University of Alabama Repository :: Browsing by Author "Sarazin, CL"

Browsing by Author "Sarazin, CL"

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Chandra observation of diffuse gas and low-mass X-ray binaries in the elliptical galaxy NGC 4649 (M60)
(IOP Publishing, 2004-01-10) Randall, SW; Sarazin, CL; Irwin, JA; University of Virginia; University of Michigan System; University of Michigan; University of Alabama Tuscaloosa
We present a Chandra X-ray observation of the X-ray bright E2 elliptical galaxy NGC 4649. In addition to bright diffuse emission, we resolve 165 discrete sources, most of which are presumably low-mass X-ray binaries. As found in previous studies, the luminosity function of the resolved sources is well-fitted by a broken power law. In NGC 4697 and NGC 1553, the break luminosity was comparable to the Eddington luminosity of a 1.4 M-. neutron star. One possible interpretation of this result is that those sources with luminosities above the break are accreting black holes and those below are mainly accreting neutron stars. The total X-ray spectrum of the resolved sources is well fitted by a hard power law, while the diffuse spectrum requires a hard and a soft component, presumably due to the relatively soft diffuse gas and the harder unresolved sources. We also find evidence for structure in the diffuse emission near the center of NGC 4649. Specifically, there appear to be bright "fingers'' of emission extending from the center of the galaxy and a 500 long bar at the center of the galaxy. The fingers are morphologically similar to radial features seen in two-dimensional hydrodynamic simulations of cooling flows in elliptical galaxies, and although their other properties do not match the predictions of the particular simulations used, we conclude that the radial fingers might be due to convective motions of hot outflowing gas and cooler inflowing gas. The bar is coincident with the central extended radio source; we conclude that the bar may be caused by weak shocks in the diffuse gas from an undetected low-luminosity active galactic nucleus.
Chandra observations of low-mass X-ray binaries and diffuse gas in the early-type galaxies NGC 4365 and NGC 4382 (M85)
(IOP Publishing, 2003-12-10) Sivakoff, GR; Sarazin, CL; Irwin, JA; University of Virginia; University of Michigan System; University of Michigan; University of Alabama Tuscaloosa
We used the Chandra X-Ray Observatory ACIS-S3 to image the X-ray-faint elliptical galaxy NGC 4365 and lenticular galaxy NGC 4382. The observations resolve much of the X-ray emission into 99 and 58 sources, respectively, most of which are low-mass X-ray binaries (LMXBs) associated with each of the galaxies. Within one effective radius of NGC 4365, about 45% of the counts are resolved into sources, 30% are attributed to unresolved LMXBs, and 25% are attributed to diffuse gas. Within two effective radii of NGC 4382, about 22% of the counts are resolved into sources, 33% are attributed to unresolved LMXBs, and 45% are attributed to diffuse gas. We identify 18 out of the 37 X-ray sources in a central field in NGC 4365 with globular clusters. The luminosity functions of the resolved sources for both galaxies are best fitted with cutoff power laws whose cutoff luminosity is approximate to (0.9-3.1) x 10(39) ergs s(-1). These luminosities are much larger than those previously measured for similar galaxies; we do not find evidence for a break in the luminosity function at the Eddington luminosity of a 1.4 M-. neutron star. The spatial distributions of the resolved sources for both galaxies are broader than the distribution of optical stars. In both galaxies, a hard powerlaw model fits the summed spectrum of all of the sources. The unresolved emission is best fitted by the sum of a soft MEKAL model, representing emission from diffuse gas, and a hard power law, presumed to be from unresolved LMXBs. There is some evidence that the temperature of the diffuse gas increases with increasing radius. A standard beta model fits the radial distribution of the diffuse gas in both galaxies. In the elliptical galaxy NGC 4365, the best-fit core radius is very small, while the SO galaxy NGC 4382 has a larger core radius. This may indicate that the gas in NGC 4382 is rotating significantly.
