---
abstract: |
    ASCA observations of NGC 4636 and a southern region have revealed extended X-ray emission to a radius of about 300 kpc from the galaxy. The symmetric nature of the observed surface brightness around NGC 4636 indicates its association to this galaxy rather than to the Virgo cluster. Model independent estimation of the gravitational mass profile shows a flattening at a radius of $20
      \sim 35$ kpc, where the total mass reaches $\sim 6\times 10^{11}
      M_\odot$ and a mass-to-light ratio of 23. The mass still increases to larger radii, reaching $9\times10^{12} M_\odot$ with a mass-to-light ratio of 300 at $\sim$ 300 kpc from NGC 4636. These features suggest presence of a galaxy group surrounding NGC 4636. If such optically dark groups are common among X-ray bright ellipticals, it would explain the very large scatter in their X-ray luminosities with similar optical luminosities.
author:
- |
    [Kyoko]{} [Matsushita]{}, [Kazuo]{} [Makishima]{}, [Yasushi]{} [Ikebe]{}\
    [Etsuko]{} [Rokutanda]{}, [Noriko]{} Y. [Yamasaki]{}, and [Takaya]{} [Ohashi]{}
title: 'Largely Extended X-ray Emission around the Elliptical Galaxy NGC 4636 Observed with ASCA'
---

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

Spatial distribution of gravitational mass around elliptical galaxies has been estimated from radial distribution of X-ray emitting hot interstellar medium (ISM) (e.g. [@3]; [@4]). However, the outermost radius of the ISM distribution has so far remained undetermined due to a lack of sensitivity. As a result, the total gravitating masses of elliptical galaxies depend heavily on the assumed overall extent of the ISM. High sensitivity X-ray observations of bright galaxies are therefore important in determing the mass distribution in these systems.

NGC 4636 is one of the nearest giant elliptical galaxies. We employ a distance of 17 Mpc ([@8]). Diffuse thermal X-rays from its ISM make it one of the X-ray brightest elliptical galaxies (e.g. [@3]; [@4]) except those located at the center of clusters. Although NGC 4636 is located in the Virgo Southern Extension ([@nol]), it is apparently clear of the large-scale X-ray emission from hot intra-cluster gas associated with the Virgo cluster ([@10]; [@11]). Trinchieri et al. (1994), using [*ROSAT*]{}, detected a largely extended X-ray emission up to 18$'$ from NGC 4636, and suggested it to be an extension of the Virgo cluster emission. Here, we present new results on the extended X-ray emission around NGC 4636 based on a very long observation towards the galaxy center and on an offset observation from ASCA. Spectral properties of NGC 4636 have been already reported by Matsushita et al. (1997).

Observation
===========

We observed NGC 4636 in the 0.5–10 keV X-rays twice with the [*ASCA*]{} observatory ([@7]). The first observation, performed for 40 ksec in 1993 July 22, was already analyzed by several authors ([@13]; [@14]; [@15]; [@16]; [@18]; [@matusita97]). The second observation was conducted from 1995 December 28 through 1996 January 4, resulting in an exceptionally long exposure for 200 ksec. We also employ [*ASCA*]{} data taken on 1994 June 15, in the field of NGC 4643, which is just to the south of NGC 4636.

Data Analysis and Results
=========================

 X-ray image and radial brightness profile
------------------------------------------

Figure 1 shows background-subtracted 0.7–2.0 keV X-ray images of the NGC 4636 field and the south-offset field, taken with the GIS (Gas Imaging Spectrometer) ([@19]; [@20]). This energy band is dominated by the ISM emission. In addition to the bright emission peaked at NGC 4636, a fainter emission spreads over the entire GIS field of $\sim 25'$ radius, and even to the south-offset field $\sim
1^\circ$ ($\sim 300$ kpc) away.

