Poster Abstract

P.2 Marta Frias Castillo (Leiden Observatory)

Kiloparsec-scale imaging of the CO(1-0)-traced cold molecular gas reservoir in a z~3.4 submillimeter galaxy

High-redshift submillimiter galaxies (SMG) are some of the most extreme systems at early cosmic times, with star formation rates of 100-1000 Msolar/year. A key to understanding how these extreme objects fit into current models of galaxy formation and how their vigorous star formation is maintained is the cold, molecular gas. The CO(J=1-0) transition is the best tracer of the full cold molecular gas reservoirs due to its low excitation requirements. However, detecting this faint CO(1-0) is challenging in high-z galaxies, with vast majority of studies using brighter, higher-excitation mid-J CO lines, but can often imply gas masses and sizes several times smaller than from CO(1-0). Observations of the CO(1-0) emission on kpc-scales are thus critical to understanding the detailed distribution of molecular gas at high-z. However, until now, no such resolved study has been made for an unlensed, high-redshift SMG.

We present a high-resolution study of the cold molecular gas as traced by CO(1-0) in the unlensed z~3.4 SMG SMM J13120+4242, using multi-configuration observations of the Karl G. Jansky Very Large Array (JVLA). These are the deepest ever observations of CO(1-0) in an unlensed SMG. The gas reservoir, imaged on 3 kpc scales, is resolved into two components separated by ~11 kpc, extending over >16 kpc scales. Despite the large spatial extent of the reservoir, the observations suggest a CO(1-0) linewidth almost four times narrower than had been previously reported in shallower data. The resolved emission reveals a chaotic velocity field, with evidence for a velocity gradient aligned approximately with the minor axis of the CO(1-0) reservoir. The disturbed morphology, intense burst of star formation (~3700 M$_\odot$ year$^{-1}$ and a comparison with resolved CO(6-5) data suggest this SMGs is a late-stage merger. The large velocity gradient modeling of the CO excitation ladder favours a single-component ISM, instead of the multicomponent ISM observed in several high-z systems with CO(1-0) observations. This indicates that copious low excitation gas reservoirs found in other CO(1-0)-observed SMGs are not an ubiquitous feature of the SMG population. Although low-J studies of high-z SMGs are paramount to characterise their ISM, the case of SMM J13120+4242 suggests that a combination of mid- and high-J CO lines may be reliable tracers of molecular gas in some high-z systems.