arXiv subjects

A. W. Blain

Publications and source records attributed to A. W. Blain.

120 records · Page 7Linked to original sources

arXiv1999

The history of star formation in dusty galaxies

A population of distant dusty galaxies emitting in the submillimetre waveband has recently been detected using the Submillimetre Common-User Bolometer Array (SCUBA) camera on the James Clerk Maxwell Telescope (JCMT). This population can be used to trace the amount of high-redshift star-formation activity that is obscured from view in the optical waveband by dust, and so is missing from existing inventories of star formation in the distant Universe. By including this population we can construct a complete and consistent picture of the history of star formation. The evolution of obscured star formation at redshifts less than unity is constrained by mid- and far-infrared counts of dusty galaxies. Activity increases with redshift z as (1+z) to a power of about 4, consistent with the form of evolution found in the optical waveband by the Canada-France Redshift Survey (CFRS) to redshifts less than about 1. The form of evolution at higher redshifts is constrained by both our faint SCUBA counts and the intensity of background radiation in the millimetre/submillimetre waveband. We find that the total amount of energy emitted by dusty galaxies is about five times greater than that inferred from restframe ultraviolet observations, and that a larger fraction of this energy is emitted at high redshifts. The simplest explanation for these results is that a large population of luminous strongly-obscured sources at redshifts less than about 5 is missing from optical surveys. We discuss the possible contribution of obscured active galactic nuclei to the submillimetre-wave background and counts. More accurate constraints on the history of star formation will be provided by determinations of the counts in several submillimetre wavebands and crucially, by a reliable redshift distribution of the detected galaxies.

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arXiv1999

Redshift Distribution of the Faint Submillimeter Galaxy Population

We present a Keck II LRIS spectroscopic follow-up study of the possible optical counterparts to a flux-limited sample of galaxies selected from an 850-micron survey of massive lensing clusters using the SCUBA bolometer array on the JCMT. These sources represent a population of luminous dusty galaxies responsible for the bulk of the 850-micron background detected by COBE and thus for a substantial fraction of the total far-infrared emission in the Universe. We present reliable redshifts for 20 galaxies and redshift limits for a further 4 galaxies selected from the error-boxes of 14 submm sources. Two other submm detections in the sample have no obvious optical counterparts, and the final submm source was only identified from imaging data after the completion of our spectroscopic observations. The optical identifications for 4 of the submm sources have been confirmed through either their detection in CO at mm-wavelengths (two pairs of galaxies at z=2.55 and z=2.80) or from the characteristics of their spectral energy distributions (two of the central cD galaxies in the lensing clusters). Plausible arguments based on the optical spectral properties (starburst or AGN signatures) of the counterparts allow us to identify a further two likely counterparts at z=1.06 and 1.16. For the remaining 8 cases, it is not always clear which, if any, of the optical sources identified are the true counterparts. Possible counterparts for these have redshifts ranging from z=0.18 to z=2.11. Working with the current identifications, we suggest that the majority of the extragalactic background light in the submm is emitted by sources at z<3 and hence that the peak activity in highly-obscured sources (both AGN and starbursts) lies at relatively modest redshifts. (Abridged)

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arXiv1999

The Detection of Dust in the Central Galaxies of Distant Cooling-Flow Clusters

We present 850-um observations with the SCUBA submillimetre (sub-mm) camera of the central galaxies in seven concentrated clusters of galaxies at redshifts between 0.19 and 0.41. We detect sub-mm emission from the central galaxies in the rich clusters A1835 and A2390, and present upper limits for the central galaxies in the remaining five clusters. The two galaxies which we detect both exhibit unusually blue UV-optical colours and lie in clusters which contain massive cooling flows, >1000 Mo/yr. Moreover, both galaxies host relatively strong radio sources. Focusing on these two systems, we present new and archival radio-optical observations to provide a detailed view of their spectral energy distributions. Our analysis indicates that sub-mm emission from the central galaxy of A1835 can be best understood as arising from dust, heated either by vigorous star formation or an obscured active galactic nucleus. For the central galaxy of A2390, the sub-mm flux is marginally consistent with an extrapolation of the centimetre-millimetre emission from the luminous radio source that lies in its core; although we cannot rule out an excess flux density from dust emission comparable to that seen in A1835. We present details of our multi-wavelength observations and discuss the implications of these data for the interpretation of star formation in cooling-flow galaxies.

