---
author:
- 'D. Trevese , V. Zitelli , F. Vagnetti , K. Boutsia , G.M. Stirpe'
title: 'Optical Spectroscopy of Active Galactic Nuclei in SA57[^1]'
---

[The cosmological evolution of X-ray-selected and optically selected Active Galactic Nuclei (AGNs) show different behaviours interpreted in terms of two different populations. The difference is evident mainly for low luminosity AGNs (LLAGNs), many of which are lost by optical photometric surveys.]{} [ We are conducting a spectroscopical study of a composite sample of AGN candidates selected in SA57 following different searching techniques, to identify low luminosity AGNs and break down the sample into different classes of objects. ]{} [ AGN candidates were obtained through optical variability and/or X-ray emission. Of special interest are the extended variable objects, which are expected to be galaxies hosting LLAGNs. ]{} [Among the 26 classified objects a fair number (9) show typical AGN spectra. 10 objects show Narrow Emission Line Galaxy spectra, and in most of them (8/10) optical variability suggests the presence of LLAGNs. ]{}

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

In recent years a growing amount of evidence suggested the existence of a link between the evolution in cosmic time of galaxy and quasar (QSO) populations. Theoretical work discusses the effect of galaxy merging on the nuclear activity, through an increment of the rate of accretion onto the massive black hole, hosted in (possibly all) galaxy nuclei [@kor95]. Observationally, the cosmic history of active galactic nuclei (AGNs) is deduced from the analysis of optical and X-ray luminosity functions (LFs) and their redshift dependence. While optical observations imply that the maximum of the QSO/AGN number density occurs at $z_M \ga2$ independently of their absolute luminosity [@wolf03], X-ray surveys indicate a “cosmic downsizing” with the epoch of maximum density going from $z_M\sim 1.5$, for bright objects ($L_X (2-10\,{\rm keV}) \sim 10^{45}  {\rm\, erg\, s}^{-1}$), to $z_M \sim 0.5$ for faint ones ($L_X (2-10\,{\rm keV})\sim 10^{42}  {\rm\, erg\, s}^{-1}$) [@ued03; @lafr05]. It has been suggested that this behaviour is a consequence of the existence of two distinct AGN populations: the first consisting of QSOs and brighter AGNs, born at high $z$ from frequent galaxy merging in high density regions, and related to the red part of the bimodal galaxy distribution, and the second population made of smaller and gas-rich galaxies still providing material for feeding smaller black holes and energizing low luminosity AGNs at later times through galaxy interactions [@cav07]. To evaluate sample completeness and selection effects, more detailed analysis of the LF evolution is necessary, in particular for low luminosity, optically selected AGNs, to accurately quantify the intrinsic evolution. Brighter AGNs are detected in the optical band, mainly by their non-stellar colour, i.e. their position outside the “stellar locus” in colour space. Fainter AGNs cannot be detected in the same way, since the observed spectral energy distribution (SED) is dominated by the host galaxy and in general is non-stellar independently of the presence of an active nucleus. Thus, probing the cosmic downsizing in optical samples requires a different selection technique. Recently @bon07 have selected a complete, volume limited sample of 130 type 1 AGNs from the catalogue of 150000 spectra obtained by the VIMOS-VLT Deep Survey, [@lefe05]. From this sample they have found a first evidence that the peak in density of lower luminosity type 1 AGNs is progressively shifted towards lower redshifts.

Another way to select AGNs is based on the detection of their variability. The method, which was proposed for the first time by @vdb73, has different completeness and reliability depending on the accuracy of photometric measurements and the distribution of sampling times. It also depends on the total duration of the observing campaign, since the r.m.s. variation increases with the lag between observations [@bono79; @gial91; @t94; @vand04; @devr05]. It has been applied in the past to various data-sets [e.g., @t89; @cris90; @ver95]. It is particularly interesting in the case of low luminosity AGNs (LLAGNs), where the image is not point-like and the colour selection fails. This search for “variable galaxies" has been experimented by @btk98 [hereinafter BTK] in the field of Selected Area 57 (SA57) where other techniques like the selection by colour and by the absence of proper motion were also applied [@kc81; @kkc86], and a few brighter candidates were confirmed spectroscopically. A similar procedure was applied to multi-epoch Hubble Space Telescope images by @sara03 [@sara06], who have also shown that some of the variability selected AGNs are not detected in X-rays. We have observed SA57 in X-rays with XMM-Newton (for 67 ks) and obtained a catalogue of 140 AGN candidates [@trev07], 98 of which are identified with optical images mainly from the KPNO survey of that field [@kron80; @koo86]. Candidates for spectroscopy are either selected from optical variability or from X-ray emission. In this paper we describe the result of a first part of this spectroscopic campaign.

