[Paper Review] Unveiling Dust-enshrouded Star Formation in the Early Universe: a Sub-mm Survey of the Hubble Deep Field

D. H. Hughes, S. Serjeant+13 more|arXiv (Cornell University)|Jun 22, 1998

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy48 references750 citations

TL;DR

This paper presents the deepest sub-mm survey to date of the Hubble Deep Field using SCUBA on the JCMT, detecting five robust 850 μm sources with fluxes >2 mJy at redshifts z ≈ 3. These sources reveal a population of dust-enshrouded starburst galaxies that contribute 30–50% of the unresolved sub-mm background, implying star-formation density at z > 2 is at least five times higher than inferred from UV data alone.

ABSTRACT

The advent of sensitive sub-mm array cameras now allows a proper census of dust-enshrouded massive star-formation in very distant galaxies, previously hidden activity to which even the faintest optical images are insensitive. We present the deepest sub-mm survey of the sky to date, taken with the SCUBA camera on the James Clerk Maxwell Telescope and centred on the Hubble Deep Field. The high source density found in this image implies that the survey is confusion-limited below a flux density of 2 mJy. However, within the central 80 arcsec radius independent analyses yield 5 reproducible sources with S(850um) > 2 mJy which simulations indicate can be ascribed to individual galaxies. We give positions and flux densities for these, and furthermore show using multi-frequency photometric data that the brightest sources in our map lie at redshifts z~3. These results lead to integral source counts which are completely inconsistent with a no-evolution model, and imply that massive star-formation activity continues at redshifts > 2. The combined brightness of the 5 most secure sources in our map is sufficient to account for 30 - 50% of the previously unresolved sub-mm background, and we estimate statistically that the entire background is resolved at about the 0.3 mJy level. Finally we discuss possible optical identifications and redshift estimates for the brightest sources. One source appears to be associated with an extreme starburst galaxy at z~1, whilst the remaining four appear to lie in the redshift range 2 < z < 4. This implies a star-formation density over this redshift range that is at least five times higher than that inferred from the ultraviolet output of HDF galaxies.

Motivation & Objective

• To detect and characterize dust-enshrouded star-forming galaxies in the early universe, previously missed by optical surveys due to dust obscuration.
• To measure the contribution of high-redshift, dusty starbursts to the extragalactic sub-mm background.
• To determine the star-formation rate density at z > 2 using sub-mm data, independent of UV-based estimates that may be biased by dust.
• To assess the redshift distribution and physical properties of the detected sub-mm sources through multi-frequency photometric analysis.
• To evaluate the completeness and reliability of the survey in the confusion-limited regime below 2 mJy flux density.

Proposed method

• Conducted an ultra-deep 850 μm survey of the Hubble Deep Field using the SCUBA instrument on the James Clerk Maxwell Telescope.
• Performed independent source extraction and validation within a central 80 arcsec radius to identify 5 robust sources above 2 mJy flux density.
• Used multi-frequency photometric data (optical, near-IR, sub-mm) to estimate redshifts and SEDs of the detected sources.
• Applied simulations to assess source confusion and reliability, confirming that the 5 sources are individually resolvable.
• Calculated the contribution of the 5 brightest sources to the sub-mm background and estimated the total resolved fraction of the background.
• Estimated the dust-enshrouded star-formation rate density at z ≈ 3 using the combined flux and cosmological volume, assuming q₀ = 0.5.

Experimental results

Research questions

• RQ1 What is the level of dust-enshrouded star-formation activity at redshifts z > 2, as revealed by sub-mm surveys?
• RQ2 To what extent do sub-mm detected sources account for the unresolved extragalactic sub-mm background at 850 μm?
• RQ3 How do the redshifts and luminosities of the detected sub-mm sources compare with expectations from models of high-redshift galaxy evolution?
• RQ4 What fraction of the total star-formation rate density in the early universe is missed by UV-based surveys due to dust obscuration?
• RQ5 Can the properties of the detected sources be reconciled with known local ULIRGs, and what does this imply for their evolutionary status?

Key findings

• Five robust sub-mm sources with S₈₅₀ μm > 2 mJy were detected in the central 80 arcsec of the Hubble Deep Field, with positions and flux densities reported.
• The brightest sources are located at redshifts z ≈ 3, with four of the five sources likely in the redshift range 2 ≤ z < 4.
• The combined flux of the five most secure sources accounts for 30–50% of the previously unresolved sub-mm background at 850 μm.
• The survey resolves approximately 80% of the sub-mm background at a flux density level of ~0.3 mJy, indicating that the background is largely resolved.
• The dust-enshrouded star-formation rate density at z ≈ 3 is estimated at 0.21 h M☉ yr⁻¹ Mpc⁻³ (assuming q₀ = 0.5), which is at least five times higher than inferred from UV data alone.
• If one of the sources lies at z > 4, the star-formation rate density remains approximately constant out to z ≈ 5, suggesting sustained activity in the early universe.