XMM-Newton observations of the starburst merger galaxies NGC 3256 & NGC 3310

Article (PDF Available)inMonthly Notices of the Royal Astronomical Society 352(4) · May 2004with40 Reads
DOI: 10.1111/j.1365-2966.2004.08025.x · Source: arXiv
Abstract
We present XMM–Newton EPIC observations of the two nearby starburst merger galaxies NGC 3256 and NGC 3310. The broad-band (0.3–10 keV) integrated X-ray emission from both galaxies shows evidence of multiphase thermal plasmas plus an underlying hard non-thermal power-law continuum. NGC 3256 is well fitted with a model comprising two mekal components (kT= 0.6/0.9 keV) plus a hard power law (Γ= 2), while NGC 3310 has cooler mekal components (kT= 0.3/0.6 keV) and a harder power-law tail (Γ= 1.8). Chandra observations of both galaxies reveal the presence of numerous discrete sources embedded in the diffuse emission, which dominate the emission above ∼2 keV and are likely to be the source of the power-law emission. The thermal components show a trend of increasing absorption with higher temperature, suggesting that the hottest plasmas arise from supernova-heated gas within the discs of the galaxies, while the cooler components arise from outflowing galactic winds interacting with the ambient interstellar medium. We find no strong evidence for an active galactic nucleus in either galaxy.
arXiv:astro-ph/0405285v1 14 May 2004
Mon. Not. R. Astron. Soc. 000, 1–13 (2003) Printed 2 February 2008 (MN L
A
T
E
X style file v2.2)
XMM-Newton observations of the starburst merger galaxies
NGC 3256 & NGC 3310
L.P. Jenkins
1
, T.P. Roberts
1
, M.J. Ward
1
, A. Zezas
2
1
X-ray & Observational Astronomy Group, Dept. of Physics & Astronomy, University of Leicester, University Road, Leicester LE1 7RH, U.K.
2
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
Accepted 2004 May 14; Received in original form 2003 December 18
ABSTRACT
We present XMM-Newton EPIC observations of the two nearby starburst merger
galaxies NGC 3256 & NGC 3310. The broad-band (0.3–10 keV) integrated X-ray emis-
sion from both galaxies shows evidence of multi-phase thermal plasmas plus an un-
derlying hard non-thermal power-law continuum. NGC 3256 is well-fit with a model
comprising two MEKAL components (kT =0.6/0.9 keV) plus a hard power-law (Γ=2),
while NGC 3310 has cooler MEKAL components (kT =0.3/0.6 keV) and a harder
power-law tail (Γ=1.8). Chandra observations of these galaxies both reveal the pres-
ence of numerous discrete sources embedded in the diffuse emission, which dominate
the emission above 2 keV and are likely to be the source of the power-law emission.
The therma l comp onents s how a trend of increasing absorption with higher tempera-
ture, suggesting that the hottest plasmas arise from supernova-heated gas within the
disks of the galaxies, w hile the cooler components arise from outflowing galactic winds
interacting with the ambient interstellar medium (ISM). We find no strong evidence
for an active g alactic nucleus (AGN) in either galaxy.
Key words:
galaxies: individual (NGC 3256 & NGC 3310) galaxies: starburst galaxies: ISM
X-rays: galaxies X-rays: binaries
1 INTRODUCTION
By virtue of their greatly enhanced star formation rates,
starburst galaxies (SBGs) are very powerful sources of X-
ray emission, and consequently exhibit complex spectral
characteristics reflecting the variety of energetic phenom-
ena related to the end points of stellar evolution. Starburst
episodes are transitory events in th e lifetimes of galaxies,
with enh anced SFRs t hat are unsustainable over periods
greater than 10
8
yr (e.g. Heckman 1998). Galaxies go-
ing through starburst phases appear to be a common phe-
nomena in the early Universe (e.g. Smail, Ivison, & Blain
1997), and local SBGs show characteristics similar to high-
redshift examples. Therefore, our understanding of the X-
ray emission processes in local systems is of fundamental
importance. For example, SBGs are believed to be the prime
source of “stellar feedback”, whereby supernova-driven out -
flows (winds) enrich both the galaxy’s interstellar medium
(ISM) and the intergalactic medium (IGM)(Strickland et al.
2000). The similarity of local and high-redshift examples
E-mail: lej@star.le.ac.uk
is further highlighted by the detection of large-scale galac-
tic winds in UV observations of star-forming Lyman break
galaxies at z 3 (Pettini et al. 2000; Pettini et al. 2001).
Mergers of galaxies appear to trigger the most ex-
treme starburst episodes. Merger systems tend to h ave ex-
tremely high infrared (IR) luminosities ( luminous IR galax-
ies (L IRG): L
IR
> 10
11
L
and ultraluminous I R galaxies
(ULIRG): L
IR
> 10
12
L
, e.g. Sanders & Mirabel 1996), the
brightest of which can even overlap the bolometric lumi-
nosities of quasars and Seyfert galaxies (L´ıpari et al. 2000;
Lira et al. 2002 and references therein). Many luminous IR
galaxies are detected in deep-IR surveys, showing that t hey
are an important constituent of the high-redshift ( z
>
1)
Universe (e.g. Franceschini et al. 2001). In th e hierarchical
merger hypothesis (e.g. Toomre 1977), collisions and merg-
ers of galaxies are thought to be one of the most domi-
nant evolutionary mechanisms for galaxies, whereby isolated
disk galaxies merge through close tidal encounters to even-
tually form elliptical galaxies (Kormendy & Sand ers 1992;
Read & Ponman 1998; Genzel et al. 2001). X-ray observa-
tions of nearby systems with obvious signatures of merg-
ing at other wavelengths provide a window through which
c
2003 RAS
2 L. Jenkins et al.
to search for the high-energy signatures of their primary
sources of power i.e. starburst activity or the presence of an
active galactic nucleus (AGN).
Prior t o the XMM-Newton and Chandra era, X-ray mis-
sion such as Einstein, ROSAT and ASCA revealed the sig-
natures of the various processes present in the closest lo-
cal SBGs. For example, ROSAT X-ray studies of the soft
(0.1–2.4 keV) emission in the two archetypal SBGs M82 and
NGC 253 showed that both had substantial extended dif-
fuse emission from the disks and halo regions that were
well modelled with thermal plasmas with kT
<
1 keV
(Strickland, Ponman, & Stevens 1997; Pietsch et al. 2000),
whereas broader band (0.5–10 keV) ASCA studies of t hese
and other SBGs showed that their spectra could be de-
scribed by models comprising one- or two-temperature ther-
mal plasma components (kT
<
1 keV) plus a harder compo-
nent fitted with either high-t emperature plasma or a non-
thermal power-law (e.g., M82 & NGC 253, Ptak et al. 1997;
NGC 3310 & NGC 3690, Zezas, Georgantopoulos & Ward
1998; NGC 3256, Moran, Lehnert & Helfand 1999).
