D. J. Butler

The Closest View of a Dwarf Galaxy: New Evidence on the Nature of the Canis Major Overdensity

We present the first deep color-magnitude diagram of the putative central region (05 × 05) of the Canis Major stellar overdensity (l, b) = (240, - 8) found recently by Martin and coworkers, which has been proposed as the remnant of a dwarf satellite accreted onto the Milky Way on a near-equatorial orbit. We find a narrow (in apparent magnitude) main sequence extending 6 mag below the turnoff to our limiting magnitude of B ~ 24.5 mag. This main sequence has very high contrast (>3) with respect to the thin/thick disk/halo background; its narrowness at brighter magnitudes clearly implies the presence of a distinct and possibly still bound stellar system. We derived the line-of-sight size (r1/2) of this system based on the B-band width of the lower main sequence, obtaining 0.94 ± 0.18 (random) ± 0.18 (systematic) kpc. That size matches a model prediction for the main body of the parent galaxy of the Monoceros tidal stream. The high-density contrast and limited spatial extent in the radial direction are very hard to reconcile with the alternative explanation put forward to explain the Canis Major stellar overdensity: a flared or warped Galactic disk viewed in projection, as found in the recent work of Momany and coworkers. We also derived a central surface brightness of μV,0 = 23.3 ± 0.1 mag arcsec-2 and an absolute magnitude of MV = -14.5 ± 0.1 mag. These values place the Canis Major object in the category of dwarf galaxy, considering the LV-size and MV - μV planes for such objects. However, like the Sagittarius dwarf, it is an outlier in the [Fe/H] - MV plane in the sense that it is too metal-rich for its estimated absolute magnitude. This suggests that the main mechanism driving its recent and current star formation history (possibly tidal stripping) is different from that of isolated dwarfs.

MAD the ESO multi-conjugate adaptive optics demonstrator

Multi-Conjugate Adaptive Optics (MCAO) is working on the principle to perform wide field of view atmospheric turbulence correction using many Guide Stars located in and/or surrounding the observed target. The vertical distribution of the atmospheric turbulence is reconstructed by observing several guide stars and the correction is applied by some deformable mirrors optically conjugated at different altitudes above the telescope. The European Southern Observatory together with external research institutions is going to build a Multi-Conjugate Adaptive Optics Demonstrator (MAD) to perform wide field of view adaptive optics correction. The aim of MAD is to demonstrate on the sky the feasibility of the MCAO technique and to evaluate all the critical aspects in building such kind of instrument in the framework of both the 2nd generation VLT instrumentation and the 100-m telescope OWL. In this paper we present the conceptual design of the MAD module that will be installed at one of the VLT unit telescope in Paranal to perform on-sky observations. MAD is based on a two deformable mirrors correction system and on two multi-reference wavefront sensors capable to observe simultaneously some pre-selected configurations of Natural Guide Stars. MAD is expected to correct up to 2 arcmin field of view in K band.

The Very Red Afterglow of GRB 000418: Further Evidence for Dust Extinction in a Gamma-Ray Burst Host Galaxy*

We report near-infrared and optical follow-up observations of the afterglow of the GRB 000418 start- ing 2.5 days after the occurrence of the burst and extending over nearly 7 weeks. GRB 000418 represents the second case for which the afterglow was initially identi—ed by observations in the near-infrared. During the —rst 10 days its R-band afterglow was well characterized by a single power-law decay with a slope of 0.86. However, at later times the temporal evolution of the afterglow —attens with respect to a simple power-law decay. Attributing this to an underlying host galaxy, we —nd its magnitude to be R \ 23.9 and an intrinsic afterglow decay slope of 1.22. The afterglow was very red with R(K B 4 mag. The observations can be explained by an adiabatic, spherical —reball solution and a heavy reddening due to dust extinction in the host galaxy. This supports the picture that (long) bursts are associated with events in star-forming regions. Subject heading: gamma rays: bursts

ON THE STELLAR POPULATIONS IN NGC 185 AND NGC 205 AND THE NUCLEAR STAR CLUSTER IN NGC 205 FROM HUBBLE SPACE TELESCOPE OBSERVATIONS