The extended emission looks azimuthally symmetric around NGC 4636, without significant brightening toward the Virgo center (to the north); along a circle of a radius $10'$ centered on NGC 4636, the brightness is constant to within 30%. Furthermore, [*Ginga*]{} showed an upper limit on the diffuse X-ray intensity at $\sim 2
^\circ$ north of NGC 4636 ([@10]) to be 10 times lower than that observed in Fig. 1 at the edge of the NGC 4636 field. We therefore conclude that the extended emission is associated with NGC 4636 rather than the Virgo cluster.

We derived radial profile of X-ray surface brightness, as shown in Fig. 2, by azimuthally averaging the intensity in the 0.7–2.0 keV energy range around NGC 4636. The two bright background sources in the north of NGC 4636 were excluded, while effects of fainter background sources are negligible ($<5$%) ([@12]). The discontinuity of the profile at $\sim 25'$, corresponding to the field boundary, is caused by the vignetting and stray light of the ASCA X-ray Telescope (XRT; [@xrt]; [@takahashi]). As shown in Table 1 (the last column), the fraction of stray light from the NGC 4636 field to the south offset field is about 47% and 42% at $25'<r<50'$ and at $50'<r<80'$ respectively, where $r$ is the radius measured from NGC 4636. If we limit the region to $60'<r<80'$, the scattered light from the $r<50'$ and $r<60'$ field amounts to 64% and 80% of the detected photons, respectively. As shown later, we conclude from the brightness profile in Fig. 2 that the X-ray emission is significant until $60'$ (300 kpc) from NGC 4636.

We fit the radial X-ray profile with $\beta$ models convolved with the instrumental response of [*ASCA*]{}. The $\beta$ model describes projected X-ray surface brightness $S(r)$ as $     S(r) = S_0 \left\lbrace 1 + \left( r/r_{\rm{core}} \right)^2
      \right\rbrace^ {-3\beta+ \frac{1}{2}} $, where $r$ is the projected radius, while $r_{\rm{core}}$ (the core radius), $S_0$, and $\beta$ are parameters (e.g. Sarazin 1988). Based on the previous [*Einstein*]{} results ([@3]; [@4]), we fixed $r_{\rm{core}}$ to $10''$ which is practically unresolved with [*ASCA*]{}. As shown in Fig. 2, the data within $r \sim 7'$ ($\sim 35$ kpc) can be approximated well by the $\beta=0.5$ model, which behaves essentially as $\propto r^{-2.0}$. In contrast, the data start deviating from the model beyond $R \sim 7'$. These properties agree very well with the [*ROSAT*]{} radial X-ray profile ([@12]). Clearly, the observed radial X-ray profile cannot be fitted with a single $\beta$-model.

We accordingly fitted the radial profile with a sum of two $\beta$ models, a compact and an extended ones, both centered on NGC 4636. Except for the core radius of the compact one, the two components were allowed to have free parameters. The radial extent of the extended component was examined by changing a cut-off radius, and models with its value less than $50'$ predicts too little flux at $r>50'$ with the reduced $\chi^2>1.75$ for 64 dof.. When we assumed the cut-off radius to be 60’ and 150’, the reduced $\chi^2$ decrease to 1.34 and 1.12, respectively. This shows that the X-ray emission is significant to 60’ from NGC 4636. We assumed the cut-off radius to be 150’ in the later analysis. As shown in Fig. 2, the double $\beta$ model with this cut-off radius can fit the observed profile. The $\beta$ values are 0.60 (0.57–0.63) and 0.40 (0.38–0.42) for the compact and extended components respectively, and $r_{\rm
core}$ of the extended component is $6'~(4'-8')$ or 30 (20–40) kpc. The two $\beta$ components cross over at $r \sim 5'$ ($\sim 25$ kpc). Since we can exclude a flat profile for the wider component, it is certain that the extended emission is localized in the region surrounding NGC 4636. When integrated up to $r= 60'$, the 0.5–4 keV luminosity carried by the extended $\beta$ component becomes $8.1 \times 10^{41}$ ergs s$^{-1}$, which exceeds that of the compact $\beta$ component by a factor of 5.

