Low-redshift Ultraluminous Infrared Galaxies Research Articles

Modelling high-resolution ALMA observations of strongly lensed dusty star-forming galaxies detected by Herschel

ABSTRACT We present modelling of ∼0.1 arcsec resolution Atacama Large Millimetre/submillimetre Array imaging of seven strong gravitationally lensed galaxies detected by the Herschel Space Observatory. Four of these systems are galaxy–galaxy strong lenses, with the remaining three being group-scale lenses. Through careful modelling of visibilities, we infer the mass profiles of the lensing galaxies and by determining the magnification factors, we investigate the intrinsic properties and morphologies of the lensed submillimetre sources. We find that these submillimetre sources all have ratios of star formation rate to dust mass that are consistent with, or in excess of, the mean ratio for high-redshift submillimetre galaxies and low redshift ultra-luminous infrared galaxies. Reconstructions of the background sources reveal that the majority of our sample display disturbed morphologies. The majority of our lens models have mass density slopes close to isothermal, but some systems show significant differences.

Modelling high-resolution ALMA observations of strongly lensed highly star-forming galaxies detected by Herschel★

We have modelled high resolution ALMA imaging of six strong gravitationally lensed galaxies detected by the Herschel Space Observatory. Our modelling recovers mass properties of the lensing galaxies and, by determining magnification factors, intrinsic properties of the lensed sub-millimetre sources. We find that the lensed galaxies all have high ratios of star formation rate to dust mass, consistent with or higher than the mean ratio for high redshift sub-millimetre galaxies and low redshift ultra-luminous infra-red galaxies. Source reconstruction reveals that most galaxies exhibit disturbed morphologies. Both the cleaned image plane data and the directly observed interferometric visibilities have been modelled, enabling comparison of both approaches. In the majority of cases, the recovered lens models are consistent between methods, all six having mass density profiles that are close to isothermal. However, one system with poor signal to noise shows mildly significant differences.

COLOR-MAGNITUDE RELATION AND MORPHOLOGY OF LOW-REDSHIFT ULIRGs IN SLOAN DIGITAL SKY SURVEY

We present color–magnitude and morphological analysis of 54 low-redshift ultraluminous infrared galaxies (ULIRGs; 0.018 < z < 0.265 with zmedian = 0.151), a subset of the Infrared Astronomical Satellite 1 Jy sample, in the Sloan Digital Sky Survey (SDSS). The ULIRGs are both bright and blue: they are on average 1 mag brighter in than the SDSS galaxies within the same redshift range, and 0.2 mag bluer in 0.1g − 0.1r. They form a group in the color–magnitude diagram distinct from both the red sequence and the blue cloud formed by the SDSS galaxies: 24 out of the 52 unsaturated objects (∼46%) lie outside the 90% level number density contour of the SDSS galaxies. The majority (47, or ∼87%) have the colors typical of the blue cloud, and only four (∼7%) sources are located in the red sequence. While ULIRGs are popularly thought to be precursors to a QSO phase, we find few (three, or ∼6%) in the "green valley" where the majority of the X-ray- and IR-selected active galactic nuclei (AGNs) are found. Moreover, none of the AGN-host ULIRGs are found in the green valley. For the 14 previously spectroscopic identified AGNs (∼28%), we perform point-spread function subtractions and find that on average the central point sources contribute less than one-third to the total luminosity, and that their high optical luminosities and overall blue colors are apparently the result of star formation activity of the host galaxies. Visual inspection of the SDSS images reveals a wide range of morphologies including many close pairs, tidal tails, and otherwise disturbed profiles, in strong support of previous studies and the general view of ULIRGs as major mergers of gas-rich disk galaxies. A detailed morphology analysis using Gini and M20 coefficients shows that slightly less than one-half (∼42% in g band) of the ULIRGs are located in the merger region defined by morphology studies of local galaxies, while the remaining sources are located in the region of late-type and irregular galaxies. The heterogeneous distribution of ULIRGs in the G–M20 space is qualitatively consistent with the results found by numerical simulations of disk–disk mergers, and our study also shows that the measured morphological parameters are systematically affected by the signal-to-noise ratio and thus the merging galaxies can appear in various regions of the G–M20 parameter space. We briefly discuss the origins of the uncertainties and note that the morphology measurements should be implemented with caution for low physical resolution images. In general, our results reinforce the view that ULIRGs contain young stellar populations and are mergers in progress, but we do not observe the concentration of ULIRGs/AGN in the green valley as found by other studies. Our study provides a uniform comparison sample for studying dusty starbursts at higher redshifts such as Spitzer MIPS 24 μm-selected ULIRGs at z = 1–2 or submillimeter galaxies.