However, subsequent to the launch of the XMM-
Newton and Chandra X-ray observatories, we are now able
to utilize their excellent complimentary spectral and imag-
ing capabilites to study the detailed spatial and spectral
characteristics of these X-ray emission processes. The sub-
arcsecond sp atial resolution of Chandra can resolve indi-
vidual compact X-ray sources and accurately determine
their positions, whereas the high throughput of XMM-
Newton can yield detailed spectral information of bright
point sources and low-surface-brightness extended emis-
sion. For example, XMM-Newton observations have given
an unprecedented wide-field view of the spectral proper-
ties of the diffuse emission in the nucleus, disk and halo
in NGC 253 (Pietsch et al. 2001), while Chandra observa-
tions gave detailed images of point sources and diffuse emis-
sion in the central regions of the galaxy (Strickland et al.
2000; Strickland et al. 2002). S imilar work h as also been
done with M82 (Stevens, Read, & Bravo-Guerrero 2003;
Matsumoto et al. 2001).
The main X-ray spectral components of SBGs have re-
cently been quantified by Persic & Rephaeli (2002), based
on the observed spectral properties of M82 & NGC 253
as derived from BeppoSAX observations and a stellar-
population evolutionary model. They assessed contributions
from the gaseous and stellar X -ray processes expected in
SBGs, i.e. from X-ray binaries (XRBs), supernova remants
(SNRs), Compton scattering of ambient far-infrared (FIR)
photons off supernova-accelerated relativistic electrons, dif-
fuse thermal plasma and a compact nucleus in the form of
a starburst or AGN. Their results show that at low ener-
gies (
<
2 keV), t he dominant component is expected to be
a low temperature (kT 6 1 keV) diffuse plasma resulting in
part from the galactic wind itself, but mainly from shock
heating via the interaction of the hot low-density wind with
the ambient h igh-density ISM ( Strickland & Stevens 2000).
At higher energies (2–10 keV), they predict t hat the spec-
trum will be dominated by emission from bright neutron star
XRBs (high- and low-mass secondaries) in the form of a cut-
off power- law with a possible contribution from non-thermal
Compton emission or an AGN if present (Persic & Rephaeli
2002).
However, recent studies of SBGs with XMM-
Newton and Chandra have shown that, in the absence of
a bright AGN, the X-ray output of starforming galax-
ies are dominated (particularly in the hard 2–10 keV
band) by small numbers of bright extra-nuclear point
sources (e.g. Kilgard et al. 2002; Colbert et al. 2004), which
show increasing evidence of being XRB systems, pos-
sessing black h ole rather than neutron star primaries
(e.g. Fabbiano & White 2004 ; Miller & Colbert 2004). The
brightest of these discrete X-ray sources are the ultralu-
minous X-ray sources (ULXs), defined as those possessing
X-ray luminosities > 10
39
erg s
1
, which exceeds the Ed-
dington limit for accretion onto a 1.4M
neutron star (see
section 5.2 for further discussion).
In this paper, we present XMM-Newton X-ray observa-
tions of the two nearby starburst merger systems NGC 3256
& NGC 3310, together with complementary published Chan-
dra data for NGC 3256 and new Chandra results for
NGC 3310 (Zezas et al. in preparation) to aid our inter-
pretation of the spectral information from XMM-Newton.
This paper is set out as follows. In sections 2.1 and 2.2,
we give a summary of the known properties of each galaxy
based on multiwavelength studies. In section 3, we outline
the XMM-Newton data reduction methods used, th en in sec-
tion 4 we present the detailed results of the spectral fitting
procedures and compare these with previous results from
ROSAT, ASCA and Chandra observations. We discuss our
results in section 5 in the context of other recent results
from studies with XMM-Newton and Chandra, and discuss
how these compare as a whole to the starburst spectral tem-
plate of Persic & Rephaeli (2002). Our conclusions are sum-
marised in section 6. The distances we adopt in this paper
are taken from S anders et al. (2003), which assume a value
of H
0
=75 km s
1
Mpc
1
.
2 THE GALAXIES
2.1 NGC 3256
NGC 3256 is a starburst merger system located at a dis-
tance of D=35.4 Mp c. It is IR-bright (L
F IR
= 2.7 ×
10
11
L
, Sanders et al. 2003), although it does not reach the
ULIRG category. Its starburst nature has been confirmed
at UV (Kinney et al. 1993), optical (L´ıpari et al. 2000), IR
(Graham et al. 1984; Doyon et al. 1994; Moorwood & Oliva
1994) and radio wavelengths (Norris & Forbes 1995). It
shows a highly disturbed structure with prominent extended
tidal tails spanning 80 kpc, which are believed to be the sig-
nature of a merger between two gas-rich galaxies of roughly
equal size (Toomre & Toomre 1972; English et al. 2003).
This merger is thought to be relatively young (5 × 10
8
yr,
L´ıpari et al. 2000 ), because within the highly disturb ed cen-
tral region two distinct nuclei have been resolved at ra-
dio (Norris & Forbes 1995), IR (Kotilainen et al. 1996) and
optical (L´ıpari et al. 2000) wavelengths. The northern nu-
cleus has been identified as a pu re starburst with a pow-
erful ouflowing galactic wind (L´ıpari et al. 2000). Despite
previous speculation based on near-I R and radio observa-
tions that the southern nucleus may host an AGN (e.g.
Kotilainen et al. 1996; Norris & Forbes 1995), this seems in-
creasingly unlikely as no unambiguous evidence for AGN
activity was found in the recent Chandra X-ray observa-
tion which resolves both nuclei (Lira et al. 2002), although
c
2003 RAS, MNRAS 000, 1–13
XMM-Newton observations of NGC 3256 & NGC 3310 3
Table 1. Details of the XMM-Newton observations.
Target Observation ID Date Useful exposure (s) Filter Source count rate (ct s
1
, 0.3–10 keV)
(yyyy-mm-dd) PN MOS PN MOS
NGC 3256 0112810201 2001-12-15 6862 11370 Thin 0.55 0.16
NGC 3310 0112810301 2001-05-11 6728 8764 Thin 0.67 0.19
a recent radio study by Neff, Ulvestad, & Campion (2003)
detected strong radio emission from both nuclear sources
as well an ULX and show that both nuclei have proper-
ties consistent with radio-loud LLAGN. The age of the star-
burst component in this galaxy is 5 25 × 10
6
yr, which
means that it must have started long after the merger be-
gan (L´ıpari et al. 2000). This galaxy was also observed with
earlier X-ray missions, i.e. ROSAT (Boller et al. 1992) and
ASCA (Moran et al. 1999), and has been found to have a
high X-ray lum inosity (observed L
X
2 × 10
41
erg s
1
in
the 0.5-10 keV band, Moran et al. 1999).
