & 9.62 & 0.033 & 3.18E+07 & 5.01E+07 & 0.00E+00 & 1.44 & 7.09 & -2.00\
NGC0514 & nuc & 5 & 34& 61 & 0.386 & 9.55 & 0.027 & 1.49E+08 & 2.33E+08 & 2.09E-03 & 0.96 & 5.68 & 0.00\
& S1 & 0 & 49& 51 & 0.658 & 9.45 & 0.023 & 3.69E+07 & 5.73E+07 & 0.00E+00 & 0.86 & 27.34 & 2.00\
NGC0674 & nuc &16 & 10& 74 & 1.641 & 9.59 & 0.023 & 1.07E+09 & 1.69E+09 & 4.25E-02 & 1.27 & 4.47 & 0.00\
NGC0783 & nuc &11 & 48& 42 & 2.001 & 9.41 & 0.024 & 2.05E+09 & 3.14E+09 & 7.39E-02 & 1.13 & 5.29 & 0.00\
& S1 & 0 &100& 0 & 1.769 & 8.87 & 0.035 & 3.56E+08 & 4.87E+08 & 0.00E+00 & 0.86 & 11.31 & 2.00\
& N1 &16 & 64& 19 & 1.922 & 9.17 & 0.033 & 3.21E+08 & 4.69E+08 & 1.74E-01 & 0.85 & 19.00 & -2.00\
NGC0864 & nuc &14 & 75& 10 & 1.210 & 8.92 & 0.033 & 3.98E+07 & 5.61E+07 & 1.85E-02 & 0.60 & 6.48 & 0.00\
& S1 & 0 & 55& 45 & 0.700 & 9.77 & 0.017 & 3.48E+07 & 5.70E+07 & 0.00E+00 & 0.90 & 18.73 & 2.00\
& N1 & 0 & 46& 54 & 0.926 & 9.48 & 0.020 & 2.11E+07 & 3.32E+07 & 0.00E+00 & 0.95 & 30.46 & -2.00\
NGC1174 & nuc &12 & 88& 0 & 2.775 & 8.52 & 0.038 & 4.28E+08 & 5.57E+08 & 5.81E-02 & 1.16 & 3.71 & 0.00\
& S1 & 0 & 42& 58 & 0.357 & 9.57 & 0.026 & 1.64E+08 & 2.55E+08 & 0.00E+00 & 0.68 & 20.17 & 3.00\
& N1 & 0 & 69& 31 & 1.338 & 9.34 & 0.033 & 1.05E+08 & 1.59E+08 & 0.00E+00 & 0.62 & 45.96 & -3.00\
NGC1232 & nuc & 0 & 2& 98 & 0.671 & 9.69 & 0.023 & 1.41E+08 & 2.23E+08 & 0.00E+00 & 0.96 & 18.83 & 0.00\
NGC1482 & nuc &30 & 70& 0 & 3.303 & 8.55 & 0.038 & 1.03E+09 & 1.39E+09 & 4.76E-01 & 1.42 & 3.56 & 0.00\
& S1 & 4 & 59& 37 & 1.518 & 9.71 & 0.039 & 1.46E+09 & 2.39E+09 & 1.28E-02 & 11.05 & 11.05 & -3.00\
NGC2339 & nuc &48 & 52& 0 & 3.589 & 8.33 & 0.035 & 7.71E+08 & 9.79E+08 & 1.23E+00 & 1.05 & 3.11 & 0.00\
& S1 & 0 & 67& 33 & 2.455 & 9.31 & 0.037 & 4.03E+08 & 6.08E+08 & 0.00E+00 & 0.81 & 7.82 & 2.40\
& N1 &42 & 58& 0 & 3.218 & 8.51 & 0.035 & 1.13E+08 & 1.48E+08 & 1.40E-01 & 0.83 & 10.38 & -2.40\