Temperature profile of the ISM
------------------------------

In order to study temperature distribution of the gas, we accumulated the GIS data in concentric annular regions as summarized in Table 1. Then we can obtain a projected (not a three-dimensional) temperature profile. Spectral fit is carried out with the R-S model with photoelectric absorption. To represent the contribution from low-mass X-ray binaries in NGC 4636 (Matsushita et al. 1994), we added a thermal bremsstrahlung with a temperature fixed at 10 keV. Since GIS data cannot constrain hydrogen column density ($N_{\rm H}$) accurately, we fixed the value to those determined by the SIS in Matsushita et al. (1997). In the same way as in Matsushita et al.(1997), the abundances of heavy elements are represented by two parameters, one for the $\alpha$-elements (O, Ne, Mg, Si, and S) and the other for the Fe (Fe and Ni) group, which are denoted as $A_{\alpha}$ and $A_{\rm{Fe}}$ respectively. At each radius we fixed $A_{\alpha}$ to the best fit value derived by Matsushita et al.(1997), and left ${A_{\rm{Fe}}}$ free. Beyond $r>12'$, we fixed $A_{\alpha}$ to 0.37 solar as derived for $r=8'-12'$ by Matsushita et al. (1997), because the field of view of the SIS does not cover this region and no good information is available.

Acceptable fits have been obtained, and the results are summarized in Table 1. The spectral fits indicate that the gas is approximately isothermal, but there is a small positive temperature gradient, from 0.7 to 0.8 keV, from the center to $\sim 10'$, and beyond which the temperature drops to 0.5 keV at the south-offset field ($r>25'$) (Table 1). The [*ROSAT*]{} data show steeper gradient within several arcminutes from the center, probably because of the higher angular resolution.

This “ring-sorted analysis” is however affected by the spectral mixing effect among different sky regions, due to the extended point spread function (PSF) of the XRT as well as the projection effect. The PSF tends to smear out a radial gradient in the ISM properties. Fraction of leakage photons from other annular regions into each ring estimated from the observed brightness distribution is about 50% (2nd column from the right in Table 1) for $r=5'\sim
50'$.

Mass estimation from the double-$\beta$ model
---------------------------------------------

The total mass $M(R)$ within a 3-dimensional radius $R$, assuming a hydrostatic equilibrium and a spherical symmetry, is given by (e.g [@22]), $M(R) = - \frac{kTR}{G \mu m_{\rm p}}
          \left( \frac{d \ln n}{d \ln R} + 
          \frac{d \ln T}{d \ln R} \right)   $, where $m_{\rm p}$ is the proton mass, $k$ is the Boltzmann constant, $G$ is the constant of gravity, and $\mu \sim 0.63$ is the mean particle mass in unit of $m_{\rm p}$.

To estimate the radial profile of the total mass, we analytically deproject the two $\beta$-model components in Fig. 2 back to 3 dimensions individually, and obtain emissivities of the compact and extended components, $\epsilon_{\rm c}$ and $\epsilon_{\rm e}$, respectively. We then calculate the ISM density profile as the sum of the two components; $ n(R) = \sqrt{ \lbrace \epsilon_{\rm c}(R)+
\epsilon_{\rm e}(R) \rbrace / \Lambda(T,A)}~~$, with $\Lambda(T,A)$ denoting the emissivity function for a temperature $T$ and metal abundance $A$. We assume the two components to have the same temperature.

Based on the projected temperature and abundance profiles as shown in Table 1, we approximate the 3-dimensional profiles for temperature by $T(R) = 0.8 - 0.3\exp(-R/R_{\rm T})$ keV with $R_{\rm T}=10$ kpc ([@12]), and for the abundance profile by $A(R) = 0.2 + 0.8
\exp(-R/R_{\rm A})$ solar with $R_{\rm A}=40$ kpc ([@matusita97]). The observed temperature drop beyond 50 kpc is modeled as $T(R)=1.1 -
0.3\exp\lbrace (R-R_{\rm T_a})/R_{\rm T_b}\rbrace$ keV with $R_{\rm
T_a}=100$ kpc and $R_{\rm T_b}=400$ kpc. These approximation are confirmed to reproduce the observed projected profiles very well.

