Showing papers by "Mike Irwin published in 1991"

Initial light curve of Q2237 + 0305

Abstract: This paper presents CCD photometry for the gravitationally lensed quasar system 2237 + 0305, in optical passbands from B through R, taken over a time period of more than 3 yr. These data provide new information about the probable microlensing event reported by Irwin et al. (1989); the rise time of this feature is approximately 26 days. Four additional independent brightness changes in the quasar images are detected.

Interpreting the Light Curve of Q2237+0305

Abstract: Theoretical modeling of the mass distribution in the lens for Q2237+0305 will not give an accurate estimate of the microlensing parameters for each of the images. It will be possible to statistically estimate microlensing parameters from the data, however the timescales over which data must be collected, may be uncomfortably long. The required timescale depends on both the mean compact objects mass and the apparent transverse velocity of the source, and is >130 (m/M ⊙ ) 0.5 (5000 km s −1 /v t ) years. The observation of the quasar continuum source crossing a caustic or cusp line gives a good estimate of the source size

A wide-field multicolor survey for high-redshift quasars, z ≥ 2.2. I, Photometric catalog and survey selection function

Abstract: The present multicolor, wide-field, faint-magnitude survey of high-redshift quasars encompasses the 16-20 magnitude range and covers a 58.6 sq deg area, for an effective area of 45.7 sq deg after quantification of sources of incompleteness. Candidates' selection proceeds according to such novel features as the treatment of multicolor data as low-resolution spectra and the adoption of a quantitative identification algorithm employing all available information. The way in which model quasar spectra are used to calculate the selection function for the survey is detailed. Computed sample completeness is presented as a function of redshift and magnitude for a range of representative quasar types. 69 refs.

Deep CCD photometry of the Sextans dwarf spheroidal galaxy

Abstract: BV CCD photometry of Sextans obtained with the prime-focus camera of the 4-m telescope at CTIO in March-April 1990 is reported. The data are presented in extensive tables and graphs and analyzed in detail. Features discussed include a main-sequence turnoff at V = about 23.3, a mainly red horizontal branch at V = 20.35 + or - 0.2, a well populated instability strip (implying up to 300 RR Lyrs and Cepheids in the galaxy), Fe/H = -1.5 + or - 0.2, (m-M)0 = 19.7 + or - 0.3 for E(B-V) = 0.02 + or - 0.02, no significant population of stars younger than 12 Gyr, central V surface brightness 17.2 + or - 0.5 mag/sq arcmin, integrated apparent V magnitude 10.3 + or - 0.7 mag, and overabundance 0.4-0.7 dex (compared to other Galactic dwarf spheroidals). 66 refs.

The radial velocity and metal abundance of the Sextans dwarf spheroidal galaxy

Abstract: The AAT FOCAP fibre system has been used to obtain spectra, centered at the Ca II IR-triplet, of the 14 stars in the field of the recently discovered Sextans dwarf spheroidal galaxy. Radial velocities derived from these spectra indicate that six of the stars observed are Sextan members. Their velocities cluster closely about a value of 230 ± 6 km/s which is taken as the heliocentric velocity of this galaxy. This velocity indicates that Sextans makes a contribution comparable to those of other dSph galaxies, such as Sculptor and Ursa Minor, to calculations of the mass of the galaxy's halo.

On measuring the deuterium abundance in QSO absorption systems

Abstract: The results of numerical simulations aimed at evaluating techniques for potential measurements of the deuterium abundance in extragalactic absorption clouds are described. The calculations provide a guide to the precision with which we may hope to measure D/H using existing observational facilities. The measurement error can be small enough to provide valuable cosmological constraints, even if the deuterium absorption cannot be seen as a separate feature in the wing of the hydrogen lines and instead is only seen as an extension in the blue wing of one or more of the hydrogen lines

Young stars between the Magellanic Clouds. II. An association on the far-west side of the LMC halo

Abstract: This paper presents CCD B,V photometry of a group of blue stars located in the southwest LMC halo, 6.4 deg from its optical center. This association, barely 100 million years old, is the last of a string of young stellar aggregates extending from the wing of the SMC to the LMC halo. Located at distance of (V-Mv)0 = 18.7, it must lie in the far side of the LMC halo. Its CMD contrasts strongly with the one of the intermediate-age population of the LMC halo, seen in adjacent frames, which appears to be older than about 0.8 Gyrs. 24 refs.

String of young clusters linking the Magellanic Clouds

Abstract: We present deep CCD photometry of three associations lying between the Magellanic Clouds and sitting on the HI ridge. ZAMS fittings show that these clusters are located at the distance of the Clouds. Furthermore, a slight distance gradient is observed. Main sequences matched to isochrones reveal that these aggregates are very young, no more than 108 years old. An age difference from east to west is also suggested. The photographic survey by Irwin et al. (1990) of the Magellanic Bridge has revealed that the Wing of the SMC extends eastward, towards the LMC halo. This is underlined by the presence of several aggregates of blue stars distributed along the Bridge, sitting on the HI ridge. We present CCD photometry of three aggregates located between the classical eastern tip of the SMC Wing and die LMC halo. Their coordinates are given in Table 1, as well as their deduced distances from appropriate ZAMS fitting, their reddening and the number of main sequence stars with V < 20 found within each CCD field. The analysis of the spatial distribution of the main sequence stars reveals that we are dealing with associations of stars, rather than clusters. Associations 5 and 6 do not show obvious central concentration and are rather loose. Although Association 2 is more concentrated, Grondin et al (1990 GDKI) showed that with a diameter of ~ 85 pc, it is larger than typical SMC clusters by an order of magnitude. We used the ZAMS of the Revised Yale Isochrones (RYI) (Green et al. 1987) to determine the distance of these associations. GDKI argued that the reddening for Association 2 is negligible. A value of E(ß V) = 0.03 is also adopted for Association 5. Demers et al. (1990) determine for Association 6 that E(B V) = 0.10. The abundance of these young stars is unknown. Following GDKI, we adopt [Fe/H] = -0.65 and select the ZAMS accordingly. The distances found are shown in Table 1. Our photometry reveals that the stellar bridge between the Clouds shows a distance gradient. Such a result is not unexpected — the bridge starts at the SMC Wing tip and ends in the LMC halo which is closer to us. An age variation, even a gradient, among the observed associations is possible. Our preliminary results cannot be compared to adequate theoretical models. It is not

The radial velocity and metal abundance of the Sextans dwarf spheroidal galaxy.

Abstract: The multi-object spectroscopic facility FOCAP at the Anglo-Australian Telescope has been used to obtain spectra centred at the Ca II IR triplet of 14 stars in the field of the Sextans dwarf spheroidal (dSph) galaxy. This satellite of our own Galaxy was recently discovered by Irwin et al. (1990) from APM measures of UK Schmidt Telescope photographic plates.