NGC2342 & nuc &26 & 74& 0 & 2.432 & 8.56 & 0.036 & 1.28E+09 & 1.68E+09 & 4.18E-01 & 1.14 & 4.39 & 0.00\
NGC2903 & nuc & 0 & 0& 100 & 1.598 & 9.70 & 0.032 & 2.30E+08 & 3.63E+08 & 1.67E-08 & 2.23 & 2.72 & 0.00\
& S1 & 6 & 0& 94 & 1.361 & 9.67 & 0.037 & 9.91E+07 & 1.57E+08 & 1.25E-03 & 1.95 & 3.27 & 1.66\
& S2 &30 & 3& 67 & 0.805 & 9.53 & 0.032 & 8.99E+07 & 1.42E+08 & 7.98E-03 & 2.18 & 2.97 & 3.58\
& S3 &19 & 0& 81 & 0.551 & 9.61 & 0.034 & 3.38E+07 & 5.36E+07 & 1.64E-03 & 1.47 & 4.68 & 4.98\
& N1 & 2 & 3& 95 & 1.090 & 9.68 & 0.037 & 9.32E+07 & 1.48E+08 & 4.15E-04 & 2.08 & 3.67 & -1.74\
& N2 &56 & 8& 36 & 0.895 & 9.27 & 0.023 & 4.34E+07 & 6.78E+07 & 1.27E-02 & 1.90 & 3.16 & -2.40\
& N3 &45 & 55& 0 & 1.873 & 8.03 & 0.036 & 1.22E+07 & 1.53E+07 & 9.41E-03 & 1.38 & 4.13 & -4.63\
& N4 &45 & 1& 54 & 1.357 & 9.43 & 0.040 & 2.72E+07 & 4.29E+07 & 4.59E-03 & 1.34 & 5.67 & -6.13\
NGC2950 & nuc & 0 & 23& 77 & 0.579 & 9.60 & 0.031 & 2.22E+09 & 3.49E+09 & 0.00E+00 & 2.41 & 2.27 & 0.00\
& S1 & 0 & 21& 79 &-0.427 & 9.61 & 0.027 & 3.89E+08 & 6.11E+08 & 0.00E+00 & 2.16 & 3.43 & 1.50\
& S2 & 0 & 22& 78 &-0.510 & 9.61 & 0.026 & 1.66E+08 & 2.59E+08 & 0.00E+00 & 1.14 & 5.06 & 2.50\
& N1 & 0 & 24& 76 & 0.500 & 9.60 & 0.028 & 4.45E+08 & 6.97E+08 & 0.00E+00 & 2.23 & 3.26 & -1.50\
& N2 & 0 & 18& 82 & 0.329 & 9.62 & 0.025 & 1.94E+08 & 3.04E+08 & 0.00E+00 & 1.27 & 5.49 & -2.50\
NGC2964 & nuc &12 & 88& 0 & 2.155 & 8.39 & 0.036 & 3.23E+08 & 4.15E+08 & 4.65E-02 & 0.97 & 5.41 & 4.98\
& S1 & 8 & 92& 0 & 1.730 & 8.28 & 0.038 & 8.66E+07 & 1.08E+08 & 5.91E-02 & 1.08 & 21.17 & 0.00\
NGC3184 & nuc &21 & 79& 0 & 1.471 & 8.27 & 0.039 & 8.97E+06 & 1.13E+07 & 1.41E-02 & 0.85 & 7.01 & 0.00\
& S1 &18 & 82& 0 & 1.155 & 7.93 & 0.040 & 3.22E+06 & 3.99E+06 & 1.22E-03 & 0.74 & 18.95 & 2.00\
& N1 & 3 & 97& 0 & 1.675 & 7.99 & 0.040 & 4.06E+06 & 5.05E+06 & 1.07E-04 & 0.75 & 24.56 & -2.00\
NGC4102 & nuc &52 & 48& 0 & 3.251 & 8.58 & 0.020 & 8.02E+08 & 1.05E+09 & 2.02E+00 & 1.00 & 2.72 & 0.00\
