(808) 956-6664 (phone) (808) (fax) email: kchib AT ifa.hawaii.edu
A newly discovered dwarf galaxy in M81, hiding behind galactic cirrus
I. Searching for resolved dwarfs in the nearby M81 group (with Brent Tully and Igor Karachentsev): Because the M81 group is so nearby (z = 0.0001), galaxies resolve into stars. Thus, any galaxy, no matter how small or faint, should be detectable through its component stellar population. In this way we can probe as far down into the faint-end of the galaxy luminosity function as there are galaxies with stars! To do this we have imaged a 65 square degree region around M81 using the CFHT/Megacam in the r' band. Candidate group members are chosen on the basis of their degree of stellar resolution and followed up using other telescopes/instruments. We have discovered 22 new candidates including 3 blue compact dwarfs, one potential tidal dwarf, and a number of dSph and dIs, the faintest of which, if real, has an absolute r' magnitude of -6.7. Follow-up imaging with the HST has confirmed 8 members, we are awaiting Cycle 17 data to complete this work. Including all candidates, we find a shallow faint-end slope of -1.28 (Chiboucas et al. 2008).
II. Going deep in Coma (with B. Tully and the ACS Coma Cluster Survey Team) : My work in the ACS Coma Cluster Treasury Survey involves studying in detail the dwarf galaxy population in this nearby rich cluster of galaxies down to B=27.3. HST images using the Advanced Camera for Surveys have been obtained of both the core and infall regions. These data are being complemented by numerous observations in other wavelength regions. Deep spectroscopic observations of the dwarf galaxes are underway using Hectospec on MMT and DEIMOS/LRIS on Keck to measure redshifts and velocity dispersions of faint low surface brightness dwarfs and candidate ultra-compact dwarfs (UCDs). We show here an LRIS spectroscopically confirmed UCD member of the Coma Cluster (of 16 discovered to date). The overall goals of this project are to study the structure and star forming properties of dwarf galaxies in such a rich environment, and to study extreme dwarfs fainter than ever studied before in a rich environment.
III. Dwarf galaxy populations in groups within the Local Supercluster (with B. Tully, N. Trentham, A. Mahdavi):
My research at Gemini Observatory and in collaboration with the Gemini/HST Cluster project is on galaxy evolution. Along with Inger Jorgensen, Jordi Barr, Marcel Bergmann, Roger Davies, Maela Collobert, and others, we are studying the star formation and assembly history of cluster galaxies as a function of redshift. Specifically we have obtained imaging and spectroscopy for galaxies in 15 galaxy clusters at redshifts ranging from z = 0 to 1, when the universe was only half its age. We are investigating the evolution of galaxy scaling relations, in particular the Fundamental Plane and line index relations. Our two z ~ 0.85 clusters exhibit a steeper FP slope than our low redshift comparison sample. We take this to be evidence of downsizing in which lower mass galaxies have undergone more recent star formation and are overluminous in comparison to their low redshift counterparts (Jorgensen et al. 2006 and 2007). The FP for our z=0.28 cluster displays a large scatter which may indicate star formation having occurred in bursts over a range of epochs in this cluster ( Barr et al. 2006). Our spectral line studies have shown that all three clusters have enhanced alpha to iron element abundance also indicative of star formation occuring within short bursts. This is difficult to reconcile with passive evolution models and it is moreover even more difficult to envision a scenario in which these higher redshift cluster galaxies will evolve to look like galaxies in the local universe (Jorgensen et al. 2005 and Barr et al. 2005).
Centaurus Cluster LF
My research at the University of Michigan with Dr. Mario Mateo involved surveying clusters of galaxies in search of low surface brightness dwarf galaxies. We measured the faint-end slope of the galaxy luminosity function in 5 nearby galaxy clusters and found at least 3 of the clusters (A262, A1367, A3526) had a similar slope of -1.4 +- 0.2. A fourth cluster (A3537) appeared to consist of a superposition of 2 or 3 groups or clusters each having a slope of ~-1.4. We do not find evidence for a very steep slope as has been found in a few earlier studies. Theoretical models predict a steep slope of -1.8. Our results indicate there is either a problem with CDM hierarchical models of galaxy formation, or with models which describe how mass will be turned into light. These models depend on mechanisms such as star formation efficiency and feedback in low mass galaxies which are not well understood. It may also be possible that later factors, such as reionization, effect the low mass galaxy populations. If theoretical models are correct, this may imply that there exists a large population of dark halos or difficult to detect very low surface brightness galaxies.
Given the shallow slope we find and assuming a M/L ratio typical of local group dwarf galaxies, dwarf galaxies and their massive dark halos cannot account for a significant fration of the dark matter content known to exist in clusters.
The slope we have found for cluster galaxies is larger than that found in most field galaxy studies. This could either be due to primordial variation or later environmental effects which either promote dwarf galaxy formation in clusters (through creation in tidal tails or transformation of more massive galaxies) or support dwarf galaxies against destruction (through pressure confinement or from earlier collapse and formation pre-reionization due to the higher density environment). Results are presented in a series of papers:
Chiboucas and Mateo 2007
Chiboucas and Mateo 2006
Chiboucas and Mateo 2008
For a somewhat technical description of that project, click here. For a very brief description of this project, you may check out the abstract we submitted to the AAS for the 1997 winter conference. To see the entire poster we presented, click here.This work was also presented at the IAU Colloquium 171, The Low Surface Brightness Universe. You may download the conference proceedings paper.
To see a color image of a dwarf, click here. The dwarf is the blue smudge at the bottom below 3 much larger galaxies.
I worked on a project in collaboration with Kaspar von Braun, Kelly Minske, Jose Salgado, and Dr. Guy Worthey to determine relationships between optical and infrared colors as a function of metallicity for metal-poor giants. You may download a copy of our paper "Color-color Relations for Red Giants in Star Clusters" published in PASP.
I am also involved in work on an image database for trouble shooting
while at the telescope. This database contains examples and explanations for
images in which something went wrong. Wherever possible, solutions for
fixing the problems are
also supplied. To check out this collection of 'lousy' images, click
here or you can download our paper which was published in the