Extremely Large Telescope

Abstract:

Submillimeter galaxies (SMGs) at $z\gtrsim1$ are luminous in the far-infrared and have star-formation rates, SFR, of hundreds to thousands of solar masses per year. However, it is unclear whether they are true analogs of local ULIRGs or whether the mode of their star formation is more similar to that in local disk galaxies. We target these questions by using Herschel-PACS to examine the conditions in the interstellar medium (ISM) in far-infrared luminous SMGs at z~1-4. We present 70-160 micron photometry and spectroscopy of the [OIV]26 micron, [FeII]26 micron, [SIII]33 micron, [SiII]34 micron, [OIII]52 micron, [NIII]57 micron, and [OI]63 micron fine-structure lines and the S(0) and S(1) hydrogen rotational lines in 13 lensed SMGs identified by their brightness in early Herschel data. Most of the 13 targets are not individually spectroscopically detected and we instead focus on stacking these spectra with observations of an additional 32 SMGs from the \herschel\ archive -- representing a complete compilation of PACS spectroscopy of SMGs. We detect [OI]63 micron, [SiII]34 micron, and [NIII]57 micron at >3sigma in the stacked spectra, determining that the average strengths of these lines relative to the far-IR continuum are $(0.36\pm0.12)\times10^{-3}$, $(0.84\pm0.17)\times10^{-3}$, and $(0.27\pm0.10)\times10^{-3}$, respectively. Using the [OIII]52/[NIII]57 emission line ratio we show that SMGs have average gas-phase metallicities $\gtrsim Z_{\rm sun}$. By using PDR modelling and combining the new spectral measurements with integrated far-infrared fluxes and existing [CII]158 micron data we show that SMGs have average gas densities, n, of $\sim10^{1-3}{\rm cm^{-3}}$ and FUV field strengths, $G_0\sim10^{2.2-4.5}$ (in Habing units: $1.6\times10^{-3}{\rm erg~cm^{-2}~s^{-1}}$), consistent with both local ULIRGs and lower luminosity star-forming galaxies.

Abstract:

In order to understand the role of radio-quiet quasars (RQQs) in galaxy evolution, we must determine the relative levels of accretion and star-formation activity within these objects. Previous work at low radio flux densities has shown that accretion makes a significant contribution to the total radio emission, in contrast with other quasar studies that suggest star formation dominates. To investigate, we use 70 RQQs from the Spitzer-Herschel Active Galaxy Survey. These quasars are all at z ∼ 1, thereby minimizing evolutionary effects, and have been selected to span a factor of ∼100 in optical luminosity, so that the luminosity dependence of their properties can be studied. We have imaged the sample using the Karl G. Jansky Very Large Array (JVLA), whose high sensitivity results in 35 RQQs being detected above 2σ. This radio data set is combined with far-infrared luminosities derived from grey-body fitting to Herschel photometry. By exploiting the far-infrared-radio correlation observed for star-forming galaxies, and comparing two independent estimates of the star-formation rate, we show that star formation alone is not sufficient to explain the total radio emission. Considering RQQs above a 2σ detection level in both the radio and the far-infrared, 92 per cent are accretion dominated, and the accretion process accounts for 80 per cent of the radio luminosity when summed across the objects. The radio emission connected with accretion appears to be correlated with the optical luminosity of the RQQ, whilst a weaker luminosity dependence is evident for the radio emission connected with star formation.