& S1 &58 & 42& 0 & 2.528 & 8.42 & 0.018 & 9.61E+07 & 1.29E+08 & 1.33E-01 & 1.04 & 3.58 & 2.00\
& N1 &38 & 62& 0 & 3.446 & 8.54 & 0.032 & 3.14E+08 & 3.97E+08 & 8.26E-01 & 1.37 & 3.30 & -2.00\
NGC4179 & nuc & 0 & 4&96 & 0.951 & 9.69 & 0.026 & 2.17E+09 & 3.42E+09 & 0.00E+00 & 1.96 & 4.24 & 0.00\
& S1 & 0 & 2&98 & 0.587 & 9.69 & 0.024 & 7.18E+08 & 1.13E+09 & 0.00E+00 & 1.45 & 6.41 & 2.00\
NGC4303 & nuc & 0 & 55&45 & 0.557 & 9.48 & 0.033 & 3.12E+08 & 4.81E+08 & 0.00E+00 & 0.87 & 2.98 & 0.00\
& S1 & 0 & 31&69 & 0.357 & 9.56 & 0.034 & 7.76E+07 & 1.22E+08 & 0.00E+00 & 1.77 & 3.45 & 1.50\
& S2 &34 & 30&36 & 0.575 & 9.30 & 0.039 & 2.91E+07 & 4.45E+07 & 2.69E-02 & 1.14 & 4.40 & 2.50\
& S3 &17 & 24&59 & 0.703 & 9.49 & 0.035 & 2.14E+07 & 3.36E+07 & 1.15E-03 & 0.70 & 6.29 & 3.50\
& N1 & 0 & 33&66 & 0.431 & 9.55 & 0.035 & 7.09E+07 & 1.11E+08 & 1.52E-05 & 1.60 & 3.63 & -1.50\
& N2 &23 & 42&35 & 1.085 & 9.30 & 0.036 & 2.76E+07 & 4.21E+07 & 1.49E-02 & 1.05 & 4.87 & -2.50\
& N3 &33 & 25&42 & 1.017 & 9.36 & 0.035 & 1.47E+07 & 2.27E+07 & 7.81E-03 & 0.81 & 9.51 & -3.50\
NGC4461 & nuc &48 & 28& 24 & 1.875 & 9.51 & 0.014 & 7.15E+08 & 1.15E+09 & 7.55E-01 & 0.53 & 4.91 & 0.00\
& S1 &24 & 60& 16 & 1.609 & 9.36 & 0.033 & 3.18E+08 & 4.82E+08 & 3.05E-01 & 0.94 & 8.11 & 2.00\
& N1 & 0 & 79& 21 & 0.546 & 9.22 & 0.030 & 1.80E+08 & 2.69E+08 & 0.00E+00 & 0.95 & 9.06 & -2.00\
NGC4845 & nuc &35 & 65& 1 & 4.000 & 8.55 & 0.029 & 5.96E+08 & 7.93E+08 & 4.63E-01 & 1.92 & 2.41 & 0.00\
& S1 & 9 & 91& 0 & 2.555 & 8.41 & 0.033 & 1.29E+08 & 1.67E+08 & 1.22E-02 & 1.24 & 3.60 & 1.50\
& S2 &25 & 75& 0 & 3.039 & 8.37 & 0.024 & 6.54E+07 & 8.16E+07 & 1.19E-01 & 0.97 & 6.52 & 2.50\
& N1 &51 & 49& 0 & 4.000 & 8.43 & 0.032 & 8.87E+07 & 1.12E+08 & 2.67E-01 & 1.08 & 4.42 & -1.50\
& N2 &50 & 50& 0 & 4.000 & 8.54 & 0.030 & 3.92E+07 & 4.90E+07 & 1.40E-01 & 0.94 & 8.52 & -2.50\
NGC5457 & nuc &64 & 28& 7 & 1.182 & 8.74 & 0.016 & 1.13E+07 & 1.60E+07 & 3.82E-02 & 0.72 & 5.83 & 0.00\