The paper is organized as follows. §2 describes spectroscopic observations and data reduction, §3 describes the results and §4 contains a discussion of the results.

We adopt the concordance cosmology, $H_o=75$ km s$^{-1}$Mpc$^{-1}$, $\Omega_m=0.3$, $\Omega_{\Lambda}=0.7$, throughout the paper.

Observations and data reduction
===============================

Observations were carried out with the fiber-fed multi-object spectrograph AF2/WYFFOS at the 4.2m William Herschel Telescope (WHT), La Palma (Canary Islands - Spain) , on 28-29 April 2006. In a single, partially clear night on April 1 2006 we obtained also the spectrum of one object with DOLORES at Telescopio Nazionale Galileo (TNG) , La Palma. The 1.6 arcsec diameter of the AF2/WYFFOS fibers requires a relatively accurate positioning. For this reason we re-computed the absolute $\alpha,\delta$ in the following way. We started from the catalogue of the Kitt Peak National Observatory (KPNO) survey of SA57 [@kron80; @koo86], based on $U, B_J, F,N$ photometry taken almost yearly between 1974 and 1989. We cross-correlated the object positions with position in the USNO-A2.0 catalogue. After a 2-$\sigma$ rejection, the IRAF [*ccmap*]{} utility provides a 4th order coordinate transformation based on 446 objects spread over the field, with $<$0.2 arcsec r.m.s. deviation in both $\alpha$ and $\delta$, with respect to USNO-A2.0. The 2-$\sigma$ rejection eliminates most of the high proper motion objects whose positions underwent significant changes between the epochs of the KPNO master catalogue (1974) and the USNO-A2.0 catalogue (1956). Proper motions in the field were published by @maj92. This allowed us to select the guiding stars among low proper motion ($< 1.2$ arcsec/yr) objects and compute their position at the epoch of observation. The result was an accurate centering of all of the selected guiding stars. Candidates were selected from three different lists: i) variable extended candidates from BTK (details on objects fainter than 22.5 were not reported in the BTK paper); ii) point-like variable objects from @t89, not yet observed spectroscopically ; iii) objects from a subsample of 98 among 140 X-ray detected objects from @trev07 (see notes to Tab. \[Tab1\] in the next section). Some previously observed objects were automatically included by the fiber positioning software in those cases in which some of the candidates were not observable due to the crowding of the field. Observations were carried out for two successive nights in April 2006. We adopted two different configurations, such that fainter objects had a total exposure of 12 hours, i.e. were present in both configurations and were observed for two nights, while brighter objects were observed for 6 hours during the first or the second night. Between 15 and 20 fibers spread over the field were dedicated to the sky spectrum. We used the grating R158B providing a dispersion of 2.0 Å/px, the detector was the 2-chip EEV mosaic, read with $2 \times 2$ binning and the resolution was 16 Å. Individual observing blocks lasted 30 minutes each and were co-added to form images of 2 hours of total exposure. For each image we recorded also arc spectra for wavelength calibration. Data reduction was carried out with standard IRAF procedures. Spectra were extracted from each (2 hours) exposure and were calibrated in wavelength. An average, and high signal to noise, spectrum of the sky was obtained by combining several sky spectra. Sky subtraction was performed after a normalization of the average sky spectrum to minimize the residuals in correspondence of the main sky emission lines. In some cases, a single sky spectrum from a fibre close to the object was subtracted instead. The spectrum of the object NSER 16338 (see §3) was also obtained with DOLORES at TNG, in long slit mode. It was obtained in a single 40 min exposure, instead of the 160 min planned, due to bad weather conditions.