Chandra X-ray observations of the X-ray faint elliptical galaxy NGC 4697
(IOP Publishing, 2001-08-01) Sarazin, CL; Irwin, JA; Bregman, JN; University of Virginia; University of Michigan System; University of Michigan; University of Alabama Tuscaloosa
A Chandra ACIS S3 observation of the X-ray faint elliptical galaxy NGC 4697 resolves much of the X-ray emission (61% of the counts from within one effective radius) into 90 point sources, of which similar to 80 are low-mass X-ray binaries (LMXBs) associated with this galaxy. The dominance of LMXBs indicates that X-ray faint early-type galaxies have lost much of their interstellar gas. On the other hand, a modest portion of the X-ray emission from NGC 4697 is due to hot gas. Of the unresolved emission, it is likely that about half is from fainter unresolved LMXBs, while the other half (similar to 23% of the total count rate) is from interstellar gas. The X-ray-emitting gas in NGC 4697 has a rather low temperature (kT = 0.29 keV). The emission from the gas is very extended, with a much flatter surface brightness profile than the optical light, and has an irregular, L-shaped morphology. The physical state of the hot gas is uncertain; the X-ray luminosity and extended surface brightness are inconsistent with a global supersonic wind, a partial wind, or a global cooling inflow. The gas may be undergoing subsonic inflation, rotationally induced outflow, or ram pressure stripping. X-ray spectra of the resolved sources and diffuse emission show that the soft X-ray spectral component, found in this and other X-ray faint ellipticals with ROSAT, is due to interstellar gas. The cumulative LMXB spectrum is well fitted by thermal bremsstrahlung at kT = 8.1 keV, without a significant soft component. NGC 4697 has a central X-ray source with a luminosity of L(X) = 8 x 10(38) ergs s(-1), which may be due to an active galactic nucleus and/or one or more LMXBs. At most, the massive black hole (BH) at the center of this galaxy is radiating at a very small fraction (less than or equal to4 x 10(-8)) of its Eddington luminosity. Three of the resolved sources in NGC 4697 are supersoft sources. In the outer regions of NGC 4697, seven of the LMXBs (about 20%) are coincident with candidate globular clusters, which indicates that globulars have a high probability of containing X-ray binaries compared to the normal stellar population. We suggest that all of the LMXBs may have been formed in globulars. The X-ray to optical luminosity ratio for the LMXBs in NGC 4697 is L(X)(LMXB, 0.3-10 keV)/L(B) = 8.1 X 10(29) ergs s(-1) L(B.)(-1), which is about 35% higher than the value for the bulge of M31. Other comparisons suggest that there are significant variations (factor of greater than or similar to2) in the LMXB X-ray-to-optical ratios of early-type galaxies and spiral bulges. The X-ray luminosity function of NGC 4697 is also flatter than that found for the bulge of M31. The X-ray luminosities (0.3-10 keV) of the resolved LMXBs range from similar to5 x 10(37) to similar to2.5 X 10(39) ergs s(-1). The luminosity function of the LMXBs has a "knee" at 3.2 X 10(38) ergs s(-1), which is approximately the Eddington luminosity of a 1.4 M. neutron star (NS). This knee appears to be a characteristic feature of the LMXB population of early-type galaxies, and we argue that it separates BH and NS binaries. This characteristic luminosity could be used as a distance estimator. If they are Eddington limited, the brightest LMXBs contain fairly massive accreting BHs. The presence of this large population of NS and massive BH stellar remnants in this elliptical galaxy shows that it (or its progenitors) once contained a large population of massive main-sequence stars.
Heating of the intracluster gas in the Triangulum Australis cluster
(University of Chicago Press, 1996-11-20) Markevitch, ML; Sarazin, CL; Irwin, JA; University of Alabama Tuscaloosa
ASCA and ROSAT X-ray data are used to obtain two-dimensional maps of the gas temperature, pressure, and specific entropy in the Triangulum Australis cluster of galaxies. We find that this hot (T-e = 10.3 +/- 0.8 keV) system probably has a temperature peak (T-e > 12 keV) at the cluster core, which approximately corresponds to the adiabatic relation. An underdense gas sector, found in the ROSAT cluster image eastward of the core, has a higher temperature than average at that radius. At this higher temperature, the gas pressure in this sector is equal to that of the rest of the cluster at the same radius, but the specific entropy of this gas is significantly higher (although the temperature difference itself is only marginally significant). We speculate that the existence of this region of underdense high-entropy gas, as well as the adiabatic central temperature peak, indicate recent or ongoing heating of the intergalactic medium in this cluster. The most probable source of such heating is a subcluster merger, for which the hydrodynamic simulations predict a qualitatively similar temperature structure. We point out that entropy maps can provide a physically meaningful way of diagnosing merging clusters and comparing the predictions of merger simulations to the data.