& S1 &54 & 46& 0 & 1.925 & 8.13 & 0.031 & 5.13E+06 & 5.97E+06 & 2.36E-02 & 0.84 & 15.96 & 2.00\
& N1 &41 & 59& 0 & 2.074 & 8.29 & 0.033 & 4.57E+06 & 5.82E+06 & 9.48E-03 & 0.87 & 20.81 & -2.00\
NGC5905 & nuc &32 & 20& 49 & 1.393 & 9.41 & 0.023 & 1.35E+09 & 2.12E+09 & 1.61E-01 & 1.32 & 3.31 & 0.00\
& S1 &52 & 23& 25 & 1.537 & 9.15 & 0.038 & 2.96E+08 & 4.42E+08 & 5.77E-01 & 0.87 & 6.97 & 2.00\
& N1 &46 & 42& 12 & 1.633 & 9.06 & 0.031 & 2.73E+08 & 3.97E+08 & 4.40E-01 & 0.92 & 7.57 & -2.00\
NGC6181 & nuc &38 & 33& 30 & 1.195 & 9.27 & 0.030 & 6.95E+08 & 1.06E+09 & 3.40E-01 & 1.10 & 3.50 & 0.00\
& S1 & 5 & 27& 67 & 0.292 & 9.59 & 0.034 & 4.72E+08 & 7.39E+08 & 6.38E-03 & 0.93 & 5.07 & 2.00\
NGC6946 & nuc &56 & 44& 0& 3.295 & 8.41 & 0.033 & 6.29E+07 & 8.29E+07 & 9.61E-02 & 2.83 & 1.33 & 0.00\
NGC7080 & nuc &16 & 67& 17 & 1.725 & 9.17 & 0.029 & 2.32E+09 & 3.44E+09 & 2.95E-01 & 1.28 & 2.54 & 0.00\
& S1 & 0 & 31& 69 & 0.890 & 9.59 & 0.036 & 1.22E+09 & 1.91E+09 & 1.49E-06 & 1.01 & 5.83 & 2.00\
& N1 & 0 & 56& 44 & 1.358 & 9.39 & 0.036 & 7.80E+08 & 1.20E+09 & 0.00E+00 & 0.98 & 7.09 & -2.00\
NGC7448 & nuc & 3 & 43& 55 & 0.196 & 9.44 & 0.036 & 1.66E+08 & 2.56E+08 & 1.45E-03 & 0.92 & 5.41 & 0.00\
& S1 & 0 & 37& 63 & 0.298 & 9.50 & 0.036 & 1.06E+08 & 1.64E+08 & 0.00E+00 & 0.75 & 8.84 & 2.00\
& N1 & 0 & 29& 71 & 0.085 & 9.55 & 0.033 & 1.06E+08 & 1.66E+08 & 0.00E+00 & 0.83 & 10.22 & -2.00\
NGC7798 & nuc &10 & 25& 65 & 1.473 & 9.53 & 0.025 & 1.20E+09 & 1.88E+09 & 3.53E-02 & 1.47 & 3.55 & 0.00\
& S1 & 0 & 10& 90 & 1.195 & 9.75 & 0.026 & 2.96E+08 & 4.77E+08 & 0.00E+00 & 0.70 & 10.28 & 2.00\
& N1 & 0 & 37& 63 & 1.828 & 9.58 & 0.011 & 2.22E+08 & 3.51E+08 & 0.00E+00 & 0.66 & 22.11 & -2.00\
NGC7817 & nuc & 0 & 15& 85 & 2.041 & 9.65 & 0.031 & 5.78E+08 & 9.09E+08 & 0.00E+00 & 1.03 & 5.35 & 0.00\
& N1 & 0 & 1& 99 & 1.538 & 9.70 & 0.022 & 1.84E+08 & 2.90E+08 & 0.00E+00 & 0.80 & 18.17 & -2.00\