Millimetre/submillimetre-wave emission-line searches for high-redshift galaxies

The redshifted spectral line radiation emitted from both atomic fine-structure and molecular rotational transitions in the interstellar medium (ISM) of high-redshift galaxies can be detected in the centimetre, millimetre and submillimetre wavebands. Here we predict the counts of galaxies detectable in an array of molecular and atomic lines. This calculation requires a reasonable knowledge of both the surface density of these galaxies on the sky, and the physical conditions in their ISM. The surface density is constrained using the results of submillimetre-wave continuum surveys. Follow-up OVRO Millimeter Array observations of two of the galaxies detected in the dust continuum have provided direct measurements of CO rotational line emission at redshifts of 2.56 and 2.81. Based on these direct high-redshift observations and on models of the ISM that are constrained by observations of low-redshift ultraluminous infrared galaxies, we predict the surface density of line-emitting galaxies as a function of line flux density and observing frequency. We incorporate the sensitivities and mapping speeds of existing and future millimetre/submillimetre-wave telescopes and spectrographs, and so assess the prospects for blank-field surveys to detect this line emission from gas-rich high-redshift galaxies.

The differential magnification of high-redshift ultraluminous infrared galaxies

ABSTRA C T A class of extremely luminous high-redshift galaxies has recently been detected in unbiased submillimetre-wave surveys using the Submillimetre Common-User Bolometer Array (SCUBA) camera at the James Clerk Maxwell Telescope. Most of the luminosity of these galaxies is emitted from warm interstellar dust grains, and they could be the high-redshift counterparts of the low-redshift ultraluminous infrared galaxies (ULIRGs). Only one ‐ SMM J02399ˇ0136 ‐ has yet been studied in detail. Three other very luminous high-redshift dusty galaxies with well-determined spectral energy distributions in the mid-infrared waveband are known ‐ IRAS F10214+4724, H1413+117 and APM 08279+5255. These were detected serendipitously rather than in unbiased surveys, and are all gravitationally lensed by a foreground galaxy. Two ‐ H1413+117 and APM 08279+5255 ‐ appear to emit a significantly greater fraction of their luminosity in the mid-infrared waveband, compared with both lowredshift ULIRGs and high-redshift submillimetre-selected galaxies. This can be explained by a systematically greater lensing magnification of hotter regions of the source compared with cooler regions: differential magnification. This effect can confuse the interpretation of the properties of distant ultraluminous galaxies that are lensed by intervening galaxies, but offers a possible way to investigate the temperature distribution of dust in their nuclei on scales of tens of pc.

The differential magnification of high-redshift ultraluminous infrared galaxies

A class of extremely luminous high-redshift galaxies has recently been detected in unbiased submillimetre-wave surveys using the Submillimetre Common-User Bolometer Array (SCUBA) camera at the James Clerk Maxwell Telescope. Most of the luminosity of these galaxies is emitted from warm interstellar dust grains, and they could be the high-redshift counterparts of the low-redshift ultraluminous infrared galaxies (ULIRGs). Only one - SMM J02399-0136 - has yet been studied in detail. Three other very luminous high-redshift dusty galaxies with well determined spectral energy distributions in the mid-infrared waveband are known - IRAS F10214+4724, H1413+117 and APM 08279+5255. These were detected serendipitously rather than in unbiased surveys, and are all gravitationally lensed by a foreground galaxy. Two - H 1413+117 and APM 08279+5255 - appear to emit a significantly greater fraction of their luminosity in the mid-infrared waveband as compared with both low-redshift ULIRGs and high-redshift submillimetre-selected galaxies. This can be explained by a systematically greater lensing magnification of hotter regions of the source as compared with cooler regions: differential magnification. This effect can confuse the interpretation of the properties of distant ultraluminous galaxies that are lensed by intervening galaxies, but offers a possible way to investigate the temperature distribution of dust in their nuclei on scales of tens of parsecs.