A search for cold molecular outflows in cosmic noon galaxies

TL;DR

This paper investigates whether cold molecular gas traced by CO(2--1) exists in the CGM of typical main-sequence star-forming galaxies at cosmic noon ($z_{med}\approx1.3$) and whether ongoing molecular outflows can be detected. By stacking ALMA Band 3 CO(2--1) maps and spectra for 26 galaxies and comparing to UltraVISTA Ks-band stellar light, the authors find CO emission extending to about $\sim12$ kpc, with a spatial extent similar to the stellar component and no robust broad wings in the stacked spectrum. A marginal blue excess at negative velocities is noted, but not conclusive for outflows; overall, the data favor weak CO-traced molecular outflows in these typical galaxies. The results imply that molecular gas largely remains within or near the stellar disc at cosmic noon and that detecting CGM CO requires deeper observations and/or uv-plane analyses or next-generation facilities like AtLAST to map diffuse gas on CGM scales.

Abstract

The flow of baryons in and out of galaxies is the primary driver for galaxy evolution. In addition to depleting the gas reservoir of galaxies, outflows also enrich their circumgalactic medium (CGM) with processed gas -- which can further impact the next stages of gas accretion, resulting in the presence of molecular gas beyond the stellar component of galaxies, out to CGM scales. Here, we aim to search for cold molecular gas (MH2) in the CGM of typical main-sequence (MS) star-forming galaxies (SFGs) at cosmic noon (zmed=1.3), where we expect outflows to be particularly prominent. Using Band 3 CO(2-1) data from the Atacama Large Millimeter and submillimeter Array (ALMA), we study the spatial extent of the MH2 of a sample of 26 SFGs, via stacking techniques. We compare this extent to that of the stacked stellar emission of our sample traced by UltraVISTA Ks band data. We also search for broad wings in the stacked spectrum which can be indicative of ongoing outflows. Within the noise level of the observations, we find that the total intrinsic MH2 of our sample spatially extends to scales of ~12 kpc, similarly to the stellar emission (~13 kpc). We do not find broad wings in the stacked spectrum that could hint at ongoing molecular outflows, but we find a tentative minor excess of CO(2-1) emission at negative velocities that might be indicative of outflows, where the redshifted gas is optically thick. The absence of high-velocity molecular gas suggests that molecular outflows traced by CO(2-1) emission are weak in MS SFGs at cosmic noon. These weak outflows thus fail to expel a significant amount of molecular gas to CGM scales, as indicated by the absence of molecular emission extending beyond the stellar emission region. This lack of CO emission at large radii could also imply that the molecular gas does not survive at such distances.

Figures (13)

• Figure 1: Distributions of the properties of the sample used in this study. Galaxies with values below (above) the median are marked in yellow (blue). The median value is shown with the dashed vertical line, except for the distance to the main sequence for which we show a cut at 3.5 times above (below) the main sequence. Top left: the stellar mass $M_\star$. Top right: the distance to the main sequence as reported in Valentino2020. Bottom left: the infrared luminosity $L_{IR}$. Bottom right: the specific star formation rate sSFR.
• Figure 2: Uniformly weighted stack of CO(2--1) intensity maps. Contours at [3, 5, 7, 9, 13, 17]$\sigma$ are shown with white contours. The beam is shown in the bottom left corner.
• Figure 3: Surface brightness radial profiles of the CO(2--1) emission (in purple) and the synthesised beam (in beige) of the uniform stacked CO(2--1) intensity map. The pink dashed-line shows the noise level of the map. The upper panel shows the profiles of the entire sample of 26 galaxies, and the intrinsic CO(2--1) emission (black line). The bottom panel shows the profiles of 500 resampling iterations by randomly removing 30% of the sample (grey thin lines) and the median of these iterations (purple line). The error bars show the median of the uncertainty associated with each iteration within each annulus and the horizontal pink dashed-line shows the median standard deviation of the 500 resampled intensity maps.
• Figure 4: Top panel: CO(2--1) intensity map resulting from the stacked cube (uniformly weighted). The beam is shown in the bottom left corner. Bottom panel: Surface brightness radial profiles of the CO(2--1) emission (in purple), the synthesised beam (in beige) of the CO(2--1) intensity map imaged from the uniformly stacked cubes, and the intrinsic CO(2--1) emission (in black).
• Figure 5: Surface brightness radial profiles of the CO(2--1) emission and corresponding synthesised beam in different galaxy property bins. The number of galaxies in each bin is shown in parentheses. Low (MS) bins are shown in yellow and high (SB) bins are shown in blue. Top left: stellar mass bin, where low mass $\leq10^{10.83} M_\odot$ and high mass $>10^{10.83} M_\odot$. Top right: infrared luminosity bin, where low $L_{IR}\leq10^{12.26} L_\odot$ and high $L_{IR}>10^{12.26} L_\odot$. Bottom left: specific star formation rate bin, where (log) low sSFR $\leq -8.5$ yr$^{-1}$ and (log) high sSFR $> -8.5$ yr$^{-1}$ . Bottom right: distance to the main sequence, where MS $\leq3.5$ times correspond to galaxies within the scatter of the MS and SB $>3.5$ times correspond to galaxies above.