We present the first detailed analysis of resolved stellar populations in the dwarf galaxies NGC 185 and NGC 205 based on archival V- and I-band WFPC2 pointings. For NGC 185 we deduce from the brightest main-sequence and blue-loop stars that star formation was probably still active about 4 × 108 yr ago and have three key abundance-related results: (1) We identify ancient stars with [Fe/H] -1.5 dex by a well-defined horizontal branch (HB). (2) We find a prominent clump/bump-like feature along the red giant branch/faint asymptotic giant branch (RGB/faint AGB) with the same mean V-band magnitude as in the HB, within uncertainties [i.e., ΔV(bump - HB) = 0]; from a comparison with theory, the implication is that ancient stars have [Fe/H] ~ -1.5 dex, with a higher abundance level for intermediate-age stars. (3) From color information we infer that the median [Fe/H] > -1.11 ± 0.08 dex for ancient stars [assuming E(B - V) = 0.18 mag]. For NGC 205 we record a new distance modulus, (m - M)0 = 24.76 ± 0.1 mag, taking E(B - V) = 0.11 mag, based on the RGB tip magnitude method in the I band. We find that stars were probably still forming less than 3 × 108 yr ago in NGC 205, which is compatible with star formation triggered by an interaction with M31. There are three key abundance-related results for NGC 205: (1) The RGB/faint AGB is significantly skewed to redder values than those for a control field in the outskirts of M31; this probably results from a relatively narrow metallicity and/or age range for a significant fraction of the dwarfs stars. (2) From a comparison with models, the most metal-rich RGB stars reach [Fe/H] -0.7 dex (0.2 Z⊙). (3) For ancient stars we infer from color information that the median [Fe/H] > -1.06 ± 0.04 dex [for E(B - V) = 0.11 mag]. We briefly compare the stellar populations of NGC 205, NGC 185, and NGC 147. Finally, we study several V- and R-band structural properties of the nuclear star cluster in NGC 205 for the first time; the apparent V- and R-band effective radii indicate a blue excess in the clusters outer region. In terms of size, the cluster is like a typical Galactic globular cluster or a nuclear cluster in a nearby late-type spiral galaxy, but it is quite bright (106 L⊙,R), unlike an ancient globular cluster, and by matching with models, the blue color hints that its stellar population is young, up to a few times 108 yr old.

Modeling the stellar populations in the canis major overdensity : The relation between the old and young populations

We analyze the stellar populations of the Canis Major stellar overdensity using quantitative color-magnitude diagram (CMD) fitting techniques. The analysis is based on photometry obtained with the Wide Field Imager at the 2.2 m telescope at La Silla for several fields near the probable center of the overdensity. A modified version of the MATCH software package was applied to fit the observed CMDs, enabling us to constrain the properties of the old and young stellar populations that appear to be present. For the old population we find [Fe/H] ~ -1.0, a distance of ~7.5 kpc, a line-of-sight depth σlos of 1.5 ± 0.2 kpc, and a characteristic age range of 3-6 Gyr. However, the spread in ages and the possible presence of a ~10 Gyr old population cannot be constrained. The young main sequence is found to have an age spread; ages must range from a few hundred Myr to 2 Gyr. Because of the degeneracy between distance and metallicity in CMDs, the estimates of these parameters are strongly correlated, and two scenarios are consistent with the data: if the young stars have a metallicity similar to the old stars, they are equidistant and therefore cospatial with the old stars; if the young stars have close to solar metallicity they are more distant (~9 kpc). The relatively low metallicity of the old main sequence favors the interpretation that CMa is the remnant of an accreted dwarf galaxy. Spectroscopic metallicity measurements are needed to determine whether the young main sequence is cospatial.

Absolute Proper Motion of the Canis Major Dwarf Galaxy Candidate

We have measured the absolute proper motion of the candidate Canis Major dwarf galaxy (CMa), at (l, b) =(240°, -8°). Likely main-sequence stars in CMa have been selected from a region in the color-magnitude diagram that has very little contamination from known Milky Way components. We obtain μl cos b = -1.47 ± 0.37 and μb = -1.07 ± 0.38 mas yr-1, on the International Celestial Reference System by means of Hipparcos stars. Together with the radial velocity of 109 km s-1 and the assumed distance of 8 kpc, these results imply a space motion of (Π, Θ, W) = (-5 ± 12, 188 ± 10, -49 ± 15) km s-1. While CMa has in-plane rotation similar to the mean of thick-disk stars, it shows significant (3 σ) motion perpendicular to the disk and differs even more (7 σ) from that expected for the Galactic warp. The W-velocity lends support to the argument that the CMa overdensity is part of a satellite-galaxy remnant.

