arXiv subjects

T. J. Davidge

Publications and source records attributed to T. J. Davidge.

82 records · Page 5Linked to original sources

arXiv1999

The Central Regions of M81

High angular resolution optical and near-infrared images are used to investigate the central regions of the nearby Sb galaxy M81 (NGC3031). The spectral-energy distribution of the circumnuclear region, which extends out to 1.5 arcsec (~ 24 pc if μ_0 = 27.5) from the nucleus, can be modelled as a combination of an old metal-rich population and emission from hot dust. Thermal emission has been detected near other AGN, and simple models indicate that hot dust can account for ~20% of the light in K within 0.5 arcsec of the M81 nucleus. An elongated structure with M_V ~ -7, which may be an area of active star formation, is detected 0.45 arcsec from the nucleus. At distances in excess of 1.5 arcsec from the nucleus the J-K color of the M81 bulge is not significantly different from what is seen in M31. The HST data are also used to search for bright globular clusters within 2kpc of the center of M81. The area within 0.26 kpc of the M81 nucleus is largely devoid of bright globular clusters, in agreement with what is seen in the central regions of the Galaxy and M31. However, our survey indicates that there may be ~45 +/- 12 globular cluster candidates with M_V \leq -7 within 2 kpc of the galaxy center, which is consistent with what would be infered from the Milky-Way cluster system.

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arXiv1998

High Angular Resolution JHK Imaging of the Centers of the Metal-Poor Globular Clusters NGC5272 (M3), NGC6205 (M13), NGC6287, and NGC6341 (M92)

The Canada-France-Hawaii Telescope (CFHT) Adaptive Optics Bonnette (AOB) has been used to obtain high angular resolution JHK images of the centers of the metal-poor globular clusters NGC5272 (M3), NGC6205 (M13), NGC6287, and NGC6341 (M92). The color-magnitude diagrams (CMDs) derived from these data include the upper main sequence and most of the red giant branch (RGB), and the cluster sequences agree with published photometric measurements of bright stars in these clusters. The photometric accuracy is limited by PSF variations, which introduce systematic errors of a few hundredths of a magnitude near the AO reference star. The clusters are paired according to metallicity, and the near-infrared CMDs and luminosity functions are used to investigate the relative ages within each pair. The near-infrared CMDs provide the tightest constraints on the relative ages of the classical second parameter pair NGC5272 and NGC6205, and indicate that these clusters have ages that differ by no more than +/- 1 Gyr. These results thus support the notion that age is not the second parameter. We tentatively conclude that NGC6287 and NGC6341 have ages that differ by no more than +/- 2 Gyr. However, the near-infrared spectral energy distributions of stars in NGC6287 appear to differ from those of stars in outer halo clusters, bringing into question the validity of this age estimate.

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arXiv1998

The Near-Infrared Photometric Properties of Bright Giants in the Central Regions of the Galactic Bulge

Images recorded through broad (J, H, K), and narrow (CO, and 2.2micron continuum) band filters are used to investigate the photometric properties of bright (K < 13.5) stars in a 6 x 6 arcmin field centered on the SgrA complex. The giant branch ridgelines in the (K, J-K) and (K, H-K) color-magnitude diagrams are well matched by the Baade's Window (BW) M giant sequence if the mean extinction is A_K ~ 2.8 mag. Extinction measurements for individual stars are estimated using the M_K versus infrared color relations defined by M giants in BW, and the majority of stars have A_K between 2.0 and 3.5 mag. The extinction is locally high in the SgrA complex, where A_K ~ 3.1 mag. Reddening-corrected CO indices, CO_o, are derived for over 1300 stars with J, H, and K brightnesses, and over 5300 stars with H and K brightnesses. The distribution of CO_o values for stars with K_o between 11.25 and 7.25 can be reproduced using the M_K versus CO_o relation defined by M giants in BW. The data thus suggest that the most metal-rich giants in the central regions of the bulge and in BW have similar photometric properties and 2.3micron CO strengths. Hence, it appears that the central region of the bulge does not contain a population of stars that are significantly more metal-rich than what is seen in BW.

