Abstract
Abstract We present results from an Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 2 program to map CO(2−1) emission in nearby early-type galaxies (ETGs) that host circumnuclear gas disks. We obtained ∼0.″3 resolution Band 6 observations of seven ETGs selected on the basis of dust disks in Hubble Space Telescope images. We detect CO emission in five at high signal-to-noise ratio with the remaining two only faintly detected. All CO emission is coincident with the dust and is in dynamically cold rotation. Four ETGs show evidence of rapid central rotation; these are prime candidates for higher-resolution ALMA observations to measure the black hole masses. In this paper, we focus on the molecular gas and continuum properties. Total gas masses and H2 column densities for our five CO-bright galaxies are on average ∼108 M ☉ and cm−2 over the ∼kpc-scale disks, and analysis suggests that these disks are stabilized against gravitational fragmentation. The continuum emission of all seven galaxies is dominated by a central unresolved source, and in five we also detect a spatially extended component. The ∼230 GHz nuclear continua are modeled as power laws ranging from to within the observed frequency band. The extended continuum profiles of the two radio-bright (and CO-faint) galaxies are roughly aligned with their radio jet and suggest resolved synchrotron jets. The extended continua of the CO-bright disks are coincident with optically thick dust absorption and have spectral slopes that are consistent with thermal dust emission.
Highlights
An early consensus held that early-type galaxies (ETGs, encompassing elliptical and S0 galaxies) were nearly devoid of gas and dust (Hubble 1926; de Vaucouleurs 1959; Sandage 1961)
Comparing the velocity profiles to the position-velocity diagram (PVD), we find that most of the contributions to the peaks of the double-horned profiles originate far ( 100 pc) from the disk centers, at which radii the extended stellar mass profiles dominate over the black hole (BH) mass
Our Cycle 2 ETGs were selected based on morphologically round dust disks and, for the CO-bright subsample, their molecular gas disks co-rotate with the stars; based on these observations alone, we find that their in situ formation is as plausible a scenario as the accretion and settling of external gas
Summary
An early consensus held that early-type galaxies (ETGs, encompassing elliptical and S0 galaxies) were nearly devoid of gas and dust (Hubble 1926; de Vaucouleurs 1959; Sandage 1961). While at much coarser spatial resolution than HST imaging, these gas observations suggested that up to a tenth of ETGs might possess molecular gas that is both detectable in CO emission and in disk-like rotation on small scales. A better understanding of BH demographics, based on a growing sample of BH masses, has shown a more complicated connection between these central masses and properties of their host galaxy, such as classical bulges vs pseudobulges and cored vs coreless ellipticals (Kormendy & Ho 2013) These BH masses are typically measured by dynamically modeling resolved stellar or gaseous kinematics. We obtain sufficiently high resolution observations to detect high-velocity gas originating from within the BH sphere of influence When such gas is found, we conduct deeper, higher-resolution ALMA observations to map out the disk kinematics within rg and accurately measure the BH masses. In a future paper we will focus on modeling the CO(2−1) kinematics for the remainder of the sample
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