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arXiv1999

Molecular Gas in the z=2.565 Submillimeter Galaxy SMM J14011+0252

We report the detection of CO(3-2) emission from the submillimeter-selected luminous galaxy SMM J14011+0252. The optical counterpart of the submillimeter source has been identified as a merger system with spectral characteristics consistent with a starburst at z=2.565. The CO emission confirms the optical identification of the submillimeter source and implies a molecular gas mass of 5x10^{10}h_{75}^{-2} M(sun), after correcting for a lensing amplification factor of 2.75. The large molecular gas mass and the radio emission are consistent with the starburst interpretation of the source. These results are similar to those found for SMM J02399-0136, which was the first submillimeter selected CO source found at high redshift. The CO detections of these two high-redshift submillimeter galaxies suggest the presence of massive reservoirs of molecular gas which is consistent with the inferred high rates of star-formation (1000 M(sun)/yr). These two systems appear to be associated with merger events which may evolve into present day luminous elliptical galaxies.

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arXiv1998

Ultraluminous infrared galaxies at high redshift: their position on the Madau plot and their fate

A major recent development in extragalactic astronomy has been the discovery of a population of galaxies that are luminous at submillimetre wavelengths. Estimates of their spectral energy distributions suggest that these galaxies are the high-redshift analogues of the ultraluminous infrared galaxies (ULIGs) observed locally. We identify plausible (but non-unique) redshift-dependent galaxy luminosity functions that are consistent with both source counts at 2800, 850, 450 and and 175 microns, and far-infrared background radiation intensities at 850, 240 and 140 microns. In all our models, most of the submillimetre-luminous sources are distant galaxies with high bolometric luminosities >= 10^12 L_sun. As for many local ULIGs it is not possible to determine whether these luminous galaxies are powered by starbursts, like the local galaxy Arp 220, or by active galactic nuclei (AGN), like the local galaxy Markarian 231. If the submillimetre-luminous galaxies are all starbursts, then we predict that the fraction of the cosmic star-formation rate in these objects is large, but but does not necessarily dominate the star-formation rate of the Universe at any redshift. Only a few per cent by mass of the present-epoch spheroidal stellar population would have been formed in such a population of star-forming galaxies, consistent with constraints on the number of galaxies with old stellar populations in the field at low and intermediate redshift derived from K-band surves. If the submillimetre-luminous galaxies are all powered by AGN, then the comoving density of supermassive black holes onto which material is accreting at high redshift probably equals no more than a few per cent of the local density of massive dark objects.

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arXiv1998

Molecular Gas in the z=2.8 Submillimeter Galaxy SMM 02399-0136

We report the detection of CO(3-2) emission from the submillimeter-selected hyperluminous galaxy SMM 02399-0136. This galaxy is the brightest source detected in the recent SCUBA surveys of the submillimeter sky. The optical counterpart of the submillimeter source has been identified as a narrow-line AGN/starburst galaxy at z=2.8. The CO emission is unresolved, theta < 5 arcsec, and is coincident in redshift and position with the optical counterpart. The molecular gas mass derived from the CO observations is 8x10^{10} h_{75}^{-2} M_{sun}, after correcting for a lensing amplification factor of 2.5. The large CO luminosity suggests that a significant fraction of the infrared luminosity of SMM 02399-0136 arises from star formation. The high inferred star-formation rate of 10^{3} M_{sun}/yr and the large gaseous reservoir may suggest that we are seeing the formation phase of a massive galaxy. Future CO observations of other galaxies detected in deep submillimeter surveys will test the generality of these conclusions for the bulk of the faint submillimeter population.