Results
=======

In the following we show the optical spectra (Figs. \[Fig1a\]-\[Fig1e\] and \[Fig2\]) and provide notes on individual objects.

Optical spectra
---------------

In Table \[Tab1\] we list the objects. The meaning of the columns is as follows: [*Column 1*]{}: serial number NSER[^2] in the KPNO survey [@kron80; @koo86]; [*Column 2 and 3*]{}: J2000 coordinates; [*Column 4 and 5*]{}: $B_J$ and F magnitudes; [*Column 4*]{}: note on variability selection; [*Column 6*]{}: identification number in the X-ray catalogue of @trev07; [*Column 7*]{}: notes on the identification; [*Column 8*]{}: redshift; [*Column 9*]{}: notes on redshift determination; [*Column 10*]{}: classification.

     NSER   RA(2000)      DEC(2000)    $B_J$     $F$     nv$^a$   SA57X$^b$   ni$^c$    $z$    nz$^d$  classification$^e$
  ------- ------------- ------------- -------- -------- -------- ----------- -------- ------- -------- --------------------
      681  13 08 05.91   +29 06 53.5   19.955   19.261     EV         -                0.403     c     BLAGN
     2688  13 09 42.82   +29 11 24.7   22.280   20.868     EV         -                0.401           NELG
     3873  13 09 36.84   +29 13 23.9   22.345   21.503     EV         -                0.298           NELG
     4326  13 08 53.67   +29 14 06.1   20.867   20.172     EV         -                0.216           NELG
     5334  13 09 44.08   +29 15 47.0   22.235   21.249     EV         -                0.298     e     NLAGN
     6825  13 08 22.19   +29 18 07.8   22.920   21.262               37                0.437           NELG
     6884  13 08 13.52   +29 18 12.5   20.833   19.006               38                0.275     a     XBONG
     7726  13 08 34.17   +29 19 27.0   21.446   20.409     EV         -                0.405           NELG
     8553  13 07 13.93   +29 20 42.1   20.946   19.675     EV         -                0.297    a,c    NELG
     8890  13 08 55.51   +29 21 10.5   20.964   19.051               61                0.330     a     galaxy
     9342  13 08 56.78   +29 21 50.5   20.885   19.140               66         A      0.246           NLAGN
     9820  13 08 56.74   +29 22 29.0   21.868   20.097               71         M      0.355     a     XBONG
    10144  13 09 00.18   +29 22 58.9   20.180   18.471               81                  -             star
    10459  13 09 34.63   +29 23 28.1   21.366   19.682     EV         -                0.324    a,c    galaxy
    10953  13 08 50.34   +29 24 15.0   20.960   20.071               90         A      0.148           NELG
    12399  13 09 51.62   +29 26 17.9   21.930   20.710     PV         -                0.424           NELG
    12472  13 07 29.20   +29 26 25.4   22.578   20.933               102               0.094    e,u    galaxy
    13310  13 07 56.71   +29 27 38.0   21.656   21.074               109        M      2.530     e     BLAGN
    13571  13 09 49.30   +29 28 00.7   21.713   20.677     EV         -                0.317           NELG
    13732  13 08 24.04   +29 28 19.2   22.763   22.640               115               0.671    e,u    BLAGN
    14260  13 07 58.37   +29 29 08.2   22.223   20.718     EV         -                0.248    a,u    galaxy
    14264  13 08 03.40   +29 29 08.8   20.391   18.782     EV        120               0.286           NLAGN
    15465  13 09 17.09   +29 31 04.3   21.886   21.151     PV        127               0.528     e     BLAGN
    16338  13 07 30.34   +29 32 22.5   22.719   21.957     EV         -                0.252     e     NLAGN
    16710  13 09 03.87   +29 33 06.3   21.690   20.119     EV         -                0.440     u     NELG
    17475  13 07 53.14   +29 34 17.0   22.551   22.410     PV         -                0.555    e,u    BLAGN

$^a$ notes on variability selection. EV: extended variable; PV: pointlike variable\
$^b$ X-ray catalog number following @trev07\
$^c$ notes on optical identification. M: marginal; A: ambiguous\
$^d$ notes on redshift. e: only emission; a: only absorption; c: confirmed redshift (see BTK); u: uncertain\
$^e$ BLAGN: Broad Line AGN; NLAGN: Narrow Line AGN; NELG: Narrow Emission Line Galaxy; XBONG: X-ray Bright Optically Normal Galaxy; galaxy: galactic spectrum with only absorption lines.

