arXiv subjects

M. R. S. Hawkins

Publications and source records attributed to M. R. S. Hawkins.

29 records · Page 2Linked to original sources

arXiv2000

Quasar variability: correlations with amplitude

The relation between quasar variability and parameters such as luminosity and redshift has been a matter of hot debate over the last few years with many papers on the subject. Any correlations which can be established will have a profound effect on models of quasar structure and evolution. The sample of quasars in ESO/SERC field 287 contains over 600 quasars in the range 0 < z < 3.5 and is now large enough to bin in luminosity and redshift, and give definitive measures of the correlations. We find no significant correlation between amplitude and redshift, except perhaps at very low redshift, but an inverse correlation between amplitude and luminosity. This is examined in the context of various models for quasar variability.

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arXiv1999

A survey of UV-excess AGNs in the South Galactic Pole

Spectra, position, magnitudes and colors are presented for 485 faint (B<20.5) emission line objects selected with the ultraviolet-excess (UVX) criterion on a area of 24.6 sq. deg in the South Galactic Pole. The objects were selected from the analysis of pixel-to-pixel stacking of COSMOS scans of UKST U, J and R plates. The candidates were observed with the Meudon-ESO Fiber Optics System (MEFOS) at the ESO 3.6m telescope. 429 type 1 AGNs have been identified (373 in the redshift range 0.3<z<2.2). This sample has allowed the measure of a difference on the QSO clustering evolution in comparison with that found for galaxies (La Franca et al 1998). The region is part of the ESO Imaging Survey (EIS) and of the 2dF QSO redshift survey.

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arXiv1999

A Survey for Cool White Dwarfs and the age of the Galactic Disc

We describe a new multi-colour proper motion survey for cool white dwarfs (CWDs). The observational database consists of ~300 digitally scanned Schmidt plates in ESO/SERC field 287. The entire survey procedure, from the raw Schmidt plate data to final white dwarf luminosity function (WDLF) is described, with special emphasis on completeness concerns. We obtain a sample of 58 WDs, for which we have follow up CCD photometry and spectroscopy of a representative sub--sample. Effective temperatures and luminosities of our sample objects are determined by comparing photometry with the model atmosphere predictions of Bergeron, Saumon and Wesemael. Space densities are calculated using an adaptation of Schmidts (1/Vmax) method, from which a WDLF is constructed. Comparison of our observational LF with the models of both Wood and Garcia-Berro et al. indicate an age for the local Galactic Disc of 10{-1}_{+3}Gyr. Importantly, we find no evidence of incompleteness in our survey sample. Proper motion number counts imply the survey is complete, and the WD sample passes the (V/V_{max}) completeness test.

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arXiv1998

Discovery of distant high luminosity infrared galaxies

We have developed a method for selecting the most luminous galaxies detected by IRAS based on their extreme values of R, the ratio of 60 micron and B-band luminosity. These objects have optical counterparts that are close to or below the limits of Schmidt surveys. We have tested our method on a 1079 deg^2 region of sky, where we have selected a sample of IRAS sources with 60 micron flux densities greater than 0.2 Jy, corresponding to a redshift limit z~1 for objects with far-IR luminosities of 10^{13} L_sun. Optical identifications for these were obtained from the UK Schmidt Telescope plates, using the likelihood ratio method. Optical spectroscopy has been carried out to reliably identify and measure the redshifts of six objects with very faint optical counterparts, which are the only objects with R>100 in the sample. One object is a hyperluminous infrared galaxy (HyLIG) at z=0.834. Of the remaining, fainter objects, five are ultraluminous infrared galaxies (ULIGs) with a mean redshift of 0.45, higher than the highest known redshift of any non-hyperluminous ULIG prior to this study. High excitation lines reveal the presence of an active nucleus in the HyLIG, just as in the other known infrared-selected HyLIGs. In contrast, no high excitation lines are found in the non-hyperluminous ULIGs. We discuss the implications of our results for the number density of HyLIGs at z<1 and for the evolution of the infrared galaxy population out to this redshift, and show that substantial evolution is indicated. Our selection method is robust against the presence of gravitational lensing if the optical and infrared magnification factors are similar, and we suggest a way of using it to select candidate gravitationally lensed infrared galaxies.

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arXiv1998

The Signature of Dark Matter in Quasar Light Curves

The idea that dark matter in the form of small compact bodies (most plausibly planetary mass primordial black holes) betrays its presence by the microlensing of quasars, has been stimulated by statistical studies of quasar light curves and gravitationally lensed quasar systems. In this paper we review this evidence, and discuss the significance of the recent detection of a planetary mass microlensing event in the Galaxy by the MACHO group. We also look in more detail at the light curves themselves, and make the case that many show the characteristic morphology of caustic crossing events associated with a large optical depth of lenses. This supports the idea that a large fraction of the critical density is in the form of planetary mass compact bodies. The nature of such bodies is discussed, and it is argued that current work on the QCD phase transition suggests they are primordial black holes, although it is just possible that they are compact gas clouds.

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arXiv1998

Caustic Crossings in Quasar Light Curves?

