Isolated Early-Type Galaxies

Early-type galaxies (ETGs) are bulge dominated systems that generally have little gas and ongoing star formation.  ETGs typically reside in cluster environments. Cluster environments are thought to nurture the bulge formation in ETGs through interactions and mergers. However, there is a subset of ETGs that are extremely isolated; these isolated early-type galaxies (IEGs) make up less than 0.01% of the total elliptical galaxy population. These rare galaxies are important to understanding the nature of how ETGs form and evolve in the absence of a cluster/group environment. 

IEGs tend to have significantly bluer colors than typical ETGs (Marcum et al. 2004; Niemi et al. 2010). The bluer colors could be due to ongoing star formation.  Stars form from gas, yet, early-type galaxies typically have very little gas content. Also, in such extreme isolation, it is unclear where IEGs would be obtaining gas with which to form stars. H I detections are not rare in non-cluster ETGs: Serra et al. (2012) detect H I in 40% of their ETGs, Morganti et al. (2006) detect H I in 8 of their 12 ETGs, and Grossi et al. (2009) detect H I in 13% of dwarf ETGs and 44% of luminous ETGs. Yet, IEGs can be so extremely isolated from other bright galaxies (Marcum et al. 2004; Fuse et al. 2012) that it would be difficult for them to obtain a gas reservoir from which they can form stars. It is possible for these IEGs to have gas if they are young and are consuming the remainder of their gas (van Driel & van Woerden 1991; Barnes 2002; Niemi et al. 2010; Serra et al. 2012). Other reservoirs of H I external to IEGs may include: extragalactic filaments (Kereš et al. 2005; Macciò et al. 2006), dwarf companion galaxies, or gas expelled from the IEG itself through a previous starburst or merging event.

I have worked with Pamela Marcum on a set of 41 IEGs.  We collected HI data on these galaxies to search for their gas reservoirs.  We also studied their dust content using their spectral energy distributions (SEDs).  Each galaxy can be modeled to determine the amount of dust attributed to each dust production process and to determine if it is likely that an early-type galaxy that is the result of a merger.

 

References
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