arXiv subjects

R. Hopwood

Publications and source records attributed to R. Hopwood.

65 records · Page 4Linked to original sources

arXiv2011

GAMA/H-ATLAS: The ultraviolet spectral slope and obscuration in galaxies

We use multiwavelength data from the Galaxy And Mass Assembly (GAMA) and Herschel ATLAS (H-ATLAS) surveys to compare the relationship between various dust obscuration measures in galaxies. We explore the connections between the ultraviolet (UV) spectral slope, $β$, the Balmer decrement, and the far infrared (IR) to $150\,$nm far ultraviolet (FUV) luminosity ratio. We explore trends with galaxy mass, star formation rate (SFR) and redshift in order to identify possible systematics in these various measures. We reiterate the finding of other authors that there is a large scatter between the Balmer decrement and the $β$ parameter, and that $β$ may be poorly constrained when derived from only two broad passbands in the UV. We also emphasise that FUV derived SFRs, corrected for dust obscuration using $β$, will be overestimated unless a modified relation between $β$ and the attenuation factor is used. Even in the optimum case, the resulting SFRs have a significant scatter, well over an order of magnitude. While there is a stronger correlation between the IR to FUV luminosity ratio and $β$ parameter than with the Balmer decrement, neither of these correlations are particularly tight, and dust corrections based on $β$ for high redshift galaxy SFRs must be treated with caution. We conclude with a description of the extent to which the different obscuration measures are consistent with each other as well as the effects of including other galactic properties on these correlations.

astro-ph.CO↗

arXiv2011

Spitzer Imaging of Herschel-ATLAS Gravitationally Lensed Submillimeter Sources

We present physical properties of two submillimeter selected gravitationally lensed sources, identified in the Herschel Astrophysical Terahertz Large Area Survey. These submillimeter galaxies (SMGs) have flux densities > 100 mJy at 500 um, but are not visible in existing optical imaging. We fit light profiles to each component of the lensing systems in Spitzer IRAC 3.6 and 4.5 um data and successfully disentangle the foreground lens from the background source in each case, providing important constraints on the spectral energy distributions (SEDs) of the background SMG at rest-frame optical-near-infrared wavelengths. The SED fits show that these two SMGs have high dust obscuration with Av ~4 to 5 and star formation rates of ~100 M_sun/yr. They have low gas fractions and low dynamical masses compared to 850 um selected galaxies.

astro-ph.CO↗

arXiv2010

GBT Zpectrometer CO(1-0) Observations of the Strongly-Lensed Submillimeter Galaxies from the Herschel ATLAS

The Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) has uncovered a population of strongly-lensed submillimeter galaxies (SMGs). The Zpectrometer instrument on the Green Bank Telescope (GBT) was used to measure the redshifts and constrain the masses of the cold molecular gas reservoirs for two candidate high-redshift lensed sources. We derive CO(1-0) redshifts of z=3.042+/-0.001 and z=2.625+/-0.001, and measure molecular gas masses of (1--3)x10^{10}Msun, corrected for lens amplification and assuming a conversion factor of alhpa=0.8 Msun(K km/s pc^2)^{-1}. We find typical L(IR)/L'(CO) ratios of 120+/-40 and 140+/-50 Lsun (K km/s pc^2)^{-1}, which are consistent with those found for local ULIRGs and other high-redshift SMGs. From analysis of published data, we find no evidence for enhanced L(IR)/L'(CO(1-0)) ratios for the SMG population in comparison to local ULIRGs. The GBT results highlight the power of using the CO lines to derive blind redshifts, which is challenging for the SMG population at optical wavelengths given their high obscuration.

astro-ph.CO↗

arXiv2010

Which halos host Herschel-ATLAS galaxies in the local Universe?

We measure the projected cross-correlation between low redshift (z < 0.5) far-IR selected galaxies in the SDP field of the Herschel-ATLAS (H-ATLAS) survey and optically selected galaxies from the Galaxy and Mass Assembly (GAMA) redshift survey. In order to obtain robust correlation functions, we restrict the analysis to a subset of 969 out of 6900 H-ATLAS galaxies, which have reliable optical counterparts with r<19.4 mag and well-determined spectroscopic redshifts. The overlap region between the two surveys is 12.6 sq. deg; the matched sample has a median redshift of z ~ 0.2. The cross-correlation of GAMA and H-ATLAS galaxies within this region can be fitted by a power law, with correlation length r_0 ~ 4.63 +/- 0.51 Mpc. Comparing with the corresponding auto-correlation function of GAMA galaxies within the SDP field yields a relative bias (averaged over 2-8 Mpc) of H-ATLAS and GAMA galaxies of b_H/b_G ~ 0.6. Combined with clustering measurements from previous optical studies, this indicates that most of the low redshift H-ATLAS sources are hosted by halos with masses comparable to that of the Milky Way. The correlation function appears to depend on the 250 um luminosity, L_250, with bright (median luminosity νL_250 ~ 1.6 x 10^10 L_sun) objects being somewhat more strongly clustered than faint (νL_250 ~ 4.0 x 10^9 L_sun) objects. This implies that galaxies with higher dust-obscured star formation rates are hosted by more massive halos.