The derived total mass of NGC 4636 is presented in Fig. 3, together with the ISM mass profile (obtained by integrating $n$) and the stellar mass profile assuming the $r^{1/4}$-law (de Vaucoulers 1948) with assuming $M/L$ (mass-to-light ratio) of stars to be 8. We also show the result for a constant temperature (0.80 keV) and abundance (1.0 solar) in Fig. 3. The total mass $M(R)$ increases roughly as $\propto R$ up to $\sim 10$ kpc, and then it once flattens at $(3-4)\times 10^{11}~ M_\odot$, producing a shoulder-like structure at $R \sim 20$ kpc. Then $M(R)$ starts increasing again beyond $R\sim
30$ kpc. This is essentially the same feature as seen in the Fornax cluster ([@24]; [@23]) and A1795 ([@25]). At $R \sim 300$ kpc, $M(R)$ reaches $9\times 10^{12}M_\odot$.

Model-independent mass estimation
---------------------------------

The step-like feature in the gravitational mass profile derived in the previous section may depend on the particular choice of the double-$\beta$ model. We need to examine if other mass profiles are consistent with the observed brightness profile. To answer this question, we estimate the total mass without assuming the double-$\beta$ model. For simplicity, we assume an isothermal gas distribution since variation of emissivity with the gas temperature in the range 0.5$\sim$0.8 keV is less than 20%, and effects of temperature and abundance gradients on the total mass were found to be relatively small. From Fig. 2, we can safely assume that beyond 20$'$ and within $r<3'$, the radial brightness is dominated by a single $\beta$ component with $\beta=0.4$ and $\beta=0.6$ with $r_{\rm core}=10''$, respectively. These features are consistent with the [*Einstein*]{} and [*ROSAT*]{} results. Therefore, in the regions $R<3'$ and $R>20'$, we assume that the gravitational mass profile is same as that from the double-$\beta$ analysis. Then, our task is to find the mass profile which can smoothly connect the two regions.

We searched for the mass profile in the $R=3'\sim20'$ range which can produce the observed brightness profile by means of Monte-Carlo simulations. We divided the radius into $n$ ($n=3,4,5,6,8,9,10$ and 16) equal intervals in a logarithmic scale and assumed that $d\log
M/d\log R$ is constant within each step. We randomly picked up a value of $d\log M/d\log R$  between $R=3'$ and $R=20'$ for $2 \times
10^6$ times for each of the $n$ radial intervals. In the actual analysis, we extended the mass profile to $R<3'$ with the same $d\log M/d\log R$ value at $R = 3'$. For each trial mass curve, we compare the implied brightness profile with the observed one through $\chi^2$ fits. We take only acceptable results at 90% confidence limits. After a number of trials, 153 acceptable mass curves have been obtained. The envelope of these curves, is plotted in Fig. 3, which would represent the acceptable range of the gravitational mass from this model independent analysis. We note that $M(R)$ for the best fit double-$\beta$ model lies within the envelope of model-independent estimation. Although the uncertainty of $M(R)$ became larger than that of double-$\beta$ modeling, variation of $M(R)$ holds some similarity. The growth rate of $M(R)$ drops at $R\sim 10-30$ kpc, and starts increasing again in the outer region.

Discussion
==========

ASCA observations have shown a very extended X-ray emission with a radius of at least $60'$ (300 kpc) surrounding NGC 4636, which is much larger than that detected by [*ROSAT*]{}. We studied radial temperature and density distributions of the gas, and determined gravitational mass profile with and without assuming a double-$\beta$ model.