2.2 NGC 3310
NGC 3310 is a closer but smaller system than NGC 3256,
located at a distance of 19.8 Mpc and classified as
type SAB(r)bc pec (de Vaucouleurs et al. 1991). I t has a
disturbed structure, and the starburst (aged 10
7
10
8
yrs, Elmegreen et al. 2002 and references therein) is
thought to have been triggered by a collision between
NGC 3310 and a dwarf companion during the last 10
8
yr
(Balick & Heckman 1981). Although no double nucleus has
been resolved in NGC 3310, further evidence of a merger
comes from kinematic peculiarities in that t he centre of ro-
tation of the emission line gas does not coincide with its nu-
cleus (van der Kruit & de Bruyn 1976; Balick & Heckman
1981). One of the most striking features of th e optical
emission is a prominent “bow & arrow” structure extending
100 arcseconds northwest of the galaxy’s centre, which is
believed to be a remnant of the merger (Balick & Heckman
1981). Its FIR luminosity (L
F IR
= 2.8 × 10
10
L
,
Sanders et al. 2003 ) is comparable to that of M82, al-
though a factor of ten less luminous than NGC 3256.
NGC 3310 has been studied in numerous multiwave-
length observations, including optical (Balick & Heckman
1981; Pastoriza et al. 1993; Mulder & van Driel
1996), ultraviolet (UV) (van der Kruit & de Bruyn
1976; Meurer et al. 1995 ; Smith et al. 1996), IR
(Telesco & Gatley 1984; Pastoriza et al. 1993), Hα
(Mulder & van Driel 1996; Conselice et al. 2000) and
radio (Balick & Heckman 1981; van der Kruit & de Bruyn
1976; Mulder, van Driel, & Braine 1995; Kregel & Sancisi
2001). Early optical and Hα images showed that the bright
inner regions are dominated by an open spiral arm pattern
(e.g. Balick & Heckman 1981), and the inner part of this
region connects to a 30 arcsecond ( 2.9 kpc) diameter
starburst ring, which in turn surrounds a blue compact
nucleus (Kregel & Sancisi 2001). Optical and near-IR spec-
trophotometric observations have shown that the nucleus
has solar abundances, whereas the circumnuclear and disk
HII regions show a lower metallicity (Pastoriza et al. 1993).
There is a “jumbo” HII region 12 arcseconds southwest of
the nucleus, whose size and Hα luminosity is comparable to
the largest extragalactic HII regions known (e.g. NGC 5471
in M101, Balick & Heckman 1981). Most recently, high-
resolution Hubble Space Telescope (HST) optical (WFPC2)
and IR (NICMOS) observations of NGC 3310 show the
presence of numerous super star clusters in the innermost
southern spiral arm and circumnuclear ring whose positions
correlate with radio and Hα peaks, with ages consistent
with their formation resulting from the cannibalism of a
dwarf galaxy in the last 10 Myr ( Elmegreen et al. 2002). It is
important to note that there is no evidence of AGN activity
in this system at any wavelength. For example, no broad
Hα line is detected by Ho, Filippenko & Sargent (1997),
which would be expected from an AGN with the hard
(2–10 keV) X-ray luminosity measured from ROSAT and
ASCA observations assuming the L
X
/L
Hα
relation of
Elvis, Soltan & Keel (1984) (c.f. Zezas et al. 1998).
3 OBSERVATIONS, DATA REDUCTION &
ANALYSIS TECHNIQUES
3.1 XMM-Newton Observations & Reduction
The details of the XMM-Newton observations are shown in
Table 1. NGC 3256 was observed for 17 ks on the 15th Dec,
2001; NGC 3310 for 19 ks on the 11th May, 2001. During
both observations the EPIC MOS-1, MOS-2 & PN cam-
eras were operated with thin filters in “Prime Full Win-
dow mode. The event lists were pipeline-processed using
the SAS (Science Analysis Software) v5.2.1 (NGC 3256) and
v5.1.0 (NGC 3310) and all data products (spectra, images
& lightcurves) were created using SAS v5.4.1 and the EPIC
calibration as of March 2003. Full-field light curves were ac-
cumulated for all exposures in the two observations to check
for high background intervals of soft proton ares. In the
case of NGC 3256, there were numerous small flares through-
out the observation and a large section of high level flaring
for t he last 3 ks. Time intervals with count rates > 50 ct
s
1
(PN) and 15 ct s
1
(MOS) were cut from subsequent
data analysis, leaving a net good time for each camera of
11 ks (MOS 1 & MOS 2) and 7 ks (PN). For NGC 3310,
there was a high level of flaring throughout almost the en-
tire observation. In order to extract any useful information
from the data, we accepted time intervals with relatively
high background count rates (< 140 ct s
1
[PN] & < 30 ct
s
1
[MOS]), providing us with relatively clean data below
6 keV. This left net good times of 9 ks (MOS 1 & MOS 2)
and 7 ks (PN).
3.2 XMM-Newton Data Analysis
The X-ray spectra of NGC 3256 & NGC 3310 were ex-
tracted using circular regions of radii 45 and 40 arcseconds
respectively. The background regions were taken close to
c
2003 RAS, MNRAS 000, 1–13
4 L. Jenkins et al.
Figure 1. [ Top] XMM-Newton EPIC soft (0.3–2 keV, left) & hard (2–10 keV, right) X-r ay images of NGC 3256. The images are the sum
of all 3 detectors, and ar e convolved wi th a 1 pixel (4 arcsecond) HWHM 2-D Gaussian mask and displayed with a logarithmic scale.
The intensity minima/maxima correspond to surface brightnesses of 3 × 10
5
/3 × 10
3
ct s
1
(soft) and 3 × 10
5
/8 × 10
4
ct s
1
(hard), and the contours correspond to 2 × 10
4
/ 1 × 10
3
ct s
1
(soft) and 8 × 10
5
/5 × 10
4
ct s
1
(hard). [Lower left] Chandra (0.3–
8 keV) linearly scaled image of the (dot-dashed) boxed region shown in the XMM-Newton image, adaptively smoothed wi th a 3σ local
significance threshold. [Lower right] Chandra X-ray contours corresponding to surface brightnesses of [0.4,0.7,1.1,1.8,2.1,3.2,4.3]×10
4
ct s
1
overlaid on an XMM-Newton OM UVW2 (λ
central
=2120
˚
A) image of NGC 3256.
each source, using as large an area as possible with approx-
imately the same DET-Y d istance as the source region (to
ensure similar low-energy noise subtraction). We used th e
SAS task ESPECGET with standard filtering to simultan-
iously extract source and background spectra as well as cre-
ate response matrices (RMFs) and ancillary response files
(ARFs) for each source. The resulting spectra were binned
to a minimum of 20 counts per bin in order to optimise the
data for χ
2
statistics.
