arXiv subjects

D. L. Clements

Publications and source records attributed to D. L. Clements.

330 records · Page 19Linked to original sources

arXiv1998

Towards Completing a Large Area ULIRG Survey

We present the latest results in our efforts to produce a complete, large sample of Ultraluminous Infrared Galaxies (ULIRGs, L60 > 10^11.4 Lsun for H0=100kms^-1 Mpc^-1, q0=0.5) for use in the study of these objects and their evolution. We have been using an optical/IR colour technique to select those objects most likely to be ULIRGs. 65 of the 198 candidate ULIRGs that do not yet have redshifts were observed. Redshifts were obtained for 51 of these objects, including 8 new ULIRGs and a further three probable ULIRGs with tentative redshifts. The new ULIRGs include one object at z=0.44 (00029-1424), the highest in the survey so far, and at least one new broad-line ULIRG (03156-1706) which appears to lie in a galaxy cluster. We discuss the properties of these objects and their local environments, concluding that three of them have high excitation spectra, and that five show disturbed morphology even in the shallow acquisition images. We also present the redshift measurements of the non-ULIRGs identified in the course of this survey.

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arXiv1998

Limits on the star formation rates of z>2 damped Ly-alpha systems from H-alpha spectroscopy

We present the results of a long-slit K-band spectroscopic search with CGS4 on UKIRT for H-alpha emission from the objects responsible for high-redshift (z > 2) damped Ly-alpha absorption systems. The objective was to measure the star-formation rates in these systems. However, no H-alpha emission was detected above our 3-sigma limits of f < 10E-19 W/m**2, corresponding to star formation rates < 10 M_sun/yr/h**2 (q_0=0.5). These upper limits are more meaningful than those from searches for Ly-alpha emission because the H-alpha line is unaffected by resonant scattering. For q_0=0.5 our limits are in conflict with the star formation rates predicted under the assumption that the high-z DLAs are the fully-formed galactic-disk counterparts of today's massive spiral galaxies. Deeper spectroscopy is needed to test this picture for q_0=0.0. A programme of NICMOS imaging observations currently underway, combined with VLT spectroscopy, will provide a detailed picture of the link between DLAs and young galaxies.

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arXiv1997

The Fate of Ultra-luminous Mergers

Essentially all Ultra-Luminous IR Galaxies (ULIRGs) are in disturbed, interacting or merging systems. It is known that interactions tend to induce galactic starbursts. Thus, elliptical galaxies which are formed in mergers will tend to have high metallicity, low dust and molecular gas content, and faint structural distortions, as observed for the bone fide elliptical galaxy population. The old stellar population in the merged galaxies will probe the new gravitational potential, relaxing rapidly to give a de Vaucouleurs surface-brightness profile if the remnant is elliptical-like. We examine the old stellar population in 10 nearby (z < 0.15) ULIRGs, using deep near-IR imaging photometry. These data reveal signs of elliptical-like structure in the near-IR one-dimensional surface brightness profiles, supporting the hypothesis that ultra-luminous mergers evolve into elliptical galaxies.

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arXiv1996

Candidate Primeval Galaxies in the Hubble Deep Field

We present the results of colour-selection of candidate high redshift (2.6 $<$ z $<$ 3.9) galaxies within the Hubble Deep Field based on the Ly-break at 912Å. We find 8 such objects in the region, giving a comoving number density comparable to that of nearby bright galaxies (for a flat q$_0$=0.5, H$_0$=100 kms$^{-1}$Mpc$^{-1}$ universe). We provide basic data on the properties of these objects, and show that despite their absolute magnitude being significantly brighter than L$^*$ (typically M$_B$ = -22), they are generally smaller than nearby galaxies. Furthermore, visual inspection of their images shows that they are all highly disturbed systems, with multiple nuclei, tails and plumes, suggesting that they are undergoing merging processes similar to most nearby starburst galaxies. Theoretical models suggest that galaxies form by accumulation of numerous subcomponents, and we suggest that we are seeing this process underway in these objects. It is thus possible that the epoch of galaxy formation might have been discovered.

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arXiv1995

Spectroscopic Evidence That the Extreme Properties of IRAS F10214+4724 are due to Gravitational Lensing

The extreme bolometric luminosity of IRAS F10214+4724, and in particular the huge mass in molecular gas inferred from the CO line fluxes have led to suggestions that this is a giant galaxy in the process of formation. An arc-like structure and the closeness of a second object suggest, however, that gravitational lensing may be responsible for its anomalously high luminosity and mass. In this paper, we use an optical spectrum taken in conditions of 0.7-arcsec seeing to provide further evidence that F10214+4724 is a gravitationally lensed system. We measure tentative redshifts of 0.896 and 0.899 for galaxies projected $\approx 1$ and $\approx 3$ arcsec from IRAS F10214+4724 respectively. Identifying the former as the lensing galaxy we obtain a mass:light ratio consistent with those derived for other lenses, and find that its luminosity is consistent with the velocity dispersion deduced from the radius of the Einstein ring. If lensed, our models suggest magnification by a factor $\sim 10$, and hence an intrinsic bolometric luminosity for F10214+4724 similar to local ULIRGs.

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arXiv1995

On Near-Infrared H-Alpha Searches for High-Redshift Galaxies

The lack of success of Lyman-alpha searches for high-redshift (z>2) field galaxies may be due to extinction by dust, suggesting that surveys based on lines of longer wavelength, particularly H-alpha, may be more effective. To test the dust hypothesis we have undertaken deep broad- (K') and narrow-band (5000 km/s, 2.177 micron) imaging of the field towards the quasar PHL957, in an attempt to detect H-alpha emission from a known galaxy of redshift z=2.313. We cover an area of 4.9 square arcmin (0.28 h^{-2}Mpc^2) to a 4-sigma limiting narrow-band flux f=2.7x10^{-16}erg/s/cm^2, a factor of several deeper than previously published surveys. We detect the H-alpha+[N II] emission line in this galaxy at the 3.3-sigma level, inferring a star formation rate of 18 h^{-2} solar masses per year. This is a factor only a few times larger than the rate seen in some Sc galaxies today. The faint flux level reached in this work demonstrates the promise of narrow-band imaging in the near-infrared as a technique for finding normal galaxies at high redshifts.

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