According to Fig. 3, the gravitational halo of NGC 4636 appears to terminate once at $R \sim 10-30$ kpc, where the total mass reaches several $ \times 10^{11} ~ M_\odot$. This inner component probably corresponds to the mass associated with the galaxy NGC 4636. Then, the total mass starts increasing again, forming a halo-in-halo structure whose total size is comparable to that of a galaxy group. The mass of the whole system and its mass-to-light ratio at 300 kpc reach $\sim 1 \times 10^{13}~
M_\odot$ and $\sim 300$, respectively, which are again comparable to those of galaxy groups ([@28]).

At $R \ge 200$ kpc, the X-ray emitting plasma becomes the dominant form of baryons, with its mass reaching 5–8% of the total gravitating mass. This value is considerably higher than those of individual elliptical galaxies, and again close to those found in galaxy groups and poor clusters ([@28]). These features all support the presence of a gravitational potential with a size of a galaxy group around NGC 4636. A further support is given by the observed abundance decrease in the X-ray emitting plasma ([@12]; [@15]; [@matusita97]; [@17]), from $\sim 1$ solar within $\sim 30$ kpc, to $\sim 0.2$ solar beyond $\sim 50$ kpc to a is typical level of groups and clusters of galaxies.

Nolthenius (1993) identified NGC 4636 as a member of Virgo Southern Extension F Cloud. However the 3-dimensional location of NGC 4636 is far offset from the center of the member galaxies in the cloud, and their velocity dispersion, 463 km s$^{-1}$, is much larger than that inferred from the observed gas temperature. The diameter of the cloud, 1.26 Mpc, is also much larger than the scale of the X-ray emission. Therefore, the relation between the X-ray halo and the galaxy cloud remains unclear, but it is very likely that there is some galaxy concentration around NGC 4636.

Extensive studies of ASCA data have revealed ([@17]) that X-ray luminous elliptical galaxies preferentially possess large-scale X-ray halos of a few 100 kpc scale, just like NGC 4636. Such a galaxy may be regarded as a dominant member of a galaxy group even though its evidence is scarce in the optical data. Thus, the X-ray emitting plasma may provide a better tracer of the total mass distribution than the light emitting matter.

We speculate that X-ray luminous elliptical galaxies may commonly possess such an extended emission with low surface brightness. X-ray luminosity of elliptical galaxies scatter by 2 orders of magnitude for the same optical luminosity, and its cause has long been a puzzle. The presence and absence of the extended emission can easily account for the large scatter in the total X-ray luminosity, and the low surface brightness of the extended emission, such as in NGC 4636, explains the previous undetection. We hope that systematic deep exposures from ASCA will solve this problem and bring us a complete understanding of the mass distribution around elliptical galaxies.

K.M. acknowledges support by the Postdoctoral Fellowship of the Japan Society for Promotion of Science.

[DUM]{}

Awaki, H. et al. 1994, , 46, L65 Böhringer, H. et al. 1994, Nature, 368, 828 de Vaucoulers 1948, Ann. d’Astrophys., 11, 247 Forman, W., Jones, C. & Tucker, W. 1985, , 293, 102 Ikebe, Y. 1995, Ph. D. Thesis, University of Tokyo Ikebe, Y. et al. 1996, Nature, 379, 427 Makishima, K. et al. 1996, , 48, 171 Matsumoto, H. et al. 1997, , 482, 133 Matsushita, K. et al. 1994, , 436, L41 Matsushita, K. 1997, Ph. D. Thesis, University of Tokyo Matsushita, K., Makishima, K., Rokutanda, E., Yamasaki, N., Y., & Ohashi, T. 1997, , 488, L125 Mulchaey, J., Davis, D., Mushotzky, R. F. & Burstein, D. 1996, , 456, 80 Mushotzky, R. F. et al. 1994, , 436, L79 Nolthenius 1993, , 85, 1 Ohashi, T. et al. 1996, , 48, 157 Sarazin, C. L. 1988, [*“X-ray emission from clusters of galaxies”*]{}, Cambridge University Press Serlemitsos, P.J., et al. 1995, , 47, 105 Takahashi, T. et al. 1995, The ASCA Newsletter, 3 Takano, S. et al. 1989, Nature, 340, 289 Tanaka, Y., Inoue, H. & Holt, S. S. 1994, , 46, L37 Trinchieri, G., Fabbiano, G. & Canizares, C. R. 1986, , 310, 673 Trinchieri, G., Kim, D.-W., Fabbiano, G. & Canizares, C. R. 1994, , 428, 555 Tully R. B. 1988, [*“Nearby Galaxy Catalog”*]{}, Cambridge University Press Xu, H. et al. 1997, , submitted