Is there molecular gas in the H I cloud between NGC 4472 and UGC 7636?
(American Institute of Physics, 1997-05) Irwin, JA; Frayer, DT; Sarazin, CL; University of Toronto; University of Alabama Tuscaloosa
We present CO observations of the H I cloud located between the Virgo elliptical galaxy NGC 4472 and its dwarf irregular companion galaxy UGC 7636. ROSAT observations of the X-ray halo of NGC 4472 show a hole in the X-ray emission at the position of the H I cloud (Irwin gi Sarazin 1996), If the hole is assumed to be the result of soft X-ray absorption by some absorbing material within the cloud, the total implied mass of the absorbing material is much larger than the measured H I mass of the cloud, This implies that a large fraction of the cloud is composed of molecular hydrogen. However, the CO observations fail to find the required amount of molecular gas if a Galactic CO-to-H-2 conversion factor is assumed. While a low density in the molecular gas might increase the CO-to-H-2 conversion factor enough to make the CO upper limit consistent with the X-ray absorption prediction too low a density would allow the H-2 to be dissociated by UV radiation. We conclude that it is unlikely that the required absorbing mass is present. In any case, these observations provide a strict upper limit on the CO content of the H I cloud. (C) 1997 American Astronomical Society.
Low-mass X-ray binaries and globular clusters in early-type galaxies
(University of Chicago Press, 2003-10-01) Sarazin, CL; Kundu, A; Irwin, JA; Sivakoff, GR; Blanton, EL; Randall, SW; University of Virginia; Michigan State University; University of Michigan System; University of Michigan; University of Alabama Tuscaloosa
A high fraction of the low-mass X-ray binaries (LMXBs) in early-type galaxies are associated with globular clusters (GCs). Here we discuss the correlations between LMXBs and GCs in a sample of four early-type galaxies with X-ray source lists determined from Chandra observations. There is some evidence that the fraction of LMXBs associated with GCs (f(X-GC)) increases along the Hubble sequence from spiral bulges ( or spheroids) to S0s to Es to cDs. On the other hand, the fraction of GCs that contain X-ray sources appears to be roughly constant at f(GC-X) similar to 4%. There is a strong tendency for the X-ray sources to be associated with the optically more luminous GCs. However, this correlation is consistent with a constant probability of finding an LMXB per unit optical luminosity; that is, it seems to result primarily from the larger number of stars in optically luminous GCs. The probability of finding a bright LMXB per unit optical luminosity in the GCs is about 1.5 x 10(-7) LMXBs per L-.,L-I for L-X greater than or similar to 1 x 10(38) ergs s(-1) (0.3 - 10 keV) and rises to about 2.0 x 10(-7) LMXBs per L-.; I at lower X-ray luminosities, L-X greater than or similar to 3 x 10(37) ergs s(-1). This frequency appears to be roughly constant for different galaxies, including the bulges of the Milky Way and M31. There is a tendency for the X-ray sources to be found preferentially in redder GCs, which is independent of optical luminosity correlation. This seems to indicate that the evolution of X-ray binaries in a GC is affected by either the metallicity or the age of the GC, with younger and/or more metal rich GCs having more LMXBs. There is no strong difference in the X-ray luminosities of GC and non-GC LMXBs. There is a weak tendency for the brightest LMXBs, whose luminosities exceed the Eddington luminosity for a 1.4 M-. neutron star, to avoid GCs. That may indicate that black hole X-ray binaries are somewhat less likely to be found in GCs, as seems to be true in our Galaxy. On the other hand, there are some luminous LMXBs associated with GCs. There is no clear evidence that the X-ray spectra or variability of GC and non-GC X-ray sources differ. We also find no evidence for a difference in the spatial distribution of GC and non-GC LMXBs. Many of these results are similar to those found in NGC 1399 and NGC 4472 by Angelini et al. and Kundu et al., respectively.
