Suppressed CO emission and high G/D ratios in z=2 galaxies with sub-solar gas-phase metallicity. (arXiv:1901.06390v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Coogan_R/0/1/0/all/0/1">R. T. Coogan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sargent_M/0/1/0/all/0/1">M. T. Sargent</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Daddi_E/0/1/0/all/0/1">E. Daddi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Valentino_F/0/1/0/all/0/1">F. Valentino</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Strazzullo_V/0/1/0/all/0/1">V. Strazzullo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bethermin_M/0/1/0/all/0/1">M. Béthermin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gobat_R/0/1/0/all/0/1">R. Gobat</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Liu_D/0/1/0/all/0/1">D. Liu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Magdis_G/0/1/0/all/0/1">G. Magdis</a>
We study a population of significantly sub-solar enrichment galaxies at
z=1.99, to investigate how molecular gas, dust and star-formation relate in
low-metallicity galaxies at the peak epoch of star-formation. We target our
sample with several deep ALMA and VLA datasets, and find no individual
detections of CO[4-3], CO[1-0] or dust, in stark contrast to the $>$60%
detection rate expected for solar-enrichment galaxies with these MS Halpha
SFRs. We find that both low and high density molecular gas (traced by CO[1-0]
and CO[4-3] respectively) are affected by the low enrichment, showing sample
average (stacked) luminosity deficits $>$0.5-0.7 dex below expectations. This
is particularly pertinent for the use of high-J CO emission as a proxy of
instantaneous star-formation rate. Our individual galaxy data and stacked
constraints point to a strong inverse dependence $propto Z^{gamma}$ of
gas-to-dust ratios (G/D) and CO-to-H$_{2}$ conversion factors (aco) on
metallicity at z$sim$2, with $gamma_{rm G/D}<$-2.2 and $gamma_{alpha_{rm
CO}}<$-0.8, respectively. We quantify the importance of comparing G/D and aco
vs. metallicity trends from the literature on a common, suitably normalised
metallicity scale. When accounting for systematic offsets between different
metallicity scales, our z$sim$2 constraints on these scaling relations are
consistent with the corresponding relations for local galaxies. However, among
those local relations, we favour those with a steep/double power-law dependence
of G/D on metallicity. Finally, we discuss the implications of these findings
for (a) gas mass measurements for sub-M$^{*}$ galaxies, and (b) efforts to
identify the characteristic galaxy mass scale contributing most to the comoving
molecular gas density at z=2.
We study a population of significantly sub-solar enrichment galaxies at
z=1.99, to investigate how molecular gas, dust and star-formation relate in
low-metallicity galaxies at the peak epoch of star-formation. We target our
sample with several deep ALMA and VLA datasets, and find no individual
detections of CO[4-3], CO[1-0] or dust, in stark contrast to the $>$60%
detection rate expected for solar-enrichment galaxies with these MS Halpha
SFRs. We find that both low and high density molecular gas (traced by CO[1-0]
and CO[4-3] respectively) are affected by the low enrichment, showing sample
average (stacked) luminosity deficits $>$0.5-0.7 dex below expectations. This
is particularly pertinent for the use of high-J CO emission as a proxy of
instantaneous star-formation rate. Our individual galaxy data and stacked
constraints point to a strong inverse dependence $propto Z^{gamma}$ of
gas-to-dust ratios (G/D) and CO-to-H$_{2}$ conversion factors (aco) on
metallicity at z$sim$2, with $gamma_{rm G/D}<$-2.2 and $gamma_{alpha_{rm
CO}}<$-0.8, respectively. We quantify the importance of comparing G/D and aco
vs. metallicity trends from the literature on a common, suitably normalised
metallicity scale. When accounting for systematic offsets between different
metallicity scales, our z$sim$2 constraints on these scaling relations are
consistent with the corresponding relations for local galaxies. However, among
those local relations, we favour those with a steep/double power-law dependence
of G/D on metallicity. Finally, we discuss the implications of these findings
for (a) gas mass measurements for sub-M$^{*}$ galaxies, and (b) efforts to
identify the characteristic galaxy mass scale contributing most to the comoving
molecular gas density at z=2.
http://arxiv.org/icons/sfx.gif