The tentative classification reported in Table \[Tab1\] is based on the appearence of the optical spectra. A distinction among starburst galaxies, Seyfert 2’s, and Low Ionization Narrow Emission Regions (LINERs) would require the knowledge of the (\[NII\] $\lambda$6583)/H${\alpha}$$\lambda$6563) ratio which is out of our spectral range for $z\ga 0.4$. Still we assume as Seyfert 2s the objects with large (\[OIII\] $\lambda$5007/H$\beta$ $\lambda$4861)$\ga$ 3 [see Fig. 1 @vo87]. These objects are classified as Narrow Line AGNs (NLAGNs). Where no broad lines are seen and (\[OIII\] $\lambda$5007/H$\beta$ $\lambda$4861) is not large enough, the object may be either a starburst galaxy or a LINER and we simply designate it as Narrow Emission Line Galaxy (NELG). Objects are classified as Broad Line AGNs whenever the broad components are detected. It should be noted, however, that in the case of LLAGNs, this classification becomes rather “fuzzy”, since the detectability of the broad lines depends on both the S/N ratio and the relative importance of the nuclear component respect to the host galaxy. Notice that this is, obviously, true for any LLAGN sample, no matter how detected.

Notes on individual objects
---------------------------

-   [*NSER 681*]{}: Broad-line AGN. The emission-line redshift $z_e=0.403$ is confirmed as previously found by BTK. We don’t see the MgII$\lambda$2798 emission line. Instead, we reveal a broad H$\beta$. Ca II K $\lambda$3934 allows determination of an absorption-line redshift $z_a=0.402$.

-   [*NSER 2688*]{}: NELG. $z=0.401$ based on \[OII\]$\lambda$3727, H$\beta$, \[OIII\]$\lambda$4959 and $\lambda$5007, and on CaII K absorption.

-   [*NSER 3873*]{}: NELG. $z=0.298$ based on \[OII\]$\lambda$3727, H$\beta$, \[OIII\]$\lambda$4959 and $\lambda$5007, and on CaII H and K absorption.

-   [*NSER 4326*]{}: NELG with a strong \[OII\]$\lambda$3727 and a relatively weak \[OIII\]$\lambda$5007. Emission and absorption features agree on a redshift $z=0.216$.

-   [*NSER 5334*]{}: Emission-line redshift $z_e=0.298$. The strong narrow emission lines \[OII\]$\lambda$3727, \[OIII\]$\lambda$4959 and $\lambda$5007, and the low H$\beta$/\[OIII\]$\lambda$5007 ratio favor a Seyfert 2 classification.

-   [*NSER 6825*]{}: $z=0.437$ based on \[OII\] $\lambda$3727 emission and on CaII K and H absorption. NELG.

-   [*NSER 6884*]{}: No emission features. $z_a=0.275$ based on CaII K and H and on H$\beta$ and MgIb $\lambda$5175.4. An Absorption-line Galaxy which can be classified XBONG due to its relatively high X-ray luminosity ($L_{2-10{\rm keV}}\la 10^{42}$ erg/s, cf. @trev07).

-   [*NSER 7726*]{}: Emission features and CaII K$\lambda$3934 absorption agree on a redshift determination $z=0.405$. Relatively strong \[OII\]$\lambda$3727 and a high H$\beta/$\[OIII\]$\lambda$5007 ratio suggest this object is a NELG.

-   [*NSER 8553*]{}: We confirm the redshift $z=0.297$ by BTK. Our spectrum has worse S/N ratio and is not reported in Fig. 1.

-   [*NSER 8890*]{}: An Absorption-line galaxy with $z_a=0.330$ based on CaII K and H and on H$\eta$ and H$\theta$.

