Research Interests
I am interested in the formation and evolution of galaxies, particularly
bulges of spiral galaxies. Dr. Jon Holtzman and I use long-slit spectra and
images from the ARC 3.5m telescope to study the stellar populations,
kinematics, and structural properties of bulges as a means of constraining
their formation scenarios.
In my PhD thesis, I presented the results of a study aimed at understanding
the formation mechanisms of bulges through optical long-slit spectroscopy and
imaging. Long-slit spectra of 38 bulges taken with the ARC 3.5m telescope were
used to obtain luminosity-weighted age and metallicity gradients as well as
line-of-sight velocity distributions. Bulge-to-disk decomposition was
performed using images obtained with the same telescope to study the
structural properties and to separate the bulge and disk contributions.
Several bulges previously found to have similar colors as their disks were
included to see if their spectral properties would reveal evidence for secular
evolution.
We found that red bulges of all Hubble types are similar to luminous
ellipticals in their central stellar populations. They have large
luminosity-weighted ages, metallicities, and alpha/Fe ratios. Blue bulges can
be separated into a metal-poor class that is restricted to late-types with
small velocity dispersion and a young, metal-rich class that includes all
Hubble types and velocity dispersions. Luminosity-weighted metallicities and
alpha/Fe ratios are sensitive to central velocity dispersion and maximum disk
rotational velocity. Red bulges and ellipticals follow the same scaling
relations. We see differences in some scaling relations between blue and red
bulges and between bulges of barred and unbarred galaxies. Most bulges have
decreasing metallicity with increasing radius; galaxies with larger central
metallicities have steeper gradients. Where positive age gradients (with the
central regions being younger) are present, they are invariably in barred
galaxies. The metallicities of bulges are correlated with those of their
disks. Nearly all our bulges rotate fast with no discernible difference in
the rotational velocities of the stellar and ionized gas components. The
ratio of maximum rotational velocity to velocity dispersion ranges from 0.5 to
1 in a majority of our bulges, with a handful of bulges lying outside these
two extremes. We see no correlation between a large ratio of rotational
velocity to velocity dispersion do not show and disk-like (young and/or
metal-poor) stellar populations. Neither do we see any correlation between
kinematics and morphology; bulges of barred galaxies and bulges that are
structurally disk-like do not appear distinct from other bulges in their
kinematics.
While some of our observations indicate that secular evolution cannot be
ignored, our results are generally consistent with the hypothesis that bulge
formation was dominated by a single mechanism, presumably mergers.
Publications
Stellar
Populations in Bulges
B. K. Moorthy & J. A, Holtzman 2006, MNRAS,
submitted
The Rotation Curves of Dwarf Galaxies: A Problem for Cold Dark Matter?
G. Rhee, O. Valenzuela, A. Klypin, J. Holtzman, & B. Moorthy 2004,
ApJ, 617, 1059
The First Data Release of the Sloan Digital Sky Survey
K. Abazajian, [92 authors], B. Moorthy, [84 authors] 2004,
AJ, 126, 2108
Meetings
July 2005: Conference on Island Universes: Structure and Evolution of Disk
Galaxies, Bulges of
Spiral Galaxies: Stellar Populations, Structure and Kinematics
B. K. Moorthy
July 2005: Conference on The Formation of Disk Galaxies, Bulges of
Spiral Galaxies: Stellar Populations, Structure and Kinematics
B. K. Moorthy
October 2003: Conference on Stellar Populations, Stellar Populations in Bulges of Spiral Galaxies
B. K. Moorthy & J. A. Holtzman
January 2002: American Astronomical Society meeting, New Results on Line Strength Gradients in Bulges
B. K. Moorthy & J. A. Holtzman
January 2001: American Astronomical Society meeting, Line Strength Gradients in Spiral Bulges
B. K. Moorthy & J. A. Holtzman
Future Work
I successfully defended my PhD Thesis, Bulges of Spiral Galaxies: Stellar
Populations, Structure, Kinematics, and Dynamics, on July 7, 2006.
Starting in August 2006, I will be an Assistant Professor at William Rainey
Harper College (outside of Chicago). I will be teaching introductory
astronomy and physical science courses, and assisting in the development of
new astronomy courses and observatory programs.
