X-RAY EMISSION FROM THE SOMBRERO GALAXY: DISCRETE SOURCES

Journal Article · Fri Oct 01 04:00:00 EDT 2010 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/721/2/1368· OSTI ID:21464780

Zhiyuan, Li; Jones, Christine; Forman, William R; Kraft, Ralph P; Stefano, Rosanne Di ^[1]; Spitler, Lee R ^[2]; Tang, Shikui; Wang, Q Daniel ^[3]; Gilfanov, Marat ^[4]; Revnivtsev, Mikhail ^[5]

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1. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
2. Centre for Astrophysics and Supercomputing, Swinburne University, Hawthorn, VIC 3122 (Australia)
3. Department of Astronomy, University of Massachusetts, 710 North Pleasant Street, Amherst, MA 01003 (United States)
4. Max-Planck-Institut fuer Astrophysik, Karl-Schwarzschild-Str 1, 85741 Garching bei Muenchen (Germany)
5. Excellence Cluster Universe, Technische Universitaet Muenchen, Boltzmannstr. 2, 85748 Garching (Germany)

We present a study of discrete X-ray sources in and around the bulge-dominated, massive Sa galaxy, Sombrero (M104), based on new and archival Chandra observations with a total exposure of {approx}200 ks. With a detection limit of L{sub X} {approx} 10{sup 37} erg s{sup -1} and a field of view covering a galactocentric radius of {approx}30 kpc (11.'5), 383 sources are detected. Cross-correlation with Spitler et al.'s catalog of Sombrero globular clusters (GCs) identified from HST/ACS observations reveals 41 X-ray sources in GCs, presumably low-mass X-ray binaries (LMXBs). Metal-rich GCs are found to have a higher probability of hosting these LMXBs, a trend similar to that found in elliptical galaxies. On the other hand, the four most luminous GC LMXBs, with apparently super-Eddington luminosities for an accreting neutron star, are found in metal-poor GCs. We quantify the differential luminosity functions (LFs) for both the detected GC and field LMXBs, whose power-law indices ({approx}1.1 for the GC-LF and {approx}1.6 for field-LF) are consistent with previous studies for elliptical galaxies. With precise sky positions of the GCs without a detected X-ray source, we further quantify, through a fluctuation analysis, the GC-LF at fainter luminosities down to 10{sup 35} erg s{sup -1}. The derived index rules out a faint-end slope flatter than 1.1 at a 2{sigma} significance, contrary to recent findings in several elliptical galaxies and the bulge of M31. On the other hand, the 2-6 keV unresolved emission places a tight constraint on the field LF, implying a flattened index of {approx}1.0 below 10{sup 37} erg s{sup -1}. We also detect 101 sources in the halo of Sombrero. The presence of these sources cannot be interpreted as galactic LMXBs whose spatial distribution empirically follows the starlight. Their number is also higher than the expected number of cosmic active galactic nuclei (52 {+-} 11 [1{sigma}]) whose surface density is constrained by deep X-ray surveys. We suggest that either the cosmic X-ray background is unusually high in the direction of Sombrero, or a distinct population of X-ray sources is present in the halo of Sombrero.

• OSTI ID: 21464780
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 721; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
BOSONS
CATALOGS
COSMIC PHOTONS
COSMIC RADIATION
COSMIC RAY SOURCES
COSMIC X-RAY SOURCES
DISTRIBUTION
DOCUMENT TYPES
ELEMENTARY PARTICLES
ELEMENTS
EMISSION
GALAXIES
GALAXY NUCLEI
IONIZING RADIATIONS
LUMINOSITY
MASSLESS PARTICLES
METALS
NEUTRON STARS
OPTICAL PROPERTIES
PHOTON EMISSION
PHOTONS
PHYSICAL PROPERTIES
PROBABILITY
RADIATIONS
SPATIAL DISTRIBUTION
STARS
X-RAY GALAXIES