The spectral analysis of the XMM-Newton data in this
study has been performed using XSPEC 11.1/11.3. All errors
are given at the 90 percent confidence level unless stated
otherwise. For NGC 3256, the MOS and PN spectra have
been fitted simultaneously in the 0.3-10 keV band, whereas
we have only fitted the NGC 3310 data in the 0.3–6 keV
range as the high-energy data were heavily contaminated
by soft proton aring. We have also included a free normal-
isation constant in the models to account for differences in
the flux calibration of the three EPIC cameras, which differ
by
<
15 percent in practise.
Following previous work in this fi eld, the two spectral
models we use to fit the data are a power-law continuum
to represent the combined emission from compact sources,
and an optically-thin t hermal plasma (MEKAL) to model
the thermal components expected in SBGs such as galactic
winds and SNRs. For this study we have chosen to x the
abundance parameter in the MEKAL models to solar val-
ues, as previous results with ROSAT and ASCA data have
shown that tting complicated multi-temperature thermal
plasmas with this type of simple spectral model tends to re-
c
2003 RAS, MNRAS 000, 1–13
XMM-Newton observations of NGC 3256 & NGC 3310 5
Table 2. Spectral fitting results for NGC 3256.
PL M
1
M
2
χ
2
/dof F
X
b
L
X
c
N
H
a
Γ N
H
a
kT (keV ) N
H
a
kT (keV ) Obs Unabs
Model 1: M+PL (wabs*po+wabs*mekal)
3.06
+0.25
0.24
2.70
+0.13
0.13
0.64
+0.02
0.02
- - 387.1/300 9.71
+0.45
0.56
1.46
+0.07
0.08
3.78
+0.10
0.44
Model 2: M+M+PL* (wabs*po+wabs*mekal+wabs*mekal)
7.66
+1.08
0.95
2.31
+0.27
0.48
Gal 0.35
+0.04
0.02
0.73
+0.05
0.04
355.7/298 9.91
+0.42
1.08
1.49
+0.06
0.16
5.74
+0.06
0.92
Model 3: M+M+PL* (wabs*po+wabs*mekal+wabs*mekal)
1.47
+1.05
0.44
1.98
+0.41
0.40
Gal 0.57
+0.05
0.11
9.47
+1.53
1.24
0.85
+0.08
0.10
343.3/297 10.00
+0.53
1.59
1.50
+0.08
0.24
4.53
+0.26
1.17
Notes: Spectral models: PL=power-law continuum model and M=MEKAL thermal plasma (solar abundances).
a
Absorption
column in units of 10
21
cm
2
.
b
Observed fluxes in the 0.3–10 keV band, in units of 10
13
erg s
1
cm
2
.
c
Observed and
unabsorbed luminosities i n the 0.3–10 keV band, in units of 10
41
erg s
1
(assuming a distance of 35.4 Mpc). Same hydrogen column
as applied to the power-law spectral component.
Mo dels with the cool MEKAL N
H
component fixed at the Galactic value. Model
parameter errors correspond to 90 percent confidence limits for 1 parameter of interest. The best-fit model is shown in bold.
sult in un realistically low metal abundances of 0.05–0.3Z
(Strickland & Stevens 2000 and references therein).
4 RESULTS
4.1 NGC 3256
In Figure 1 we show XMM-Newton EPIC images of
NGC 3256 in soft (0.3–2 keV, top left) and hard ( 2–10 keV,
top right) bands, plus an adaptively smoothed archival
Chandra image of the central region [lower left]. No X-ray
structure is resolved in the main body of the galaxy in the
XMM-Newton data, but one discrete source is detected to
the southeast ( labelled XMM-1, see section 4.1.2). How-
ever, the Chandra images do resolve the bulk of t he emis-
sion into several discrete sources embedded within diffuse
emission. The results of the Chandra analysis are presented
in Lira et al. (2002), where it is shown that the integrated
emission from the galaxy is best fit with a model comprising
two MEKAL components plus a power-law continuum. This
study shows that the hard power-law component with an
index of Γ 2 is mainly (75–80 percent) due to the four-
teen compact discrete sources (including the two galactic
nuclei), which contribute
>
20 percent of the total emis-
sion of the galaxy in the 0.5–10 keV range. Lira et al. (2002)
specu late that the remainder of the hard emission is likely
to come from unresolved XRBs and supern ovae distributed
throughout the galaxy, and that one of the thermal compo-
nents (0.6 keV) is the signature of the hot superwind, while
the other (0.9 keV) arises from SNR heated gas within a
much smaller volume near the starburst nucleus. Figure 1
[lower right] shows the Chandra X-ray contours overlaid
on an XMM-Newton Optical Monitor (OM) UV image of
NGC 3256 of the same scale. This illustrates that the UV
emission, which typically arises in starforming regions, cor-
relates well with the Chandra X-ray data, although a sec-
tion of the UV emission is obscured by the presence a large
dust lane in the southwest of the galaxy (see Figure 10 in
Lira et al. 2002).
Since we do not spatially resolve any X-ray structure
in the centre of N GC 3256 with XMM, we can only fit the
spectrum of th e integrated emission. We began by fitting
the d ata with single- and two-component spectral models
with combinations of power-law and MEKAL components,
but we rejected these due to unacceptable values of chi-
squared (χ
2
ν
> 2), although a M+PL model with equal
absorbing columns did yield a reasonable fit as shown in
Table 2. In order to improve th is, we went on to t the data
with three-component models (M+M+PL) to better repre-
sent the multi-temperature gas expected in this system. We
also included separate absorbing N
H
columns for each of t he
three components, but fixed the absorption in the coolest
MEKAL component to the Galactic value (9.5×10
20
cm
2
,
Dickey & Lockman 1990) as fitting this parameter resulted
in N
H
values below the Galactic value. Following the work
of Lira et al. (2002), we tried a model where both absorbing
N
H
components were fitted independently, and one where
they were tied together. The results are shown in Table 2:
the best-fit model ( sh own in bold) is the one with inde-
pendent absorbing columns with kT =0.57/0.85 keV, Γ=1.98
and χ
2
ν
= 1.16, which is a > 99.9 percent improvement over
the M+PL model according to the F-test statistic. The PN
and MOS spectra are shown with the best-fit model in Fig-
ure 2, and Table 3 lists the observed fluxes, unabsorbed lu-
minosities and overall percentage contributions to the total
of each from the three spectral components in the XMM-
Newton fit.
