Bibcode
Ghosh, Kajal K.; Saripalli, Lakshmi; Gandhi, Poshak; Foellmi, Cédric; Gutiérrez, Carlos M.; López-Corredoira, Martin
Bibliographical reference
The Astronomical Journal, Volume 137, Issue 2, pp. 3263-3285 (2009).
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2
2009
Citations
11
Refereed citations
10
Description
A few nearby interacting galaxies are known that host elevated numbers
of ultraluminous X-ray sources. Here we report the results of a
multiwavelength study of the X-ray source population in the field of the
interacting pair of galaxies NGC 5774/5775. A total of 49 discrete
sources are detected, including 12 ultraluminous X-ray source candidates
with luminosities above 1039 erg s-1 in the
0.5-8.0 keV X-ray band. X-ray source positions are mapped onto optical
and radio images to search for potential counterparts. Twelve sources in
the field have optical counterparts. Optical colors are used to
differentiate these sources, which are mostly located outside the
optical extent of the interacting galaxies, as potential globular
clusters (2), one compact blue dwarf galaxy, and quasars (5). We
obtained optical spectra of two of the latter, which confirm that they
are background quasars. We expect three background sources in the field
of these two galaxies. These results are used to determine the true
X-ray population of these two interacting galaxies, which are connected
with two bridges. Two high-mass X-ray binaries are detected on these two
bridges, suggesting their formation through the interaction-induced star
formation episode. NGC 5774 is an extremely low star forming galaxy with
five X-ray sources plus three ultraluminous X-ray source candidates.
Observed X-ray population of this galaxy does not scale with the star
formation rate (SFR) alone but it may scale jointly with the mass of the
galaxy and the SFR. Twenty-four X-ray sources (excluding the active
galactic nucleus, AGN) are detected in NGC 5775. and its X-ray
luminosity function is consistent with that of other interacting
galaxies, suggesting that these galaxies have comparable numbers of
luminous sources. No X-ray point source was detected at the center of
this galaxy to a limiting luminosity of 3 × 1037 erg
s-1. Wind/outflow is detected from the central region of NGC
5775. Subsolar diffuse gas with temperature ~0.31 ± 0.04 keV is
present in this galaxy, which suggest that NGC 5775 is in the beginning
of the evolutionary process. Twelve ultraluminous X-ray source
candidates are detected within the D25 isophotes of NGC
5774/5775. Several of them are highly variable X-ray sources that fall
below the detection levels in one of two X-ray observations spaced 15
months apart. Two ultraluminous X-ray sources are located in the halo of
NGC 5775 and one of them is hosted in a globular star cluster. Four of
the remaining 10 candidates have powerlaw X-ray spectra with photon
indices around 1.8 and are extremely luminous with no optical
counterparts. One of these four objects is the brightest
(~1041 erg s-1) with a possible 6.2 hr period and
it varied by more than a factor of 500. Two of the rest six
ultraluminous X-ray source candidates are having steep-powerlaw X-ray
spectra and are embedded in diffuse Hα emission, which are
probably ionized nebulae. These nebulae could be due to energetic
supernova explosions or to continuous inflation by jets. Rest four
ultraluminous X-ray source candidates are flat-powerlaw X-ray sources
hosted in either young star clusters or bright star forming complexes.
Two of them are radio sources. Finally, we find that the number of
ultraluminous X-ray source candidates in interacting/merging galaxies
are correlated with the far-infrared, K-band, and UV luminosities of
their host galaxies, suggesting that the formation and evolution of
ultraluminous X-ray sources depend not only on the SFR but also on the
mass of their host galaxies.