Resolving the mystery of X-ray-faint elliptical galaxies: Chandra X-ray observations of NGC 4697
(University of Chicago Press, 2000-12-01) Sarazin, CL; Irwin, JA; Bregman, JN; University of Virginia; University of Michigan System; University of Michigan; University of Alabama Tuscaloosa
Chandra observations of the X-ray-faint elliptical galaxy NGC 4697 resolve much of the X-ray emission (61% within one effective radius) into similar to 80 point sources, of which most are low-mass X-ray binaries (LMXBs). These LMXBs provide the bulk of the hard emission and much of the soft emission as well. Of the remaining unresolved emission, it is likely that about half is from fainter LMXBs, while the other half (similar to 23% of the total emission) is from interstellar gas. Three of the resolved sources are supersoft sources. In the outer regions of NGC 4697, eight of the LMXBs (about 25%) are coincident with candidate globular clusters, indicating that globulars have a high probability of containing X-ray binaries compared with the normal stellar population. The X-ray luminosities (0.3-10 keV) of the resolved LMXBs range from similar to5 x 10(37) to similar to2.5 x 10(39) ergs s(-1). The luminosity function of the LMXBs has a "knee" at 3.2 x 10(38) ergs s(-1), which is roughly the Eddington luminosity of an 1.4 M-. neutron star (NS); this knee might be useful as a distance indicator. The highest luminosity source has the Eddington luminosity of an similar to 20 M-. black hole (BH). The presence of this large population of NS and massive BH stellar remnants in this elliptical galaxy shows that it (or its progenitors) once contained a large population of massive main-sequence stars.
ROSAT HRI X-ray observations of the open globular cluster NGC 288
(IOP Publishing, 1999-10-10) Sarazin, CL; Irwin, JA; Rood, RT; Ferraro, FR; Paltrinieri, B; University of Virginia; University of Michigan System; University of Michigan; Istituto Nazionale Astrofisica (INAF); European Southern Observatory; Sapienza University Rome; University of Alabama Tuscaloosa
A ROSAT HRI X-ray image was obtained of the open globular cluster NGC 288, which is located near the South Galactic Pole. This is the first deep X-ray image of this system. We detect a low-luminosity globular cluster X-ray source (LLGCX) RX J005245.0-263449 with an X-ray luminosity of (5.5 +/- 1.4) x 10(32) ergs s(-1) (0.1-2.0 keV), which is located very close to the cluster center. There is evidence for X-ray variability on a timescale of less than or similar to 1 day. The presence of this LLGCX in such an open globular cluster suggests that dense stellar systems with high interaction rates are not needed to form LLGCXs. We also searched for diffuse X-ray emission from NGC 288. Upper limits on the X-ray luminosities are L-X(h) < 9.5 x 10(32) ergs s(-1) (0.52-2.02 keV) and L-X(s) < 9.3 x 10(32) ergs s(-1) (0.11-0.41 keV), These imply upper limits to the diffuse X-ray-to-optical light ratios in NGC 288, which are lower than the values observed for X-ray faint early-type galaxies. This indicates that the soft X-ray emission in these galaxies is due either to a component that is not present in globular clusters (e.g., interstellar gas or a stellar component that is not found in low-metallicity Population II systems) or to a relatively small number of bright low-mass X-ray binaries.
