Vz-GAL: Probing Cold Molecular Gas in Dusty Star-forming Galaxies at z=1-6

TL;DR

Vz-GAL presents a large VLA CO(1-0) survey of 92 Herschel-selected DSFGs at z = 1–6, complemented by NOEMA z-GAL high-J CO data, to robustly calibrate molecular gas masses and probe ISM excitation. Molecular gas masses are derived via $M_{\rm H_2} = \alpha_{\rm CO} L'_{\rm CO(1-0)}$ with $\alpha_{\rm CO} = 4$, and line luminosities are computed with $L'_{\rm CO} = 3.25 \times 10^{7} \times \frac{I_{\rm CO} D_L^{2}}{\nu^{2}_{\rm CO,rest} (1+z)}$, revealing $\,\mu L'_{\rm CO(1-0)} \sim (0.5-5)\times 10^{11}$ and $\mu M_{\rm H_2} \sim (2-20)\times 10^{11} (\alpha_{\rm CO}/4)$ M$_{\odot}$ across the sample. The study finds L'CO(1-0)–LIR correlations consistent with local ULIRGs, a relatively constant depletion time $\tau_{\rm dep}$ of order $50-600$ Myr with hints of shorter values at $z>3.5$, and sub-thermal CO excitation with measured line ratios such as $r_{21}=0.88\pm0.25$ and $r_{61}=0.28\pm0.13$, as well as a [CI]/CO constraint of $\log\left(L'_{\rm [CI](^3P_1-^3P_0)}/L'_{\rm CO(1-0)}\right)=-0.71\pm0.12$ for a subsample. Overall, the results support common cold-gas ISM conditions in high-z DSFGs and a self-regulated star formation regime across cosmic time, while enabling precise gas-mass calibrations via CO(1-0).

Abstract

We present the first results of Vz-GAL, a high-redshift CO(J=1-0) large survey with the Karl G. Jansky Very Large Array, targeting 92 Herschel-selected, infrared-luminous, dusty star-forming galaxies (DSFGs) at redshifts 1 to 6. These sources are selected based on having redshifts and mid/high-J CO transitions from the NOrthern Extended Millimeter Array z-GAL survey. We successfully detect CO(J=1-0) emission in 90/92 galaxies at the expected positions and redshifts, including 9 tentative detections at $2σ- 3σ$ significance, and CO(J=2-1) emission in 10 of these galaxies. The CO(J=1-0) luminosities suggest apparent gas masses in the range $μ{M}_{\rm H_2}$ = $(2-20) \times {10}^{11}~(α_{CO}/{4.0})~\mathrm{M_{\odot}}$, which implies gas depletion times of $(50-600)$ Myr. These timescales show similar spread as local ULIRGs, suggesting a self-regulatory mechanism that maintains a consistent SFR per unit gas mass in starbursts across redshifts. To quantify the contribution of "excitation correction" factors to gas mass estimates, we calculate median CO line brightness temperature ratios of $r_{21}=0.88\pm0.25$, $r_{31}=0.61\pm0.22$, $r_{41}=0.49\pm0.15$, $r_{51}=0.47\pm0.13$, and $r_{61}=0.28\pm0.13$. Accounting for these corrections results in a reduced scatter in 'gas mass$-$star formation rate' relations. We also find a median log(${L}^{\prime}_{\mathrm{[CI]}(^{3}P_1 - ^{3}P_0)}/{L}^{\prime}_{\mathrm{CO}(J=1-0)})=-0.71\pm0.12$ for a subsample of 23 sources, consistent with the ratios derived for local star-forming galaxies. Together, our findings are in agreement with common conditions in the cold gas reservoirs among star-forming galaxies over a broad range in star formation modes, efficiencies, and scales.

Figures (4)

