High-Resolution Velocity Fields of Low-Mass Disk Galaxies

This study aims to examine the relative distributions of dark and baryonic matter as a function of star formation history in a representative sample of low mass disk galaxies. In this study, we present the high-resolution 12CO(J=1 → 0) interferometry for a sample of 26 nearby dwarf galaxies, which were obtained from the Combined Array for Research in Millimeter-wave Astronomy (CARMA). Among these 26 galaxies, 14 have good CO detec- tions, including 6 galaxies previously detected in single-dish CO measurements and 8 newly detected ones. We find a linear correlation between the CO flux and the mid- and far-IR flux from the WISE and IRAS catalogs. Compared to the far-IR flux, the mid-IR flux may be a better indication of whether a galaxy contains sufficient CO for detection at the level of instrument sensitivity of CARMA. This correlation might prove to be useful in future studies to help choosing other CO targets for observation. The median molecular mass (including helium and H2) of our galaxies is 2.8 × 108M⊙, which is consistent with past ob- servations for dwarf galaxies. The molecular content is weakly correlated with the dynamical mass, r-band luminosity and size of the galaxies. The median ratios of molecular mass vs. dynamical mass and molecular mass vs. r-band luminosity are Mmol/Mdyn ≈ 0.035 and Mmol/Lr ≈ 0.078M⊙/Lr,⊙, respectively, which are also consistent with past observations for dwarf galaxies. In addition, we present the rotation curves of these 14 galaxies. To examine the dark matter distribution in their central regions, the reduced CO data were fitted with simple kinematic models using two different algorithms. For all 14 galaxies, despite inhomo- geneous distribution of molecular gas in some of the sources, robust kinematic results were obtained for all sources. Most galaxies show approximately zero or small noncircular mo- tions, particularly the ones with smooth spatial distributions of CO emission. Furthermore, consistent rotation curves are obtained using both algorithms. In general, the CO rotation curves are consistent with Hα rotation curves of the same galaxies. Using the CO rotation curves, along with information on the stellar distribution from optical and infrared imaging, we model the velocity data including contributions from stars and the dark matter halo, which we parameterized using a generalized Navarro, Frenk & White profile. The results show that the inner power-law slope α of the density profile varies over a large range from below 0.38 (cored) to 1.76 (cuspy), with a mean value of 0.58 ± 0.45 (mean ± scatter) or 0.64 ± 0.49 if we assume the stellar distribution derived from r-band data or IRAC 4.5- micron data, respectively. The density profile slope is generally robust as the baryonic M/L is varied from minimum- to maximum-disk estimates. Our galaxies show low stellar mass- to-light ratios: M∗/L = 0.10±0.02 for IRAC channel-2 data and 0.49±0.33 for r-band data. Considering our results in combination with recent studies from the literature, we find weak correlations between the dark matter profile inner slope, dynamical mass and distance of the galaxies, where more distant and more massive galaxies have steeper slopes. In addition, we find no statistically significant correlation between the slope and the stellar mass, which may suggest that baryonic feedback models alone cannot fully explain the flattening behavior of the inner profiles of dwarf galaxies.

Advisor: Blitz, Leo
Lee, Adrian