Showing papers by "Mike Irwin published in 1995"

Sagittarius: the nearest dwarf galaxy

Abstract: We have discovered a new Galactic satellite galaxy in the constellation of Sagittarius. The Sagittarius dwarf galaxy is the nearest galaxy known, subtends an angle of $> 10$ degrees on the sky, lies at a distance of $24 \kpc$ from the Sun, $\sim 16 \kpc$ from the centre of the Milky Way. Itis comparable in size and luminosity to the largest dwarf spheroidal, has a well populated red horizontal branch with a blue HB extension; a substantial carbon star population; and a strong intermediate age stellar component with evidence of a metallicity spread. Isodensity maps show it to be markedly elongated along a direction pointing towards the Galactic centre and suggest that it has been tidally distorted. The close proximity to the Galactic centre, the morphological appearance and the radial velocity of 140 km/s indicate that this system must have undergone at most very few close orbital encounters with the Milky Way. It is currently undergoing strong tidal disruption prior to being integrated into the Galaxy. Probably all of the four globular clusters, M54, Arp 2, Ter 7 and Ter 8, are associated with the Sagittarius dwarf galaxy, and will probably share the fate of their progenitor.

Globular clusters with tidal tails: deep two-color star counts

Abstract: We examine the outer structure of 12 Galactic globular clusters using star-count analyses. Deep, two-color, photographic photometry is used to select and count stars with colors and magnitudes consistent with cluster-specific, color-magnitude sequences. The resulting reduction in the number of contaminating foreground stars allows us to push the star counts to significantly lower surface densities than has previously been possible. We find that most of our sample clusters show extra-tidal wings in their surface density profiles. The form of the surface density profiles is consistent with recent numerical studies of the tidal stripping of globular clusters. Two-dimensional surface density maps for several clusters are consistent with the expected appearance of tidal tails, with allowance for the effects of orbit shape, orbital phase, and orientation of our line of sight. We identify the extra-tidal material with stars which are still in the process of being removed from the clusters. The extra-tidal stars effectively limit the accuracy to which we can resolve the ``tidal'' radii of globular clusters. However, by tracing the orbital paths of their parent clusters, these stars may also facilitate a renewed attack on the problem of determining globular cluster orbits and the shape of the Galactic potential field.

Globular Clusters with Tidal Tails: Deep Two-Color Star Counts

Abstract: We examine the outer structure of 12 Galactic globular clusters using star-count analyses. Deep, two-color, photographic photometry is used to select and count stars with colors and magnitudes consistent with cluster-specific, color-magnitude sequences. The resulting reduction in the number of contaminating foreground stars allows us to push the star counts to significantly lower surface densities than has previously been possible. We find that most of our sample clusters show extra-tidal wings in their surface density profiles. The form of the surface density profiles is consistent with recent numerical studies of the tidal stripping of globular clusters. Two-dimensional surface density maps for several clusters are consistent with the expected appearance of tidal tails, with allowance for the effects of orbit shape, orbital phase, and orientation of our line of sight. We identify the extra-tidal material with stars which are still in the process of being removed from the clusters. The extra-tidal stars effectively limit the accuracy to which we can resolve the ``tidal'' radii of globular clusters. However, by tracing the orbital paths of their parent clusters, these stars may also facilitate a renewed attack on the problem of determining globular cluster orbits and the shape of the Galactic potential field.

A Supernova at z = 0.458 and implications for measuring the cosmological deceleration

Abstract: We have begun a program to discover high-redshift supernovae ($z \approx$ 0.25--0.5), and study them with follow-up photometry and spectroscopy. We report here our first discovery, a supernova at $z = 0.458$. The photometry for this supernova closely matches the lightcurve calculated for this redshift from the template of well-observed nearby Type Ia supernovae. We discuss the measurement of the deceleration parameter $q_0$ using such high-redshift supernovae, and give the best fit value assuming this one supernova is a normal, unextincted Type Ia. We describe the main sources of error in such a measurement of $q_0$, and ways to reduce these errors.

The statistics of microlensing light curves – I. Amplification probability distributions

Abstract: The passage of stars through the beam of a lensed quasar can induce violent fluctuations in its apparent brightness. The fluctuations observed in the Huchra lens, (2237+0305), are taken to be the first evidence of this ``microlensing'' occurring in lensing systems. Subsequent microlensing events observed in this system and in other gravitational lenses illustrate that microlensing should be a common phenomenon in lensed quasars. The statistical properties of the component light curves will be defined to a large extent by the mass distribution of the microlensing objects and the internal light distribution of the lensed quasar. We present statistics of a large sample of hypothetical microlensing light curves, for a number of lensing situations, generated using an efficient numerical technique. These artificial light curves show that the amplification probability distributions are independent of the mass function of the compact lensing objects. The amplification probability distributions for the Huchra lens system are presented.

GB 1508+5714 : a radio-loud quasar with Z = 4.30 and the space density of high-redshift, radio-loud quasars.

Abstract: We report the discovery of a radio loud quasar with a redshift of 4.30. This object, which is the first radio selected quasar with a redshift greater than four, was discovered during an observational investigation into the evolution of the luminosity function of radio loud quasars. Here we describe results based on a sample of ~300, flat spectrum radio sources with S(5GHz) > 200mJy. In this study, rather than carry out an indiscriminate redshift campaign on all the radio sources, we have used the APM POSS catalogue to preselect a subset of stellar-like optical counterparts with red optical colours. Such a subsample is expected to contain a high fraction of high redshift quasars. 10 of the ~300 sources were selected for follow-up optical spectroscopy and three of these are identified as quasars with redshifts greater than 3.0. One of the radio sources, GB1508+5714, is a quasar with z=4.30 and m(R)~19.