-   [*NSER 9342*]{}: A Seyfert 2 with redshift $z=0.264$ based on \[OII\]$\lambda$3727, strong \[OIII\]$\lambda$4959, $\lambda$5007, and on CaII H and K absorption.

-   [*NSER 9820*]{}: No emission features. Absorption-line Galaxy with $z_a=0.355$ based on H$\theta$, HeI $\lambda$3889, CaII K and H, CaI $\lambda$4226.7 and MgIb $\lambda$5175.4. Its relatively high X-ray luminosity ($L_{2-10{\rm keV}}\la 10^{42}$ erg/s, cf. @trev07) puts it in the range of XBONGs.

-   [*NSER 10144*]{}: A stellar M-type spectrum with TiO molecular bands.

-   [*NSER 10459*]{}: Absorption-line Galaxy with $z_a=0.324$ in agreement with the previous determination by BTK.

-   [*NSER 10953*]{}: Emission lines \[OII\]$\lambda$3727, \[OIII\] $\lambda$5007 and absorption features CaII K and H, H$\eta$ and H$\theta$ agree with a redshift $z=0.148$. NELG.

-   [*NSER 12399*]{}: $z_e=0.425$ based on \[OII\]$\lambda$3727, H$\beta$ and \[OIII\]$\lambda$5007. $z_a=0.424$ based on CaII H, H$\eta$ and H$\theta$. NELG.

-   [*NSER 12472*]{}: No emission features. Tentative absorption redshift $z_a=0.094$ based on CaII K and H.

-   [*NSER 13310*]{}: Quasar with $z_e=2.53$ based on broad emission-lines Ly$\alpha$, NV$\lambda$1240, CIV$\lambda$1549 and CIII\] $\lambda$1909.

-   [*NSER 13571*]{}: We confirm the tentative redshift determination by BTK $z=0.317$, based mainly on \[OII\]$\lambda$3727 emission and CaII H and K absorption. NELG.

-   [*NSER 13732*]{}: A single broad emission-line at $\lambda_o=4675$, tentatively assigned to MgII$\lambda$2798, with a resulting redshift $z_e=0.671$.

-   [*NSER 14260*]{}: Galaxy without emission features. Tentative absorption redshift $z_a=0.248$ based on CaII K and H, and on CH-G band $\lambda$4304.4.

-   [*NSER 14264*]{}: Narrow-line AGN. $z_e=0.286$ based on \[OII\]$\lambda$3727, \[OIII\]$\lambda$4959 and $\lambda$5007. $z_a=0.287$ based on CaII K $\lambda$3934 and H $\lambda$3969. Approximate agreement with previous determination by BTK ($z=0.287$).

-   [*NSER 15465*]{}: Broad-line AGN with $z_e=0.528$ based on broad MgII$\lambda$2798 and forbidden \[OII\] $\lambda$3727.

-   [*NSER 16338*]{}: Very weak continuum with strong narrow emission lines, \[OII\]$\lambda$3727, H$\beta$, \[OIII\]$\lambda$4959 and $\lambda$5007. A Seyfert 2 galaxy with $z_e=0.252$. This object has been observed with both the WHT and TNG (see figures \[Fig1e\] and \[Fig2\]).

-   [*NSER 16710*]{}: NELG. $z_e=0.439$ based on \[OII\]$\lambda$3727. $z_a=0.440$ based on CaII H and K.

-   [*NSER 17475*]{}: Broad-line AGN, with a single feature at $\lambda_o$=4352, which we identify with MgII $\lambda$2798, with a resulting redshift $z_e=0.555$.