astro-ph.CO↗

arXiv2010

The Detection of a Population of Submillimeter-Bright, Strongly-Lensed Galaxies

Gravitational lensing is a powerful astrophysical and cosmological probe and is particularly valuable at submillimeter wavelengths for the study of the statistical and individual properties of dusty starforming galaxies. However the identification of gravitational lenses is often time-intensive, involving the sifting of large volumes of imaging or spectroscopic data to find few candidates. We used early data from the Herschel Astrophysical Terahertz Large Area Survey to demonstrate that wide-area submillimeter surveys can simply and easily detect strong gravitational lensing events, with close to 100% efficiency.

astro-ph.CO↗

arXiv2010

Herschel-ATLAS: Statistical Properties of Galactic Cirrus in the GAMA-9 Hour Science Demonstration Phase Field

We study the Spectral Energy Distribution (SED) and the power spectrum of Galactic cirrus emission observed in the 14 deg^2 Science Demonstration Phase field of the Herschel-ATLAS using Herschel and IRAS data from 100 to 500 um. We compare the SPIRE 250, 350 and 500um maps with IRAS 100um emission, binned in 6' pixels. We assume a modified black-body SED with dust emissivity parameter beta (F ~ lambda^(-beta)) and a single dust temperature T_d, and find that the dust temperature and emissivity index varies over the science demonstration field as 10< T_rm < 25 K and 1 < beta< 4. The latter values are somewhat higher than the range of beta often quoted in the literature (1< beta< 2). We estimate the mean values of these parameters to be T_d=19.0 +/- 2.4 K and beta = 1.4 +/- 0.4. In regions of bright cirrus emission, we find that the dust has similar temperatures with T_d = 18.0 +/- 2.5 K, and similar values of beta, ranging from 1.4 +- 0.5 to 1.9+/- 0.5. We show that T_d and beta associated with diffuse cirrus emission are anti-correlated and can be described by the relationship: beta(T_d) = NT_d^alpha with [N=116+/-38, alpha=-1.4+/1 0.1]. The strong correlation found in this analysis is not just limited to high density clumps of cirrus emission as seen in previous studies, but is also seen in diffuse cirrus in low density regions. To provide an independent measure of $T_{\rm d}$ and $β$, we obtain the angular power spectrum of the cirrus emission in the {\it IRAS} and SPIRE maps, which is consistent with a power spectrum of the form P(k)=P_0(k/k_0)^gamma where gamma = â H R2.6+/-m 0.2 for scales of 50-200' in the SPIRE maps. The cirrus rms fluctuation amplitude at angular scales of 100' is consistent with a modified blackbody SED with T_d = 20.1+/- 0.9 K and beta = 1.3+/- 0.2, in agreement with the values obtained above.

astro-ph.CO↗

arXiv2010

The first release of data from the Herschel ATLAS: the SPIRE images

We have reduced the data taken with the Spectral and Photometric Imaging Receiver (SPIRE) photometer on board the Herschel Space Observatory in the Science Demonstration Phase (SDP) of the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS). We describe the data reduction, which poses specific challenges, both because of the sheer size of the data, and because only two scans are made for each region. We implement effective solutions to process the bolometric timelines into maps, and show that correlations among detectors are negligible, and that the photometer is stable on time scales up to 250 s. This is longer than the time the telescope takes to cross the observed sky region, and it allows us to use naive binning methods for an optimal reconstruction of the sky emission. The maps have equal contribution of confusion and white instrumental noise, and the latter is estimated to 5.3, 6.4, and 6.7 mJy/beam (1-σ), at 250, 350, and 500 μ{m}, respectively. This pipeline is used to reduce other H-ATLAS observations, as they became available, and we discuss how it can be used with the optimal map maker implemented in the Herschel Interactive Processing Environment (HIPE), to improve computational efficiency and stability. The SDP dataset is available from http://www.h-atlas.org/.