\[specfitresult\]

Comparison with higher resolution models
----------------------------------------

Because higher resolution stellar population models are available in the literature for the wavelength range covered by our observations, it is important to test these models and compare the output with that of the lower resolution. In principle, one would expect that higher resolution models produce a higher accuracy in the output since the signatures we are looking for should be more conspicuous. and clear. The main limitation, however, is that @maraston+11 models are available only for solar metallicity. They were constructed using the same recipe as the ones from @maraston05, but using different observables. The set of models that extends to the NIR are the ones that use the @pickles98’s stellar spectral library and above $\sim$ 1 $\mu$m, nearly half of the spectra from that library lack of spectroscopic observations. In order to solve this problem, the author constructed a smooth energy distribution from broadband photometry. It may imply that some NIR absorption features may not be well resolved, even for these higher resolution models.

We applied the synthesis population fitting method to our galaxies using these high resolution SSPs for the base, using the same ages as described in section 3. These results were compared with the low resolution models. However, to be certain that differences were due only to model differences and not metallicity differences, we performed the stellar population synthesis with the low resolution models again, but with a base of SSP containing only solar metallicity models. This will ensure that any differences found between the results were not due to any age-metallicity degeneracy effect. The comparison between the results of the low and high resolution SSPs can be seen in Figure \[comp\_lowhires\]. Note that only nuclear apertures are shown in the plot because of their higher S/N, reducing uncertainties in the fit. The dotted line in Figure \[comp\_lowhires\] marks the one to one relationship. The size of each point in these plots is inversely proportional to the $adev$ value found by the fitting procedure, meaning that the larger the point, the smaller the error.

It can be seen from Figure \[comp\_lowhires\] that the fraction of old population found for each galaxy does not change significantly when using the high resolution models instead of the low resolution ones (bottom left panel). The same can be said about the reddening (bottom right panel). Regarding the fraction of intermediate age stellar populations (top right panel), discrepancies are clearly observed, but still most points are distributed along the one-to-one relationship. The test fails, as expected, for the fraction of young stellar population (top left panel).

Indeed, @maraston+11 found that the larger discrepancies between the old models and the new ones are in the NIR, for ages around 10 $-$ 20 Myr, where the red supergiants strongly contributes. Due to the short duration of their phase, a large scatter is expected for models that try to predict their contribution to the continuum. Besides, since the higher resolution models do not have the features expected for this wavelength region, they tend to be bluer than the low resolution models. This would explain why the synthesis with these models find a smaller contribution of x$_Y$ to the spectra.

Given the limitations of the spectral library used for the high resolution models in the NIR, it is not clear that they bring an improvement to the method that compensates the fact that they are available only for solar metallicity. For this reason, we consider that the low resolution models of @maraston05 are still the best option of SSPs for this wavelength range.

![image](compare_lowhires.ps){width="162mm"}

Spectral Synthesis and the NIR Indexes
--------------------------------------

Since the development of stellar population models that take into account more rigorously the TP-AGB phase, the CN molecular band has been advertised as an unambiguous evidence of intermediate-age stellar population [@maraston05; @riffel+07; @riffel+09]. This issue, however, has not yet been fully assessed on observational grounds, mostly because the results gathered up to now are based on samples containing only a few star-forming galaxies or active galactic nuclei.

@riffel+09, for example, presented a histogram comparing the fraction of intermediate age stellar population of Seyfert galaxies (obtained by stellar population synthesis) with the presence or absence of the CN band. They found a slight tendency of the galaxies that display CN to show a higher fraction of intermediate-age stellar population. Note, however, that they also report galaxies with no CN and a high fractions of x$_I$ as well as galaxies with CN and low fractions of x$_I$. Later, @zibetti+12 claimed that the two main signatures of the presence of TP-AGB stars predicted by the @maraston05’s models, CN at 1.41$\mu$m and C$_2$ at 1.77$\mu$m, are not detected in a sample of 16 post-starburst galaxies.

Figure \[CN\_comp\] shows the relation between the presence of the CN band and the intermediate age population for our galaxies. Only nuclear apertures were considered. The values of the CN index (equivalent width as defined by @riffel+07) were obtained from @martins+13, with an average value of 18.3 Å. We divided our sample into two groups: galaxies with low values of CN (values below the average) and high values of CN (values above the average).