The errors on the fit parameters in Table 2 corre-
spond to 90% confidence limits for 1 parameter of inter-
est (∆χ
2
=2.71). However, the temperatures and absorption
values for multiple thermal components in models such as
these are strongly correlated. The absorption column as-
sociated with the cool thermal component is fixed in the
best-fit model, but more realistic errors for the warm com-
ponent i.e. for 2 parameters of interest for a 90% con-
c
2003 RAS, MNRAS 000, 1–13
6 L. Jenkins et al.
Table 3. Fluxes, luminosities and percentage of total emission
from the three components of the best-fit M+M+PL model of
NGC 3256.
Component F
X
% of total L
X
% of total
Cool MEKAL 2.50 25 0.53 12
Warm MEKAL 3.35 33 3.16 70
Power-Law 4.15 42 0.84 18
Total 10.00 100 4.53 100
Notes: Observed fluxes in the 0.3–10 keV band, in units of
10
13
erg s
1
cm
2
. Unabsorbed luminosities in the
0.3–10 keV band, in units of 10
41
erg s
1
(assuming a
distance of 35.4 Mpc).
fidence limit ( ∆χ
2
=4.61) are practically unchanged with
N
H
= 8.011.4×10
21
cm
2
and kT =0.74–0.95 keV, demon-
strating that these parameters are well constrained.
The PN data also show possible line emission at 6–
7 keV, which could be a result of either neutral 6.4 keV Fe-K
emission from an AGN or helium-like Fe 6.7 keV emission
from a population of type Ib/IIa SNRs residing in regions
of star-formation ( e.g. Behar et al. 2001). We have t herefore
fitted a narrow gaussian line to the power-law continuum of
the best-fit model, which results in an unconstrained line
energy of 6.5 keV with an equivalent width of 250 eV.
Although the addition of the gaussian component only im-
proves the fit statistics at the 1σ level, t he 90 percent upper
limit of the line normalisation allows us to constrain an up-
per limit of 1045 eV for the equivalent width of the line. We
will discuss the implications of this possible line emission in
section 5.
4.1.1 Comparison with previous work
Reassuringly, the XMM-Newton results are remarkably sim-
ilar to the results of the 30 ks Chandra observation reported
by Lira et al. (2002), where the best-fit model for the inte-
grated emission from NGC 3256 is also a M+M+PL mo del
with kT =0.58/0.92 keV and Γ=1.99. In addition, the ab-
sorbing hydrogen columns are consistent between the two
datasets within the 90 percent confidence limits. However,
the observed flux of the Chandra model in the 0.5–10 keV
range ( 1.2 × 10
12
erg cm
2
s
1
) is 20 percent higher
than the equivalent XMM-Newton model in the same energy
range ( 1.0 × 10
12
erg cm
2
s
1
). In order to try to un-
derstand this difference, we have compared the relative con-
tributions from the three model components in the XMM-
Newton data (Table 3) with those found in the Chandra data
based on the model p arameters and component normalisa-
tions quoted in Table 7 of Lira et al. (2002). This compar-
ison shows that, while the summed contributions from the
two thermal components agree to within 10 percent, there
is a much lower contribution from the h ard power-law com-
ponent in the XMM-Newton observation (65 percent of
the Chandra flux). This is not an aperture affect, as the
XMM-Newton and Chandra extraction regions are virtually
the same size (45 and 40 arcsecond radii respectively). It is
much more likely that the difference in t he power-law flux is
a real variation in the summed flux of the ULX population. If
the brightest compact X-ray source (the northern nucleus,
Figure 2. XMM-Newton spectra and folded best-fit m odel
(M+M+PL) of NGC 3256: PN [top], MOS 1 [middle], MOS 2
[bottom]. The model components are denoted by dashed (power-
law), dotted (cool MEKAL) & dot–dashed (warm MEKAL) lines.
see Table 3 in Lira et al. 2002) were to “switch off”, this
would result in a 21 percent d rop in the power-law flux,
which would explain the majority of this difference. The al-
ternative is a situation in which there is a coincidental drop
from several sources, although this is much less likely.
The results of the fits to the ASCA data for NGC 3256
rep orted by Moran et al. (1999) showed similar results
and model parameters, with the best-fit model compris-
ing two Raymond-Smith (RS) plasma components p lus a
hard power-law (kT =0.29/0.80 keV, Γ=1.68). Even though
the relative contributions of the ASCA model components
differ from both the Chandra and XMM-Newton results,
the overall observed ux in the 0.5–10 keV range ( 1.3 ×
10
12
erg cm
2
s
1
) agrees with the observed Chandra flux
c
2003 RAS, MNRAS 000, 1–13
XMM-Newton observations of NGC 3256 & NGC 3310 7
to within 5 percent (Lira et al. 2002), though it is 30
percent higher than the XMM-Newton flux.
4.1.2 Discrete Source Properties
The discrete source in the XMM-Newton data (named
XMM-1) at 10
h
27
m
55.3
s
, 43
54
46
′′
is coincident with
source 14 in the Chandra data (Lira et al. 2002). Since this
source does not have sufficient counts for spectral fitting, we
have computed its hardness ratios using the PN counts in
three different energy bands using a similar method to t hat
described in Lira et al. (2002) in order t o compare its prop-
erties in both observations. We compute log(0.3–1 keV)/(1–
2 keV)=-0.26 and log(2–7 keV)/(1–2 keV)=-0.32, which, in
terms of the XMM-Newton instrument response, is equiva-
lent to a photon index of Γ 2.5 and an absorbing hydrogen
column of N
H
5 × 10
21
cm
2
. This is softer than source
14 in the Chandra data, which shows Γ 2 (see Figure
4 in Lira et al. 2002), although the absorbing columns are
similarly high.
If we assume a power-law photon index of Γ=2.5 and
adopt an absorbing column of N
H
5× 10
21
cm
2
(as mea-
sured from the both the XMM-Newton and Chandra hard-
ness ratios), we get an observed flux in the 0.5–10 keV range
of F
X
2.5 × 10
14
erg s
1
cm
2
and an u nabsorbed lu-
minosity of L
X
7 × 10
39
erg s
1
, within 20 percent of
the Chandra luminosity for the same parameters. This lumi-
nosity p laces this source into the ULX regime, although this
value is highly dependent on the large absorption correction
estimated from the hardness ratios alone.