Numerical simulations and theoretical studies of the gravitational microlensing effect of a population of small bodies distributed along the line of sight to a compact light source such as a quasar indicate that caustic crossing effects will be present given a sufficiently large optical depth to lensing. These events will produce characteristic patterns in the quasar light curve. In this paper we use a large sample of quasar light curves to search for features with the properties of caustic crossing events. Two good candidates are presented which it is argued are not easily explained as intrinsic variation of the uasars. The relation between these events and microlensing featurees seen in multiple quasar systems is discussed, as well as the implications for the dark matter problem of a population of microlensing bodies.

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arXiv1998

The Direct Detection of Non-Baryonic Dark Matter in the Galaxy?

It has been argued in a number of recent papers that dark matter is in the form of Jupiter mass primordial black holes which betray their presence by microlensing quasars. This lensing accounts for a number of characteristic properties of quasar light curves, in both single quasars and gravitationally lensed multiple systems, which are not explained on the basis of intrinsic variation. One prediction of this idea is that Jupiter mass bodies will be detected by the MACHO experiment as short events of about 2 days duration, although the expected frequency of detection is still very hard to estimate. However, the recent report by the MACHO group of the detection of a Jupiter mass body in the direction of the Galactic bulge is consistent with this prediction, and is possibly the first direct detection of non-baryonic matter in the Galaxy.

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arXiv1997

Dark Galaxies -- the Dominant Population?

This paper argues that there is a large population of dark galaxies which reveals its presence by the gravitational lensing of quasars, and outnumbers normal galaxies by around 3:1. There are 8 double quasars with a separation greater than 2 arcseconds which have been classified as probable or certain gravitational lenses. Lensing galaxies have only been found for 2 of these systems, and analysis of the remainder has led in each case to the conclusion that they are best explained by the gravitational lensing effect of a `dark galaxy'. Here we examine the ensemble properties of this sample and conclude that there is overwhelming evidence that the systems are indeed being gravitationally lensed by dark galaxies or perhaps dark matter galactic halos. The existence and nature of such objects raises some intriguing questions, as well as having profound implications for large scale structure. Several of the quasar systems show strong evidence for microlensing. It is argued that this implies a substantial component of dark matter in the form of compact bodies, either in the halo of the lensing galaxy or more generally along the line of sight.

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arXiv1997

The Double Quasar Q2138-431: Lensing by a Dark Galaxy?

We report the discovery of a new gravitational lens candidate Q2138-431AB, comprising two quasar images at a redshift of 1.641 separated by 4.5 arcsecs. The spectra of the two images are very similar, and the redshifts agree to better than 115 km.sec$^{-1}$. The two images have magnitudes $B_J = 19.8$ and $B_J = 21.0$, and in spite of a deep search and image subtraction procedure, no lensing galaxy has been found with $R < 23.8$. Modelling of the system configuration implies that the mass-to-light ratio of any lensing galaxy is likely to be around $1000 M_{\odot}/L_{\odot}$, with an absolute lower limit of $200 M_{\odot}/L_{\odot}$ for an Einstein-de Sitter universe. We conclude that the most likely explanation of the observations is gravitational lensing by a dark galaxy, although it is possible we are seeing a binary quasar.

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arXiv1997

Dark galaxies, spin bias and gravitational lenses

Gravitational lensing studies suggest that the Universe may contain a population of dark galaxies; we investigate this intriguing possibility and propose a mechanism to explain their nature. In this mechanism a dark galaxy is formed with a low density disk in a dark halo of high spin parameter; such galaxies can have surface densities below the critical Toomre value for instabilities to develop, and following Kennicutt's work we expect these galaxies to have low star formation rates. The only stellar component of the galaxies is a halo system, formed during the collapse of the proto-galactic cloud. We compute synthetic stellar population models and show that, at a redshift $z=0.5$, such galaxies have apparent magnitudes $B \simeq 28, R \simeq 26$ and $I \simeq 25$, and could be unveiled by deep searches with the Hubble Space Telescope. Dark galaxies have an initial short blue phase and then become essentially invisible, therefore they may account for the blue population of galaxies at high redshift. We find a strong mass-dependence in the fraction of dark galaxies, and predict that spiral galaxies will not be found in halos with masses less than about $10^9 M_\odot$, if $Ω=1$. Above about $10^{12}M_\odot$, all halos can produce luminous disks. The mass-dependence of the galaxy-formation efficiency introduces the possibility of `spin bias' -- luminous galaxies being associated preferentially with strongly-clustered high-mass halos. A further prediction is that the slope of the faint-end luminosity function for galaxies will be flatter than the associated halo mass function.

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arXiv1994

Gravitational Lensing of Variability-slected QSO by Galaxy Clusters

Distribution and properties of QSOs behind galaxy clusters in the UKJ287 field were studied. QSOs were selected using variability criteria and are confined to $z \ge 0.4$ and $m_B \le 19.5$. No reddening or obscuration of background QSOs due to dust in clusters is detected. A statistically significant positive angular correlation between clusters ($\langle z\rangle \simeq 0.15$) and QSOs ($0.4 \le z \le 2.2$) on scales of several arcminutes is found. This number overdensity of QSOs behind clusters is ascribed to weak gravitational lensing by foreground galaxy clusters. The overdensity of QSOs increases as their limiting magnitude gets brighter in a way that would be expected from a simple lensing model derived using Boyle, Shanks \& Peterson (1988) QSO number counts and an average amplification of $A=2$ due to clusters. The implied amplification is substantially larger than would be expected from isothermal sphere clusters with a velocity dispersion $σ_v \sim 1000$ km/s.

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