Discussion
==========

Figure \[Fig3\] shows the distribution in the luminosity-redshift ($L-z$) plane of all objects with known redshift in the field of SA 57, including data from the literature and the objects of the present spectroscopic campaign (larger symbols); this contains candidates from both the X-ray catalogue of @trev07 and the lists of variable objects of @t89 [@t94] and BTK. The luminosity reported is computed in the $F$ band, without K-correction. Galaxy redshifts are taken from the KPNO redshift survey of SA 57 by @munn97. Crowding of points around $z\sim 0.125$ corresponds to a known galaxy overdensity [@koo84; @koo87] and at $z \sim 0.24$ corresponds to the cluster Zw 1305.4+2941 analyzed in @kkntv88 and @gas07. The variability-detected extended objects populate a limited region of the $L-z$ plane. This is consistent with the expectation, since the optical survey is limited to $F \la 22$. The galactic component becomes fainter and fainter at high $z$, and is swamped by the nuclear luminosity for $L_F  \ga 3\times 10^{43}$ erg s$^{-1}$, thus appearing as point-like. It should be noted, however, that this does not reduce the interest of variability detection because: i) the main aim of the present survey is to detect faint AGNs at low redshift to verify the possible dependence of the evolution on intrinsic luminosity; ii) variability can detect also AGNs at higher redshift, though they appear as point-like and thus are detectable also by colour techniques. X-ray candidates span a wide range of redshift and luminosity ($\Delta \log L_F \sim 4$), from relatively faint X-ray sources like starburst galaxies, to bright QSOs. Despite the spectroscopic campaign being still incomplete, so that a discussion of the LF evolution at low luminosity is still unfeasible, we find some new interesting objects indicating that the low luminosity part of our sample of AGN candidates consists of a mix of different object types. In fact, while at higher luminosities ($L_F \ga 10^{43}$ erg s$^{-1}$) most objects are broad line AGNs, at lower luminosities most objects are either narrow line AGNs or NELGs.

Figure \[Fig4\] shows the optical $F$ band versus the X-ray luminosity for the objects of the present spectroscopic campaign plus X-ray detected objects by @trev07 with previously known redshift. The 3-$\sigma$ upper limits in the X-ray luminosity correspond to objects not detected in the 2-10 keV band which may be either selected through optical variability and not detected in X-rays, or detected in X-rays but in a different band.

Most of the objects whose redshift has been determined in the present work have a relatively low X-ray (2-10 keV) luminosity. Moreover, most of the objects selected through variability are not detected in X-rays, i.e. they likely have a low value of the X/O ratio, defined as the ratio of the X-ray flux $f_X(2-10 \,{\rm keV})$ and the $F$-band flux $f_F$. They could be faint AGNs with typical nuclear X/O ($\log {X/O} \sim 1$), but with the host galaxy contributing to the observed optical luminosity. Other objects, detected in X-rays, show optical spectra without emission lines, consistent with normal galaxies and luminosities $L_X(2-10{\rm keV}) \sim  10^{42}$ erg s$^{-1}$. Thus they can be classified as X-ray Bright Optically Normal Galaxies (XBONGs) described by @fior00 and @com02a [@com02b]. Different scenarios have been proposed to interpret these objects: i) selection effects hampering line detection [@hor05]: ii) heavy nuclear absorption [@com02a]; iii) heavy extra-nuclear absorption by the host galaxy dust [@rig06]; iv) strong dilution by the host galaxy light [@geor05]; v) radiatively inefficient accretion flow in low luminosity active nuclei [@yua04]. However all the above interpretations are based on the presence of an active nucleus. In particular @yua04 postulate the existence of a transition radius in the accretion disk, below which a radiatively inefficient accretion flow (RIAF) occurs. According to @ho99 a RIAF can also explain the absence of a big blue bump in the spectra of LINERs [see however @mao07]. This suggests a relation between the two classes of objects in spite of the differences in their optical and X-ray properties. The completion of the spectroscopic follow-up of our candidates will provide further data to investigate this issue.

Variability detected narrow emission line objects deserve further discussion. Let us consider first the objects we classified as Seyfert 2s on the basis of their high \[OIII\]/H$\beta$ ratio. According to the classic unified model [@anto85] the nuclear component should be hidden by the absorbing torus. At the same time the size of the narrow line region is such that line variability should be strongly reduced. Thus, in our case, the origin of variability is unclear. Notice that variability in some type 2 objects has been observed by @kle07. A possible explanations might be that the broad line region is not obscured, but intrinsically lacking, so that the variable continuum can be seen [@gho07]. Moreover some objects exhibit extreme spectral variations such that they appear of different type depending on the observing epoch (see for instance @cze04 and refs. therein) as was already noted in the case of NSER 4326 (104326 in BTK). An object of this type could be NSER 16338, which was selected on the basis of its variability during the photometric campaign in the years 1974-1989, when its magnitude was $B \sim 22.7$ and, on the basis of the present observations (April 2006), shows a very low optical continuum with only strong emission lines. Its spectrum is shown in both Figures \[Fig1e\] and \[Fig2\], as observed with WHT and TNG respectively. It would be interesting to monitor this object to detect possible long time scale spectral variations.

