arXiv subjects

Carl J. Grillmair

Publications and source records attributed to Carl J. Grillmair.

63 records · Page 4Linked to original sources

arXiv1998

The Luminosity Function and Mass Function in the Galactic Bulge

We present deep photometry obtained with the Hubble Space Telescope (HST) in a field in Baade's Window in the Galactic bulge. We derive a luminosity function down to I ~ 24.3, or V ~ 27.5, corresponding to M ~ 0.3 Msun. The luminosity function from the turnoff down to this level appears remarkably similar to that observed in the solar neighborhood. We derive a mass function using both an empirical local mass-luminosity relation and a mass-luminosity relation from recent stellar model calculations, allowing for the presence of binaries and photometric errors. The mass function has a power law form with dN/dM proportional to M^{-2.2} for M >~ 0.7 Msun. However, we find strong evidence for a break in the mass function slope around 0.5-0.7 Msun, with a significantly shallower slope at lower masses. The value of the slope for the low masses depends on the assumed binary fraction and the accuracy of our completeness correction. This mass function should directly reflect the initial mass function.

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arXiv1997

Probing the Galactic Halo with Globular Cluster Tidal Tails

We discuss recent observational and numerical work on tidal tails in globular clusters. Evidence of tidal tails has now been found in 16 Galactic globulars and 4 globulars in M31. Simulations indicate that mapping of these tidal tails over most of a typical cluster's orbit may be quite feasible. A relatively modest effort in this regard would yield reliable space velocities for those clusters whose tidal tails could be traced to angular separations of a few degrees or more. Mining of forthcoming databases should allow us to trace the individual orbital paths of globular clusters over large portions of the sky. This would enable us to better constrain models of the collapse of the Galaxy and to build up an accurate picture of the distribution of matter in the Galactic halo.

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arXiv1997

Stellar Populations in Three Outer Fields of the LMC

We present HST photometry for three fields in the outer disk of the LMC extending approximately four magnitudes below the faintest main sequence turnoff. We cannot detect any strongly significant differences in the stellar populations of the three fields based on the morphologies of the color-magnitude diagrams, the luminosity functions, and the relative numbers of stars in different evolutionary stages. Our observations therefore suggest similar star formation histories in these regions, although some variations are certainly allowed. The fields are located in two regions of the LMC: one is in the north-east field and two are located in the north-west. Under the assumption of a common star formation history, we combine the three fields with ground-based data at the same location as one of the fields to improve statistics for the brightest stars. We compare this stellar population with those predicted from several simple star formation histories suggested in the literature, using a combination of the R-method of Bertelli et al (1992) and comparisons with the observed luminosity function. The only model which we consider that is not rejected by the observations is one in which the star formation rate is roughly constant for most of the LMC's history and then increases by a factor of three about 2 Gyr ago. Such a model has roughly equal numbers of stars older and younger than 4 Gyr, and thus is not dominated by young stars. This star formation history, combined with a closed box chemical evolution model, is consistent with observations that the metallicity of the LMC has doubled in the past 2 Gyr.

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arXiv1997

Keck Spectroscopy of Globular Clusters around NGC 1399

We report moderate resolution, high signal-to-noise spectroscopy of globular clusters around NGC 1399, the central cD galaxy in the Fornax cluster. We address issues as diverse as element abundances of globular clusters versus stellar populations in ellipticals, blue horizontal branches in metal-rich globular clusters, broad-band colors as metallicity tracers, possible overestimation of the age-metallicity degeneracy in globular clusters, and dark matter in the halo of NGC 1399. (...) The derived element abundances for the globular clusters span the entire range observed in the Milky Way and M31, with a mean metallicity of our sample around [Fe/H] ~ -0.8 dex. This implies that the two major sub-populations reported from photometry could have formed by the same processes as the ones that formed halo and disk/bulge globular clusters in the Local Group spirals. Two globular clusters (that we associate with a group of very red globular clusters, representing about 5% of the total system) clearly stand out and exhibit metal abundances as high as observed for stellar populations in giant ellipticals. In addition, they display surprisingly high Hbeta and Hgamma indices that are not explained by any age/metallicity combination of existing models. The high Mg and Hbeta values in these clusters could, however, be explained by the presence of blue horizontal branches. (...)

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arXiv1997

HST Imaging of the Globular Clusters in the Fornax Cluster: NGC 1379

We present B and I photometry for ~300 globular cluster candidates in NGC 1379, an E0 galaxy in the Fornax Cluster. Our data are from both Hubble Space Telescope (HST) and ground-based observations. The HST photometry (B only) is essentially complete and free of foreground/background contamination to ~2 mag fainter than the peak of the globular cluster luminosity function. Fitting a Gaussian to the luminosity function we find =24.95+-0.30 and σ_B = 1.55+-0.21. We estimate the total number of globular clusters to be 436+- 30. To a radius of 70 arcsec we derive a moderate specific frequency, S_N=3.5 +- 0.4. At radii r~ 3-6 kpc the surface density profile of the globular cluster system is indistinguishable from that of the underlying galaxy light. At r \ltsim 2.5 kpc the profile of the globular cluster system flattens, and at r \ltsim 1 kpc, the number density appears to decrease. The (B-I) colour distribution of the globular clusters (from ground-based data) is similar to that for Milky Way globulars, once corrected for background contamination. It shows no evidence for bimodality or for the presence of a population with [Fe/H]\gtsim -0.5. Unlike in the case of larger, centrally located cluster ellipticals, neither mergers nor a multiphase collapse are required to explain the formation of the NGC 1379 globular cluster system. We stress the importance of correcting for background contamination: the colour distribution of background galaxies is strongly peaked slightly bluer than the peak of a typical globular cluster distribution. This can create the impression of skewed colour distributions, or bimodality where none exists.

