Mousumi Das

CO Detection and Millimeter Continuum Emission from Low Surface Brightness Galaxies

We present BIMA and IRAM CO (1–0) observations of seven low surface brightness (LSB) galaxies, including three large spiral galaxies with faint disks but prominent bulges and four relatively small LSB galaxies with irregular disks. The giant LSB galaxies are UGC 5709, UGC 6614, and F568-6 (Malin 2). The smaller LSB galaxies are NGC 5585, UGC 4115, UGC 5209, and F583-1. The galaxies were selected based on their relatively high metallicity and apparent signs of star formation in their disks. The BIMA maps suggested the presence of molecular gas in two of the giant LSB galaxies: F568-6 and UGC 6614. Using the 30 m IRAM telescope, we detected CO (1–0) emission in the disks of both galaxies and in the nucleus of F568-6. The molecular gas in these galaxies is clearly offset from the nucleus and definitely associated with the LSB disk. In addition, we detected a millimeter continuum source in the center of UGC 6614. When compared with Very Large Array (VLA) 1.5 GHz observations of the galaxy, the emission was found to have a flat spectrum, indicating that the millimeter continuum emission is most likely due to an active galactic nucleus (AGN) in the galaxy. Our results show that giant LSB spiral galaxies may contain significant quantities of molecular gas in their disks and also harbor radio-bright AGNs in their centers. Subject headingg galaxies: individual (F568-6, UGC 6614) — galaxies: ISM — ISM: kinematics and dynamics — ISM: molecules — radio lines: galaxies

Millimeter Observations of GRB 030329: Continued Evidence for a Two-Component Jet

We present the results of a dedicated campaign on the afterglow of GRB 030329 with the millimeter interferometers of the Owens Valley Radio Observatory (OVRO) and the Berkeley-Illinois-Maryland Association (BIMA), and with the MAMBO-2 bolometer array on the IRAM 30 m telescope. These observations allow us to trace the full evolution of the afterglow of GRB 030329 at frequencies of 100 and 250 GHz for the first time. The millimeter light curves exhibit two main features: a bright, constant flux density portion and a steep power-law decline. The absence of bright, short-lived millimeter emission is used to show that the GRB central engine was not actively injecting energy well after the burst. The millimeter data support a model, advocated by Berger et al., of a two-component jetlike outflow in which a narrow-angle jet is responsible for the high-energy emission and early optical afterglow, and a wide-angle jet carrying most of the energy is powering the radio and late optical afterglow emission.

Central Mass Concentration and Bar Dissolution in Nearby Spiral Galaxies

We use data from the BIMA Survey of Nearby Galaxies (SONG) to investigate the relationship between ellipticity and central mass concentration in barred spirals. Existing simulations predict that bar ellipticity decreases as inflowing mass driven by the bar accumulates in the central regions, ultimately destroying the bar. Using the ratio of the bulge mass to the mass within the bar radius as an estimate of the central mass concentration, we obtain dynamical mass estimates from SONG CO 1-0 rotation curve data. We find an inverse correlation between bar ellipticity and central mass concentration, consistent with simulations of bar dissolution.

Starbursts triggered by central overpressure in interacting galaxies

We propose a physical mechanism for the triggering of a burst of massive-star formation in the central regions of interacting spiral galaxy pairs. Our mechanism is based on the detailed evolution of the interstellar medium in a galaxy following an encounter. We show that as a disk giant molecular cloud tumbles into the central region of the galaxy following a galaxy encounter, it undergoes a radiative shock compression by the preexisting high pressure of the central molecular intercloud medium. When the growth time for the gravitational instabilities in the shocked outer shell of a cloud becomes smaller than the crossing time of the shock, the shell becomes gravitationally unstable, which results in a burst of star formation. The resulting luminosity depends linearly on the fraction of the cloud mass that is shock compressed, the efficiency of star formation in the shocked gas, and the gas infall rate from the galactic disk. If a galaxy with preencounter gas parameters as in the Galaxy were to undergo distant atidal encounter, our mechanism yields a lower limit to the expected central infrared luminosity of

BIMA MILLIMETER-WAVE OBSERVATIONS OF THE CORE-JET AND MOLECULAR GAS IN THE FR I RADIO GALAXY NGC 3801

\sim2-6 x 10^9 L_\odot

The Central Velocity Field in NGC 253: Possible Indication of a Bar

and the infrared luminosity-to-gas mass ratio of

CHANDRA OBSERVATIONS OF NUCLEAR X-RAY EMISSION FROM LOW SURFACE BRIGHTNESS GALAXIES

\sim

Detection of extended molecular gas in the disk of the LSB galaxy Malin 2

a few

Internal heating of molecular clouds by tidal fields

L_\odot/M_\odot

Collisional buildup of molecular clouds: mass and velocity spectra

. These results are in reasonable agreement with observations of central starbursts in interacting galaxies. The evolved mergers of galaxies, with their higher central gas concentrations, would yield higher values of