Part of the objects in Table \[Tab1\] are generically classified as NELGs since we do not observe \[NII\]/H${\alpha}$, while \[OIII\]/H$\beta$ is not large enough to indicate the AGN character. These objects could be either starburst galaxies, or LINERs or transition objects (TOs). However, we can look at their variability. Among the 10 NELGs in Table \[Tab1\], only 2 were below the variability selection threshold and were selected solely on the basis of their X-ray emission. The other 8 NELGs were selected through their variability, and this is an evidence in favour of their AGN character. In fact @mao05, on the basis of [*Hubble Space Telescope*]{} monitoring of a sample of LINERs, conclude that UV variability is detected in most, if not all, objects of this type. Moreover they conclude that this [*murmur*]{} of a sleeping black hole cannot be entirely explained by the variability of luminous stars but it implies the presence of a non-stellar component. According to @cid04 and @flo06, even if the AGN is present in LINERs, often its luminosity is insufficient to explain the observed emission-line intensity. In any case our results on variability selected objects favour the presence of low luminosity AGNs in at least some narrow emission line galaxies.

In summary :

-   [We have obtained the spectra of a composite sample of variability-selected and X-ray-selected AGN candidates;]{}

-   [some candidates show typical AGN spectra;]{}

-   [one variability selected object has strong emission lines but undetected continuum, suggesting extreme spectral variation;]{}

-   [2 objects are classified as XBONGs;]{}

-   [most of the other objects are classified as NELGs, i.e. starbursts or LINERs;]{}

-   [most NELGs were variability selected suggesting that they are LINERs hosting a LLAGN.]{}

We are continuing our spectroscopic campaign in SA57 with various telescopes, and we plan to obtain spectra of a larger sample of faint AGN candidates selected both through variability and X-ray emission. This will allow us to evaluate the relevance of the different selection effects in the study of the evolution of the faint end of the AGN luminosity function. A collection of a larger sample of variability-selected LINERs will provide a better understanding of the role of the hosted AGNs.

We thank A. Cavaliere for useful comments. We acknowledge partial support of Agenzia Spaziale Italiana and Istituto Nazionale di Astrofisica by the grant ASI/INAF n. I/023/05/0.

