arXiv subjects

Mark Sullivan

Publications and source records attributed to Mark Sullivan.

95 records · Page 6Linked to original sources

arXiv2004

Discovery of a Transient U-band Dropout in a Lyman-Break Survey: A Tidally-Disrupted Star at z = 3.3?

We report the discovery of a transient source in the central regions of galaxy cluster Abell 267. The object, which we call "PALS1", was found in a survey aimed at identifying highly-magnified Lyman-break galaxies in the fields of intervening rich clusters. At discovery, the source had U>24.7 (2-sigma; AB), g=21.96, and very blue g-r and r-i colors; i.e., PALS1 was a U-band drop-out, characteristic of star-forming galaxies and quasars at z~3. However, three months later the source had faded by more than three magnitudes. Further observations showed a continued decline in luminosity, to R>26.4 seven months after discovery. Though the apparent brightness is suggestive of a supernova at roughly the cluster redshift, we show that the photometry and light curve argue against any known type of supernova at any redshift. The spectral energy distribution and location near the center of a galaxy cluster are consistent with the hypothesis that PALS1 is a gravitationally-lensed transient at z~3.3. If this interpretation is correct, the source is magnified by a factor of 4-7 and two counterimages are predicted. Our lens model predicts time delays between the three images of 1-10 years and that we have witnessed the final occurrence of the transient. The intense luminosity (M(AB) ~ -23.5 after correcting for lensing) and blue UV continuum (implying T>50,000 K) argue the source may have been a flare resulting from the tidal disruption of a star by a 10^6-10^8 solar-mass black hole. Regardless of its physical nature, PALS1 highlights the importance of monitoring regions of high magnification in galaxy clusters for distant, time-varying phenomena.

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arXiv2003

An ultraviolet-selected galaxy redshift survey - III: Multicolour imaging and non-uniform star formation histories

(abridged) We present panoramic u' and optical ground-based imaging observations of a complete sample of low-redshift (0<z<0.4) galaxies selected in the ultraviolet (UV) at 2000A using the balloon-borne FOCA instrument of Milliard et al. This survey is highly sensitive to newly-formed massive stars, and hence to actively star-forming galaxies. We use the new data to further investigate the stellar population and star formation properties of this sample. When combined with our earlier spectroscopic surveys, these new data allow us to compare star-formation measures derived from aperture-corrected Ha line fluxes, UV(2000A) and u'(3600A) continuum fluxes on a galaxy-by-galaxy basis. As expected from our earlier studies, we find broad correlations between the dust-corrected star-formation diagnostics, though the scatter is larger than that from observational errors, with significant offsets from trends expected according to simple star-formation history (SFH) models. Varying metallicities and/or initial mass functions seem unable to explain the observed discrepancies. We investigate the star-formation properties further by modelling the observed spectroscopic and photometric properties of the galaxies in our survey. Nearly half of the galaxies surveyed possess features that appear incompatible with simple constant or smoothly declining SFHs, favouring instead irregular or temporally-varying SFHs. We demonstrate how this can reconcile the majority of our observations, and determine empirical corrections to calculate intrinsic star formation rates from measures based on UV (or u') observations alone. We discuss the broader implications of our finding that a significant fraction of star-forming galaxies have complex SFHs, particularly in the context of recent determinations of the cosmic SFH.

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arXiv2001

A Comparison of Independent Star Formation Diagnostics for a UV-Selected Sample of Nearby Galaxies

We present results from a decimetric radio survey undertaken with the Very Large Array as part of a longer term goal to inter-compare star formation and dust extinction diagnostics, on a galaxy by galaxy basis, for a representative sample of nearby galaxies. For our survey field, Selected Area 57, star formation rates derived from 1.4GHz luminosities are compared with earlier nebular emission line and ultraviolet (UV) continuum diagnostics. We find broad correlations, over several decades in luminosity, between H-alpha, the UV continuum and 1.4GHz diagnostics. However, the scatter in these relations is found to be larger than observational errors, with offsets between the observed relations and those expected assuming constant star-formation histories and luminosity-independent extinction models. We investigate the physical origin of the observed relations, and conclude the discrepancies between different star-formation diagnostics can only be partly explained by simple models of dust extinction in galaxies. These models cannot by themselves explain all the observed differences, introducing the need for temporally varying star-formation histories and/or more complex models of extinction, to explain the entire dataset.

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arXiv2000

Verifying the use of supernovae as probes of the cosmic expansion

We present preliminary results of a follow-up survey which aims to characterise in detail those galaxies which hosted Type Ia supernovae found by the Supernova Cosmology Project (SCP). Our survey has two components: Hubble Space Telescope imaging with STIS and Keck spectroscopy with ESI, the goal being to classify each host galaxy into one of three broad morphological/spectral classes and hence to investigate the dependence of supernovae properties on host galaxy type over a large range in redshift. Of particular interest is the supernova Hubble diagram characterised by host galaxy class which suggests that most of the scatter arises from those occurring in late-type irregulars. Supernovae hosted by (presumed dust-free) E/S0 galaxies closely follow the adopted SCP cosmological model. Although larger datasets are required, we cannot yet find any significant difference in the light curves of distant supernovae hosted in different galaxy types.

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arXiv2000

A strategy for finding gravitationally-lensed distant supernovae

Distant Type Ia and II supernovae (SNe) can serve as valuable probes of the history of the cosmic expansion and star formation, and provide important information on their progenitor models. At present, however, there are few observational constraints on the abundance of SNe at high redshifts. A major science driver for the Next Generation Space Telescope (NGST) is the study of such very distant supernovae. In this paper we discuss strategies for finding and counting distant SNe by using repeat imaging of super-critical intermediate redshift clusters whose mass distributions are well-constrained via modelling of strongly-lensed features. For a variety of different models for the star formation history and supernova progenitors, we estimate the likelihood of detecting lensed SNe as a function of their redshift. In the case of a survey conducted with HST, we predict a high probability of seeing a supernova in a single return visit with either WFPC-2 or ACS, and a much higher probability of detecting examples with z>1 in the lensed case. Most events would represent magnified SNe II at z~1, and a fraction will be more distant examples. We discuss various ways to classify such events using ground-based infrared photometry. We demonstrate an application of the method using the HST archival data and discuss the case of a possible event found in the rich cluster AC 114 (z=0.31).

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