A Herschel view of the far-infrared properties of submillimetre galaxies (1202.0761v1)

Abstract: We study a sample of 61 submillimetre galaxies (SMGs) selected from ground-based surveys, with known spectroscopic redshifts and observed with Herschel as part of the PACS Evolutionary Probe (PEP) and the Herschel Multi-tiered Extragalactic Survey (HerMES) key programmes. We use the broad far-infrared wavelength coverage (100-600um) provided by the combination of PACS and SPIRE observations. Using a power-law temperature distribution model to derive infrared luminosities and dust temperatures, we measure a dust emissivity spectral index for SMGs of beta=2.0+/-0.2. Our results unveil the diversity of the SMG population. Some SMGs exhibit extreme infrared luminosities of ~10^13 Lsun and relatively warm dust components, while others are fainter (~10^12 Lsun) and are biased towards cold dust temperatures. The extreme infrared luminosities of some SMGs (LIR>10^12.7 Lsun, 26/61 systems) imply SFRs of >500Msun yr^-1. Such high SFRs are difficult to reconcile with a secular mode of star formation, and may instead correspond to a merger-driven stage in the evolution of these galaxies. Another observational argument in favour of this scenario is the presence of dust temperatures warmer than that of SMGs of lower luminosities (~40K as opposed to ~25K), consistent with observations of local ULIRGs triggered by major mergers and with results from hydrodynamic simulations of major mergers combined with radiative transfer calculations. Luminous SMGs are also offset from normal star-forming galaxies in the stellar mass-SFR plane, suggesting that they are undergoing starburst events with short duty cycles, compatible with the major merger scenario. On the other hand, a significant fraction of the low infrared luminosity SMGs have cold dust temperatures, are located close to the main sequence of star formation, and thus might be evolving through a secular mode of star formation. [abridged]

• The paper employs a power-law temperature distribution model to extract FIR properties, identifying that high-luminosity SMGs exhibit dust temperatures around 40K.
• It reveals that SMGs with luminosities near 10^13 L⊙ have star formation rates exceeding 500 M⊙/yr, suggesting merger-driven bursts.
• The study demonstrates how wide spectral coverage from 100-600 μm refines our understanding of SMG evolution and star formation mechanisms.

Analysis of Far-Infrared Properties of SMGs Observed by Herschel

This paper utilizes the Herschel Space Observatory to examine the far-infrared (FIR) characteristics of a sample of 61 submillimetre galaxies (SMGs) with known spectroscopic redshifts. These observations were conducted under the PACS Evolutionary Probe (PEP) and the Herschel Multi-tiered Extragalactic Survey (HerMES) guaranteed time key programs. The paper's goal is to elucidate the diverse FIR properties of SMGs, leveraging the broad wavelength coverage from 100-600 μm made possible by Herschel's instruments.

The research highlights the methodological use of a power-law temperature distribution model to deduce infrared luminosities and dust temperatures. The dust emissivity spectral index for the SMGs is measured to be β = 2.0 ± 0.2. This paper addresses the heterogeneity within the SMG population: some exhibit extreme infrared luminosities near ~10^13 L_⊙ with warm dust, while others are less luminous, with infrared outputs around a few times 10^12 L_⊙ and biased toward colder dust temperatures.

Significant results emerge from the paper: at a redshift of approximately 2, classical SMGs typically show large infrared luminosities (~10^13 L_⊙). However, when selected purely by submm flux densities without redshift information, these galaxies can span a broader range of dust temperatures and infrared luminosities. The analysis suggests that the high infrared luminosities observed imply star formation rates (SFRs) exceeding 500 M_⊙ yr^-1, assuming a Chabrier initial mass function (IMF) without significant AGN contribution. This observation challenges the presumption that such high SFRs could result from steady star formation, suggesting instead the influence of merger-driven evolutionary phases.

Luminous SMGs demonstrate dust temperatures (~40K) warmer than their less luminous counterparts (~25K), akin to local ultra-luminous infrared galaxies often triggered by major mergers. This finding aligns with results from hydrodynamic simulations indicating that major mergers, associated with short duty cycle starburst events, likely drive the observed luminosity in these systems. Conversely, many of the lower luminosity SMGs with colder dust appear near the main sequence of star formation, suggesting a secular, non-violent evolutionary mode.

The paper indicates that the diversity observed within SMGs can enrich our understanding of their formation and the role of environmental and interaction-driven star formation processes across cosmic time. Furthermore, the detailed data gathered on SMGs' FIR properties enhance our ability to test pre-Herschel infrared luminosity estimates derived from monochromatic extrapolations more rigorously.

In terms of broader implications, these findings provide a robust framework for interpreting the complex FIR behavior inherent in the SMG population, contributing to the characterization of early-universe star formation dynamics. The versatility of Herschel observations underscores the critical need for wide spectral coverage in disentangling the diverse physical processes governing galaxy evolution. Future research may build on this work by exploring the molecular gas content and dynamics of SMGs to further unravel the interplay between star formation and galactic interactions.