CO in MASsive Spirals (CO-MASS): an IRAM 30m CO emission line survey of the CGM-MASS sample

TL;DR

This study investigates why extremely massive spiral galaxies in isolation exhibit low star formation despite substantial stellar and gas reservoirs. Using IRAM 30m CO$(J=1-0,2-1)$ observations of five CGM-MASS galaxies and a MW-like CO-to-H$_2$ conversion, the authors quantify total molecular gas and compare star formation efficiency to the Kennicutt–Schmidt relation, also examining the baryonic Tully–Fisher relation. They find systematically low SFEs and low $M_{\mathrm{H_2}}/M_{\mathrm{HI}}$ in these systems, with line ratios $R_{21}$ typically $\lesssim 0.7$, suggesting cooler, extended molecular gas; stellar mass loss and hot-mode accretion cannot sustain the observed $M_{\mathrm{H_2}}$, implying a past-merger/starburst origin for the gas. The CGM-MASS galaxies remain offset from the canonical BTFR, indicating either a turnover at the high-mass end or hidden baryons beyond the virial radius, underscoring the role of assembly history and morphology in regulating star formation and baryon content in massive disks.

Abstract

There exist extremely massive spiral galaxies in isolated environments, with stellar masses several times that of the Milky Way, yet their star formation rates (SFRs) are comparable to or even lower than that of the Milky Way. In this paper, we investigate the molecular gas properties of such galaxies to better understand the origin of their low SFRs. We present IRAM 30m CO observations of five extremely massive spirals from the CGM-MASS sample. We compare their star formation efficiencies (SFEs) with the Kennicutt-Schmidt relation and find that these massive spirals generally exhibit low efficiency in converting molecular gas into stars. We further compare their molecular gas masses with their atomic gas and stellar masses, and also include the CHANG-ES sample galaxies observed with the IRAM 30m telescope in a similar manner for comparison. Our sample galaxies show low efficiency in converting atomic to molecular gas and have lower molecular gas fractions, suggesting that their suppressed star formation stems from both limited gas supply and inefficient star formation. Considering potential cold gas sources in massive spirals, we argue that their current reservoirs likely originate from past starburst or merger events rather than ongoing accretion in present isolated environments. Finally, we examine the location of these galaxies on the baryonic Tully-Fisher relation, finding them baryon-deficient and deviating from the trend of lower-mass galaxies. This suggests either a significant undetected baryonic component or a flattening/turnover of the relation at the high-mass end, consistent with the stellar mass-halo mass relation.

Figures (14)

• Figure 1: left panel: DSS r-band image displays an $8.5^{\prime}~\times~8.5^{\prime}$ centered at UGCA 145. The solid circles are the location of IRAM 30m beams for the $^{12}\mathrm{CO}~J=1-0$ band with a diameter of $21.4^{\prime\prime}$. right panel: The integrated intensities of the $^{12}\mathrm{CO}~J=1-0$ (top row) and $^{12}\mathrm{CO}~J=2-1$ (bottom row) lines along the galaxy disk. The right y-axes of the top panel shows the column density of the molecular gas in these regions.
• Figure 2: Comparison of $\mathrm{^{12}CO}~J=1\text{--}0$ and $\mathrm{^{12}CO}~J=2\text{--}1$ integrated intensities. $I_{\rm^{12}CO_{10}}$ is corrected for beam dilution. The dashed and dotted lines indicate constant intensity ratios $R_{21}=1$ and $0.7$, respectively. Different symbols denote different galaxies.
• Figure 3: $\Sigma_{\mathrm{SFR}}$ v.s. $\Sigma_{\mathrm{H_2}}$. The solid line and the shaded area represents the best fitting of the star formation law and the 1 $\sigma$ uncertainty of the fitting provided in querejeta21. The dashed lines from top to bottom represent gas depletion time of $10^7$, $10^8$, $10^9$, and $10^{10}$ years, respectively.
• Figure 4: The distribution of $M_{\mathrm{HI+H_2}}$ with $M_{\star}$. The dashed line and shaded area represent the best-fitting linear relation and the 1$\sigma$ uncertainty of the fitting using the CHANG-ES data from Yan25.
• Figure 5: The distribution of $M_{\mathrm{H_2}}/M_{\mathrm{HI}}$ ratio with $M_{\star}$. The dashed line represents the result fitted using the CHANG-ES data from Yan25.