The molecular and atomic hydrogen gas content of the Bootes Void galaxy CG 910

Abstract

Context. Void galaxies are located in the most under-dense 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. Although the stellar properties of void galaxies have been studied, very little is known about their cold gas content, both molecular (H2) gas and atomic hydrogen (H I) gas. Aims. We present CO (1─0) observations of the H2 gas disk in CG 910. CG 910 lies in the Boötes Void, one of the largest nearby voids, and is at relatively low redshifts (∼0.04─0.05). We selected CG 910 as it is a massive disk galaxy and early single-dish CO observations indicate that it has a high H2 gas mass. However, the H I content was not studied. Therefore, our aim was to map the cold disk, estimate the H I mass (and hence the total gas mass) in CG 910, and study the CO gas distribution along with the velocity field. Methods. We used the Combined Array for Research in Millimetre 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 stellar content of the galaxy and the star formation rate were derived using archival optical data. Results. The CO(1─0) observations from CARMA reveal a molecular gas disk with a H2 mass of ∼12.0 ± 1.1 × 109 M⊙ and a diameter of 7 kpc. The CO velocity field shows a regularly rotating disk with a flat rotation velocity of 256 kms−1 and 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 H I disk with a H I mass of ∼3.1 ± 0.8 × 109 M⊙, which is relatively low compared to the galaxy stellar mass (M★) of ∼21.5 × 109 M⊙. The total gas mass fraction, (M(H2)+M(HI))/M★, and the atomic gas mass fraction, M(H I)/M★, for CG 910 are 0.70 and 0.14, respectively. Conclusions. We conclude that CG 910 has a regularly rotating but massive molecular gas disk, which is more massive than the H I disk. The lower atomic gas mass fraction and star formation rate indicate a longer gas depletion timescale, confirming that, like most void galaxies, CG 910 is evolving more slowly than normal disk galaxies.