Blue Compact Dwarf Galaxies

Star formation in dwarf galaxies is not well understood. Generally, galaxies require gas to fuel star formation and a disturbance to their gas to form high density regions which then collapse and form stars. However, dwarf galaxies are stable against 2D instabilities (e.g. spiral arms); typical dwarf irregular galaxies rely on 3D processes to trigger their star formation (Elmegreen & Hunter 2015). Yet, there are dwarf galaxies with high star formation rates (compared to typical dwarf galaxies; Thuan et al. 1999) that are thought to require a large disturbance (e.g. a dwarf-dwarf merger) to trigger their burst of star formation. 

Haro 29.  Image credit: LITTLE THINGS

Haro 29.  Image credit: LITTLE THINGS

It is often suggested that the enhanced star formation rates in BCDs come from interactions with other galaxies or that they are the result of dwarf-dwarf mergers (Taylor 1997; Noeske et al. 2001; Pustilnik et al. 2001; Bekki 2008; Martínez-Delgado et al. 2012). Yet, there are still many BCDs that are relatively isolated with respect to other galaxies, making an interaction or merger scenario less likely (Taylor 1997; Nicholls et al. 2011; Simpson et al. 2011; Ashley et al. 2013). Other methods for triggering the burst of star formation in BCDs have been suggested, from accretion of intergalactic medium (IGM) to material sloshing in dark matter potentials (Wilcots & Miller 1998; Brosch et al. 2004; Simpson et al. 2011; Helmi et al. 2012; Verbeke et al. 2014), but what has triggered the burst of star formation in a majority of BCDs remains unknown. 

Determining how the bursts of star formation are triggered in BCDs is important for understanding dwarf galaxy evolution.  Many models have attempted to place BCDs on an evolutionary path to/from other types of dwarfs but have been largely unsuccessful in replicating the properties of observed dwarf galaxies (Papaderos et al. 1996; van Zee 2001; Tajiri & Kamaya 2002; Gil de Paz & Madore 2005). There are models that do replicate the properties of BCDs using methods of galaxy merging and consumption of IGM accretion (Bekki 2008 and Verbeke et al. 2014, respectively), however, these processes have not yet been observationally confirmed. If BCDs are formed through IGM accretion or dwarf-dwarf mergers, then they would be useful analogs for galaxy formation in the early universe. 

Haro 36.  Image credit: LITTLE THINGS

Haro 36.  Image credit: LITTLE THINGS

Part of my role as a LITTLE THINGS team member has been to study the atomic hydrogen (HI) Very Large Array (VLA) Telescope data of the six BCDs in the survey (Haro 29, Haro 36, Mrk 178, VII Zw 403, IC 10, NGC 3738).  The high velocity and angular resolution of the VLA HI data allows me to study the kinematics and morphology of the inner gaseous disk.  There I look for signatures of past interactions, mergers, consumption of intergalactic medium, and ram pressure stripping, such as: tidal tails, counter-rotating gaseous cores, external gas clouds.  Each of these processes could trigger a burst of star formation in the BCD. 

I have also collected Green Bank Telescope (GBT) atomic hydrogen data on each of the LITTLE THINGS BCDs.  The GBT data has a lower angular resolution than the VLA data, however, it has a much higher sensitivity than the VLA data.  The GBT is also able to reach low levels of atomic hydrogen surface density in a short time period.  Therefore, with the GBT data, I have searched a large region around the six BCDs for nearby gas clouds, extended gaseous emission in the disks of the BCDs, and faint, gaseous companions to the BCDs.  Nearby gas clouds could indicate that the BCD is consuming exterior gas to fuel its star formation.  The extended disk emission may contain faint signatures of past interactions, such as a tidal tail feature.  Finally, faint gaseous companions may indicate that the BCD has had a past interaction with the companion.  Interactions may disturb the gas enough to enhance star formation.

I am also leading observations using the Ultra Violet Imaging Telescope (UVIT) on AstroSat to search for faint young stellar features in the LITTLE THINGS BCDs.  Faint young stars in the extended emission of BCDs could help distinguish between features such as tidal tails (which would be expected to have a young stellar population: Neff et al. 2005; Smith et al. 2010) and intergalactic medium (which would not be expected to have a detectable young stellar population).

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 am working with Pamela Marcum on a set of 41 IEGs.  We are collecting HI data on these galaxies to search for their gas reservoirs.  We are also studying 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.


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