Rosat X-ray colors and emission mechanisms in early-type galaxies
(IOP Publishing, 1998-06-01) Irwin, JA; Sarazin, CL; University of Virginia; University of Alabama Tuscaloosa
The X-ray colors and X-ray-to-optical luminosity ratios (L-X/L-B) of 61 early-type galaxies observed with the ROSAT PSPC are determined. The colors indicate that the X-ray spectral properties of galaxies vary as a function of L-X/L-B. The brightest X-ray galaxies have colors that are consistent with thermal emission from hot gas with roughly the same metallicity of 50% solar. The spatial variation of the colors indicates that the gas temperature in these galaxies increases radially. Galaxies with medium L-X/L-B also have spectral properties consistent with emission from hot gas. If a simple one-component thermal model is assumed to describe the 0.1-2.0 keV X-ray emission in these galaxies, then one possible explanation for the progressive decrease in L-X/L-B among galaxies of this class could be the progressive decrease in metal abundance of the X-ray-emitting gas contained by the galaxies. However, stellar X-ray emission may become a complicating factor for the fainter galaxies in this medium-L-X/L-B class. Galaxies with the lowest L-X/L-B values appear to be lacking a hot interstellar component. Their X-ray colors are consistent with those derived from the bulges of the spiral galaxies M31 and NGC 1291. In M31, the X-ray emission is resolved into discrete sources and is apparently due primarily to low-mass X-ray binaries (LMXBs). We therefore suggest that the bulk of the X-ray emission in the faintest elliptical galaxies is also due to LMXBs. Previously, the X-ray spectra of X-ray-faint galaxies had been found to be described by a hard component, which was attributed to LMXB emission, and a very soft component of unknown origin. We show that the very soft component also likely results from LMXBs, as a very soft component is seen in the X-ray spectra of the nearby LMXB Her X-1 and LMXBs in the bulge of M31. If the X-ray emission in X-ray-faint galaxies is primarily from stellar sources, then a range in L-X/L-B among these galaxies suggests that the stellar X-ray luminosity does not scale with optical luminosity, at least for galaxies of low optical luminosities. This could be the result of a decrease in the proportion of LMXBs with decreasing optical luminosity and/or the effects of fluctuations in the small number of LMXBs expected.
ROSAT X-RAY OBSERVATIONS OF THE 2A-0335+096 CLUSTER OF GALAXIES
(University of Chicago Press, 1995-12-20) Irwin, JA; Sarazin, CL; California Institute of Technology; University of Texas System; University of Texas Austin; University of Alabama Tuscaloosa
The 2A 0335+096 cluster of galaxies was observed in X-rays with the Position Sensitive Proportional Counter (PSPC) and the High Resolution Imager (HRI) on ROSAT. Spatially resolved X-ray spectra show a decrease in the ambient temperature of the X-ray emitting gas in the inner regions of the cluster, indicative of a cooling flow. The spectra also confirm the presence of an absorbing column density in excess of the measured Galactic value toward the cooling how region, which must be intrinsic to the cluster. The mass of this absorbing matter is 7.4 x 10(11) M. within 170 kpc of the center of the cluster. A deprojection of the surface brightness profile combined with the observed temperature profile yields electron density, gas and gravitational mass, and mass deposition rate profiles. Within 1.2 Mpc the total gravitating mass is 1.9 x 10(14) M.. A cooling rate of 400(-65)(+90) M. yr(-1) is found within the cooling radius of 166(-18)(+28) kpc. This value is consistent with the cooling rate obtained by fitting a cooling flow model to the spectrum of the central 166 kpc. Although the cooling rate is proportional to radius for the inner 60 kpc, it is essentially constant beyond this point, which suggests that the gas cools homogeneously in this region. A similar result was found for the NGC 5044 group of galaxies by David et al. (1994). This disagrees with earlier observations of other clusters, which suggested that the gas within the entire cooling flow region was inhomogeneous.
