Zhongli Zhang, Marat Gilfanov, Akos Bogdan
We investigate the dependence of the LMXB population in early-type galaxies
on the stellar age. We selected 20 massive nearby early-type galaxies from the
Chandra archive occupying relatively narrow range of masses and spanning broad
range of ages, from 1.6 Gyr to more than 10 Gyrs, with the median value of 6
Gyrs. With ~ 2000 X-ray point sources detected in total, we correlated the
specific number of LMXBs in each galaxy with its stellar age and globular
cluster content. We found a correlation between the LMXB population and the
stellar age -- older galaxies tend to possess about ~ 50% more LMXBs (per unit
stellar mass) than the younger ones. The interpretation of this dependence is
complicated by large scatter and a rather strong correlation between the
stellar age and the globular cluster content of galaxies in our sample. We
present evidence suggesting that the more important factor is the evolution of
the LMXB population with time. Its effect is further amplified by the larger
globular content of older galaxies and correspondingly, larger numbers of
dynamically formed binaries in them. We also found clear evolution of the X-ray
luminosity function with age, that younger galaxies have more bright sources
and fewer faint sources per unit stellar mass. The luminosity function of LMXBs
in younger galaxies appears to extend significantly beyond E39 erg/s. Such
bright sources seem to be less frequent in older galaxies. We found that 3 out
of ~ 8 (ultra-) luminous sources are located in globular clusters.
View original:
http://arxiv.org/abs/1202.2331
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