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Register a new userThe far-infrared/radio correlation as probed by Herschel
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R. J. Ivison
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We set out to determine the ratio, q(IR), of rest-frame 8-1000um flux, S(IR), to monochromatic radio flux, S(1.4GHz), for galaxies selected at far-IR and radio wavelengths, to search for signs that the ratio evolves with redshift, luminosity or dust temperature, and to identify any far-IR-bright outliers - useful laboratories for exploring why the far-IR/radio correlation is generally so tight when the prevailing theory suggests variations are almost inevitable. We use flux-limited 250-um and 1.4-GHz samples, obtained in GOODS-N using Herschel (HerMES; PEP) and the VLA. We determine bolometric IR output using ten bands spanning 24-1250um, exploiting data from PACS and SPIRE, as well as Spitzer, SCUBA, AzTEC and MAMBO. We also explore the properties of an L(IR)-matched sample, designed to reveal evolution of q(IR) with z, spanning log L(IR) = 11-12 L(sun) and z=0-2, by stacking into the radio and far-IR images. For 1.4-GHz-selected galaxies, we see tentative evidence of a break in the flux ratio, q(IR), at L(1.4GHz) ~ 10^22.7 W/Hz, where AGN are starting to dominate the radio power density, and of weaker correlations with z and T(d). From our 250-um-selected sample we identify a small number of far-IR-bright outliers, and see trends of q(IR) with L(1.4GHz), L(IR), T(d) and z, noting that some of these are inter-related. For our L(IR)-matched sample, there is no evidence that q(IR) changes significantly as we move back into the epoch of galaxy formation: we find q(IR) goes as (1+z)^gamma, where gamma = -0.04 +/- 0.03 at z=0-2; however, discounting the least reliable data at z<0.5 we find gamma = -0.26 +/- 0.07, modest evolution which may be related to the radio background seen by ARCADE2, perhaps driven by <10uJy radio activity amongst ordinary star-forming galaxies at z>1.
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We present the cross-correlation between the far-infrared background fluctuations as measured with the Herschel Space Observatory at 250, 350, and 500 {mu}m and the near-infrared background fluctuations with Spitzer Space Telescope at 3.6 {mu}m. The cross-correlation between far and near-IR background anisotropies are detected such that the correlation coefficient at a few to ten arcminute angular scales decreases from 0.3 to 0.1 when the far-IR wavelength increases from 250 {mu}m to 500 {mu}m. We model the cross-correlation using a halo model with three components: (a) far-IR bright or dusty star-forming galaxies below the masking depth in Herschel maps, (b) near-IR faint galaxies below the masking depth at 3.6 {mu}m, and (c) intra-halo light, or diffuse stars in dark matter halos, that likely dominates fluctuations at 3.6 {mu}m. The model is able to reasonably reproduce the auto correlations at each of the far-IR wavelengths and at 3.6 {mu}m and their corresponding cross-correlations. While the far and near-IR auto-correlations are dominated by faint dusty, star-forming galaxies and intra-halo light, respectively, we find that roughly half of the cross-correlation between near and far-IR backgrounds is due to the same galaxies that remain unmasked at 3.6 {mu}m. The remaining signal in the cross-correlation is due to intra-halo light present in the same dark matter halos as those hosting the same faint and unmasked galaxies. In this model, the decrease in the cross-correlation signal from 250 {mu}m to 500 {mu}m comes from the fact that the galaxies that are primarily contributing to 500 {mu}m fluctuations peak at a higher redshift than those at 250 {mu}m.
We investigate the correlation between far-infrared (FIR) and radio luminosities in distant galaxies, a lynchpin of modern astronomy. We use data from the Balloon-borne Large Aperture Submillimetre Telescope (BLAST), Spitzer, the Large Apex BOlometer CamerA (LABOCA), the Very Large Array (VLA) and the Giant Metre-wave Radio Telescope (GMRT) in the Extended Chandra Deep Field South (ECDFS). For a catalogue of BLAST 250-micron-selected galaxies, we re-measure the 70--870-micron flux densities at the positions of their most likely 24-micron counterparts, which have a median [interquartile] redshift of 0.74 [0.25, 1.57]. From these, we determine the monochromatic flux density ratio, q_250 = log_10 (S_250micron / S_1400MHz), and the bolometric equivalent, q_IR. At z ~= 0.6, where our 250-micron filter probes rest-frame 160-micron emission, we find no evolution relative to q_160 for local galaxies. We also stack the FIR and submm images at the positions of 24-micron- and radio-selected galaxies. The difference between q_IR seen for 250-micron- and radio-selected galaxies suggests star formation provides most of the IR luminosity in ~< 100-uJy radio galaxies, but rather less for those in the mJy regime. For the 24-micron sample, the radio spectral index is constant across 0 < z < 3, but q_IR exhibits tentative evidence of a steady decline such that q_IR is proportional to (1+z)^(-0.15 +/- 0.03) - significant evolution, spanning the epoch of galaxy formation, with major implications for techniques that rely on the FIR/radio correlation. We compare with model predictions and speculate that we may be seeing the increase in radio activity that gives rise to the radio background.
