The Herschel Multi-Tiered Extragalactic Survey: SPIRE-mm Photometric Redshifts

Monthly Notices of The Royal Astronomical Society - MON NOTIC ROY ASTRON SOC 09/2011; DOI: 10.1111/j.1365-2966.2011.19827.x
Source: arXiv

ABSTRACT We investigate the potential of submm-mm and submm-mm-radio photometric
redshifts using a sample of mm-selected sources as seen at 250, 350 and 500
{\mu}m by the SPIRE instrument on Herschel. From a sample of 63 previously
identified mm-sources with reliable radio identifications in the GOODS-N and
Lockman Hole North fields 46 (73 per cent) are found to have detections in at
least one SPIRE band. We explore the observed submm/mm colour evolution with
redshift, finding that the colours of mm-sources are adequately described by a
modified blackbody with constant optical depth {\tau} = ({\nu}/{\nu}0)^{\beta}
where {\beta} = +1.8 and {\nu}0 = c/100 {\mu}m. We find a tight correlation
between dust temperature and IR luminosity. Using a single model of the dust
temperature and IR luminosity relation we derive photometric redshift estimates
for the 46 SPIRE detected mm-sources. Testing against the 22 sources with known
spectroscopic, or good quality optical/near-IR photometric, redshifts we find
submm/mm photometric redshifts offer a redshift accuracy of |z|/(1+z) = 0.16 (<
|z| >= 0.51). Including constraints from the radio-far IR correlation the
accuracy is improved to |z|/(1 + z) = 0.15 (< |z| >= 0.45). We estimate the
redshift distribution of mm-selected sources finding a significant excess at z
> 3 when compared to ~ 850 {\mu}m selected samples.

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    ABSTRACT: We present Herschel far-IR photometry and spectroscopy as well as ground based CO observations of an intermediate redshift (0.21 < z < 0.88) sample of Herschel-selected (ultra)-luminous infrared galaxies (L_IR > 10^11.5L_sun). With these measurements we trace the dust continuum, far-IR atomic line emission, in particular [CII]\,157.7microns, as well as the molecular gas of z~0.3 (U)LIRGs and perform a detailed investigation of the interstellar medium of the population. We find that the majority of Herschel-selected intermediate redshift (U)LIRGs have L_CII/L_FIR ratios that are a factor of about 10 higher than that of local ULIRGs and comparable to that of local normal and high-$z$ star forming galaxies. Using our sample to bridge local and high-z [CII] observations, we find that the majority of galaxies at all redshifts and all luminosities follow a L_CII-L_FIR relation with a slope of unity, from which local ULIRGs and high-z AGN dominated sources are clear outliers. We also confirm that the strong anti-correlation between the L_CII/L_FIR ratio and the far-IR color L_60/L_100 observed in the local Universe holds over a broad range of redshifts and luminosities, in the sense that warmer sources exhibit lower L_CII/L_FIR at any epoch. Intermediate redshift ULIRGs are also characterised by large molecular gas reservoirs and by lower star formation efficiencies compared to that of local ULIRGs. The high L_CII/L_FIR ratios, the moderate star formation efficiencies (L_LIR/L_CO or L_IR/M_gas) and the relatively low dust temperatures of our sample (which are also common characteristics of high-z star forming galaxies with ULIRG-like luminosities) indicate that the evolution of the physical properties of (U)LIRGs between the present day and z > 1 is already significant by z ~ 0.3.
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    ABSTRACT: We present the most complete study to date of the X-ray emission from star-formation in high redshift (median z=0.7; z<1.5), IR-luminous (L_IR=10^10-10^13 L_sun) galaxies detected by Herschel's PACS and SPIRE instruments. For our purpose we take advantage of the deepest X-ray data to date, the Chandra deep fields (North and South). Sources which host AGN are removed from our analysis by means of multiple AGN indicators. We find an AGN fraction of 18+/-2 per cent amongst our sample and note that AGN entirely dominate at values of log[L_X/L_IR]>-3 in both hard and soft X-ray bands. From the sources which are star-formation dominated, only a small fraction are individually X-ray detected and for the bulk of the sample we calculate average X-ray luminosities through stacking. We find an average soft X-ray to infrared ratio of log[L_SX/L_IR]=-4.3 and an average hard X-ray to infrared ratio of log[L_HX/L_IR]=-3.8. We report that the X-ray/IR correlation is approximately linear through the entire range of L_IR and z probed and, although broadly consistent with the local (z<0.1) one, it does display some discrepancies. We suggest that these discrepancies are unlikely to be physical, i.e. due to an intrinsic change in the X-ray properties of star-forming galaxies with cosmic time, as there is no significant evidence for evolution of the L_X/L_IR ratio with redshift. Instead they are possibly due to selection effects and remaining AGN contamination. We also examine whether dust obscuration in the galaxy plays a role in attenuating X-rays from star-formation, by investigating changes in the L_X/L_IR ratio as a function of the average dust temperature. We conclude that X-rays do not suffer any measurable attenuation in the host galaxy.
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