Untangling the X-ray emission from the Sa galaxy NGC 1291 with Chandra
(IOP Publishing, 2002-05-01) Irwin, JA; Sarazin, CL; Bregman, JN; University of Michigan System; University of Michigan; University of Virginia; University of Alabama Tuscaloosa
We present a Chandra ACIS-S observation of the nearby bulge-dominated Sa galaxy NGC 1291. The X-ray emission from the bulge resembles the X-ray emission from a subclass of elliptical and S0 galaxies with low X-ray-to-optical luminosity ratios. The X-ray emission is composed of a central pointlike nucleus, similar to50 point sources that are most likely low-mass X-ray binaries (LMXBs), and diffuse gas detectable out to a radius of 120" (5.2 kpc). The diffuse gas has a global temperature of 0.32(-0.03)(+0.04) and metallicity of 0.06+/-0.02 solar, and both quantities marginally decrease with increasing radius. The hot gas fills the hole in the H I distribution, and the softening of the spectrum of the X-ray gas with radius might indicate a thermal coupling of the hot and cold phases of the interstellar medium as previously suggested. The integrated X-ray luminosity of the LMXBs, once normalized by the optical luminosity, is a factor of 1.4 less than in the elliptical galaxy NGC 4697 or S0 galaxy NGC 1553. The difference in L-X,L-stellar/L-B between the galaxies appears to be because of a lack of very bright sources in NGC 1291. No sources above 3x10(38) ergs s(-1) were found in NGC 1291 when similar to7 were expected from scaling from NGC 4697 and NGC 1553. The cumulative L-X,L-stellar/L-B value including only sources below 1.0x10(38) ergs s(-1) is remarkably similar between NGC 1291 and NGC 4697, if a recent surface brightness fluctuation-determined distance is assumed for NGC 4697. If this is a common feature of the LMXB population in early-type systems, it might be used as a distance indicator. Finally, a bright, variable [(1.6=3.1) x 10(39) ergs s(-1)] source was detected at the optical center of the galaxy. Its spectrum shows excess soft emission superposed on a highly absorbed power-law component, similar to what has been found in several other low-luminosity active galactic nuclei. However, the soft component does not vary in intensity like the hard component, indicating that the soft component is not reprocessed hard component emission.
X-ray evidence for the interaction of the giant elliptical galaxy NGC 4472 with its virgo cluster environment
(IOP Publishing, 1996-11-10) Irwin, JA; Sarazin, CL; University of Alabama Tuscaloosa
We analyze X-ray spatial and spectral data on the giant elliptical galaxy NGC 4472, the brightest galaxy in the Virgo cluster. The X-ray contours of NGC 4472 are elongated in the northeast-southwest direction, perhaps as a result of motion through the Virgo intracluster gas. A bow shock-like structure is evident on the galaxy's north side. The temperature at a given radius in this bow shock region is slightly higher than the temperature at the same radius on the galaxy's southwest side. Away from this bow shock region, the surface brightness profile of NGC 4472 can be traced out to a radius of 260 kpc in the southwest direction. Beyond 260 kpc, we find evidence for emission from both the Virgo cluster and the Galactic North Polar Spur (believed to be the rim of a hot Galactic superbubble). NGC 4472 is interacting with the dwarf irregular galaxy UGC 7636. We do not detect any excess or deficit in the X-ray emission toward this galaxy. An H I cloud, detected previously in the 21 cm line and located midway between the two galaxies, appears to have been removed from the irregular galaxy through either tidal interaction or ram pressure stripping. We find a marginally significant hole in the ROSAT HRI and PSPC X-ray images at the position of this cloud, suggesting that the cloud lies at the front side of NGC 4472. If the hole in the X-ray images is due to soft X-ray absorption, the total gaseous mass of the cloud must be at least 1.7 x 10(9) M., far greater than its 21 cm H I mass. This suggests that the majority of the material in the cloud is molecular.
The X-ray-faint early-type galaxy NGC 4697
(IOP Publishing, 2000-11-20) Irwin, JA; Sarazin, CL; Bregman, JN; University of Michigan System; University of Michigan; University of Virginia; University of Alabama Tuscaloosa
We analyze archival ROSAT HRI, ROSAT PSPC, and ASCA data of the X-ray-faint early-type galaxy NGC 4697. The joint ROSAT PSPC + ASCA spectrum is fitted by a two-component thermal model, a MEKAL model with kT(MEKAL) = 0.26(-0.03)(+0.04) keV with low metallicity and a bremsstrahlung model with kT(BREM) = 5.2(-1.6)(+3.0) keV. A similar model was found to fit the spectra of another faint early-type galaxy (NGC 4382) and the bulge of M31. We interpret this soft emission as a combination of emission from a soft component of low-mass X-ray binaries (LMXBs) and from a low-temperature interstellar medium, although the relative contributions of the two components could not be determined. Twelve point sources were identified within 4 ' of NGC 4697, of which 11 are most likely LMXBs associated with the galaxy. The soft X-ray colors of four of the LMXBs in NGC 4697 support the claim that LMXBs possess a soft spectral component. Finally, we present a simulation of what we believe the Chandra data of NGC 4697 will look like.

Browsing by Author "Sarazin, CL"

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