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arXiv1998

Confusion noise in millimetre/submillimetre surveys

The flux density from undetected sources in the observing beam of a telescope produces source confusion noise in the resulting maps of the sky, and thus limits sensitivity. In a recent paper we discussed this effect in the millimetre/submillimetre waveband, using a simple model of galaxy evolution that could account for the gravitationally lensed images of distant dusty galaxies newly discovered in the fields of clusters of galaxies. New models explain all the available mm, submm and far-infrared data. The associated predictions of source confusion noise are presented here. The new and old estimates agree to within a factor of two on angular scales between about 0.3 and 50 arcmin, and are most similar on angular scales of about 1 arcmin. The new estimates are greater/less than the old ones on larger/smaller angular scales.

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arXiv1998

Strong gravitational lensing by spiral galaxies

We investigate gravitational lensing by a realistic model of disk galaxies. Most of the mass is contained in a large spherical isothermal dark matter halo, but the potential is modified significantly in the core by a gravitationally-dominant exponential disk. The method used is adapted from a very general multi-lens ray-tracing technique developed by Moeller. We investigate the effects of the disk-to-halo mass ratio, the disk scale length, the disk inclination to the line of sight and the lens redshift on two strong-lensing cross sections: the cross section for multiple imaging, and the cross section for large magnifications, in excess of a factor of 10. We find that the multiple-imaging cross section can be enhanced significantly by an almost edge-on Milky Way disk as compared with a singular isothermal sphere (SIS) in individual cases; however, when averaged over all disk inclinations, the cross section is only increased by about fifty per cent. These results are consistent with other recent work. The presence of a disk, however, increases the inclination-averaged high-magnification cross section by an order of magnitude as compared with a SIS. This result has important implications for magnification bias in future lens surveys, particularly those in the submillimetre waveband, where dust extinction in the lensing galaxy has no effect on the brightness of the images.

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arXiv1998

The Faint Sub-mm Galaxy Population: HST Morphologies and Colors

We present optical morphologies obtained from deep HST and ground-based images for galaxies selected from the first sub-millimeter survey of the distant Universe. Our sample comprises galaxies detected in deep 850-micron continuum maps of seven massive clusters, obtained using SCUBA, the new bolometer camera on the JCMT. The survey covers a total area of 0.01 square degrees to 1-sigma noise levels of about 2 mJy/beam. We detect a total of 25 sources at 850 microns, of which 17 and 10 are brighter than the respective 50% and 80% completeness limits. Optical counterparts are identified for 14 of the 16 sources in the f(50%) sample and for 9 of the 10 sources in the f(80%) sample that lie within our optical fields. The morphologies of those galaxies for which we have HST imaging fall into three broad categories: faint disturbed galaxies and interactions; faint galaxies too compact to classify reliably; and dusty, star-forming galaxies at intermediate redshifts. The disturbed and interacting galaxies constitute the largest class, which suggests that interactions remain an important mechanism for triggering star formation and the formation of ultraluminous galaxies in the distant Universe. The faint, compact galaxies may represent a later evolutionary stage in these mergers, or more centrally-concentrated starbursts. It is likely that some of these will host AGN. Analysis of the colors of our sample allow us to estimate a crude redshift distribution: >75% have z<5.5 whilst >50% lie at z<4.5, suggesting that the luminous sub-mm population is coeval with the more modestly star-forming galaxies selected by UV/optical surveys of the distant Universe. This imposes important constraints on models of galaxy formation and evolution.