![Histogram comparing the intermediate age component (x$_I$) for the nuclear aperture of the galaxies with low values of CN (crosshatch) and high values of CN (shaded in gray). []{data-label="CN_comp"}](cn_histogram.ps){width="86mm"}

From the histogram, there is no clear trend between the CN index and the fraction of intermediate stellar population within a galaxy. We found galaxies with a very high fraction of intermediate-age stellar population with no measured CN and galaxies where the CN is very strong but only a small fraction of intermediate-age stars was found. As pointed out by @riffel+09, that result does not necessarily imply that the CN is not a suitable tracer of intermediate-age stellar population. The proximity of strong permitted lines to the CN band may hinder, for instance, its observation. This is the case of galaxies where He[i]{} 1.081$\mu$m and Pa$\gamma$ in emission are strong. The red wings of these two lines will partially or totally fill up the CN band. As a result, the derived CN index will be small. This problem is particularly relevant in AGNs with broad permitted lines, which is not our case. Another possibility is that for galaxies with low $z$ ($z<$0.01), the CN band falls in a region of strong telluric absorption. Residuals left after the division by the telluric standard star may affect the S/N around 1.1 $\mu$m. In this case, the measurement of the CN band may be subjected to large uncertainties.

For our sample, we estimate that low S/N around 1.1 $\mu$m can be an issue in NGC1232 (x$_{\rm i}$=97.5%), NGC2964 (x$_{\rm i}$=87.5%), NGC3184 (x$_{\rm i}$=78.8%), NGC4461 (x$_{\rm i}$=27.8%) and NGC7817 (x$_{\rm i}$=15%). Note, however, that NGC2964 and NGC3184 display emission lines in their NIR spectra, giving additional support to the result of a large fraction of intermediate-age stars in their nuclei (see next section).

We found that although the CN band is a potential tracer of the intermediate age population, in practice its usefulness can be limited by the proximity of emission lines and residuals left after the division by the telluric standard.

Stellar population synthesis results and the presence of emission lines
-----------------------------------------------------------------------

Although no correlation was found between the stellar population synthesis and the NIR indexes, we were able to identify a relationship between the stellar population and the presence of emission lines in the galaxy sample. Galaxies that display no emission lines in the NIR were found to have, in general, very old stellar populations. Assuming that the spectra recorded for these sources is representative of the bulk of their stellar populations, these objects are either dominated by x$_O$ or it represents a large fraction of the emitted light. The weak nebular component found in the optical region contributes little to the NIR spectrum and/or the regions that emit these lines were not present inside the slit. Most of the important young population contributions were found in apertures with strong emission lines. Only a few were found in off-nuclear apertures with no signs of emission lines, but this happens when the S/N of the spectrum is very low, which means larger uncertainties in the fit. This is observed by the high [*adev*]{} value of these fits.

@martins+13 compared emission line properties from the optical observations and from the NIR. They found that for a subsample of galaxies the emission lines in the NIR were much weaker than in the optical, sometimes even absent. They concluded that this was mainly due to the differences in aperture between these two sets of data - the aperture in the optical has 5 times the area than the aperture in the NIR. In many of these galaxies the star-formation is probably not nuclear, but circumnuclear, or located in hot spots outside the nucleus. Based on this comparisons, they classified the galaxy sample in four classes:

-   Weak emission lines in the optical, no emission lines in the NIR, either at the nucleus or in the extended region (class 1): NGC514, NGC674, NGC6181, NGC7448.

-   Strong emission lines in the optical, no emission lines in the NIR, either at the nucleus or in the extended region (class 2): NGC278, NGC7817

-   Strong emission lines in the optical, evidence of weak to moderate-intensity lines in the NIR (nucleus or/and extended region - class 3): NGC783, NGC864, NGC1174, NGC2964, NGC3184, NGC4303, NGC4845, NGC5457, NGC5905, NGC7080, NGC7798

-   Strong emission lines in the optical, moderate to strong emission lines in the NIR (class 4): NGC2339, NGC2342, NGC2903, NGC4102, NGC6946

Figure \[synperclass\] shows the synthesis results for each of these classes. Only results for which the [*adev*]{} value was smaller than 5.0 were included in these plots. This figure shows the distribution of x$_Y$, x$_I$, x$_O$, the average age (t$_{av}$) and average metallicity (Z$_{av}$) of the stellar population and the star formation rate (SFR$_{100}$) in the last 100 Myrs, as found by the synthesis.

