A Chandra Observation of the Nearby Lenticular Galaxy NGC 5102: Where are the X-ray Binaries?

Abstract

We present results from a 34 ks Chandra/ACIS-S observation of the nearby (d=3.1 Mpc) lenticular galaxy NGC 5102, previously shown to have an unusually low X-ray luminosity. We detect eleven X-ray point sources within the the $D_{25}$ optical boundary of the galaxy (93% of the light), one third to one half of which are likely to be background AGN. One source is coincident with the optical nucleus and may be a low-luminosity AGN. Only two sources with an X-ray luminosity greater than 10$^{37}$ ergs s$^{-1}$ in the 0.5-5.0 keV band were detected, one of which is statistically likely to be a background AGN. We expected to detect 6 such luminous sources if the XRB population scales linearly with optical magnitude of the host galaxy. NGC 5102 has an unusually low number of XRBs. NGC 5102 is unusually blue for its morphological type, and has undergone at least two recent bursts of star formation. We present the results of optical/UV spectral synthesis analysis and demonstrate that a significant fraction ($>$50%) of the stars in this galaxy are comparatively young ($<3\times10^9$ years old). If the lack of X-ray binaries is related to the relative youth of most of the stars, this would support models of LMXB formation and evolution that require wide binaries to shed angular momentum on a timescale of Gyrs. We find that NGC 5102 has an unusually low specific frequency of globular clusters ($S_N\sim$0.4), which could also explain the lack of LMXBs. We also detect diffuse X-ray emission in the central $\sim$1 kpc of the galaxy. This hot gas is most likely a superbubble created by multiple supernovae of massive stars born during the most recent star burst, and is driving the shock into the ISM which was inferred from optical observations. Comment: 33 pages, 7 figures, 6 tables - Accepted for publication in the Astrophysical Journal

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arXiv:astro-ph/0505408v1 19 May 2005
A Chandra Observation of the Nearby Lenticular Galaxy NGC
5102: Where are the X-ray Binaries?
R. P. Kraft
Harvard/Smithsonian Center for Astrophysics, 60 Garden St., MS-67, Cambridge, MA
02138
L. A. Nolan, T. J. Ponman
School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, UK
C. Jones
Harvard/Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138
S. Raychaudhury
School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, UK
ABSTRACT
We present results from a 34 ks Chandra/ACIS-S observation of the LMXB
population and the hot ISM in the nearby (d=3.1 Mpc) lenticular galaxy NGC
5102, previously shown to have an unusually low X-ray luminosity. We detect
eleven X-ray point sources within the the D25 optical boundary of the galaxy
(93% of the light), one third to one half of which are likely to be background
AGN. One of the X-ray sources is coincident with the optical nucleus and may
be a low-luminosity AGN. Only two sources with an X-ray luminosity greater
than 1037ergs s−1in the 0.5-5.0 keV band were detected, one of which is statisti-
cally likely to be a background AGN. We expected to detect 7 or 5 such luminous
sources if the XRB population scales linearly with the B band or J band mag-
nitudes, respectively, of the host galaxy. By this measure, NGC 5102 has an
unusually low number of XRBs. The deficit of LMXBs is even more striking
because some of these sources may in fact be HMXBs. NGC 5102 is unusually
blue for its morphological type, and has undergone at least two recent bursts of
star formation only ∼1.5×107and ∼3×108years ago. We present the results of
optical/UV spectral synthesis analysis and demonstrate that a significant frac-
tion (>50%) of the stars in this galaxy are comparatively young (<3×109years
old). We discuss the relationship between the XRB population, the globular

