The Molecular and Atomic Hydrogen Gas Content of the Bo\"otes Void galaxy CG 910

Abstract

Void galaxies are located in the most underdense environments of the Universe, where the number density of galaxies is extremely low. They are, hence, good targets for studying the secular evolution of galaxies and the slow buildup of stellar mass through star formation. To date, very little is known about their cold gas content, both molecular (H$_2$) gas and atomic hydrogen (HI) gas. We present CO (1--0) observations of the H$_2$ gas disk in CG 910, which lies in the Bo\"otes void, one of the largest nearby voids at relatively low redshifts (z$\sim$0.04-0.05). We used the Combined Array for Research in Research in Millimeter Astronomy (CARMA) to study the CO(1-0) distribution and gas kinematics in CG 910. We also carried out atomic hydrogen observations of the galaxy using the Robert C. Byrd Green Bank Telescope (GBT). The CARMA CO(1-0) observations reveal a molecular gas disk of mass, M(H$_{2}$) $\sim$12.0$\pm$1.1$\times$10$^{9}$M$_{\odot}$ and diameter 7 kpc. The CO velocity field shows a regularly rotating disk with a flat rotation velocity of 256 kms$^{-1}$ with no clear signatures of interaction or gas accretion. This is possibly the first CO (1-0) map of a void galaxy, and hence, important for understanding the molecular gas distribution and kinematics in void galaxies. The GBT observations reveal a HI disk of mass, M(HI) $\sim$3.1$\pm$0.8$\times$10$^{9}$M$_{\odot}$, which is relatively small compared to its stellar mass of M$_{\star}\sim$21.5$\times$10$^{9}M_{\odot}$. The total gas mass fraction, (M(H$_2$)+M(HI))/$M_{\star}$ and the atomic gas mass fraction, M(HI)/M$_{\star}$ for CG 910 are 0.70 and 0.14, respectively. We conclude that CG 910 has a regularly rotating but massive molecular gas disk. The lower atomic gas mass fraction and star formation rate indicate a longer gas depletion timescale, confirming that CG 910 is slowly evolving compared to normal disk galaxies.