All class 1 and class 2 objects have very old average age. For all objects the synthesis found a very old stellar population in the nucleus. The lack of emission lines in the NIR was interpreted by @martins+13 as a difference between the slit sizes of the optical observations and the ones done in the NIR. Since the slit used for the NIR observations was much smaller, the ionisation source was missed by these observations, and that agrees with the old ages found by the synthesis. NGC 6181 seems to be a particular exception since a contribution of x$_Y$ to the nucleus larger than 30$\%$ was found. However, its average age is still high. For the off-nuclear apertures of NGC 514 and NGC 674 the spectra were very noisy and the synthesis results cannot be really trusted.

Class 3 and 4 objects have in general much lower average ages. All class 4 and most of class 3 objects have significant contribution of a young stellar population. It is also very interesting that this contribution tends to be higher in the apertures where the emission lines are stronger. This young stellar population should be the responsible for the gas ionisation. We also found higher star formation rates in the last 100 Myrs in objects from classes 3 and 4. There are no significant differences in metallicity between the classes. However, the non-star-forming galaxies seem to have lower metallicities than the galaxies classified as starburst or star-forming.

It is also interesting to comment on the results found for the non-star-forming galaxies NGC 221, NGC 1232, NGC 2950, NGC 4179 and NGC 4461. The first four galaxies, as expected, are dominated by x$_O$. The lack of young stars able to ionise the gas should explain the absence of emission lines in their spectra. However NGC 4461 has an important contribution of x$_Y$. The result for this galaxy is somewhat puzzling because the quality of the fit, as measured by the [*adev*]{} value is not bad (at least for the nuclear aperture). We do not expect this galaxy to have any contribution of a young, ionising stellar population to its continuum, otherwise strong emission lines should be seen. One possibility is that this young population is indeed real and the galaxy has no gas (or not enough to have strong emission lines).

Taking all this together, we found that overall the stellar population synthesis is recovering what would be expected of star-forming galaxies. However, individual results have to be carefully verified.

![image](syn_per_class.ps){width="178mm"}

Stellar Extinction
------------------

@martins+13 calculated the gas extinction in the NIR using the hydrogen ratio Pa$\beta$/Br$\gamma$, and found it to be generally larger than the gas extinction calculated from optical emission lines. This result agreed with previous results from the literature, and is explained by the fact that the NIR should probe larger optical depths than the optical range. One of the outputs from the stellar populations synthesis is A$_V$, the extinction from the continuum. Figure \[extinction\] shows the comparison of the values we obtained with the gas extinction from @martins+13. Filled circles represent the nuclear and open circles represent the off-nuclear apertures.

The extinction we found from the stars in the NIR traces the extinction from the gas in the optical. This is somewhat expected. The extinction obtained from the gas in the optical is higher than the one derived from the stars in this same wavelength region because the gas emission comes from a dustier region than the stars. However, the NIR emission from the stars should also probe dustier regions than the optical emission from the stars, which means that the emission from the gas in the optical and the continuum emission from the stars in the NIR should probe similar optical depths. However, one has to keep in mind that they do not have to be associated with the same spatial positions, as they differ from the extinction from the stars in the optical for different reasons - the gas emission intrinsically arrises from dustier positions while the stars in the NIR just probe higher optical depths.

![image](extinction_complete_calzetti.ps){width="182mm"}

@martins+13 also showed that the continuum of the galaxies in the NIR have a diversity of shapes and steepness. In Figure 6 of their paper, they show the nuclear spectra of the galaxies, organised by their steepness. They suggest that the shapes are related to the presence or absence of dust and the young stellar population. Here we present in Figure \[ordercont\] the average age and the stellar reddening obtained by the synthesis for the galaxies, now organised in steepness order, from the steepest continua (left) to flatter ones (right).

From this figure it is clear that the continuum shape is closely related to the stellar extinction (bottom panel), and that the presence of younger stellar populations make the continuum flatter.