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arXiv1997

Globular Clusters in the Sombrero Galaxy (NGC 4594)

The Sombrero galaxy, NGC 4594, contains the most numerous globular cluster system of any nearby spiral. It is an ideal candidate in which to study the globular clusters and contrast them with those in Local Group spirals. Here we present B and I imaging from the CTIO Schmidt telescope which gives a field-of-view of 31' x 31'. Using DAOPHOT we have detected over 400 globular clusters and derived their magnitudes, B--I colors and photometric metallicities. We have attempted to separate our sample into disk and bulge/halo globular cluster populations, based on location in the galaxy. There is some evidence that the disk population is more metal--rich than the bulge/halo globular clusters, however contamination, dust reddening and small number statistics makes this result very tentative. We find that the median metallicity of the bulge/halo globular clusters is [Fe/H] = -0.8. This metallicity is consistent with previous estimates based on smaller samples. It is also similar to the metallicity predicted by the globular cluster metallicity -- galaxy luminosity relation. As with our Galaxy, there is no radial metallicity gradient in the halo globular clusters. This suggests that the spheriodal component of NGC 4594 did not form by a dissipational process.

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arXiv1996

The Nuclear Region of M51 Imaged with the HST Planetary Camera

We present high-resolution, broad- and narrow-band, pre-refurbishment images of the central region of M51 taken with the Planetary Camera of the Hubble Space Telescope. The V-band images show a rather chaotic distribution of dust lanes, though some are oriented radially, roughly aligned with the major axis of the bar, and may be transporting gas to the AGN in the nucleus. The dust lane obscuring the nucleus of the galaxy, which was previously thought to be an edge-on accretion disk feeding the AGN, is not centered on the nucleus. It is unlikely that this is a stable configuration, suggesting that the material has only recently entered the nuclear region. The nucleus is contained within a cluster of stars having a total luminosity of order 5x10^7 L_0. Fitting a King model to the least obscured portions of the cluster yields a maximum core radius of 14 pc. The morphology apparent in the forbidden-line images of the extra-nuclear cloud is consistent with a narrow jet striking and scattering off the boundary of a relatively dense cocoon of gas in the disk of the galaxy. The emission-line regions are concentrated along the inner borders of dust filaments, supporting the view that the nuclear jet is ramming into and stirring up the ISM of the disk.

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arXiv1996

Hubble Space Telescope Observations of M32: The Color-Magnitude Diagram

We present a V-I color-magnitude diagram for a region 1'-2' from the center of M32 based on Hubble Space Telescope WFPC2 images. The broad color-luminosity distribution of red giants shows that the stellar population comprises stars with a wide range in metallicity. This distribution cannot be explained by a spread in age. The blue side of the giant branch rises to M_I ~ -4.0 and can be fitted with isochrones having [Fe/H] ~ -1.5. The red side consists of a heavily populated and dominant sequence that tops out at M_I ~ -3.2, and extends beyond V-I=4. This sequence can be fitted with isochrones with -0.2 < [Fe/H] < +0.1, for ages running from 15 Gyr to 5 Gyr respectively. We do not find the optically bright asymptotic giant branch stars seen in previous ground-based work and argue that the majority of them were artifacts of crowding. Our results are consistent with the presence of the infrared-luminous giants found in ground-based studies, though their existence cannot be directly confirmed by our data. There is little evidence for an extended or even a red horizontal branch, but we find a strong clump on the giant branch itself. If the age spread is not extreme, the distribution of metallicities in M32 is considerably narrower than that of the closed-box model of chemical evolution, and also appears somewhat narrower than that of the solar neighborhood. Overall, the M32 HST color-magnitude diagram is consistent with the average luminosity-weighted age of 8.5 Gyr and [Fe/H] = -0.25 inferred from integrated spectral indices.

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arXiv1996

Hubble Space Telescope Planetary Camera Images of NGC 1316

We present HST Planetary Camera V and I~band images of the central region of the peculiar giant elliptical galaxy NGC 1316. The inner profile is well fit by a nonisothermal core model with a core radius of 0.41" +/- 0.02" (34 pc). At an assumed distance of 16.9 Mpc, the deprojected luminosity density reaches $\sim 2.0 \times 10^3 L_{\sun}$ pc$^{-3}$. Outside the inner two or three arcseconds, a constant mass-to-light ratio of $\sim 2.2 \pm 0.2$ is found to fit the observed line width measurements. The line width measurements of the center indicate the existence of either a central dark object of mass $2 \times 10^9 M_{\sun}$, an increase in the stellar mass-to-light ratio by at least a factor of two for the inner few arcseconds, or perhaps increasing radial orbit anisotropy towards the center. The mass-to-light ratio run in the center of NGC 1316 resembles that of many other giant ellipticals, some of which are known from other evidence to harbor central massive dark objects (MDO's). We also examine twenty globular clusters associated with NGC 1316 and report their brightnesses, colors, and limits on tidal radii. The brightest cluster has a luminosity of $9.9 \times 10^6 L_{\sun}$ ($M_V = -12.7$), and the faintest detectable cluster has a luminosity of $2.4 \times 10^5 L_{\sun}$ ($M_V = -8.6$). The globular clusters are just barely resolved, but their core radii are too small to be measured. The tidal radii in this region appear to be $\le$ 35 pc. Although this galaxy seems to have undergone a substantial merger in the recent past, young globular clusters are not detected.

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