[rlclcrrr]{} r & $kT$ & $A_{\alpha} $ (fix) & $A_{\rm{Fe}}^*$ & $N_H (fix)$ & $\chi^2/\nu$ & 1 & 2\
(arcmin) &(keV)& (solar) & (solar) & ($10^{22}\rm{cm^{-2}}$) &\
0–5 & 0.74 $^{+0.01}_{-0.02}$&1.19 &0.80 $^{+0.06}_{-0.04}$ & 0.02& 115.3/87 & 8% & 0.4%\
5–10 &0.78 $^{+0.02}_{-0.02}$&0.53 &0.38 $^{+0.05}_{-0.04}$ & 0.00 & 121.7/77 & 48% & 2.5%\
10-15 &0.75 $^{+0.04}_{-0.05}$&0.37 &0.27 $^{+0.07}_{-0.07}$ & 0.00 & 192.0/163& 45% & 7.2%\
15–20 &0.79 $^{+0.03}_{-0.05}$&0.37 &0.34 $^{+0.10}_{-0.09}$ & 0.00 &151.2/161& 49% &13%\
20–25 &0.70 $^{+0.04}_{-0.05}$&0.37 &0.25 $^{+0.05}_{-0.06}$ & 0.00 & 255.2/258& 43% &22%\
25–50 &0.64 $^{+0.10}_{-0.09}$ & 0.37 & 0.19 $^{+0.09}_{-0.08}$&0.00 & 99.6/99 & 52% & 47%\
50–80 &0.44 $^{+0.14}_{-0.16}$ & 0.37 & 0.40 $^{+0.47}_{-0.25}$ &0.00 &54.8/75 & 74% & 42%\

\* The definition of solar iron abundance: Fe/H=3.24$\times10^{-5}$ (by number)\
1: Fraction of leakage photons from other annular regions into each ring.\
2: Fraction of stray light, or scattered photons from outside of the field of view into each ring.\

Fig. 1 : The 0.7–2.0 keV background-subtracted image of the NGC 4636 field and the south-offset field, obtained with the GIS. The contour levels are logarithmically spaced by a factor of 1.5, with the lowest contour corresponding to 25% of the brightness of the cosmic X-ray background. The image is not corrected for the telescope vignetting. One-dimensional X-ray brightness profile, obtained by slicing the image along the dotted region, is also shown.

Fig. 2: The 0.7–2.0 keV X-ray radial count-rate profile of NGC 4636. Typical $\beta$-model profiles convolved with the instrumental response are shown (thin dotted lines). Beyond $\sim 24'$, the profile shows the data from the south-offset field. We have also shown the profile (thick solid line) fitted with a sum of two $\beta$-model components (thick dotted line and thick dashed line). The best-fit model parameters are indicated in the figure.

Fig. 3: Radial mass profiles of the total gravitating matter, the X-ray emitting plasma, and the stellar component, in the NGC 4636 system. The solid curves for the total mass and the plasma mass assume a constant temperature (0.80 keV) and a constant abundance (1.0 solar), while the dotted curves take into account the radial gradients in temperature and abundance, both for a double $\beta$-model. The dotted region shows acceptable mass curves based on the Monte-Carlo simulation.