The PYXIS Cluster: A Newly Identified Galactic Globular Cluster

Abstract: One of the "interesting" objects identified by Weinberger during an eyeball search of sky survey plates for planetary nebulae is a previously unrecognized distant Galactic halo globular cluster. Deep B, R, and I CCD frames of the object to R = 23 reveal it to be a cluster of stars some 2' in core size, with a well-defined main sequence with a turnoff at R = 22 and a sparsely populated subgiant branch leading to a stubby red horizontal branch at R = 18.7. With a reddening-corrected distance modulus of 18.0, corresponding to a distance of 40 kpc, the Pyxis globular cluster is the latest addition to an exclusive club of distant Galactic satellites. Pyxis is similar in morphological appearance to other outer halo globular clusters and lies tantalizingly close to the plane of the Magellanic Clouds orbit (Jones and coworkers), suggesting it might be a detached cluster of this system.

High Redshift Lyman Limit and Damped Lyman-Alpha Absorbers

Abstract: We have obtained high signal:to:noise optical spectroscopy at 5A resolution of 27 quasars from the APM z>4 quasar survey. The spectra have been analyzed to create new samples of high redshift Lyman-limit and damped Lyman-a absorbers. These data have been combined with published data sets in a study of the redshift evolution and the column density distribution function for absorbers with logN(HI)≥ 17.5, over the redshift range 0.01 < z < 5. The main results are: (i) Lyman limit systems: The data are well fit by a power law N(z) = N 0(1+ z)γ for the number density per unit redshift. For the first time intrinsic evolution is detected in the product of the absorption cross-section and comoving spatial number density for an Ω = 1 Universe. We find γ = 1.55 (γ = 0.5 for no evolution) and N 0 = 0.27 with >99.7% confidence limits for γ of 0.82 & 2.37. (ii) Damped Lyα systems: The APM QSOs provide a substantial increase in the redshift path available for damped surveys for z > 3. Eleven candidate and three confirmed damped Lyα absorption systems, have been identified in the APM QSO spectra covering the redshift range 2.8 ≤ z ≤ 4.4 (11 with z > 3.5). Combining the APM survey confirmed and candidate damped Lya absorbers with previous surveys, we find evidence for a turnover at z~3 or a flattening at z~2 in the cosmological mass density of neutral gas, Ω g. The Lyman limit survey results are published in Storrie-Lombardi, et al., 1994, ApJ, 427, L13. Here we describe the results for the DLA population of absorbers.

High Redshift Lyman Limit and Damped Lyman-Alpha Absorbers

Abstract: We have obtained high signal:to:noise optical spectroscopy at 5\AA\ resolution of 27 quasars from the APM z$>$4 quasar survey. The spectra have been analyzed to create new samples of high redshift Lyman-limit and damped Lyman-$\alpha$ absorbers. These data have been combined with published data sets in a study of the redshift evolution and the column density distribution function for absorbers with $\log$N(HI)$\ge17.5$, over the redshift range 0.01 $ $99.7\% confidence limits for $\gamma$ of 0.82 \& 2.37. \item Damped \lya systems: The APM QSOs provide a substantial increase in the redshift path available for damped surveys for $z>3$. Eleven candidate and three confirmed damped Ly$\alpha$ absorption systems, have been identified in the APM QSO spectra covering the redshift range $2.8\le z \le 4.4$ (11 with $z>3.5$). Combining the APM survey confirmed and candidate damped \lya absorbers with previous surveys, we find evidence for a turnover at z$\sim$3 or a flattening at z$\sim$2 in the cosmological mass density of neutral gas, $\Omega_g$. \end{itemize} The Lyman limit survey results are published in Storrie-Lombardi, et~al., 1994, ApJ, 427, L13. Here we describe the results for the DLA population of absorbers.

The PYXIS cluster: a newly identified galactic globular cluster

Abstract: One of the "interesting" objects identified by Weinberger during an eyeball search of sky survey plates for planetary nebulae is a previously unrecognized distant Galactic halo globular cluster. Deep B, R, and I CCD frames of the object to R = 23 reveal it to be a cluster of stars some 2' in core size, with a well-defined main sequence with a turnoff at R = 22 and a sparsely populated subgiant branch leading to a stubby red horizontal branch at R = 18.7. With a reddening-corrected distance modulus of 18.0, corresponding to a distance of 40 kpc, the Pyxis globular cluster is the latest addition to an exclusive club of distant Galactic satellites. Pyxis is similar in morphological appearance to other outer halo globular clusters and lies tantalizingly close to the plane of the Magellanic Clouds orbit (Jones and coworkers), suggesting it might be a detached cluster of this system.

The distant supernova search and implications for the cosmological deceleration

Abstract: We have begun a program to discover high-redshift supernovae ( z ≈ 0.2–0.6), and study them with follow-up photometry and spectroscopy. We report here our first discovery, a supernova at z = 0.458 (SN1992bi) and the ongoing analysis on several new supernova candidates found in a recent search (Dec 93 – March 94). The photometry for SN1992bi closely matches the lightcurve calculated for its redshift from the template of well-observed nearby Type la supernovae giving a best fit value for the deceleration parameter: q o = 0.1 ± 0.3 (±0.55) (with Λ = 0), where the first uncertainty is due to the photometry of the distant supernova and the second reflects the estimated intrinsic width of the Type la absolute magnitude distribution based on nearby supernovae. If there is significant host galaxy extinction then q o would be larger than this best fit value. We describe the main sources of measurement error inherent in such a measurement of qo , and ways to reduce these errors.