A Dust-Obscured Massive Maximum-Starburst Galaxy at a Redshift of 6.34 (1304.4256v1)

Abstract: Massive present-day early-type (elliptical and lenticular) galaxies probably gained the bulk of their stellar mass and heavy elements through intense, dust-enshrouded starbursts - that is, increased rates of star formation - in the most massive dark matter halos at early epochs. However, it remains unknown how soon after the Big Bang such massive starburst progenitors exist. The measured redshift distribution of dusty, massive starbursts has long been suspected to be biased low in redshift owing to selection effects, as confirmed by recent findings of systems out to redshift z~5. Here we report the identification of a massive starburst galaxy at redshift 6.34 through a submillimeter color-selection technique. We unambiguously determined the redshift from a suite of molecular and atomic fine structure cooling lines. These measurements reveal a hundred billion solar masses of highly excited, chemically evolved interstellar medium in this galaxy, which constitutes at least 40% of the baryonic mass. A "maximum starburst" converts the gas into stars at a rate more than 2,000 times that of the Milky Way, a rate among the highest observed at any epoch. Despite the overall downturn of cosmic star formation towards the highest redshifts, it seems that environments mature enough to form the most massive, intense starbursts existed at least as early as 880 million years after the Big Bang.

• The paper identifies HFLS3 using full-frequency scans and multiple spectral lines, precisely determining its redshift of 6.34.
• The paper reveals an extreme star formation rate of 2,900 M☉ yr⁻¹ and a stellar mass of ~3.7×10¹⁰ M☉, exemplifying a maximum starburst.
• The paper details a massive interstellar medium with gas masses reaching 1.0×10¹¹ M☉, underscoring significant early chemical enrichment and dynamic mass assembly.

Analysis of "A Dust-Obscured Massive Maximum-Starburst Galaxy at a Redshift of 6.34"

The paper by Riechers et al. identifies a galaxy, HFLS3, at a notably high redshift of 6.34, utilizing a submillimeter color-selection technique. This places HFLS3 in the early universe, approximately 880 million years post-Big Bang. It offers a substantial contribution to the understanding of star formation and molecular gas in the formative stages of galaxy development.

Key Findings

1. Identification and Methodology: The discovery of HFLS3 was made possible by full-frequency scans of the 3-mm and 1-mm bands, combined with optical to radio wavelength observations. The redshift was precisely determined using a suite of spectral lines including seven CO lines, H2O, and [CII], among others.
2. Star Formation Rate and Stellar Mass: HFLS3 is described as hosting an intense starburst with an SFR of 2,900 M☉ yr^-1. This rate is over 2,000 times that of the Milky Way, underscoring its classification as a "maximum starburst." Moreover, the galaxy's stellar mass is documented at ~3.7 x 10^10 M☉, suggesting rapid assembly of stellar material.
3. Interstellar Medium and Gas Content: The interstellar medium of HFLS3 is both massive and chemically evolved. It holds molecular gas masses of M_gas = 1.0 x 10^11 M☉ and atomic gas masses M_HI = 2.0 x 10^10 M☉. These significant quantities indicate an early formation and enrichment process, supported by supernovae.
4. Comparison with Local and Historical Galactic Counts: The mass and luminosity of HFLS3 are compared to local ULIRGs like Arp 220. Its FIR luminosity exceeds common quasar hosts at similar epochs, reflecting extreme conditions typical of the early universe.
5. Spatial Distribution: The findings demonstrate a dense, yet spatially extended dust and gas environment with notable velocity gradients detectable in CO line profiles. This denotes a dynamic mass of 2.7 x 10^11 M☉, where a substantial proportion is attributed to gas.

Implications

• Galaxy Formation: HFLS3 represents a key piece in understanding massive star-forming galaxies in the early universe. Its properties challenge models of galaxy formation and the timeframes of mass and chemical enrichment post-Big Bang.
• Cosmic Reionization: Despite its significant star-forming activity, HFLS3 and similar ultrared sources do not contribute dominantly to the UV photon budget necessary for cosmic reionization, suggesting more diffuse or less obscured systems played pivotal roles.
• Future Research Directions: This paper highlights the necessity for further exploration into high-z galaxy environments. Upcoming studies are urged to explore the prevalence and characteristics of such extreme starbursts, potentially offering insights into primordial galaxy clusters.

Riechers et al.'s findings represent a milestone in observational cosmology, pushing the boundaries of current models regarding starburst activity and cosmic structure during the universe's infancy. This paper provides an exceptional basis for the ongoing debate regarding early star formation and galaxy assembly.