The Intense Starburst HDF850.1 in a Galaxy Overdensity at z=5.2 in the Hubble Deep Field

Abstract: The Hubble Deep Field (HDF) is a region in the sky that provides one of the deepest multi-wavelength views of the distant universe and has led to the detection of thousands of galaxies seen throughout cosmic time. An early map of the HDF at a wavelength of 850 microns that is sensitive to dust emission powered by star formation revealed the brightest source in the field, dubbed HDF850.1. For more than a decade, this source remained elusive and, despite significant efforts, no counterpart at shorter wavelengths, and thus no redshift, size or mass, could be identified. Here we report, using a millimeter wave molecular line scan, an unambiguous redshift determination for HDF850.1 of z=5.183. This places HDF850.1 in a galaxy overdensity at z~5.2 in the HDF, corresponding to a cosmic age of only 1.1 Gyr after the Big Bang. This redshift is significantly higher than earlier estimates and higher than most of the >100 sub-millimeter bright galaxies identified to date. The source has a star formation rate of 850 M_sun/yr and is spatially resolved on scales of 5 kpc, with an implied dynamical mass of ~1.3x10^11 M_sun, a significant fraction of which is present in the form of molecular gas. Despite our accurate redshift and position, a counterpart arising from starlight remains elusive.

• The paper establishes a precise redshift of z=5.2 for HDF850.1 using millimeter wave molecular line scans within a dense galaxy environment.
• It reports an exceptional star formation rate of 850 M☉/yr and a molecular gas mass of 1.3×10¹¹ M☉, highlighting vigorous early-universe starburst activity.
• Spatially resolved [CII] imaging and LVG modeling of CO transitions reveal detailed morphological characteristics that inform models of early cosmic structure formation.

Analysis of HDF850.1: A High-Redshift Submillimeter Source in a Galaxy Overdensity at z=5.2

The observation and characterization of HDF850.1 present a significant contribution to our understanding of sub-millimeter bright galaxies at high redshift. Identified within the Hubble Deep Field (HDF), HDF850.1 is situated in a galaxy overdensity at a redshift of z=5.183, corresponding to a cosmic age of merely 1.1 Gyr post-Big Bang. This paper employs millimeter wave molecular line scans to firmly establish the redshift of this elusive source, revealing a higher redshift than what has been detected for the majority of similar sub-millimeter sources.

Key Findings

• Redshift and Environment: The unequivocal determination of the redshift z=5.183 for HDF850.1 places it among the highest detected so far for sub-millimeter bright galaxies. This underscores its inclusion in a galaxy overdensity, a rare spatial configuration indicative of significant early cosmic structure formation.
• Star Formation Rate and Molecular Gas Content: HDF850.1 exhibits a prodigious star formation rate of 850 M☉ yr⁻¹, facilitated by its substantial molecular gas reservoir. The estimated dynamical mass, largely composed of molecular gas, is approximately 1.3 x 10¹¹ M☉. Yet, no active galactic nucleus counterparts were identified, which suggests the source operates within a non-quasar paradigm typical of star-forming regions.
• Morphological Characteristics: The spatially resolved [CII] emission suggests an object with an extent of roughly 5.7 kpc, a feature seldom resolved in such high-redshift systems. Additionally, the source aligns with a universal star formation law, balancing molecular gas density against star formation rate densities, which is more moderate than those seen in contemporary merging systems.
• CO Line Luminosities and Implications: With CO(6-5) and CO(2-1) line luminosities, the inferred sub-thermal excitation levels deviate from the norms found in galactic nuclei. The methodologies relied upon large velocity gradient (LVG) models, further refining the understanding of molecular gas properties in these early-universe structures.
• Environmental Influence and Gravitational Lensing: The potential gravitational lensing effect by an intervening elliptical galaxy amplifies its brightness, though not enough to significantly alter derived mass or other fundamental properties of HDF850.1.

Implications and Future Considerations

The findings presented have profound implications for theoretical models surrounding early galaxy formation and the mechanisms governing star formation under dust-obscured environments. As one of the first sources detected, HDF850.1 provides critical empirical data supporting the existence of sub-millimeter luminous galaxies at high redshift, emphasizing the need for revisiting population estimates of such objects. The research highlights the utility of extensive blind line surveys in unveiling these obscured systems, ensuring continuous refinement of cosmic evolution models.

In future work, further observational campaigns at similar or shorter wavelengths with advanced facilities could achieve higher resolution imaging capable of better resolving complex regions within HDF850.1. This work shall sharpen insights into the role of these extreme starbursts during cosmological epochs that precede extensive quasar formation, particularly how they contribute to mass assembly in the burgeoning universe. The integration of comprehensive multi-wavelength datasets beyond FIR and CO wavelengths is imperative in refining our understanding of these early galaxies’ formation, dynamical processes, and star-formation histories.