An ALMA Survey of Submillimetre Galaxies in the Extended Chandra Deep Field South: The Far-Infrared Properties of SMGs (1310.6362v2)

Abstract: We exploit ALMA 870um (345GHz) observations of submillimetre sources in the Extended Chandra Deep Field South to investigate the far-infrared properties of high-redshift submillimetre galaxies (SMGs). Using the precisely located 870um ALMA positions of 99 SMGs, together with 24um and radio imaging of this field, we deblend the Herschel/SPIRE imaging of this region to extract their far-infrared fluxes and colours. The median photometric redshifts for ALMA LESS (ALESS) SMGs which are detected in at least two SPIRE bands increases with wavelength of the peak in their SEDs, with z=2.3+/-0.2, 2.5+/-0.3 and 3.5+/-0.5 for the 250, 350 and 500-um peakers respectively. We find that 34 ALESS SMGs do not have a >3-sigma counterpart at 250, 350 or 500-um. These galaxies have a median photometric redshift of z=3.3+/-0.5, which is higher than the full ALESS SMG sample; z=2.5+/-0.2. Using the photometric redshifts together with the 250-870um photometry, we estimate the far-infrared luminosities and characteristic dust temperature of each SMG. The median infrared luminosity of the S_870um>2mJy SMGs is L_IR=(3.0+/-0.3)x10^{12}Lo(SFR=300+/-30Mo/yr). At a fixed luminosity, the characteristic dust temperature of these high-redshift SMGs is 2-3K lower than comparably luminous galaxies at z=0, reflecting the more extended star formation occurring in these systems. By extrapolating the 870um number counts to S_ 870um=1mJy, we show that the contribution of S_870um>1mJy SMGs to the cosmic star formation budget is 20% of the total over the redshift range z~1-4. We derive a median dust mass for these SMGs of M_d=(3.6+/-0.3)x10^8Mo and by adopting an appropriate gas-to-dust ratio, we estimate an average molecular mass of M_H2=(4.2+/-0.4)x10^{10}Mo. Finally, we use our estimates of the H2 masses to show that SMGs with S_870um>1mJy contain ~10% of the z~2 volume-averaged H2 mass density at this epoch.

• The paper establishes that SMGs have a median redshift of approximately 2.5 with high infrared luminosities, indicating star formation rates around 300 M☉/year.
• It employs precise 870 μm ALMA imaging to deblend Herschel/SPIRE data, enabling accurate measurement of dust properties and spectral energy distributions.
• These findings demonstrate that SMGs account for nearly 20% of the cosmic star formation rate density between redshifts 1 and 4, underscoring their pivotal role in galaxy evolution.

An ALMA Survey of Sub-millimetre Galaxies: Unveiling the Far-Infrared Properties

The paper "An ALMA Survey of Sub-millimetre Galaxies in the Extended Chandra Deep Field South: The Far-Infrared Properties of SMGs" provides a comprehensive analysis of 99 sub-millimetre galaxies (SMGs) using high-resolution 870 μm observations from the Atacama Large Millimeter/submillimeter Array (ALMA). The authors aim to explore the far-infrared characteristics of SMGs, shedding light on their star formation rates, dust properties, and contributions to cosmic star formation over significant redshift ranges.

Methodological Approach

The paper utilizes precise ALMA positions to overcome the limitations of previous single-dish observations which suffered from poor spatial resolution. By pinpointing 870 μm positions, the team accurately deblends the Herschel/SPIRE imaging data, obtaining reliable far-infrared fluxes and spectral energy distributions (SEDs) for these distant galaxies. Coupled with multi-wavelength data from radio and 24μm imaging, the research constructs a detailed picture of SMG properties.

Key Findings

1. Redshift and Dust Characteristics:
   • The SMGs exhibit a median redshift of z ≈ 2.5, with those peaking at longer infrared wavelengths generally situated at higher redshifts. For instance, the 500 μm peakers show an average redshift of z = 3.5 ± 0.5.
   • SMGs without SPIRE counterparts tend to have even higher redshifts, z = 3.3 ± 0.5, suggesting a significant population of distant, potentially colder sources previously undetected.
2. Luminosity and Star Formation:
   • The paper finds median infrared luminosities (L_IR) of around 3.0 × 10^12 L_☉, corresponding to high star formation rates approximately 300 M_☉/year.
   • A distinct characteristic of high-redshift SMGs is their cooler dust temperatures compared to similar luminosity galaxies at z = 0. This indicates more extended star formation regions in these systems, as inferred by their characteristic dust temperatures being 3–5 K lower, on average.
3. Cosmic Star Formation Contribution:
   • The analysis reveals that SMGs contribute approximately 20% of the total cosmic star formation rate density between z ≈ 1 and z ≈ 4.
   • By extrapolating 870 μm number counts to 1 mJy, the authors estimate this substantial contribution, emphasizing the crucial role of SMGs during critical epochs of galaxy assembly.
4. Dust and Molecular Gas Masses:
   • The SMGs have median dust masses of 3.6 × 10^8 M_☉. With an assumed gas-to-dust ratio, the typical molecular gas mass for an SMG is determined to be about 4.2 × 10^10 M_☉.
   • These estimates point toward a significant molecular gas reservoir within SMGs, constituting about 10% of the volume-averaged molecular gas mass density at z ≈ 2.

Theoretical Implications

This work advances our understanding of SMG properties and their evolution, demonstrating how these systems may bridge the connection between enhanced star formation activity and the growth of massive galaxies at early cosmic times. The observed cool dust temperatures at high luminosities could be indicative of different galaxy formation processes at high redshifts compared to the local universe, with implications for models of galaxy evolution, especially in addressing discrepancies observed between simulation predictions and empirical data.

Conclusion

This paper presents an insightful examination of the far-infrared characteristics of SMGs, offering valuable data that reinforce the significant role these galaxies play in the cosmic star formation landscape at high redshifts. The research suggests further investigations into the physical mechanisms driving such efficiency in star formation, potentially leveraging the resolving power of ALMA for even more detailed studies of the submillimetre universe. Future developments in high-redshift galaxy surveys, alongside complementary simulations, are likely to build upon these findings, refining models of galaxy formation and evolution in the early universe.