An ALMA survey of Sub-millimeter Galaxies in the Extended Chandra Deep Field South: Physical properties derived from ultraviolet-to-radio modelling (1504.04376v1)

Abstract: [abridged] The ALESS survey has followed-up a sample of 122 sub-millimeter sources in the Extended Chandra Deep Field South at 870um with ALMA, allowing to pinpoint the positions of sub-millimeter galaxies (SMGs) to 0.3'' and to find their precise counterparts at different wavelengths. This enabled the first compilation of the multi-wavelength spectral energy distributions (SEDs) of a statistically reliable survey of SMGs. In this paper, we present a new calibration of the MAGPHYS modelling code that is optimized to fit these UV-to-radio SEDs of z>1 star-forming galaxies using an energy balance technique to connect the emission from stellar populations, dust attenuation and dust emission in a physically consistent way. We derive statistically and physically robust estimates of the photometric redshifts and physical parameters for the ALESS SMGs. We find that they have a median stellar mass $M_\ast=(8.9\pm0.1)\times10^{10} M_\odot$, SFR$=280\pm70 M_\odot$/yr, overall V-band dust attenuation $A_V=1.9\pm0.2$ mag, dust mass $M_\rm{dust}=(5.6\pm1.0)\times10^8 M_\odot$, and average dust temperature Tdust~40 K. The average intrinsic SED of the ALESS SMGs resembles that of local ULIRGs in the IR range, but the stellar emission of our average SMG is brighter and bluer, indicating lower dust attenuation, possibly because they are more extended. We explore how the average SEDs vary with different parameters, and we provide a new set of SMG templates. To put the ALESS SMGs into context, we compare their stellar masses and SFRs with those of less actively star-forming galaxies at the same redshifts. At z~2, about half of the SMGs lie above the star-forming main sequence, while half are at the high-mass end of the sequence. At higher redshifts (z~3.5), the SMGs tend to have higher SFR and Mstar, but the fraction of SMGs that lie significantly above the main sequence decreases to less than a third.

• The paper introduces a calibrated energy balance model linking UV-optical and FIR-radio emissions to derive robust SMG properties.
• The model yields key measurements including a median stellar mass of 8.9×10¹⁰ M⊙, SFR of 280 M⊙/yr, and dust temperature around 40 K.
• Findings indicate many SMGs exceed the star-forming main sequence at z≈2, underscoring their significant role in galaxy evolution.

Overview of the Paper on ALMA Survey of Sub-millimeter Galaxies

The paper presents a comprehensive paper of sub-millimeter galaxies (SMGs) observed in the Atacama Large Millimeter/submillimeter Array (ALMA) survey within the Extended Chandra Deep Field South. The survey, referred to as ALESS, provides high precision data on the positions and counterparts of SMGs, enabling an unprecedented multi-wavelength characterization of these galaxies from ultraviolet to radio wavelengths.

Methodology and Model Calibration

A significant contribution of the paper is the development of a calibrated modelling technique tailored for SMGs. This model consistently links the ultraviolet and optical stellar emission to the far-infrared and radio dust emission through an energy balance approach. The modelling prioritizes physical solutions that are statistically robust, accommodating the complex interrelations among stellar populations, dust attenuation, and dust re-emission.

Key parameters such as photometric redshifts, stellar masses, star formation rates (SFRs), and dust properties are derived. The paper highlights a median stellar mass of about $8.9 \times 10^{10} M_\odot$ and median SFR of approximately $280 M_\odot \mathrm{yr}^{-1}$. The effectiveness of the energy balance model allows for a self-consistent treatment of these multi-wavelength properties, revealing detailed insights into the nature of SMGs.

Key Findings

• Stellar and Dust Properties: The results indicate a high degree of dust content in SMGs, with median estimates for the dust attenuation and mass being $A_V = 1.9$ mag and $5.6 \times 10^8 M_\odot$, respectively. The average dust temperature is around 40 K.
• Spectral Energy Distributions: The average spectral energy distributions (SEDs) of SMGs resemble those of local ultra-luminous infrared galaxies (ULIRGs), although differences in the stellar emission spectrum suggest lower dust attenuation likely due to more extended structures.
• Star-forming Context: By comparing SMGs to the star-forming main sequence, the findings suggest that at redshifts $z\simeq2$, about half of the SMGs are above the main sequence with SFRs three times that of typical galaxies of similar mass. However, this fraction decreases significantly at higher redshifts ($z\simeq3.5$).

Implications and Future Directions

The paper reinforces the complexity of SMGs and their pivotal role in understanding galaxy evolution. The ability to derive photometric redshifts alongside detailed physical properties using the updated modelling technique mark a significant advancement in our approach to these enigmatic galaxies.

The findings have implications for the evolutionary pathways of massive galaxies, suggesting that SMGs contribute significantly to the mass assembly at high redshifts. These insights provide a strong foundation for future observational campaigns and theoretical models aiming to refine our understanding of galaxy formation and evolution.

Future developments may focus on utilizing this methodology to explore the distribution of SMGs across different cosmic environments and to investigate potential AGN contributions to their energy outputs. High-resolution morphological studies, combined with the derived physical properties, will further elucidate the structure and evolution implications for high-redshift star-forming galaxies.

In conclusion, this paper represents a vital step in comprehensively characterizing SMGs with broad implications for the paper of galaxy evolution across cosmic time, expanding the scope of what can be inferred from extragalactic sub-millimeter observations.