Flat Rotation Curves found in Merging Dusty Starbursts at $z=2.3$ through Tilted-Ring Modeling

Abstract

The brightest 500$\hat{I}¼$m source in the XMM field, HXMM01, is a rare merger of luminous starburst galaxies at $z=2.3$ with a dust-obscured star-formation rate of 2,000$M_{\odot }{\mathrm{yr}}^{-1}$. Here we present high-resolution spectroscopic observations of HXMM01 with the Atacama Large Millimeter/submillimeter Array (ALMA). We detect line emission from $\mathrm{CO}J=7\to 6$, [C I]${}^3{P}_2\to {}^3{P}_1$, and p-${\mathrm{H}}_2\mathrm{O}{2}_{11}\to 2_{02}$ and continuum emission at $230$GHz. At a spatial resolution of 0.2" and a spectral resolution of 40$\mathrm{km}{\mathrm{s}}^{-1}$, the source is resolved into three distinct components, which are spatially and dynamically associated within a projected radius of 20kpc and a radial velocity range of 2,000$\mathrm{km}{\mathrm{s}}^{-1}$. For two major components, our Bayesian-based tilted-ring modeling of the ALMA spectral cubes shows almost flat rotation curves peaking at $\sim 500\mathrm{km}{\mathrm{s}}^{-1}$ at galactocentric distances between 2 and 5kpc. Each of them has a dynamical mass of $\sim 10^{11}{M}_{\odot }$. The combination of the dynamical masses and the archival $\mathrm{CO}J=1\to 0$ data places strong upper limits on the CO$\to$H${}_2$ conversion factor of $\hat{I}{\pm }_{\mathrm{CO}}\lesssim 1.4-2.0M_{\odot }(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^2{)}^{-1}$. These limits are significantly below the Galactic inner disk $\hat{I}{\pm }_{\mathrm{CO}}$ value of $4.3M_{\odot }(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^2{)}^{-1}$ but are consistent with those of local starbursts. Therefore, the previously estimated short gas depletion timescale of $\sim 200$Myr remains unchanged.

Accepted for publication in ApJL