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cluster population, and the relative youth of the majority of stars in this galaxy.
If the lack of X-ray binaries is related to the relative youth of most of the stars,
this would support models of LMXB formation and evolution that require wide
binaries to shed angular momentum on a timescale of Gyrs. We have also ana-
lyzed archival HST images of NGC 5102, and find that it has an unusually low
specific frequency of globular clusters (SN∼0.4). The lack of LMXBs could also
be explained by the small number of GCs. We have also detected diffuse X-ray
emission in the central ∼1 kpc of the galaxy with an X-ray luminosity of 4.1×1037
ergs s−1in the 0.1-2.0 keV band. This hot gas is most likely a superbubble cre-
ated by multiple supernovae of massive stars born during the most recent star
burst, and is driving the shock into the ISM which was inferred from previous [O
III] λ5007 and Hαobservations.
Subject headings: galaxies: individual (NGC 5102) - X-rays: galaxies - galaxies:
ISM
1.Introduction
Early-type galaxies have been known to be luminous X-ray sources since the Einstein era.
The X-ray emission from the most massive elliptical galaxies is dominated by emission from
thermal coronae. These coronae may be primordial and enriched with metals from outflows
created by supernovae or stellar winds, or they may be entirely built up from such outflows,
with the primordial gas being blown off during a large, early burst of star formation (David,
Forman, & Jones 1990, 1991; O’Sullivan, Forbes, & Ponman 2001). There is a well known
correlation between the X-ray luminosity of an elliptical galaxy and its total mass (quantified
by its blue magnitude), although there is more than a factor of 100 dispersion around the
mean. For less massive galaxies, and for massive galaxies that have lost much of their corona
through tidal or ram pressure stripping, the contribution of the population of X-ray binaries
(XRB) to the total X-ray luminosity becomes increasingly important. The contribution of
the XRB population to the X-ray spectra of early galaxies was noted in Einstein and ASCA
observations (Kim, Fabbiano, & Trinchieri 1992; Matsumoto et al. 1997). The Chandra
X-ray Observatory (CXO) has observed a large number of elliptical and S0 galaxies and has
resolved the brightest XRBs from the hot ISM (e.g. Sarazin, Irwin, & Bregman (2000);
Kraft et al. (2001); Kundu, Maccarone, & Zepf (2002) among many others). The XRB
populations of galaxies with a range of masses and morphological types have now been
studied in detail, and it is clear that the variance in the number of XRBs per unit optical
luminosity in early galaxies is larger than that expected based on counting statistics alone.

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This variance is likely related to the origin and evolution of the XRBs, subjects on which
there is still considerable debate.
There is general agreement that the creation of XRBs must be a continuous, ongoing
phenomenon in the life cycle of early galaxies because the lifetime of all XRBs is considerably
less (∼109yrs for low mass X-ray binaries (LMXBs) and ∼107yrs for high mass X-ray
binaries) than the age of their host galaxy (Grimm, Gilfanov, & Sunyaev
specific mechanism for XRB formation is still a subject of contention. Some have argued
that most or all of the XRBs in elliptical galaxies originate in globular clusters because they
are the only regions where the stellar density is sufficient for neutron stars to be captured
in N-body interactions. Recent Chandra observations of NGC 4472, NGC 1399, and other
massive early galaxies have shown that a significant (40-60%) fraction of the XRBs are
coincident with globular clusters, supporting this idea. In this scenario, the variance in the
number of XRBs can then be explained as differences in the globular cluster populations.
However, the ellipticals for which a large XRB/GC correlation have been demonstrated are
located in rich environments and have exceptionally large globular cluster specific frequencies
(GCSFs). Even in early-type galaxies with unusually large GCSFs, at least half of the XRB
population is not directly accounted for by this GC/XRB correlation. Others have argued
that the XRB population is related to the bulk of the stars in the galaxy, and that the
variance in the XRB populations is related to the star formation history of the host galaxy
(White & Ghosh 1998; Wu 2001). The fact that the majority of the LMXBs in the galactic
center region of the Milky Way and the M31 bulge do not appear to be associated with GCs
supports this argument. Recent analysis of the X-ray point source population from a sample
of early galaxies suggests both mechanisms are important (Irwin & Bregman 2004).
2002). The
In this paper, we present results from a 34 ks Chandra/ACIS-S observation of the S0
galaxy NGC 5102. NGC 5102 is a small (MB=-17.45) nearby (d=3.1±0.15 (McMillan,
Ciardullo, & Jacoby 1995)) lenticular galaxy and is a member of the Centaurus A group
(Israel 1998). The luminosity of this galaxy, and therefore presumably the mass, is similar
to the LMC. NGC 5102 has been previously observed by Einstein and ROSAT (Forman,
Jones, & Tucker 1985; Irwin & Sarazin 1998; O’Sullivan, Forbes, & Ponman 2001), where
it was noted that its integrated X-ray luminosity is considerably less than one would expect
from a simple extrapolation of its mass compared with the more luminous ellipticals. The
bulge of this galaxy is unusually blue for its morphological type, and it is believed to have
undergone one or more recent bursts of star formation (within ∼108years ago), although
spectral synthesis modeling suggests that most or all of the stars are relatively young (<5×109
yrs) (Pritchet 1979; Rocca-Volmerange & Guiderdoni 1987; Deharveng et al. 1997). Note
that unlike most other early-type galaxies, it is possible that NGC 5102 has a significant
HMXB population because of the recent star formation. In order to better understand the