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arXiv1998

The Evolved Red Stellar Contents of the Sculptor Group Galaxies NGC55, NGC300, and NGC7793

Deep J, H, and K images are used to probe the evolved stellar contents in the central regions of the Sculptor group galaxies NGC55, NGC300, and NGC7793. The brightest stars are massive red supergiants (RSGs) with K ~ 15 - 15.5. The peak RSG brightness is constant to within ~0.5 mag in K, suggesting that NGC55, NGC300, and NGC7793 are at comparable distances. Comparisons with bright RSGs in the Magellanic Clouds indicate that the difference in distance modulus with respect to the LMC is = 7.5. A rich population of asymptotic giant branch (AGB) stars, which isochrones indicate have ages between 0.1 and 10 Gyr, dominates the (K, J-K) color-magnitude diagram (CMD) of each galaxy. The detection of significant numbers of AGB stars with ages near 10 Gyr indicates that the disks of these galaxies contain an underlying old population. The CMDs and luminosity functions reveal significant galaxy-to-galaxy variations in stellar content. Star-forming activity in the central arcmin of NGC300 has been suppressed for the past Gyr with respect to disk fields at larger radii. Nevertheless, comparisons between fields within each galaxy indicate that star-forming activity during intermediate epochs was coherent on spatial scales of a kpc or more. A large cluster of stars, which isochrones suggest has an age near 100 Myr, is seen in one of the NGC55 fields. The luminosity function of the brightest stars in this cluster is flat, as expected if a linear luminosity-core mass relation is present.

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arXiv1997

The Stellar Content Near the Galactic Center

High angular resolution J, H, K, and L' images are used to investigate the stellar content within 6 arcsec of SgrA*. The data, which are complete to K ~ 16, are the deepest multicolor observations of the region published to date.

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arXiv1997

Near Infrared Imaging of the Proposed z=2 Cluster Behind CL 0939+4713 (Abell 851)

We have obtained deep J and K' images of a 90 arcsec square field containing the z=2.0 QSO that is viewed through the z=0.4 rich cluster A851. Brightnesses of individual galaxies are measured from these data, and archival F702W WFPC2 images from HST. The results are used to construct 2-colour diagrams, colour-magnitude diagrams, and luminosity functions of galaxies in A851 and the faint objects discovered by Dressler et al (1993) near the QSO. The QSO faint companions appear to form a separate population from other faint galaxies in the field. Comparisons with the GISSEL models covering redshifts from 0 to 3 indicate that (1) the A851 galaxies have ages between 3 and 10 Gyr, and (2) if they are at the QSO redshift, the QSO companion galaxies are within 1Gyr of cessation of star-formation.

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arXiv1995

NEAR-INFRARED PHOTOMETRY OF THE CORE-COLLAPSED METAL-POOR GLOBULAR CLUSTERS NGC5946 AND NGC7099

Moderately deep near-infrared images are used to investigate the photometric properties and spatial distribution of bright giants near the centers of the core-collapsed globular clusters NGC5946 and NGC7099. The former cluster is located at a low Galactic latitude and is heavily reddened; nevertheless, the K, J-K color-magnitude diagram (CMD) shows a well-defined giant branch, with a width +/- 0.05 magnitudes in J-K. Most of the observed width is due to differential reddening. The upper giant branch is relatively steep, and comparisons with less reddened clusters indicate that (1) NGC5946 must be very metal-poor, with [Fe/H] ~ -2, and (2) E(B-V) ~ 0.6. The distance to NGC5946 derived using the near-infrared brightness of the giant branch tip is in excellent agreement with that computed from the V brightness of the HB if E(B-V) = 0.6. The NGC7099 giant branch terminates ~ 1 mag fainter in K than the expected tip brightness. The integrated J-K color of NGC7099 changes with radius, such that Delta (J-K) /Delta log(r) = 0.05 - 0.09, depending on the technique used to measure colors. Similar radial variations in stellar content might be expected in NGC5946; however, the integrated J-K color of this cluster remains constant with radius and, after removing the brightest resolved stars, a color gradient is seen that is in the opposite sense to that in NGC7099. The relative density of bright giant branch stars also remains constant within 30 arcsec of the center of NGC5946. Therefore, it is concluded that NGC5946 does not contain stellar content gradients similar to those seen in NGC7099 and other post core-collapsed clusters.

