The z-structure of disk galaxies towards the galaxy planes

We present a detailed study of a statistically complete sample of highly inclined disk galaxies in the near-infrared K' band. Since the K'-band light is relatively insensitive to contamination by galactic dust, we have been able to follow the vertical light distributions all the way down to the galaxy planes. The mean levels for the sharpness of the K'-band luminosity peaks indicate that the vertical luminosity distributions are more peaked than expected for the intermediate sech(z) distribution, but rounder than exponential. Since projection of not completely edge-on galaxies onto the plane of the sky causes vertical luminosity profiles to become rounder, we have performed simulations that show that it is possible that all our galaxies can have intrinsically exponential vertical surface brightness distributions. We find that the profile shape is independent of galaxy type, and varies little with position along the major axis. The fact that we observe this in all our sample galaxies indicates that the formation process of the galaxy disks perpendicular to the galaxy planes is a process intrinsic to the disks themselves.Comment: 23 pages, LaTeX, includes 12 figures, accepted for publication in Astronomy and Astrophysics. Fig. 1 will be sent on reques

The Stellar Populations of Pixels and Frames

Derived from first physical principles, a few simple rules are presented that can help in both the planning and interpretation of CCD and IR-array camera observations of resolvable stellar populations. These rules concern the overall size of the population sampled by a frame as measured by its total luminosity, and allow to estimate the number of stars (in all evolutionary stages) that are included in the frame. The total luminosity sampled by each pixel (or resolution element) allows instead to estimate to which depth meaningful stellar photometry can be safely attempted, and below which crowding makes it impossible. Simple relations give also the number of pixels (resolution elements) in the frame that will contain an unresolved blend of two stars of any kind. It is shown that the number of such blends increases quadratically with both the surface brightness of the target, as well as with the angular size of the pixel (or resolution element). A series of examples are presented illustrating how the rules are practically used in concrete observational situations. Application of these tools to existing photometric data for the inner parts of the bulge of M31, M32 and NGC 147 indicates that no solid evidence has yet emerged for the presence of a significant intermediate age population in these objects.Comment: 28 pages, LaTeX file using aasms4.sty, 2 postscript figures To appear on: The Astronomical Journa

Early Type Galaxies in the Mid Infrared: a new flavor to their stellar populations

The mid infrared emission of early type galaxies traces the presence of intermediate age stellar populations as well as even tiny amounts of ongoing star formation. Here we discuss high S/N Spitzer IRS spectra of a sample of Virgo early type galaxies, with particular reference to NGC 4435. We show that, by combining mid infrared spectroscopic observations with existing broad band fluxes, it is possible to obtain a very clean picture of the nuclear activity in this galaxy.Comment: 4 pages; proceedings of IAU Symposium No. 241, "Stellar Populations as Building Blocks of Galaxies", editors A. Vazdekis and R. Peletie

Color Gradients in Early-Type Galaxies in Clusters at the Redshift from 0.37 to 0.56

Color gradients in elliptical galaxies in distant clusters ($z=0.37-0.56$) are examined by using the archival deep imaging data of Wide Field Planetary Camera 2 (WFPC2) on-board the Hubble Space Telescope (HST). Obtained color gradients are compared with the two model gradients to examine the origin of the color gradients. In one model, a color gradient is assumed to be caused by a metallicity gradient of stellar populations, while in the other one, it is caused by an age gradient. Both of these model color gradients reproduce the average color gradient seen in nearby ellipticals, but predict significantly different gradients at a redshift larger than $\sim$0.3. Comparison between the observed gradients and the model gradients reveals that the metallicity gradient is much more favorable as the primary origin of color gradients in elliptical galaxies in clusters. The same conclusion has been obtained for field ellipticals by using those at the redshift from 0.1 to 1.0 in the Hubble Deep Field-North by Tamura et al. (2000). Thus, it is also suggested that the primary origin of the color gradients in elliptical galaxies does not depend on galaxy environment.Comment: 23 pages LaTeX, 5 PostScript figures, accepted for publication in The Astronomical Journa

