Molecular Gas Detections in Eight Faint DSFGs with Red NIR Colors at z = 1.2-2.5

TL;DR

This study targets eight faint DSFGs at $z \in [1.2,2.5]$ using NOEMA to detect CO(3-2)/CO(4-3) and [CI](1-0) by leveraging a NIR color selection that links SCUBA-2 sources to spectroscopic redshifts. Seven CO detections (plus a serendipitous [CI] detection) yield molecular gas masses around $M_{\rm mol} \sim 10^{10.8-11.9}\,M_\odot$ (assuming $\alpha_{\rm CO}=3.6$) and a roughly constant depletion time $t_{\rm dep} \sim 500$ Myr, with starburst systems exhibiting elevated gas fractions relative to scaling relations. A joint analysis of UV-to-mm SEDs (via BAGPIPES) and FIR/MBB fits indicates SFRs of a few hundred $M_\odot\,\mathrm{yr}^{-1}$ and stellar masses $M_\star \sim 10^{10.4}-10^{11.5}\,M_\odot$, highlighting a population of low-$M_\star$, high-$\Delta\mathrm{MS}$ DSFGs that are underrepresented in previous CO surveys. The results suggest possible systematic overestimation of $M_{\rm mol}$ in these galaxies due to $\alpha_{\rm CO}$ and/or CO excitation effects, and they underscore the need for self-consistent, multiwavelength analyses when comparing to molecular gas scaling relations. The work also discusses how deeper, wide-area dust continuum surveys paired with NIR-based counterpart matching could enable accessing CO in low-$M_\star$ main-sequence DSFGs, enabling more complete mapping of gas budgets across cosmic noon.

Abstract

We present a NOEMA survey of CO(3-2), CO(4-3), and [C I]($^3$P$_1$-$^3$P$_0$) in eight faint (average $S_{\rm 850 μm} = 2.3$ mJy) dusty star-forming galaxies (DSFGs) at $z = 1.2-2.5$. We used a NIR flux-color cut to match faint SCUBA-2 sources to red stellar counterparts with existing spectroscopic redshifts, allowing us to target CO lines at known frequencies. We obtained seven new CO detections and a serendipitous [C I] detection in an off-axis source, and measured molecular gas masses of $M_{\rm mol} = (6-22)\times10^{10}~(α_{\rm CO}/3.6)~{\rm M}_\odot$ from these lines. We performed UV-to-mm SED fits to measure the SFRs and stellar masses of our sample, and compared these with two other $z = 1-3$ CO samples from the literature. The CO detections have constant depletion times of $t_{\rm dep} \sim 500$ Myr, with no evidence for correlation between $t_{\rm dep}$ and redshift or main-sequence offset. We find that low-mass ($M_\star \lesssim 10^{11}~{\rm M}_\odot$), starbursting galaxies have gas fractions and depletion times twice as high as predicted by molecular gas scaling relations, which may indicate that $M_{\rm mol}$ is systematically over-estimated in this population, possibly due to decreased $α_{\rm CO}$ or increased CO excitation compared to the well-studied massive and/or main-sequence DSFG population.

Figures (8)

• Figure 1: HST/WFC3 F160W and Spitzer/IRAC 4.5 $\mu$m color-magnitude diagram showing the criterion used for our sample selection. The circles in this figure are Barro19 CANDELS galaxies within 5$"$ of a Cowie17 850 $\mu$m position. The red solid lines show our red selection (Equation \ref{['eqn:red']}), with red and gray points showing galaxies that do and do not satisfy this selection, respectively. Filled and open circles are points with and without spectroscopic redshifts, respectively. The large blue circles show CANDELS galaxies wtihin $1"$ of an SMA position from Cowie17. Our NOEMA targets are shown as green stars, while the cyan diamond shows the serendipitous detection GN-8051 (see Section \ref{['sec:other_sources']}). The black dotted line shows an arbitrary cutoff of 1 $\mu$Jy (23.9 AB mag) in F160W, below which very few sources have spectroscopic redshifts.
• Figure 2: HST three-color thumbnails of our 8 original targets and the serendipitous NOEMA detection GN-8051 (rightmost panel; see Section \ref{['sec:other_sources']}). All cutouts use the HST filters $rgb$ = F160W/F814W/F435W, with a contour of the Spitzer/IRAC 4.5 $\mu$m emission overlaid in white. Each cutout is $15"$ on a side, centered on the SCUBA-2 position, and the green dashed circle of radius $5"$ is the region within which we search for OIR counterparts. The red square in each thumbnail shows the sole red-selected (Equation \ref{['eqn:red']}) CANDELS galaxy within this region. The SCUBA-2 ID and 850 $\mu$m flux Cowie17 are shown in the bottom right corner of each thumbnail, while the CANDELS ID number Barro19 and spectroscopic redshift Kodra23 are shown in the top left corner.
• Figure 3: NOEMA 2 mm continuum images with serendipitous off-axis detections. In both cases, the main target that the pointing is centered on is marked by the green star, as in Figure \ref{['fig:selection']}, and the black dashed circle shows the extent of the primary beam. Contours are at ${\rm S/N} = -2$ (dashed), 2, 3 and 5. Left: Continuum image of the USB, centered on the CANDELS position of GN-19876, with the off-axis source GN-19084 marked by a red square. Right: Same, but for the image centered on GN-7630, with GN-8051 marked by the cyan diamond, as in Figure \ref{['fig:selection']}.
• Figure 4: NOEMA spectra and images of the nine detected emission lines in our sample. In each spectrum, the CANDELS ID of the source and the targeted line are given in the top left corner, the black line is the continuum-subtracted and primary beam-corrected spectrum, the gray line is the rms noise per channel, the vertical dashed green line is the OIR spectroscopic redshift (see Table \ref{['tab:sample']}), and the red curve is the best-fitting Gaussian. Blue and orange shading show the line channels ($\pm2\sigma$ from the center) for CO and [C i] emission, respectively. All spectra show a bandwidth of 2500 km s$^{-1}$ centered on the Gaussian. To the right of each spectrum is a $12"\times12"$ thumbnail showing the cleaned line-only images produced from these channels, with the NOEMA synthesized beam shape shown in the bottom right corner. Each thumbnail is centered on the CANDELS position, which is shown by the red circle and cross. The 2D S/N of these images is given in the top left corner of each thumbnail. Contours are at 2, 4, 6, 8, 10, and 12$\sigma$.
• Figure 5: SEDs of the the nine faint DSFGs with spectroscopic redshifts. The CANDELS ID and redshift are shown in the top-left corner of each panel. The gray curve and shaded region in each panel show the median posterior SED and the 16th to 84th percentile range, respectively. The orange curve and shaded region show the median optically-thin MBB SED and the 16th to 84th percentile range, respectively. The square points indicate detections, while the triangles indicate $3\sigma$ upper limits. All vertical errorbars show $3\times$ the measured error.