Lira et al. (2002) speculate that the luminous X-ray
sources in NGC 3256 are likely to be high-mass XRBs
(HMXBs). They detect no short-term variability in the
Chandra data expected from an accreting source, although
variability on short timescales is not generally detected in
Chandra observations of ULXs (c.f. Roberts et al. 2004).
The position is coincident with an outer spiral arm of the
galaxy and offset by 4 arcseconds from an apparent fore-
ground star, but given that there is no evidence of either
significant long- or short-term variability, th is source cou ld
be an obscured b ackground AGN. U sing t he log(N)–log(S)
relation of Hasinger et al. (2001), we calculate that in an an-
nular region of 1.2 arcminute radius centred on NGC 3256,
excluding the 0.45 arcminute radius of the galaxy itself, we
expect to detect 0.02 background AGN at the 2–10 keV flux
of this source (F
X
1.6×10
14
erg s
1
cm
2
). This suggests
that the source is likely to be associated with NGC 3256.
4.2 NGC 3310
Figure 3 [top] shows the soft (0.3–2 keV, left) and hard (2–
6 keV, right) XMM-Newton EPIC images of NGC 3310. The
majority of th e X-ray emission comes from the bright cen-
tral region of the galaxy, corresponding to the nucleus, star-
burst ring, inner disk, and outer spiral arm. There are also
two symmetrically-positioned discrete sources of X-ray emis-
sion to the northwest and southeast of the nucleus (dis-
cussed below), which we denote as X-2 (south) following
Roberts & Warwick (2000), and X-3 (north) (although X-3
was not included in that paper due to an error in the origi-
nal data analysis). X-3 was also detected in the Zezas et al.
(1998) ROSAT study (J103843.2+533107), although X-2
was not.
A recent 30 ks Chandra observation of NGC 3310
(Zezas et al. in preparation) has revealed the presence of 24
discrete sources with X-ray luminosities ranging from 10
37
10
40
erg s
1
, most of which rather spectacularly trace the
central starburst ring (Figure 3 [lower left]). There is also ev-
idence in the Chandra images of extended diffuse emission.
Figure 3 [lower right] shows the Chandra X-ray contours
overlaid on an XMM-Newton OM UV image of NGC 3310
showing again, as in the case of NGC 3256, that the UV and
X-ray emission are well correlated.
We have fitted the MOS and PN spectra for the
main body of the galaxy ( excluding the two discrete
sources) following the same fitting procedures for the XMM-
Newton sp ectra as used for NGC 3256, albeit only in the
0.3–6 keV range due to t he high level of background flaring
in this observation (see section 3.2). The results are shown
in Table 4, with fluxes and luminosities extrapolated to the
0.3–10 keV band. For this galaxy we have also fitted the N
H
column associated with the coolest MEKAL component, as
leaving this parameter free does not result in unrealistic sub-
Galactic values as was the case for NGC 3256.
To begin with, we fitted the data u sing models where all
N
H
columns were left as free parameters. This metho d pro-
duced very good ts to the data with both M+PL (χ
2
ν
=0.89)
and M+M+PL (χ
2
ν
=0.87) models, with a statistical im-
provement at the 98.2 percent level for the addition of
the second thermal component. However, the fitted power-
law component had very little intrinsic column (3×10
20
cm
2
) compared to the thermal comp onents (5×10
21
and
4×10
22
cm
2
for the cool and warm components respec-
tively). Chandra observations of starforming galaxies have
shown that point sources typically have non-negligible ab-
sorption e.g. the NGC 3256 discrete sources have columns in
the range 10
21
to 10
22
cm
2
(see Table 3, Lira et al. 2002) .
Additionally, if we assume that the point sources are re-
lated to the same stellar population as the thermal com-
ponents, we would not expect to see such large differences
in their associated column densities. These models also im-
plied that at energies < 0.5 keV, the emission was dominated
by the power-law component from point sources, but this is
inconsistent with soft (0.3–2 keV) and h ard (2–8 keV) Chan-
dra images, which show a large contribution from apparently
diffuse emission at soft energies.
In order to achieve a more physically realistic fit, we
re-fitted the data with models where the power-law ab-
sorption comp onent was fixed. Firstly, we fix ed it to the
value obtained from the simple power-law plus absorption
model (N
H
=1.19×10
21
cm
2
), and fitted the remainder of
the spectrum with one and two thermal components (mod-
els 1 & 2 in Table 4). Both models gave good fits to the data,
with a > 99 percent improvement with the addition of the
second thermal component. The absorption values obtained
with these fits are much more realistic than our initial fits,
with N
H
values in the range of 0.4–3×10
21
cm
2
.
Secondly we tried a model in which the power-law ab-
sorption comprised one N
H
component fixed at 1×10
21
cm
2
, and a partial covering absorption component fixed
at 1×10
22
cm
2
with a covering fraction of 0.5, i.e. ef-
fectively some p ower-law emission experiencing moderate
columns and some experiencing higher absorption. This re-
c
2003 RAS, MNRAS 000, 1–13
8 L. Jenkins et al.
Figure 3. [Top] XMM-Newton EPIC soft (0.3–2 keV, left) & hard (2–6 keV, right) X-ray images of NGC 3310. The images are the
sum of all 3 detectors, and are convolved wi th a 1 pixel (4 arcsecond) HWHM 2-D Gaussian mask and displayed with a logarithmic
scale. The intensity minima/maxima correspond to surface brightnesses of 5 × 10
5
/4 × 10
3
ct s
1
(soft) and 9 × 10
5
/8 × 10
4
ct s
1
(hard), and the contours corr espond to 4 × 10
4
/ 1 × 10
3
ct s
1
(soft) and 3 × 10
4
/6 × 10
4
ct s
1
(hard). [Lower left]
Chandra (0.3–8 keV) linearly scaled image of the (dot-dashed) boxed region shown in the XMM-Newton image, adaptively smoothed
with a 3σ local significance threshold. [Lower right] Chandra X-ray contours corresponding to s urface brightnesses of [2,4,6,16]×10
5
ct
s
1
overlaid on an XMM-Newton OM UVW1 (λ
central
=2910
˚
A) image of NGC 3310.
sulted in improved ts (models 3 & 4 in Table 4). Again, the
absorption values measured with these fits are reasonable
with N
H
6 × 10
20
for t he cool MEKAL component and
N
H
2 × 10
21
for the warm MEKAL component. We there-
fore adopt this M+M+PL model (model 4, shown in bold in
Table 4) as the best-fit model for this data. In this case, if we
consider more realistic model errors i.e. 90% confidence lim-
its for 2 parameters of interest (∆χ
2
=4.61), the absorption
and temperature parameters for the thermal components are
slightly less well constrained than those of NGC 3256. While
we can only gain an upper limit to the N
H
component tied
to the warm thermal component (N
H
<
4 × 10
21
cm
2
),
the constraints on the temperature remain unchanged. The
parameters for the cool component are only slightly less well
constrained with N
H
= 0.3 3.3 × 10
21
cm
2
and kT =0.14–
0.30 keV.