Infrared-detected AGNs in the local Universe

Spitzer/IRAC color selection is a promising technique to identify hot accreting nuclei, that is to say AGN, in galaxies. We demonstrate this using a small sample of SAURON galaxies, and explore this further. The goal of this study is to find a simple and efficient way to reveal optically obscured nuclear accretion in (nearby) galaxies. We apply an infrared selection method to the Spitzer Survey of Stellar Structures in Galaxies (S4G) sample of more than 2500 galaxies, together with its extension sample of more than 400 galaxies. We use the Spitzer colors to find galaxies in the S$^{4}$G survey containing a hot core, suggesting the presence of a strong AGN, and study the detection fraction as a function of morphological type. We test this infrared color selection method by examining the radio properties of the galaxies, using the VLA NVSS and FIRST surveys. Using the radio data, we demonstrate that galaxies displaying hot mid-infrared nuclei stand out as being (candidate) active galaxies. When using, instead of Spitzer, colors from the lower spatial resolution WISE mission, we reproduce these results. Hence multi-band infrared imaging represents a useful tool to uncover optically obscured nuclear activity in galaxies.Comment: 11 pages, accepted in Astronomy and Astrophysic

Radial Age and Metal Abundance Gradients in the Stellar Content of M32

We present long-slit spectroscopy of the elliptical galaxy M32, obtained with the 8-m Subaru telescope at Mauna Kea, the 1.5-m Tillinghast telescope at the F. L. Whipple Observatory, and the 4-m Mayall telescope at the Kitt Peak National Observatory. The spectra cover the Lick index red spectral region as well as higher order Balmer lines in the blue. Spectra have been taken with the slit off-set from the nucleus to avoid scattered light contamination from the bright nucleus of M32. An analysis of numerous absorption features, particularly involving the H$\gamma$ and H$\beta$ Balmer lines, reveals that systematic radial trends are evident in the integrated spectrum of M32. Population synthesis models indicate a radial change in both the age and chemical composition of the light-weighted mean stellar population in M32, from the nucleus out to 33", i.e., approximately 1.0 effective radius, R_e. Specifically, the light-weighted mean stellar population at 1 R_e is older, by \~3 Gyr, and more metal-poor, by ~-0.25 dex in [Fe/H], t han the central value of ~4 Gyr and [Fe/H]~0.0. We show that this apparent population trend cannot be attributed to a varying contribution from either hot stars or emission line contamination. The increase in age and decrease in metal-abundance with radius are sufficiently well-matched to explain the flat radial color profiles previously observed in M32. In addition, the ratio of Mg to Fe abundance, [Mg/Fe], increases from ~-0.25 in the nucleus to ~-0.08 at 1 R_e. Finally, we find spuriously pronounced line strength gradients in the Mayall data that are an artifact of scattered light from the bright nucleus. Scattered light issues may explain the lack of consistency among previously published studies of radial line strength gradients in M32.Comment: 25 pages, 14 figures, 3 tables, accepted for publication in the Astronomical Journa

A SAURON look at galaxy bulges

Kinematic and population studies show that bulges are generally rotationally flattened systems similar to low-luminosity ellipticals. However, observations with state-of-the-art integral field spectrographs, such as SAURON, indicate that the situation is much more complex, and allow us to investigate phenomena such as triaxiality, kinematic decoupling and population substructure, and to study their connection to current formation and evolution scenarios for bulges of early-type galaxies. We present the examples of two S0 bulges from galaxies in our sample of nearby galaxies: one that shows all the properties expected from classical bulges (NGC5866), and another case that presents kinematic features appropriate for barred disk galaxies (NGC7332).Comment: 4 pages, 3 figures, accepted for publishing in AN (refereed conf. proc. of the Euro3D Science workshop, IoA Cambridge, May 2003