Herschel reveals the obscured star formation in HiZELS Hα emitters at z = 1.47

Ibar, E. and Sobral, D. and Best, P. N. and Ivison, R. J. and Smail, I. and Arumugam, V. and Berta, S. and Béthermin, M. and Bock, J. and Cava, A. and Conley, A. and Farrah, D. and Geach, J. and Ikarashi, S. and Kohno, K. and Le Floc'h, E. and Lutz, D. and Magdis, G. and Magnelli, B. and Marsden, G. and Oliver, S. J. and Page, M. J. and Pozzi, F. and Riguccini, L. and Schulz, B. and Seymour, N. and Smith, A. J. and Symeonidis, M. and Wang, L. and Wardlow, J. and Zemcov, M. (2013) Herschel reveals the obscured star formation in HiZELS Hα emitters at z = 1.47. Monthly Notices of the Royal Astronomical Society, 434 (4). pp. 3218-3235. ISSN 0035-8711

Abstract

We describe the far-infrared (far-IR; rest-frame 8-1000-μm) properties of a sample of 443 Ha- selected star-forming galaxies in the Cosmic Evolution Survey (COSMOS) and Ultra Deep Survey (UDS) fields detected by the High-redshift Emission Line Survey (HiZELS) imaging survey. Sources are identified using narrow-band filters in combination with broad-band photometry to uniformly select Ha (and [O II] if available) emitters in a narrow redshift slice at z = 1.47 ± 0.02.We use a stacking approach in Spitzer-MIPS mid-IR, Herschel-PACS/SPIRE far-IR [from the PACS Evolutionary Prove (PEP) and Herschel Multi-tiered Extragalactic Survey (HerMES)] and AzTEC mm-wave images to describe their typical far-IR properties. We find that HiZELS galaxies with observed Ha luminosities of L(Hα)obs ≈ 108.1-9.1 L⊙ (≈1041.7-42.7 erg s-1) have bolometric far-IR luminosities of typical luminous IR galaxies, L(8-1000 μm) ≈ 1011.41+0.04 -0.06 L⊙. Combining the Ha and far-IR luminosities, we derive median star formation rates (SFRs) of SFRHα, FIR = 32 ± 5 M⊙ yr-1 and Ha extinctions of AHα= 1.0 ± 0.2 mag. Perhaps surprisingly, little difference is seen in typical HiZELSextinction levels compared to local star-forming galaxies. We confirm previous empirical stellar mass (M*) to AHa relations and the little or no evolution up to z = 1.47. For HiZELS galaxies (or similar samples) we provide an empirical parametrization of the SFR as a function of rest-frame (u - z) colours and 3.6-μm photometry - a useful proxy to aid in the absence of far-IR detections in high-z galaxies. We find that the observed Ha luminosity is a dominant SFR tracer when rest-frame (u - z) colours are 9.7M⊙. We do not find any correlation between the [OII]/Ha and far-IR luminosity, suggesting that this emission line ratio does not trace the extinction of the most obscured star-forming regions, especially in massive galaxies where these dominate. The luminosity-limited HiZELS sample tends to lie above of the so-called main sequence for star-forming galaxies, especially at low stellar masses, indicating high star formation efficiencies in these galaxies. This work has implications for SFR indicators and suggests that obscured star formation is linked to the assembly of stellarmass, with deeper potential wells in massive galaxies providing dense, heavily obscured environments in which stars can form rapidly.