The PN and MOS spectra are plotted with the best-fit
model in Figure 4, and Table 5 lists the observed fluxes, un -
absorbed luminosities and overall percentage contributions
to the total of each from the three spectral components.
The power-law continuum is the dominant component over
the 0.3–10 keV range, contributing 80 percent of the total
observed ux and unabsorbed luminosity of the galaxy. We
note that we were unable to test for the presence of Fe-K
lines after excluding data above 6 keV.
c
2003 RAS, MNRAS 000, 1–13
XMM-Newton observations of NGC 3256 & NGC 3310 9
Table 4. Spectral fitting results for NGC 3310.
PL M
1
M
2
χ
2
/dof F
X
b
L
X
c
N
H
a
Γ N
H
a
kT (keV ) N
H
a
kT (keV ) Obs Unabs
Model 1: M+PL (wabs*po+wabs*mekal)
1.19
1.70
+0.07
0.08
2.97
+1.21
0.78
0.26
+0.03
0.03
- - 322.8/348 17.16
+0.67
1.84
0.81
+0.03
0.09
1.40
+0.06
0.47
Model 2: M+M+PL (wabs*po+wabs*mekal+wabs*mekal)
1.19
1.56
+0.10
0.07
0.42
+4.10
0.39
0.24
+0.04
0.09
0.86
+2.00
0.45
0.63
+0.07
0.05
288.2/345 18.13
+0.30
2.05
0.85
+0.01
0.10
0.99
+0.01
0.09
Model 3: M+PL (wabs*pcfabs*po+wabs*mekal)
1/10
1.98
+0.07
0.07
4.17
+0.74
0.83
0.24
+0.03
0.02
- - 309.6/348 16.53
+0.70
2.07
0.78
+0.03
0.10
2.21
+0.11
0.89
Model 4: M+M+PL (wabs*pcfabs*po+wabs*mekal+wabs*mekal)
1/10
1.84
+0.11
0.13
0.61
+1.84
0.13
0.25
+0.04
0.08
2.19
+2.08
1.06
0.62
+0.06
0.05
283.1/345 17.30
+0.46
1.82
0.81
+0.02
0.09
1.26
+0.01
0.18
Notes: Spectral models: PL=power-law continuum model and M=MEKAL thermal plasma (solar abundances).
a
Absorption
column in units of 10
21
cm
2
.
b
Observed fluxes in the 0.3–10 keV band, in units of 10
13
erg s
1
cm
2
.
c
Observed and
unabsorbed luminosities in the 0.3–10 keV band, in units of 10
41
erg s
1
(assuming a distance of 19.8 Mpc).
N
H
component fixed.
N
H
components fixed at 10
21
cm
2
and 10
22
cm
2
for wabs and pcfabs components respectively. Errors correspond to 90 percent
confidence limits for 1 parameter of interest. The best-fit model is shown in bold.
Table 5. Fluxes, luminosities and percentage of total emission
from the three components of the best-fit M+M+PL model of
NGC 3310.
Component F
X
% of total L
X
% of total
Cool MEKAL 1.50 9 1. 09 9
Warm MEKAL 2.00 11 1.98 16
Power-Law 13.80 80 9.48 75
Total 17.30 100 12.55 100
Notes: Observed fluxes in the 0.3–10 keV band, in units of
10
13
erg s
1
cm
2
. Unabsorbed luminosities in the
0.3–10 keV band, in units of 10
40
erg s
1
(assuming a
distance of 19.8 Mpc).
4.2.1 Comparison with previous work
In the ROSAT and ASCA observations of NGC 3310 re-
ported by Zezas et al. (1998), the combined spectrum of
the integrated emission from NGC 3310 was well-fit with
either a double RS thermal plasma (k T =0.80/14.98 keV)
or one comprised of a power-law plus RS thermal plasma
(Γ=1.44, kT =0.81 keV), with both components affected by
a single absorbing column. These p arameters are p artly
consistent with the XMM-Newton results; the power-law
slope is slightly harder and the thermal plasma component
similar (to within the 90 percent confidence limits) to the
warm MEKAL component in our best-fit M+M+PL model.
However, the main difference is that whereas th ey fitted
a hot thermal plasma plus a low absorbing hydrogen col-
umn (N
H
0.17 × 10
21
cm
2
), the improved quality of
the d ata in the XMM-Newton observation at the soft en-
ergies means that we are able instead to model the soft
emission with a cool thermal p lasma with a higher absorb-
ing column. There is also a marked difference in the mea-
sured flux es and luminosities in ROSAT /ASCA and XMM-
Newton datasets. Even if we consider the nearly identical
two-component PL+M/PL+RS models, there is a 40 p er-
cent reduction in t he observed flux in the XMM-Newton data
compared with the ROSAT /ASCA analysis ( 3 × 10
12
erg s
1
cm
2
). It is likely that this is due to the difference
in size of the extraction regions used. The ROSAT spectra
were extracted using a 1.5 arcminute radius region and the
ASCA regions were larger at 2.7 and 5.5 arcminutes radius
for the SIS and GIS respectively (Zezas et al. 1998), whereas
we have used a 40 arcsecond radius circular source region to
avoid contamination from t he two discrete sources.
In order to make a direct comparison of the
observed fluxes, we have attempted to replicate th e
ASCA /ROSAT results by extracting XMM-Newton spec-
tra in an aperture with t he same radius as the largest
ASCA aperture (5.5 arcminutes) together with as large an
annular background region as possible given th e size of the
MOS and PN chip arrays. However, this turns out to be
impractical, as the XMM-Newton spectra are dominated by
the high background flux (especially at energies > 2 keV),
resulting in low-quality background-subtracted data which
cannot constrain the spectral shape, and hence cannot con-
strain the ux from a larger region.