astro-ph.IM↗

arXiv2010

Herschel-ATLAS: far-infrared properties of radio-selected galaxies

We use the Herschel-ATLAS science demonstration data to investigate the star-formation properties of radio-selected galaxies in the GAMA-9h field as a function of radio luminosity and redshift. Radio selection at the lowest radio luminosities, as expected, selects mostly starburst galaxies. At higher radio luminosities, where the population is dominated by AGN, we find that some individual objects are associated with high far-infrared luminosities. However, the far-infrared properties of the radio-loud population are statistically indistinguishable from those of a comparison population of radio-quiet galaxies matched in redshift and K-band absolute magnitude. There is thus no evidence that the host galaxies of these largely low-luminosity (Fanaroff-Riley class I), and presumably low-excitation, AGN, as a population, have particularly unusual star-formation histories. Models in which the AGN activity in higher-luminosity, high-excitation radio galaxies is triggered by major mergers would predict a luminosity-dependent effect that is not seen in our data (which only span a limited range in radio luminosity) but which may well be detectable with the full Herschel-ATLAS dataset.

astro-ph.CO↗

arXiv2010

Herschel-ATLAS: The far-infrared--radio correlation at z<0.5

We use data from the Herschel-ATLAS to investigate the evolution of the far-infrared--radio correlation over the redshift range 0 5sigma sources in the Herschel-ATLAS Science Demonstration Field and cross-matching these data with radio data from the Faint Images of the Radio Sky at Twenty-Centimetres (FIRST) survey and the NRAO VLA Northern Sky Survey (NVSS), we obtain 104 radio counterparts to the Herschel sources. With these data we find no evidence for evolution in the far-infrared--radio correlation over the redshift range 0<z<0.5, where the median value for the ratio between far-infrared and radio luminosity, $q_{\rm IR}$, over this range is $q_{\rm IR} = 2.40\pm 0.12$ (and a mean of $q_{\rm IR}=2.52 \pm 0.03$ accounting for the lower limits), consistent with both the local value determined from {\em IRAS} and values derived from surveys targeting the high-redshift Universe. By comparing the radio fluxes of our sample measured from both FIRST and NVSS we show that previous results suggesting an increase in the value of $q_{\rm IR}$ from high to low redshift may be the result of resolving out extended emission of the low-redshift sources with relatively high-resolution interferometric data, although AGN contamination could still play a significant role. We also find tentative evidence that the longer wavelength, cooler dust is heated by an evolved stellar population which does not trace the star-formation rate as closely as the shorter wavelength $\ltsim 250~μ$m emission or the radio emission, supporting suggestions based on detailed models of individual galaxies.

astro-ph.CO↗

arXiv2010

H-ATLAS: PACS imaging for the Science Demonstration Phase

We describe the reduction of data taken with the PACS instrument on board the Herschel Space Observatory in the Science Demonstration Phase of the Herschel-ATLAS (H-ATLAS) survey, specifically data obtained for a 4x4-deg^2 region using Herschel's fast-scan (60 arcsec/s) parallel mode. We describe in detail a pipeline for data reduction using customised procedures within HIPE from data retrieval to the production of science-quality images. We found that the standard procedure for removing Cosmic-Ray glitches also removed parts of bright sources and so implemented an effective two-stage process to minimise these problems. The pronounced 1/f noise is removed from the timelines using 3.4- and 2.5-arcmin boxcar high-pass filters at 100 and 160-um. Empirical measurements of the point-spread function (PSF) are used to determine the encircled energy fraction as a function of aperture size. For the 100- and 160-um bands, the effective PSFs are ~9 and ~13 arcsec (FWHM), and the 90-per-cent encircled energy radii are 13 and 18 arcsec. Astrometric accuracy is good to ~<2 arcsec. The noise in the final maps is correlated between neighbouring pixels and rather higher than advertised prior to launch. For a pair of cross-scans, the 5-sigma point-source sensitivities are 125-165 mJy for 9-13-arcsec radius apertures at 100-um and 150-240 mJy for 13-18-arcsec radius apertures at 160-um.

astro-ph.IM↗

arXiv2010

Ultra deep AKARI observations of Abell 2218: resolving the 15 um extragalactic background light

We present extragalactic number counts and a lower limit estimate for the cosmic infrared background at 15 um from AKARI ultra deep mapping of the gravitational lensing cluster Abell 2218. This data is the deepest taken by any facility at this wavelength, and uniquely samples the normal galaxy population. We have de-blended our sources, to resolve photometric confusion, and de-lensed our photometry to probe beyond AKARI's blank-field sensitivity. We estimate a de-blended 5 sigma sensitivity of 28.7 uJy. The resulting 15 um galaxy number counts are a factor of three fainter than previous results, extending to a depth of ~ 0.01 mJy and providing a stronger lower limit constraint on the cosmic infrared background at 15 um of 1.9 +/- 0.5 nW m^-2 sr^-1.

astro-ph.CO↗