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link between the XRB populations, the GC population, and the star formation history of
the stellar population, we have analyzed archival HST data to search for GCs and made a
spectral stellar synthesis study of the optical/UV spectra of the galaxy to constrain the star
formation history.
The purpose of this observation was two-fold. The primary goal was to resolve the XRB
population from the hot gas and quantify the nature of the X-ray deficit in this unusually
blue galaxy. Because of the proximity of the galaxy and the sensitivity of the CXO, we
have been able to detect point sources below LX∼1036ergs s−1, much deeper than most
Chandra observations of more massive early-type galaxies at the distance of the Virgo cluster.
We wanted to compare the XRB population of this galaxy with other early-type galaxies
observed with Chandra. Naively, one would expect that the recent bursts of star formation
might enhance the XRB population relative to older ellipticals due to the presence of the
additional population of HMXBs. On the contrary, there are surprisingly few XRBs. A
secondary goal of this observation was to resolve any hot gas that may be present in the
galaxy from the XRB population, and to determine its thermodynamic state.
This paper is organized as follows. Section 2 contains a brief discussion of the observation
and data preparation. The X-ray point source (XPS) population and its luminosity function
are presented in section 3. Spectral and spatial analysis of the hot ISM is described in section
4. A discussion of the significance of the lack of XRBs, particularly as it relates both to
the stellar ages and star formation history as derived from optical/UV spectral synthesis
and to the GC population, is presented in section 5. We conclude with a brief discussion
and implications of our results in section 6. We assume a distance to NGC 5102 of 3.1 Mpc
throughout this paper (McMillan, Ciardullo, & Jacoby 1995), and J2000 coordinates are
used in all figures and tables.
2. Observation Log
The lenticular galaxy NGC 5102 was observed by the Chandra X-ray Observatory for
34217 s with the ACIS-S detector in very faint (VF) mode. A summary of the optical
properties of NGC 5102 and the Chandra observing log are contained in Table 1. Very faint
mode event filtering was applied to the data to reduce the background to the lowest possible
levels. Hot columns, events near node boundaries, short term transients, and the streaks on
the S4 chip were also removed. The background above 2 keV was examined for periods of
flaring and none were found. The level of the background in the 2-10 keV band agrees well
with the nominal blank-sky ACIS-S background. A 2MASS J-band image with the FOV of
the ACIS S2, S3, and S4 CCDs overlaid is shown in Figure 1.

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3.X-ray Point Sources
Individual point sources were detected with the CIAO program wavdetect in the 0.5-
5.0 keV band. The detection sensitivity was chosen so that there is approximately one
false detection for every million pixels. The source list generated by wavdetect was visually
compared with a raw image to confirm that every source it detected was indeed a source. We
likewise inspected the raw image to ensure that all sources visible by eye were detected by
the program. We detected 55 sources within the intersection of a 12.2′×13.0′box centered
on the galaxy and the ACIS-S FOV (see Figure 1). Based on the sensitivity, we expect one
of these to be a false source. To convert the observed count rate to flux, we have assumed
a 5 keV thermal bremsstrahlung spectrum with galactic (NH=5×1020cm−2) absorption, a
spectrum typical of LMXBs. For this spectrum, a count rate of 10−3cts s−1in the 0.5-5 keV
band corresponds to a flux (unabsorbed) of 7.04×10−15ergs cm−2s−1in the same band. We
define the boundary of the galaxy to be the D25isophote (de Vaucouleurs et al. 1991). The
major and minor effective diameters at this isophote are 8.7′and 2.8′, respectively, and the
apparent diameter of the effective aperture is 0.79′. Assuming that the light is distributed
according to the de Vaucouleurs R1/4law, this ellipse contains approximately 93% of the
light. The positions, count rates and uncertainties, and X-ray luminosities (if the source is
located within the D25boundary) of the sources are shown in Table 2. The D25isophote is
contained entirely within the S3 chip. Several of the sources outside this boundary appear
to be coincident with foreground stars in 2MASS and DSS images. We note that several of
the sources (labeled with a dagger in Table 2) were detected on the S2 and S4 chips. For
these sources (assuming the same spectrum and correcting for vignetting), a count rate of
10−3cts s−1in the 0.5-5 keV band corresponds to a flux (unabsorbed) of 1.06×10−14ergs
cm−2s−1in the same band.
Only eleven X-ray point sources (XPSs) are contained within the D25boundary of the
galaxy (all on the S3 chip), and only two sources with LX ≥1037ergs s−1in the 0.5-5.0
keV band. A 2MASS J band image with the positions of all of the sources overplotted is
shown in Figure 2. The positions of HII regions and planetary nebulae (PN) have also been
overplotted for visual comparison (McMillan, Ciardullo, & Jacoby 1995). There are no
co-incidences between the XPSs and any of the HII regions or PN. Two of the XPSs are
located near the optical center of the galaxy. One of the sources is one of the two sources
with LX>1037ergs s−1, and the other, less luminous source is coincident with the optical
center/nucleus. It is possible that this second source is a low-luminosity AGN (LLAGN),
although no other evidence of nuclear activity has been reported in the literature. There
is a group of sources approximately 3.5′north of the center of the galaxy, but whether this
group is related to NGC 5102 is not known.