Atmosphere-like turbulence generation with surface-etched phase-screens

We built and characterized an optical system that emulates the optical characteristics of an 8m-class telescope like the Very Large Telescope. The system contains rotating glass phase-screens to generate realistic atmosphere-like optical turbulence, as needed for testing multiconjugate adaptive optics systems. In this paper we present an investigation of the statistical properties of two phase-screens etched on glass-plate surfaces, obtained from Silios Technologies. Those etched screens are highly transmissive (above 85%) from 0.45 to 2.5 microm. From direct imaging, their Fried parameter r0 values (0.43+/-0.04 microm and 0.81+/-0.03 microm, respectively, at 0.633 microm) agree with the expectation to within 10%. This is also confirmed by a comparison of measured and expected Zernike coefficient variances. Overall, we find that those screens are quite reproducible, allowing sub-millimetre r0 values, which were difficult to achieve in the past. We conclude that the telescope emulator and phase-screens form a powerful atmospheric turbulence generator allowing systematic testing of different kinds of AO instrumentation.

Broadband, static wave-front generation: Na-Ag ion-exchange phase screens and telescope emulation.

We test the statistical properties of static, atmospherelike wave fronts in glass that allow repeatable testing of astronomical adaptive optics instrumentation. The technology is mask-structured ion exchange (MSI) in glass and has significant advantages over other transmissive technologies. The screens are easy to clean, are insensitive to ambient temperature changes, and have high optical-to-near-infrared transmission. However, the effective coherence length (r0) on each of the fabricated screens is systematically too large or, equivalently, the fabricated aberrations are too weak. Despite this strong caveat, the screens appear to be quite useful: Long-exposure point-spread functions have realistic shapes, and power spectrum indices closely match those of the computer-generated wave fronts. Most significant, stacking screens with similar r0 values reduced r0 by the amount predicted by turbulence theory. The refractivity of MSI screens remains unmeasured. Finally, we present our design of an optical system that emulates the key characteristics of the Very Large Telescope, made to contain glass phase screens and to mimic an array of stars for multiconjugate adaptive optics system testing.

The ALFA Laser Guide Star: Operation and Results

The importance of laser guide stars to the practical usefulness ofadaptive optics cannot be understated, and yet there are veryfew working systems.This contribution discusses the current status of the ALFAlaser guide star, with regard to the particular difficulties encounteredwhile observing as well as both the expected performance and that so farachieved from scientific observations.A description is given of a number of experiments involving ALFA whichaim to determine the atmospheric turbulence and sodium layercharateristics, and demonstrate the possibility for tilt recovery fromlaser guide stars.

THE STELLAR CONTENT AND STAR FORMATION HISTORY OF THE LATE-TYPE SPIRAL GALAXY NGC 300 FROM HUBBLE SPACE TELESCOPE OBSERVATIONS

We present the first Hubble Space Telescope (HST) WFPC2 V and I photometry for the Sculptor Group galaxy NGC 300 in four fields ranging from the center to the outer edge. We have made the first measurement of the star formation histories in two disk fields: the oldest stars were born at similar epochs and formation activity increased but at different mean rates. The main disk stellar population is predominantly old, consisting of red giant branch (RGB) and asymptotic giant branch (AGB) stars, based on a synthetic color-magnitude diagram analysis. The metallicity Z is found to have been less than 0.006 (or 0.33 Z⊙), with no evidence for significant change in mean Z over time in both disk fields. In the central region, we find a dearth of bright stars with respect to the two disk fields that cannot be explained by observational effects. Taken at face value, this finding would agree with the Davidge report of suppressed star formation there during the past 109 yr with respect to his disk fields at larger radii; but the possibility remains that significant central extinction affects our finding. We have also determined the first distance modulus estimate based on the tip of the red giant branch method. On the Cepheid distance scale of Ferrarese et al., we find (m-M)0 = 26.56 ± 0.07(±0.13) mag and a similar value from the Cepheid-independent empirical method of Lee, Freedman, & Madore, both in good agreement with the Cepheid distance determined by Freedman et al. A discrepancy between this value and the theoretical calibration of the red giant branch tip magnitude method remains. Finally, we report a newly detected young (up to about 10 Myr) stellar association of about average size (~140 pc) in one of the disk fields.