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arXiv1998

A hyperluminous galaxy at z = 2.8 found in a deep submillimetre survey

We present a detailed study of SMM 02399-0136, a hyperluminous, active galaxy selected from a sub-mm survey of the distant Universe. This galaxy is the brightest source in the fields of seven rich, lensing clusters - total area 0.01 deg^2 - that we have mapped with a sensitivity of ~2 mJy/bm at 850um. We identify a compact optical counterpart with B ~ 23 and a LSB companion 3" away. Our spectroscopy shows that components have the same redshift; z = 2.803 +/- 0.003. The emission lines widths, FWHM ~ 1000-1500 km/s, and line ratios, along with the compact morphology and high luminosity (M_B ~ -24.0) of the galaxy indicate that SMM 02399-0136 contains a rare dust-embedded, narrow-line or type-2 AGN. The source is lensed by the foreground cluster, amplifying its apparent luminosity by a factor of 2.5. Taking this into account, we estimate that SMM 02399-0136 is five times more luminous than F10214+4724. Its far-IR and H-alpha luminosities and LSB radio emission are indicative of an extremely high SFR - several thousand Mo/yr. This assumes that a starburst is the dominant source of energy, but we cannot yet determine the relative contributions of the starburst and the buried AGN. A dust mass of 5-7 x 10^8 Mo is indicated by our data for T(dust) ~ 40-50K, independent of whether the dominant energy source is an AGN or a starburst. We estimate the possible space density of such luminous sub-mm sources and find that while a very large population of these obscured sources could be detected in future wide-field sub-mm surveys, they are unlikely to dominate the faint counts in this waveband. Galaxies such as SMM 02399-0136 and F10214+4724 cannot be easily detected in conventional AGN/QSO surveys, and so estimates of the prevalence of AGN in the early Universe may require significant revision.

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arXiv1998

Lensed submillimetre-wave foregrounds and the CMBR

Samples of high-redshift galaxies are easy to select in the millimetre/submillimetre (mm/submm) waveband using sensitive telescopes, because their flux density-redshift relations are expected to be flat, and so the selection function is almost redshift-independent at redshifts greater than 0.5. Source counts are expected to be very steep in the mm/submm waveband, and so the magnification bias due to gravitational lensing is expected to be very large, both for lensing by field galaxies and for lensing by clusters. Recent submm-wave observations of lensed images in clusters have constrained the submm-wave counts directly for the first time. In the next ten years our knowledge of galaxy evolution in this waveband will be greatly enhanced by the commissioning of sensitive new instruments and telescopes, including the CMBR imaging space mission Planck Surveyor. This paper highlights the important features of gravitational lensing in the submm waveband and discusses the excellent prospects for lens searches using these forthcoming facilities.

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arXiv1997

A Deep Sub-millimeter Survey of Lensing Clusters: A New Window on Galaxy Formation and Evolution

We present the first results of a sub-millimeter survey of distant clusters using the new Sub-mm Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. We have mapped fields in two massive, concentrated clusters, A370 at z=0.37 and Cl2244-02 at z=0.33, at wavelengths of 450 and 850-um. The resulting continuum maps cover a total area of about 10 sq. arcmin to 1 sigma noise levels less than 14 and 2 mJy per beam at the two wavelengths, 2-3 orders of magnitude deeper than was previously possible. We have concentrated on lensing clusters to exploit the amplification of all background sources by the cluster, improving the sensitivity by a factor of 1.3--2 as compared with a blank-field survey. A cumulative source surface density of (2.4+/-1.0) x 10^3 per sq. degree is found to a 50% completeness limit of ~4 mJy at 850-um. The sub-mm spectral properties of these sources indicate that the majority lie at high redshift, z>1. Without correcting for lens amplification, our observations limit the blank-field counts at this depth. The surface density is 3 orders of magnitude greater than the expectation of a non-evolving ber density of strongly star-forming galaxies in the high-redshift Universe and suggest that optical surveys may have substantial underestimated the star formation density in the distant Universe. Deeper sub-mm surveys with SCUBA should detect large numbers of star-forming galaxies at high redshift, and so provide strong constraints on the formation of normal galaxies.

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