4.2.2 Discrete Source Properties
The two discrete sources to the south (X-2) & north (X-3) of
the main body of the galaxy have insufficient counts for de-
tailed spectral fi tting (130 and 110 in each MOS camera
respectively). However, to estimate their spectral shapes,
we have co-added and tted the MOS spectra (X-3 is lo-
cated on a chip gap in the PN) with simple power-law plus
Galactic absorption models. Their XMM-Newton positions,
c
2003 RAS, MNRAS 000, 1–13
10 L. Jenkins et al.
Figure 4. XMM-Newton spectra and folded best-fit model
(M+M+PL) of NGC 3310: PN [top], MOS 1 [middle], MOS 2
[bottom]. The model components are denoted by dashed (p ower-
law), dotted (cool MEKAL) & dot–dashed (warm MEKAL) lines.
power-law ph oton indices, observed fluxes and unabsorbed
luminosities are shown in Table 6. The X-ray luminosities of
these sources are high (7 × 10
39
erg s
1
& 5 × 10
39
erg s
1
,
0.3–10 keV), putting them well into the ULX regime. Al-
though not shown in the OM image (Figure 3 [lower right]),
they are both coincident with regions of UV emission, which
is indicative of star formation activity though there are no
obvious UV counterparts in this data. Although this XMM-
Newton observation is very flare-contaminated, we tested
the variability of each source in the XMM-Newton data by
deriving short-term light curves. The data from the MOS
cameras were co-added to improve the signal-to-noise ratio,
and the data were tailored so that each bin had at least 20
counts after background subtraction. We performed χ
2
tests
to search for large amplitude variability against the hypoth-
Table 6. Discrete Sources in NGC 3310.
Source Position (J2000) Γ F
X
L
X
X-2 10
h
38
m
50.2
s
+53
29
25
′′
1.49
+0.96
0.72
1.47 7.00
X-3 10
h
38
m
43.3
s
+53
31
00
′′
1.66
+0.88
0.75
1.05 5.01
Notes: Observed fluxes in the 0.3–10 keV band, in units of 10
13
erg s
1
cm
2
. Unabsorbed luminosities in the 0.3–10 keV band,
in units of 10
39
erg s
1
(assuming a distance of 19.8 Mpc).
esis of a constant count rate, but neither source showed any
significant variability (
>
3σ). I n order to search for longer-
term variability, we have made a direct comparison between
their observed fluxes in this XMM-Newton observation and
the 1995 ROSAT HRI 41 ks observation, both in the 0.3–
2.4 keV band as this is the energy range covered by both
mission. We derived fl uxes for the ROSAT HRI observa-
tion with webPIMMS using the observed count rate from
Roberts & Warwick (2000) for X- 2, and a new count rate
estimate for X-3 from the archival HRI data, assuming th e
absorbed power-law continuum slope measured in the XMM-
Newton observation and normalized to the 0.3–2.4 keV band.
During this 6 year period, the flux for X-3 has decreased
by 57 percent, while that of X-2 has increased by 65
percent. Variability on this t imescale is consistent with that
found for other ULX (c.f. Roberts et al. 2004), and supports
the scenario that these sources are single black-hole XRBs
rather than concentrated regions of star formation, as multi-
ple sources such as a cluster of XRBs would not be expected
to all change their output simultaneously.
5 DISCUSSION
Our results are consistent with other local SBGs stud-
ied with XMM-Newton and Chandra, e.g. N GC 253
(Pietsch et al. 2001; Strickland et al. 2000; Strickland et al.
2002); M82, (Stevens et al. 2003; Matsumoto et al.
2001) and the closest merger system NGC 4038/4039
(The Antennae), (Fabbiano, Zezas, & Murray 2001;
Fabbiano et al. 2003; Zezas & Fabbiano 2002; Zezas et al.
2002a; Zezas et al. 2002b). The X-ray emission from these
systems are well-fitted with thermal components ranging
between 0.2–0.9 keV plus harder (> 2 keV) emission
dominated by power-law continua from discrete sources.
Even with the relatively short ex posures of these XMM-
Newton observations, especially in the case of NGC 3310
where there was substantial background flare contamina-
tion in the data, we are able to improve the definition of the
plasma temperatures and power-law slopes compared to ear-
lier ROSAT and ASCA data for these galaxies. Additionally,
the information from the spatially-resolved Chandra data
has allowed us to deconvolve this spectral information and
properly assess the contributions from point sources and dif-
fuse components.
An interesting result of this stud y are th e different tem-
peratures of the thermal components in NGC 3256 and
NGC 3310. To investigate the reasons for this, we have
summarized the known prop erties of each system in Ta-
ble 7. We have also included d etails for the Antennae system
(NGC 4038/39), as this is the best example of a close merger
system of two equal-mass galaxies. For th e purposes of this
c
2003 RAS, MNRAS 000, 1–13
XMM-Newton observations of NGC 3256 & NGC 3310 11
Table 7. Comparison of the properties of NGC 3256, NGC 3310 and the Antennae (NGC 4038/9).
Parameters NGC 3256 NGC 3310 Antennae (NGC 4038/9) Refs
Distance (Mpc, H
0
=75 km s
1
Mpc) 35.4 19.8 19.3 1,2
Merger Type Major merger - two Merger with dwarf Major merger - two 3,4,6
equal mass galaxies companion equal mass galaxies
Merger Age 5 × 10
8
yr
<
10
8
yr 2 5 × 10
8
yr 3,4,5,6
Starburst Age 5 25 × 10
6
yr 10
7
10
8
yr 5 100 × 10
6
yr 3,5,7
Dynamical Mass (10
10
M
) 5 2.2 8 3,8
L
F IR
(L
)
1.8 × 10
11
2.0 × 10
10
2.9 × 10
10
1
SFR
IR
(M
yr
1
)
32.6 3.6 5.2
L
X
( erg s
1
) 4.5×10
41
1.3×10
41
6.7×10
40
M
1
(kT /keV ) 0.6 (12%) 0.3 (9%) 0.4 (17%)
M
2
(kT /keV ) 0.9 (70%) 0.6 (16%) 0.7 (14%)
Γ
2.0 (18%) 1.8 (75%) 1.6 (69%)
References: (1) Sanders et al. (2003); (2) Zezas et al. 2002a; (3) L´ıpari et al. (2000); (4) Balick & Heckman (1981);
(5) Whitmore et al. (1999); (6) Barnes (1988); (7) Elm egreen et al. (2002); (8) Kregel & Sancisi (2001).
FIR luminosities calculated
using FIR fluxes derived from the 60µ and 100µ IR flux densities of Sanders et al. (2003) using the relation o f
Helou, Soifer, & Rowan-Robinson (1985): F IR = 1.26 × 10
11
(2.58S
60µ
+ S
100µ
) erg cm
2
s
1
.
SFRs determined using the relation
of Kennicutt (1998): SFR=L
F IR
/2.2 × 10
43
M
yr
1
.
Unabsorb