Abstract
We examine X-ray emission produced from hot gas during collisions and mergers of disk galaxies. To study this process, we employ simulations that incorporate cosmologically motivated disk-galaxy models and include the effects of radiative cooling, star formation, supernova feedback, and accreting supermassive black holes. We find that during a merger, the colliding gas in the disks is shock heated to X-ray-emitting temperatures. The X-ray luminosity is spatially extended, rises during the initial stages of the merger, and peaks when the galactic centers coalesce. When a physical model for accreting black holes is included, the resulting feedback can drive powerful winds that contribute significantly to the amount and metallicity of hot gas, both of which increase the X-ray luminosity. In terms of their stellar kinematics and structural properties, the merger remnants in our simulations resemble elliptical galaxies. We find that the X-ray luminosities of the remnants with B-band luminosities in the range LB ~ 1010-1011 L☉ are consistent with observations, while remnants with smaller or larger masses are underluminous in X-rays. Moreover, because the majority of the merger remnants are broadly consistent with the observed scaling relations between temperature, B-band luminosity, and X-ray luminosity, we conclude that major mergers are a viable mechanism for producing the X-ray halos of large, luminous elliptical galaxies.
Highlights
It is well established that elliptical galaxies contain substantial quantities of hot gas
Because the majority of the merger remnants are broadly consistent with the observed scaling relations between temperature, B-band luminosity and X-ray luminosity we conclude that major mergers are a viable mechanism for producing the X-ray halos of large, luminous elliptical galaxies
Detailed studies of the X-ray emission produced by this hot gas have revealed that its luminosity is correlated with the gas temperature, B-band luminosity, and stellar velocity dispersion of the host elliptical galaxy (O’Sullivan, Forbes, & Ponman 2001a; O’Sullivan, Ponman, & Collins 2003)
Summary
It is well established that elliptical galaxies contain substantial quantities of hot gas (see Mathews & Brighenti 2003, and references therein). Humphrey & Buote (2006) analyzed a sample of 29 X-ray luminous elliptical galaxies and found that their two-phase models for the hot Xray emitting plasma yielded metallicities that were typically solar or even higher and generally correlated with the stellar metallicity This result indicates that the gas that makes up the hot coronae surrounding ellipticals has been recycled from star-forming gas and likely once resided within the dense central regions of the galaxies. The ability of this hot gas to produce X-rays, the luminosity of the X-ray emission, and the correlation to the remnant stellar properties are all open questions These studies did not attempt to model all feedback processes in detail.
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