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arXiv1995

STELLAR POPULATION GRADIENTS IN BRIGHT CLUSTER GALAXIES AT z=0.2

Slit spectra, covering the rest frame near-ultraviolet and blue wavelength regions, are combined with moderately deep g and R images to investigate radial population gradients in the brightest components of six z=0.2 galaxy clusters selected by x-ray brightness. We conclude that the brightest cluster galaxies (BCGs) of the EMSS0440+02, EMSS0906+11 and EMSS1231+15 clusters have global stellar contents resembling nearby elliptical galaxies. The BCGs in the clusters EMSS0839+29, EMSS1455+22 and Abell2390 are complicated by ionized gas, which is detected over large portions of all three systems. Radial color variations are detected in the majority of galaxies and, for those systems not showing extended [OII] emission, the weakening of D4000 (which measures the 4000 Angstrom break in magnitudes) with radius falls in the upper range of what is seen among nearby ellipticals. However, there is no correlation between local D4000 values and (g-r), suggesting that these quantities are sensitive to different parameters. We estimate the metallicity sensitivity of D4000 using a combination of observational and theoretical data, and find metallicity gradients for BCGs which are relatively steep, and in qualitative agreement with predictions for galaxies which experience a single monolithic dissipative collapse. Line gradients measured with respect to g surface brightness show excellent galaxy-to-galaxy consistency, and are in good agreement with what has been measured in Coma cluster galaxies. The presence of significant stellar population gradients in the BCGs suggests that the material from which they formed experienced dissipation during the collapse phases, and it seems likely that these galaxies could not have formed solely from the merging of gas-poor galaxies.

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arXiv1995

DEEP IR IMAGING OF GLOBULAR CLUSTERS. III. M13

We have used the CFHT REDEYE infrared camera to obtain deep J and K' images of four fields in the globular cluster M13. The composite K, J-K color-magnitude diagram (CMD) extends from the upper red giant branch (RGB) to 2 magnitudes below the main sequence turn-off (MSTO). Selected [Fe/H]~ -1.6 isochrones from Bergbusch & VandenBerg (1992, ApJS, 81, 163) and Straniero & Chieffi (1991, ApJS, 76, 525) are transformed onto the near-infrared observational plane, and these suggest an age for M13 in the range 14-16 Gyr. We emphasize that any effort to estimate ages should be considered as tentative given uncertainties in the input physics; however, these uncertainties notwithstanding, comparisons between the near-infrared isochrones indicate that age differences of order +/-2 Gyr should be detectable among metal-poor clusters on the K, J-K plane. Building on this result, we find that the difference in J-K color between the MSTO and the base of the RGB in M13 is the same as in M4, and conclude that these clusters have similar ages. This conclusion is verified by comparing (1) the K brightnesses of the MSTO, and (2) the morphologies of optical CMDs. Finally, we investigate the mass function of main sequence stars in M13 with distances between 3 and 6 core radii from the cluster center. The mass functions in the interval 0.55 and 0.8 solar masses are relatively flat, independent of radius. Optical studies at larger radii have found non-zero mass function indices in this same mass interval, and we attribute this radial variation in mass function morphology to dynamical evolution.

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arXiv1994

V and I Photometry of Bright Giants in the Central Regions of NGC 147

Deep V and I CCD images with sub-arcsec spatial resolution are used to investigate the stellar content of the central regions of the Local Group dwarf elliptical galaxy NGC147. Red giant branch (RGB) stars are resolved over the entire field, and the RGB-tip occurs at I ~ 20.5, suggesting that the distance modulus is 24.3. A comparison with globular cluster sequences indicates that the center of NGC147 is moderately metal-poor, with [Fe/H] ~ -1. This is not significantly different from what was found in the outer regions of the galaxy by Mould, Kristian & Da Costa (1983, ApJ, 270, 471). Moreover, the width of the V-I color distribution at I = 21.0 indicates that a spread in metallicity is present, with sigma[Fe/H] ~ +/- 0.3. There is no evidence of a component more metal-poor than [Fe/H] ~ -1.3. A small population of moderately bright asymptotic giant branch (AGB) stars has also been detected, and the AGB-tip occurs near M{bol} ~ -5.0, indicating that an intermediate-age population is present. It is estimated that the intermediate-age population contributes ~ 2-3% of the V light from NGC147.

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