CGs represent ideal systems to study galaxy evolution due to their intense gravitational interactions compared to galaxies in the field. In some cases, these interactions also cause large masses of gas to be stripped out of the galaxies and deposited into the space between them. Here we have presented an overview of gas processing in the CG environ- ment, beginning with a theoretical background covering the mass-fraction, distribution, temperature, and phase of the gas in galaxies and their environments. We then discussed observational results of studies concerning the cold molecular (H2), cool neutral (H i), warm ionized, and hot ionized gas in these systems. However, there are several outstand- ing questions with respect to the hot gas that must be addressed to better understand how gas is processed in the CG environment.
1. What is the distribution of the X-ray emitting gas? As described by Tamburri et al. (2012), the hot gas in HCG 79 seems to be isolated to the individual galaxies. Is this the case in most if not all CGs? The result of this question o↵ers insight into the importance of the individual galaxies in heating the IGM in low-mass systems.
2. How important are the tidal interactions in these groups in the buildup of the hot IGM? Strong interactions can liberate vast amounts of cool gas from the disks of gas-rich spirals (e.g., Verdes-Montenegro et al. 2001), but it is unclear how the CG environment heats that gas to X-ray temperatures, i.e., is virialization, tidal heating, or winds from galaxies the primary heating mechanism, or do we need to consider a combination of these?
3. What is the fraction of baryons contained in the hot IGM? In galaxy clusters, the baryon fraction of the stars is estimated to be 10–20% (Balogh et al. 2001; Lin et al.
2003), while a significant amount of mass is tied up in the hot ICM. Are groups, and in particular CGs, similar, or is the phase of the baryons much di↵erent from clusters owing to the lower masses of the systems? Tracking the baryons and their physical properties helps to put constraints on the formation and evolution of these systems.
4. What e↵ect, if any, does the intragroup gas have on galaxy evolution? CG galaxies have been found to have several unusual properties compared to other samples (e.g., the mid-infrared color bimodality). If the environment is the cause, then perhaps there is a link between the hot gas properties of the CGs and the galaxies that reside within them. For example, ram-pressure stripping may cause accelerated evolution in CGs if the hot gas in the intragroup medium has a high density. Comparing the group hot gas luminosity with the aggregate CG galaxy properties may elucidate the importance of the hot intragroup medium on galaxy evolution in dense environments.
Analysis of data from modern X-ray observatories such asChandraandXMM-Newton, with their superior spatial resolution and sensitivity to low-temperature di↵use gas, re- spectively, will give further insight into the properties of the hot gas in CGs. We can then begin to address these questions and link the hot X-ray properties to the gas in other phases to study the evolution of gas in the low-mass, CG environment. In Chapters 2 and 3, we will discuss theChandraobservations of di↵use hot gas in CGs and study how it relates to other observed properties of these systems, e.g., the cool gas fraction, star formation rates, and baryonic masses.
Furthermore, we can use these facilities to explore the X-ray properties of galaxies in dense environments. Doing so may reveal more about the star formation histories and buildup of stellar mass in such environments and help us to answer lingering questions about the evolution of galaxies in these systems. We may then ask ourselves:
5. How do the X-ray luminosity scaling relations with stellar mass and star formation rate compare for galaxies in dense environments, i.e., CGs or the Coma cluster infall region, against the expected relations from other environments? Does the compact environment impact the X-ray emission in CG galaxies as it does at other wavelengths (e.g., the mid-infrared color gap)? What other physical aspects of the galaxies, if any, may a↵ect the X-ray emission? These questions in particular shed light on the importance of environment on the X-ray observables in galaxies.
6. How does the AGN fraction of CG galaxies compare to other environments? Tzanavaris et al. (2014) recently examined the X-ray identified AGNs in CGs, while other papers (e.g., Gallagher et al. 2008; Mart´ınez et al. 2010; Sohn et al. 2013) have examined the AGN populations of CGs using mid-infrared and optical diagnostics. If one uses multi-wavelength selection methods in a similar environment, i.e., the Coma cluster infall region, how do the AGN fractions, and the properties of the AGN themselves, compare with the CG environment?
In Chapter 4 we will explore the X-ray properties of galaxies in the Coma cluster, which has already been used several times as a comparison sample for galaxies in CGs, and investigate how the X-ray emission scales with stellar mass and star formation rate. We also study the AGN fraction and properties of galaxies identified as AGN in the Coma infall region and compare them against CGs.
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