• Figure 1: Herschel/SPIRE $500\mu m$ flux densities ($S_{500\mu m}$) of the Vz--GAL DSFGs (this work) compared to the pilot program stanley+2023 with respect to their spectroscopic redshifts based on the z--GAL project. The histograms show fluxes of the Vz--GAL sample with shown in gray the remaining observations not executed to date. In the central portion, the filled area in silver color ($S_{500\mu m}$$\mathrm{>100~mJy}$) indicates the flux selection criteria for the HeLMS & HerS targets. The area in yellow (i.e., $S_{500\mu m}$$\mathrm{>80~mJy}$) shows the lower flux cut that was used to select the HerBS sources. Orange data points and arrows (the latter for $S_{500\mu m}$$\mathrm{>180~mJy}$) represent sources from this work in comparison to the DSFGs from pilot sample shown as blue crosses. Sources not yet observed are mostly in the $S_{500\mu m} = \mathrm{80-100~mJy}$ regime and hence selected from the GAMA fields. This incompleteness is largely due to scheduling restrictions in certain LST ranges and thus random beyond the $S_{500\mu m}$ distribution. As a result, based on the homogeneous selection of the parent sample, the survey is representative and nearly complete down to $S_{500\mu m}$ = $\mathrm{100~mJy}$, while the redshift--$S_{500\mu m}$ space is relatively evenly incomplete in the $S_{500\mu m} = \mathrm{80-100~mJy}$ regime based on the observations not yet executed.
• Figure 2: (Upper panel) A proxy for the Kennicutt-Schmidt (KS) relation of the Vz--GAL sample in terms of the infrared ($\rm 8-1000\, \mu m$) luminosity berta+2023 and the CO(1--0) line luminosity stanley+2023. Dashed diagonals present constant star formation efficiency (SFE) lines with SFR [${\rm M}_{\odot}~{\rm year}^{-1}$] = $1.09 \times {10}^{-10}$${L}_{\rm IR} [{\rm L}_{\odot}]$ and ${\alpha}_{\rm CO} = \mathrm{4 ~{M}_{\odot}~{\left(K~km~{s}^{-1}~{pc}^{2} \right)}^{-1}}$. Here, we highlight the Vz--GAL pilot sample from stanley+2023 separately to illustrate its consistency with our measurements. However, in the subsequent statistical analyses and corresponding figures, the 14 pilot galaxies are combined with our 92 DSFGs and treated as a single, unified Vz--GAL sample. (Lower panel) Ratio of the infrared to CO(1--0) luminosity, a proxy for star formation efficiency (SFE), versus the CO line luminosity (without lensing correction) as a probe for molecular gas mass. Relations derived by genzel2010 are shown with dash-dotted lines for comparison, up-scaling their far-infrared luminosities $L_{\rm FIR}$ ($\rm 50-300 \, \mu m$) to $L_{\rm IR}$ ($\rm 8-1000 \, \mu m$) using $L_{\rm IR}$/$L_{\rm FIR}$$\sim 1.3$graciacarpio2008 to match our plot. In both panels, we also show literature data points with available CO(1--0) observations: high-zHerschel-selected DSFGs riechers+2011lensedharris+2012fu2013ivison+2013harrington19leung19bakx+2020lensed2, other distant DSFGs and sub-millimeter galaxies (SMGs) greve04riechers08carilli10harris10ivison+2011emonts11riechers+2011sled1riechers+2011sled2swinbank11thomson12riechers+2013sharon13emonts14sharon15aravena+2016sharon+2016harrington17dannerhr19friascastillo23, high-z AGNs/QSOs carilli02lewis02riechers06aravena08riechers09wagg10lestrade11riechers11sharon+2016Riechers2020, distant MS galaxies aravena12aravena14rudnick17gomezgujjaro+2019kaasinen19Riechers2020, local ULIRGs solomon97chung09combes11gowardhan18, and nearby massive MS galaxies geach11Saintonge+2017villanueva17dunne21.
• Figure 3: (Upper panel) Redshift evolution of the total infrared ($8-1000 \mu m$) to CO(1--0) luminosity ratio $L_{\rm IR}/L^\prime_{\rm CO(1-0)}$ as a proxy for SFE. The linear fit shown with a dashed black line considers all the high-z DSFGs and local ULIRGs, which shows no confirmation of a linear trend. We choose to plot the redshift in log-scale here to highlight the variation across MS galaxies and ULIRGs at $z\sim0$. (Lower panel) Variation of the gas depletion time ($\tau_{\rm dep}$) as a function of redshift when assuming ${M}_{\rm H_2} = 4 {{L}^{\prime}}_{\rm CO(1-0)}$ (see Section \ref{['subsec:lum']}) and SFR = $1.09 \times {10}^{-10}$${L}_{\rm IR}$ (see Section \ref{['subsec:sfe']}). The dashed red line is the median $\tau_{dep}$ for the Vz--GAL sources, which is compared to that of local ULIRGs (cyan dotted line) and all DSFGs including the Vz--GAL sample (dashed orange line). The SFR$-M_{\ast}$ main-sequence (MS) shown as magenta dashed line is based on Figure 7 from tacconi2020, which varies as $\tau_{\rm dep}=1.6 \times {(1+z)}^{-1}$. Both panels include the literature data points mentioned in the caption of Figure \ref{['fig:KSrelation']}. Please note that a gap in CO(1--0) observations (rest frame $115~{\rm GHz}$) around $z=1$ is due to the current observational limitation at corresponding redshifted frequencies.
• Figure 4: CO-line ratio histograms for the Vz--GAL sample. The shaded histograms in gray are with the sources detected above 5$\sigma$ significance and the white histograms show the rest of our targets that have SNR$>$2. The ratios show mostly sub-thermal excitation of the mid/high-$J$ CO lines, indicating the importance of CO(1--0) as an anchor point and as a robustly calibrated molecular gas mass tracer. The values compiled by carilliwalter2013 are plotted for comparison (red dotted lines). Optically thick and thermalized gas with ${r}_{J1}=1$ is shown as black dashed lines in each subplots.