Antonucci, R. & Miller, J. S. 1985, ApJ, 297, 621 Bershady, M. A., Trevese, D., & Kron, R. G. 1998, ApJ, 496, 103 (BTK) Bongiorno, A., Zamorani, G., Gavignaud, I. et al. 2007, A&A, 472, 443 Bonoli, F., Braccesi, A., Federici, L., Zitelli, V. & Formiggini, L., 1979, A&ASupp, 35, 391 Cavaliere, A., Menci, N. 2007, ApJ, 664, 47 Cid-Fernandes, R., et al., 2004, ApJ, 605, 105 Comastri, A., Mignoli, M., Ciliegi, P., et al., 2002, ApJ, 571, 771 Comastri, A., Brusa, M., Ciliegi, P., et al., 2002, in New Visions of the XÐray Universe in the XMMÐNewton and Chandra Era, ESA SPÐ488, ed. F. Jansen Cristiani, S., Vio, R., & Andreani, P., 1990, AJ, 100, 56 Czerny, B., 2004, ASP.Conf.Ser., 360, 265 de Vries, W.H., Becker, R.H., White, R.L., &Loomis, C., 2005, AJ, 129, 615 Fiore, F., La Franca, F., Vignali, C. et al., 2000, NewA, 5, 143 Flohic, H.M.L.G., Eracleous, M., Chartas, G., Shields, J.C., & Moran, E.C., 2006, ApJ, 647, 140 Gastaldello, F., Trevese, D., Vagnetti, F., Fusco-Femiano, R. 2007, ApJ (in press), arXiv:0709.3168 Geha, M., Alcock, C., Allsman, R. A., et al. 2003, ApJ 125,1 Georgantopulos, I., & Georgakakis, A., 2005, MNRAS, 358, 131 Ghosh, H., Pogge, R.W., Mathur, S., Martini, P., & Shields, J.C., 2007, ApJ, 656, 105 Giallongo, E., Trevese, D., & Vagnetti, F., 1991, ApJ, 377, 345 Ho, L. C., 1999, ApJ, 516, 672 Hornschemeier, A.E., Heckman,T. M., Ptak, A.F., Tremonti, C. A., & Colbert, E.J.M., 2005, ApJ, 129, 86 Klesman, A., & Sarajedini, V., 2007, ApJ, 665, 225 Koo, D. C., Kron, R. G., Nanni, D., Trevese, D., Vignato, A., 1984, in Clusters and Groups of Galaxies, F. Mardirossian et al. Eds. (Dordrecht: Reidel), p.159 Koo, D.C., 1986, ApJ, 311, 651 Koo, D. C., Kron, R. G., & Cudworth, K. M. 1986, PASP 98, 285 Koo, D. C., Kron, R. G., Nanni, D., Trevese, D., & Vignato, A. 1988, ApJ, 333, 586 Koo, D.C., & Kron, R.G., 1987, IAU Symp., 124, 383 Kormendy, J., & Richstone, D. 1995, ARA&A,33, 581 Kron, R. G., 1980, ApJS, 43, 305 Kron, R. G., & Chiu, L. G. 1981, PASP, 93, 397 La Franca, F., Fiore, F., Comastri, A. et al., 2005, ApJ, 635, 864 Le Fèvre, O., Vettolani, G., Garilli, B., et al. 2005, A&A, 439, 845 Majewski, S.R. 1992, ApJSupp 78, 87 Maoz, D., Nagar, N.M., Falcke, H., & Wilson, A.S. 2005, ApJ 625, 699 Maoz, D., 2007, MNRAS, 377,1696 Munn, J. A., Koo, D. C., Kron, R. G., Majewski, S. R., Bershady, M. A., Smetanka, J. J., 1997, ApJS, 109, 45 Rigby, J.R., Rieke, G.H., Donley, J.L., Alonso-Herrero, A., & Perez-Gonzalez, P. G. 2006, ApJ, 645, 115 Sarajedini, V.L., Gilliland, R.L., & Kasm, C., 2003, ApJ, 599, 173 Sarajedini, V.L., Koo, D.C., Phillips, A.C., et al. 2006, ApJSupp 166, 69 Trevese, D., Pittella G., Kron R. G.,Koo D. C., & Bershady M. A. 1989, AJ, 98, 108 Trevese, D., Kron, R. G., Majewski, S. R., Bershady, M. A., Koo, D. C. 1994, ApJ, 433, 494 Trevese, D., Vagnetti, F., Puccetti, S., Fiore, F., Tomei, M., Bershady, M. A. 2007, A&A, 469, 1211 Ueda, Y., Akiyama, M., Ohta, K., Miyaji, T. 2003, ApJ 598, 886 Vanden Berk, D.E., Wilhite, B. C., Kron, R. G., et al., 2004, ApJ, 601, 692 van den Bergh, S., Herbst, E., Pritchet, C., 1973, AJ, 78, 375 Veilleux and Osterbrock 1987, ApJS, 63, 295 Véron, P., & Hawkins, M.R.S., 1995, A& A, 296, 665 Wolf, C., Wisotzki, L., Borch, A., et al., 2003, A&A, 408, 499 Yuan, F. & Narayan, R., 2004, ApJ, 612, 724

[^1]: Based on observations made with the William Herschel Telescope (WHT), operated by the ING, and with the Italian Telescopio Nazionale Galileo (TNG), operated on the island of La Palma by the Fundaci—n Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica), both at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.

[^2]: Notice that in some publications [@munn97 BTK] the same objects are designed by a serial number equal to (NSER+100,000)
