Prof. Dimitra Rigopoulou

Abstract:

Emission at far-infrared wavelengths makes up a significant fraction of the total light detected from galaxies over the age of Universe. Herschel provides an opportunity for studying galaxies at the peak wavelength of their emission. Our aim is to provide a benchmark for models of galaxy population evolution and to test pre-existing models of galaxies. With the Herschel Multi-tiered Extra-galactic survey, HerMES, we have observed a number of fields of different areas and sensitivity using the SPIRE instrument on Herschel. We have determined the number counts of galaxies down to ~20 mJy. Our constraints from directly counting galaxies are consistent with, though more precise than, estimates from the BLAST fluctuation analysis. We have found a steep rise in the Euclidean normalised counts <100 mJy. We have directly resolved ~15% of the infrared extra-galactic background at the wavelength near where it peaks. © 2010 ESO.

Abstract:

We report on the sensitivity of SPIRE photometers on the Herschel Space Observatory. Specifically, we measure the confusion noise from observations taken during the science demonstration phase of the Herschel Multi-tiered Extragalactic Survey. Confusion noise is defined to be the spatial variation of the sky intensity in the limit of infinite integration time, and is found to be consistent among the different fields in our survey at the level of 5.8, 6.3 and 6.8 mJy/beam at 250, 350 and 500 μm, respectively. These results, together with the measured instrument noise, may be used to estimate the integration time required for confusion limited maps, and provide a noise estimate for maps obtained by SPIRE. © 2010 ESO.

Abstract:

We present colours of sources detected with the Herschel/SPIRE instrument in deep extragalactic surveys of the Lockman Hole, Spitzer-FLS, and GOODS-N fields in three photometric bands at 250, 350 and 500 μm. We compare these with expectations from the literature and discuss associated uncertainties and biases in the SPIRE data. We identify a 500 μm flux limited selection of sources from the HerMES point source catalogue that appears free from neighbouring/blended sources in all three SPIRE bands. We compare the colours with redshift tracks of various contemporary models. Based on these spectral templates we show that regions corresponding to specific population types and redshifts can be identified better in colour-flux space. The redshift tracks as well as the colour-flux plots imply a majority of detected objects with redshifts at 1 < z < 3.5, somewhat depending on the group of model SEDs used. We also find that a population of sources with S250/S 350 < 0.8 at fluxes above 50 mJy as observed by SPIRE are not well represented by contemporary models and could consist of a mix of cold and lensed galaxies. © 2010 ESO.

Abstract:

The Herschel Space Observatory enables us to accurately measure the bolometric output of starburst galaxies and active galactic nuclei (AGN) by directly sampling the peak of their far-infrared (IR) emission. Here we examine whether the spectral energy distribution (SED) and dust temperature of galaxies have strongly evolved over the last 80% of the age of the Universe. We discuss possible consequences for the determination of starformation rates (SFR) and any evidence for a major change in their star-formation properties. We use Herschel deep extragalactic surveys from 100 to 500 μm to compute total IR luminosities in galaxies down to the faintest levels, using PACS and SPIRE in the GOODS-North field (PEP and HerMES key programs). An extension to fainter luminosities is done by stacking images on 24 μm prior positions. We show that measurements in the SPIRE bands can be used below the statistical confusion limit if information at higher spatial resolution is used, e.g. at 24 μm, to identify isolated ± galaxies whose flux is not boosted by bright neighbors. Below ∼1.5, mid-IR extrapolations are correct for star-forming galaxies with a dispersion of only 40% (0.15 dex), therefore similar to ∼0 galaxies, over three decades in luminosity below the regime of ultra-luminous IR galaxies (ULIRGs, LIR ¡Ý 1012 L). This narrow distribution is puzzling when considering the range of physical processes that could have affected the SED of these galaxies. Extrapolations from only one of the 160 μm, 250 μm or 350 μm bands alone tend to overestimate the total IR luminosity. This may be explained by the lack of far-IR constraints around and above ∼150 μm (rest-frame) before Herschel on those templates. We also note that the dust temperature of luminous IR galaxies (LIRGs, LIR 10 11 L) around ∼1 is mildly colder by 10̈C15% than their local analogs and up to 20% for ULIRGs at ∼1.6 (using a single modified blackbody-fit to the peak far-IR emission with an emissivity index of ≥ = 1.5). Above z = 1.5, distant galaxies are found to exhibit a substantially larger mid-over far-IR ratio, which could either result from stronger broad emission lines or warm dust continuum heated by a hidden AGN. Two thirds of the AGNs identified in the field with a measured redshift exhibit the same behavior as purely star-forming galaxies. Hence a large fraction of AGNs harbor coeval star formation at very high SFR and in conditions similar to purely star-forming galaxies. © 2010 ESO.

Abstract:

The Herschel Space Observatory enables us to accurately measure the bolometric output of starburst galaxies and active galactic nuclei (AGN) by directly sampling the peak of their far-infrared (IR) emission. Here we examine whether the spectral energy distribution (SED) and dust temperature of galaxies have strongly evolved over the last 80% of the age of the Universe. We discuss possible consequences for the determination of starformation rates (SFR) and any evidence for a major change in their star-formation properties. We use Herschel deep extragalactic surveys from 100 to 500 μ m to compute total IR luminosities in galaxies down to the faintest levels, using PACS and SPIRE in the GOODS-North field (PEP and HerMES key programs). An extension to fainter luminosities is done by stacking images on 24 μ m prior positions. We show that measurements in the SPIRE bands can be used below the statistical confusion limit if information at higher spatial resolution is used, e.g. at 24 μ m, to identify "isolated" galaxies whose flux is not boosted by bright neighbors. Below z ∼ 1.5, mid-IR extrapolations are correct for star-forming galaxies with a dispersion of only 40% (0.15 dex), therefore similar to z ∼ 0 galaxies, over three decades in luminosity below the regime of ultra-luminous IR galaxies (ULIRGs, LIR ≥ 1012 L⊙). This narrow distribution is puzzling when considering the range of physical processes that could have affected the SED of these galaxies. Extrapolations from only one of the 160 μ m, 250 μ m or 350 μ m bands alone tend to overestimate the total IR luminosity. This may be explained by the lack of far-IR constraints around and above ∼150 μ m (rest-frame) before Herschel on those templates. We also note that the dust temperature of luminous IR galaxies (LIRGs, LIR ≥ 1011 L⊙) around z ∼ 1 is mildly colder by 10-15% than their local analogs and up to 20% for ULIRGs at z ∼ 1.6 (using a single modified blackbody-fit to the peak far-IR emission with an emissivity index of β = 1.5). Above z = 1.5, distant galaxies are found to exhibit a substantially larger mid- over far-IR ratio, which could either result from stronger broad emission lines or warm dust continuum heated by a hidden AGN. Two thirds of the AGNs identified in the field with a measured redshift exhibit the same behavior as purely star-forming galaxies. Hence a large fraction of AGNs harbor coeval star formation at very high SFR and in conditions similar to purely star-forming galaxies. © 2010 ESO.