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. 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 include accretion of intergalactic medium (IGM) and 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, observationally it remains unclear what the starburst trigger is for all BCDs. 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 much higher sensitivity than the VLA data.  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).

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