My research focuses on investigating interacting galaxies and their interaction-induced star formation, through both observations and theories. These galaxies play a very important role in galaxy formation and evolution. I am currently working with Dr. Joshua E. Barnes on my dissertation- Star Formation Histories in Merging Galaxies. My dissertation combines two fundamental areas of astrophysics- stellar evolution and galactic dynamics- to investigate the triggers of star formation in merging galaxies. I use Low Resolution Imaging and Spectrograph (LRIS) on Keck Telescope on Mauna Kea to observe star clusters and young globular clusters in merging galaxies as part of my dissertation. I am also a team member of the Great Observatory LIRGS Survey (GOALS), collaborating with Prof. Aaron S. Evans at the University of Virginia. I also work with Dr. Lisa J. Kelwey in understanding the metallicity gradient and the chemical evolution in merging galaxies.
Last Updated: January 8, 2009
Modeling The Atoms of peace galaxy- NGC 7252
NGC 7252 is a very well studied proto-elliptical merger remnant observationally. It has two long tidal tails, many shells and ripples around its center and a central molecular disk with violent star formation in its central regions. We present a new dynamical model with two star formation prescriptions- density-dependent and shock-induced star formation rules. In our models, two similar gas-rich disk galaxies fell together ~620 Myr ago. In general the two star formation rules predict significantly different star formation histories and using young clusters in NGC 7252 (Schweizer & Seitzer 1998) helps us determine which mechanism is dominant in triggering star formation in NGC 7252. Our analysis shows that shock-induced star formation plays an important role in its history (Chien & Barnes 2009).
Ages and metallicities in a series of merging galaxies
NGC 4676 is a merging system which past its first encounter but has not yet had its second encounter. Its nice long tails and its appearance earned a pretty name of The Playing Mice Galaxies. I have obtained spectra of 12 young clusters in the Mice and they yield reliable age and metallicity determinations (Chien et al. 2007). Among the ages we obtained two are ~170 Myr old, which suggests that they likely formed during the first passage (Barnes 2004). These two older objects are located in the tidal tails of the pair, which is consistent with the spatial distribution of star formation predicted by shock-induced models.
Multi-object spectroscopy of young star clusters
I have obtained cluster spectra for a series of merging galaxies, ranging from early stages (Arp 256, NGC 7469) through merging (Arp 299) to fully merged (NGC 2623, IC 883) systems. Top figure shows spectra of 6 YGCs in the merged system IC 883. These YGCs have ages of ~100-300 Myr (Chien et al. 2009), generally older than the YGCs in NGC 4676, which has not yet had a second encounter. On the other hand, young clusters in early interacting systems such as Arp 256 and NGC 7469 show ages of <50 Myr. These ages will provide a crucial way to discriminate between the alternate star formation histories predicted by the two rules. Detailed analysis of ages and metallicities of young star clusters in these galaxies will soon be published (Chien et al. 2009).
Optical and UV properties of merging galaxies
Bolometric luminosity is a good tracer of the star formation rate since it comes largely from young massive stars. Different encounter geometries (colors) yield different star formation histories, but choice of star formation rules is clearly a more important factor. In shock-induced simulations (top), a global burst is triggered by large-scale shocks during the first passage at T=0; later bursts of star formation, concentrated within the central regions, occur at the second passage and merger ~500-600 Myr. In contrast, density-dependent models (bottom) generally predict a rather gradual increase within the central regions of the galaxies following the first passage; only the low-inclination passage (black) shows a starburst before the galaxies fall back together and merge.