![Relation between the average age (top panel) and the continuum extinction in the NIR (bottom panel) with the continuum steepness. The abscissa shows the galaxies from the sample in order of continuum steepness, from steepest ones (left) to the flatter ones (right).[]{data-label="ordercont"}](order_continuum.ps){width="86mm"}

Comparison with Far-Infrared Tracers 
-------------------------------------

Galaxies with active star-formation present a peak of emission around 60 $\mu$m, which is the maximum temperature of the dust heated by this process. The young OB stars that dominate the starburst radiate primarily in the optical and ultraviolet, but surrounding gas and dust reprocesses this radiation and thus strongly radiates at thermal wavelengths in the far-infrared. Far-IR luminosity (LIR) is thus indicative of the magnitude of recent star formation activity [@telesco88; @lonsdale+84].

We can then try to correlate the LIR with our results. Again, one has to be careful with the aperture sizes in each of the observations, to be sure results are consistent. We obtained IRAS (Infrared Astronomical Satellite) IR fluxes from @surace+04, and calculated the LIR luminosity as defined in @sanders+96.

As expected, no direct correlation was found between these values and the fractions of x$_Y$ or x$_I$ found by the synthesis. This is directly related to the fact that for many of the galaxies in our sample the star-formation is not a nuclear phenomenon. Figure \[lirdistrib\] shows the LIR distribution separated by the classes as defined by @martins+13. From this figure it is clear that classes 3 and 4, which had the strongest emission lines and higher fractions of younger populations, present also higher LIR values in comparison with classes 1 and 2. Non-star-forming galaxies clearly have the lowers LIR luminosities, as expected.

![image](LIR_per_class_new.ps){width="172mm"}

Summary and conclusions
=======================

We performed stellar population synthesis in a sample of long-slit NIR spectra of 23 star-forming and 5 non-star-forming galaxies to test the predictions of stellar population models that are available in this wavelength region. The stellar population synthesis code used for this work was STARLIGHT. The chosen SSP models used as a base for STARLIGHT were the ones from @maraston05, because they include the effect of the TP-AGB phase, crucial to model the stellar population in this wavelength region. So far, these (and their most recent version in @maraston+11) are the only ones published that take this effect into account.

We compared the synthesis results from older version of the models [@maraston05] with the new version @maraston+11. We found discrepancies for the fraction of young stellar populations, which were mentioned by the own authors, due to the uncertainties in the presence of the red supergiants. Given the limitations of these models, we believe that the lower resolution models from @maraston05 are still the best set of models for stellar population synthesis in the NIR.

No hot dust contribution was found for any galaxy, ruling out the presence of a possible hidden AGN in any of them. We found no correlation between the synthesis results and the NIR indexes measured by @martins+13. Although some of the signatures, like the CN band, can be potential traces of the intermediate age population, we believe that the use of them in practice is still complicated, mostly because of observational limitations.

The results from the synthesis seem to be in very good agreement with the emission line measurements. For galaxies with no emission lines detected in the NIR, no significant young stellar population was found. They have their continuum dominated by old stellar populations. In addition, the apertures with strong emission lines have large contribution of a young stellar population. This becomes more obvious when comparing the results from the synthesis separated by the classes defined by @martins+13. The classes were created based on the strength of the emission lines in the NIR, with class 1 objects presenting no detected emission lines in the NIR and class 4 having the strongest emission lines measured. Classes 1 and 2 objects have higher contribution of older stellar populations and higher average ages. Classes 3 and 4 objects have higher contribution of the young stellar population and lower average ages.

From the synthesis we also obtained the extinction from the continuum in the NIR, which we found to trace the extinction from the gas in the optical. We also find that the continuum steepness is related to this extinction, in the sense that the steepest continua tend to have smaller extinction values that the flatter ones.

Acknowledgments {#acknowledgments .unnumbered}
===============

The authors thank the reviewer of the paper for his valuable comments. This research has been partially supported by the Brazilian agency FAPESP (2011/00171-4). ARA thanks to CNPq for financial support through grant 307403/2012-2. RR thanks to FAPERGs (ARD 11/1758-5) and CNPq (304796/2011-5) for financial support.

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[^1]: E-mail: lucimara.martins@cruzeirodosul.edu.br

[^2]: The code and its manual can be downloaded from http://astro.ufsc.br/starlight/node/1
