A deep ALMA image of the Hubble Ultra Deep Field (1606.00227v2)

Abstract: We present the results of the first, deep ALMA imaging covering the full 4.5 sq arcmin of the Hubble Ultra Deep Field (HUDF) as previously imaged with WFC3/IR on HST. Using a mosaic of 45 pointings, we have obtained a homogeneous 1.3mm image of the HUDF, achieving an rms sensitivity of 35 microJy, at a resolution of 0.7 arcsec. From an initial list of ~50 >3.5sigma peaks, a rigorous analysis confirms 16 sources with flux densities S(1.3) > 120 microJy. All of these have secure galaxy counterparts with robust redshifts (<z> = 2.15), and 12 are also detected at 6GHz in new deep JVLA imaging. Due to the wealth of supporting data in this unique field, the physical properties of the ALMA sources are well constrained, including their stellar masses (M*) and UV+FIR star-formation rates (SFR). Our results show that stellar mass is the best predictor of SFR in the high-z Universe; indeed at z > 2 our ALMA sample contains 7 of the 9 galaxies in the HUDF with M* > 2 x 10^10 Msun and we detect only one galaxy at z > 3.5, reflecting the rapid drop-off of high-mass galaxies with increasing redshift. The detections, coupled with stacking, allow us to probe the redshift/mass distribution of the 1.3-mm background down to S(1.3) ~ 10 micro-Jy. We find strong evidence for a steep `main sequence' for star-forming galaxies at z ~ 2, with SFR \propto M* and a mean specific SFR = 2.2 /Gyr. Moreover, we find that ~85% of total star formation at z ~ 2 is enshrouded in dust, with ~65% of all star formation at this epoch occurring in high-mass galaxies (M* > 2 x 10^10 Msun), for which the average obscured:unobscured SF ratio is ~200. Finally, we combine our new ALMA results with the existing HST data to revisit the cosmic evolution of star-formation rate density; we find that this peaks at z ~ 2.5, and that the star-forming Universe transits from primarily unobscured to primarily obscured thereafter at z ~ 4.

• The paper demonstrates that deep ALMA imaging bridges optical/UV and far-infrared views to provide a holistic perspective on high-redshift galaxy evolution.
• The paper employs a 45-point mosaic with 35 μJy sensitivity and 0.7 arcsec resolution, validating 16 robust source detections above 120 μJy.
• The paper establishes stellar mass as a predictor of star formation rate, showing that approximately 85% of star formation at z≈2 is obscured by dust.

Deep ALMA Observations of the Hubble Ultra Deep Field: An Analysis

The Hubble Ultra Deep Field (HUDF) is one of the most meticulously studied regions in the sky, providing expansive insights into the high-redshift Universe. The research paper presents a comprehensive analysis based on deep Atacama Large Millimeter/submillimeter Array (ALMA) imaging of the HUDF, which aimed to bridge the gap between optical/UV and far-infrared/mm views of the Universe, thereby offering a more holistic approach to studying galaxy formation and evolution.

Key Observational Insights

Using a 45-pointing mosaic, the researchers achieved an unprecedented homogeneous 1.3-mm image of the HUDF with a sensitivity of approximately 35 ${\rm \mu Jy}$ and a resolution of roughly 0.7 arcseconds. This imaging effort is significant as it taps into the sub-mm regime, traditionally hampered by high background noise and poor angular resolution when observed with single-dish telescopes.

From the initial list of over 50 potential sources showing peaks above 3.5$\sigma$, a refined analysis validated 16 robust detections with flux densities above 120 ${\rm \mu Jy}$. Each of these sources was paired with secure galaxy counterparts possessing robust redshifts, averaging around $z \approx 2.15$.

Stellar Mass and Star-Formation Correlation

The analysis established stellar mass as an effective predictor of star formation rate (SFR) in the high-redshift Universe. Specifically, the ALMA sample encompasses 7 out of the 9 galaxies in the HUDF with stellar masses $M_* \geq 2 \times 10^{10} {\rm M_{\odot}}$ at redshifts $z \geq 2$. Notably, only a single galaxy was detected beyond $z = 3.5$, underscoring the rapid decline of high-mass galaxies with increasing redshift, as corroborated by the evolving galaxy mass function.

Distribution Across the 1.3-mm Background

Through detected sources and stacking techniques, the paper tracked the distribution of the 1.3-mm background, reaching depths of $S_{1.3} \simeq 10\,{\rm \mu Jy}$. Findings indicate significant contribution to dusty star formation at these epochs, with $\sim$85% of total star formation at $z \simeq 2$ shrouded in dust. Of particular note, about 65% of the star formation during this time occurs in high-mass galaxies with average obscured:unobscured ratios near 200.

New Understanding of the Star-Forming Main Sequence

The paper presents solid evidence for a steep star-forming main sequence at $z \simeq 2$, characterized by SFR $\propto M_*$, with a mean specific SFR around 2.2 Gyr$^{-1}$. This correlation between star formation and stellar mass provides insights into the mechanisms driving galaxy evolution during the peak era of cosmic star formation.

Re-evaluating Cosmic Star Formation History

By integrating ALMA observations with optical data, the research offers a revised understanding of cosmic star formation rate density ($\rho_{\rm SFR}$). Results indicate a peak in $\rho_{\rm SFR}$ around $z \simeq 2.5$. The paper also marks a transition from predominantly unobscured star formation at $z \gtrsim 4$ to obscured star formation at lower redshifts due to an increase in high-mass star-forming galaxies.

Future Directions in Sub-mm Astronomy

This work underscores ALMA's capabilities in expanding our understanding of the formation and evolution of high-redshift galaxies. The outcomes align with existing literature while offering refined metrics based on direct observation rather than extrapolation. Future studies could capitalize on these methodologies to explore fainter populations, potentially shifting our understanding of the very early Universe. As observational depth and resolution improve, further clarifications on the roles of stellar mass and specific sub-mm fluxes in cosmic evolution are anticipated.