We report the first systematic study of the submillimeter water vapor rotational emission lines in infrared (IR) galaxies based on the Fourier Transform Spectrometer (FTS) data of {it Herschel} SPIRE. Among the 176 galaxies with publicly available FTS data, 45 have at least one H$_2$O emission line detected. The H$_2$O line luminosities range from $sim 1 times 10^5$ $L_{odot}$ to $sim 5 times 10^7 L_{odot}$ while the total IR luminosities ($L_mathrm{IR}$) have a similar spread ($sim 1-300 times 10^{10} L_{odot}$). In addition, emission lines of H$_2$O$^+$ and H$_2^{18}$O are also detected. H$_2$O is found, for most galaxies, to be the strongest molecular emitter after CO in FTS spectra. The luminosity of the five most important H$_2$O lines is near-linearly correlated with $L_mathrm{IR}$, no matter whether strong active galactic nucleus signature is present or not. However, the luminosity of H$_2$O($2_{11}-2_{02}$) and H$_2$O($2_{20}-2_{11}$) appears to increase slightly faster than linear with $L_mathrm{IR}$. Although the slope turns out to be slightly steeper when $zsim 2-4$ ULIRGs are included, the correlation is still closely linear. We find that $L_mathrm{H_2O}/L_mathrm{IR}$ decreases with increasing $f_{25}/f_{60}$, but see no dependence on $f_{60}/f_{100}$, possibly indicating that very warm dust contributes little to the excitation of the submillimeter H$_2$O lines. The average spectral line energy distribution (SLED) of the entire sample is consistent with individual SLEDs and the IR pumping plus collisional excitation model, showing that the strongest lines are H$_2$O($2_{02}-1_{11}$) and H$_2$O($3_{21}-3_{12}$).
We present a multi-wavelength analysis of star-forming galaxies in the massive cluster MS0451.6-0305 at z $sim$ 0.54 to shed new light on the evolution of the far-infrared-radio relationship in distant rich clusters. We have derived total infrared luminosities for a spectroscopically confirmed sample of cluster and field galaxies through an empirical relation based on $Spitzer$ MIPS 24 $mu$m photometry. The radio flux densities were measured from deep Very Large Array 1.4 GHz radio continuum observations. We find the ratio of far-infrared to radio luminosity for galaxies in an intermediate redshift cluster to be $q_{rm FIR}$ = 1.80$pm$0.15 with a dispersion of 0.53. Due to the large intrinsic dispersion, we do not find any observable change in this value with either redshift or environment. However, a higher percentage of galaxies in this cluster show an excess in their radio fluxes when compared to low redshift clusters ($27^{+23}_{-13}%$ to $11%$), suggestive of a cluster enhancement of radio-excess sources at this earlier epoch. In addition, the far-infrared-radio relationship for blue galaxies, where $q_{rm FIR}$ = 2.01$pm$0.14 with a dispersion of 0.35, is consistent with the predicted value from the field relationship, although these results are based on a sample from a single cluster.
(abridged) We present the first study of the farIR properties of high redshift, radio-selected ULIRGs using deep observations obtained with SPIRE from the Herschel Multi-tiered Extragalactic Survey (HerMES). These galaxies span a large range of 850um fluxes from submillimetre-luminous ~10mJy SCUBA galaxies -- SMGs to ~1.5mJy from stacked SCUBA non-detections, thus likely representing a complete distribution of ULIRG spectral energy distributions. From Keck spectroscopic surveys in the Lockman-North field we identified a sample of 31 SMGs and 37 submillimetre-faint, optically-faint radio galaxies (OFRGs), all with radio-inferred IR luminosities >10^12 Lsun. These galaxies were cross-identified with SPIRE 250, 350 and 500um catalogs based on fluxes extracted at 24um positions in the SWIRE survey, yielding a sample of more than half of the galaxies well detected in at least two of the SPIRE bandpasses. By fitting greybody dust models to the SPIRE photometry together with SCUBA 850um measurements, we infer dust temperatures and far-infrared luminosities. The OFRGs detected by SPIRE have median <T_d>= 41+-5 K and the SMGs have <T_d>= 34+-5 K, both in reasonable agreement with previous (pre-Herschel) estimates, reaffirming that the local FIR/radio correlation holds (at least for this subset of high-z ULIRGs) at high redshift. Our observations firstly confirm that a substantial fraction of OFRGs exhibit large infrared luminosities corresponding to SFRs of ~400 Msun/yr. The SPIRE observations secondly confirm the higher dust temperatures for these OFRGs than similarly selected SMGs, consistent with early predictions of the submm-faint radio populations. Our observations also clearly confirm the large infrared luminosities of most SMGs selected with S850um>5 mJy and radio and strong 24um detections, corresponding to SFRs of ~700 Msun/yr.
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