A dominant population of optically invisible massive galaxies in the early Universe (1908.02372v1)

Abstract: Our current knowledge of cosmic star-formation history during the first two billion years (corresponding to redshift z >3) is mainly based on galaxies identified in rest-frame ultraviolet light. However, this population of galaxies is known to under-represent the most massive galaxies, which have rich dust content and/or old stellar populations. This raises the questions of the true abundance of massive galaxies and the star-formation-rate density in the early universe. Although several massive galaxies that are invisible in the ultraviolet have recently been confirmed at early epochs, most of them are extreme starbursts with star-formation rates exceeding 1000 solar masses per year, suggesting that they are unlikely to represent the bulk population of massive galaxies. Here we report submillimeter (wavelength 870um) detections of 39 massive star-forming galaxies at z > 3, which are unseen in the spectral region from the deepest ultraviolet to the near-infrared. With a space density of about $2 \times 10^{-5}$ per cubic megaparsec (two orders of magnitudes higher than extreme starbursts) and star-formation rates of 200 solar masses per year, these galaxies represent the bulk population of massive galaxies that have been missed from previous surveys. They contribute a total star-formation-rate density ten times larger than that of equivalently massive ultraviolet-bright galaxies at z >3. Residing in the most massive dark matter halos at their redshifts, they are probably the progenitors of the largest present-day galaxies in massive groups and clusters. Such a high abundance of massive and dusty galaxies in the early universe challenges our understanding of massive-galaxy formation.

• The paper identifies 39 submillimeter galaxies as massive, dusty objects previously undetected in optical surveys.
• The paper uses ALMA 870 μm observations to determine star-formation rates of about 200 solar masses per year and a space density of 2×10⁻⁵ Mpc⁻³.
• The paper challenges existing galaxy formation models by linking these galaxies to the progenitors of today’s massive ellipticals in high-redshift dark matter halos.

A Dominant Population of Optically Invisible Massive Galaxies in the Early Universe

The paper presents a comprehensive paper of a population of massive galaxies in the early universe that have eluded detection in optical surveys. These galaxies have been identified through submillimeter observations at a wavelength of 870 μm, unveiling a significant population that is optically invisible. This research bridges a crucial gap in our understanding of the formation and evolution of massive galaxies by revealing a substantial number of such entities at redshifts $z > 3$.

The paper reveals the presence of 39 submillimeter galaxies within a sample of 63 H-band dropouts, which are galaxies characterized by a lack of detection in the H-band and a consequential detection at longer wavelengths. Their detection was achieved using the Atacama Large Millimeter/submillimeter Array (ALMA), which facilitated high-resolution observations allowing for the confirmation of these galaxies' dusty and massive nature. The space density of these galaxies is about $2 \times 10^{-5}$ per cubic megaparsec, surpassing the space densities of extreme starbursts by two orders of magnitude.

These observed galaxies exhibit star-formation rates around 200 solar masses per year. This activity suggests that the submillimeter galaxies constitute the bulk of the massive galaxy population at high redshifts, contrasting with previously known extreme starbursts which have much higher star-formation rates but are less numerous. Importantly, this population accounts for a significant fraction—ten times the star-formation rate density—of equivalently massive ultraviolet-bright galaxies at the same redshifts, highlighting that prior surveys have significantly underestimated the prevalence of massive galaxies in the early universe.

A critical element in this work is the assessment of the galaxies' contribution to the cosmic star-formation-rate density and their stellar mass function. The analysis demonstrates that these galaxies are likely progenitors of the most massive contemporary galaxies found in clusters and groups, suggesting that they reside in some of the most massive dark matter halos at their respective redshifts. The clustering analysis further supports this hypothesis, aligning their biased distribution with that expected for galaxies destined to become massive ellipticals and central cluster galaxies.

Moreover, this discovery challenges existing theories and models of galaxy formation, which lack the capacity to predict such a high abundance of massive, dusty galaxies at $z > 3$. The conventional models, including both semi-analytical models and hydrodynamic simulations, fall short in accounting for these findings, necessitating potential revisions to our understanding of galaxy evolution, particularly under the Lambda Cold Dark Matter (LCDM) cosmological framework.

Future prospects discussed in the paper emphasize the importance of further spectroscopic follow-ups to solidify these findings. The advent of instruments like the James Webb Space Telescope (JWST) promises enhanced mid-infrared spectral capabilities, which will be instrumental in further characterizing these populations and refining redshift estimates.

This paper contributes significantly to the field of astrophysics by providing a new perspective on the scale and nature of dusty, massive galaxies in the early universe, challenging previously held conceptions about early galaxy formation and evolution.