We map the neutral atomic gas content of M33 using high resolution VLA and GBT observations and fit a tilted ring model to determine the orientation of the extended gaseous disk and its rotation curve. The disk of M33 warps from 8 kpc outwards withou
t substantial change of its inclination with respect to the line of sight. Rotational velocities rise steeply with radius in the inner disk, reaching 100 km/s in 4 kpc, then the rotation curve becomes more perturbed and flatter with velocities as high as 120-130 km/s out to 23 kpc. We derive the stellar mass surface density map of M33s optical disk, via pixel -SED fitting methods based on population synthesis models, which highlights variations in the mass-to-light ratio. The stellar mass surface further out is estimated from deep images of outer disk fields. Stellar and gas maps are then used in the dynamical analysis of the rotation curve to constrain the dark matter distribution which is relevant at all radii. A dark matter halo with a Navarro-Frenk-White density profile in a LCDM cosmology, provides the best fit to the rotation curve for a dark halo concentration C=10 and a total halo mass of 4.3 10^{11}Msun. This imples a baryonic fraction of order 0.02 and the evolutionary history of this galaxy should account for loss of a large fraction of its original baryonic content.
We present new spectro-photometric NIR observations of 16 post-starburst galaxies especially designed to test for the presence of strong carbon features of thermally pulsing AGB (TP-AGB) stars, as predicted by recent models of stellar population synt
hesis. Selection based on clear spectroscopic optical features indicating the strong predominance of stellar populations with ages between 0.5 and 1.5 Gyr and redshift around 0.2 allows us to probe the spectral region that is most affected by the carbon features of TP-AGB stars (unaccessible from the ground for z~0 galaxies) in the evolutionary phase when their impact on the IR luminosity is maximum. Nevertheless, none of the observed galaxies display such features. Moreover the NIR fluxes relative to optical are consistent with those predicted by the original Bruzual & Charlot (2003) models, where the impact of TP-AGB stars is much lower than has been recently advocated.
A large body of evidence has demonstrated that the global rest-frame optical and IR colours of galaxies correlate well with each other, as well as with other galactic properties such as surface brightness and morphology. However the processes that le
ad to the observed correlations are contrary; the stellar light that contributes to the optical is readily absorbed by dust which emits in the IR. Thus on small scales we expect these correlations to break down. We examine seven nearby galaxies ranging from early- to late-types, on a pixel-by-pixel basis and we demonstrate that there is disconnect between the optical and IR when normalized to the near-IR (H-band). We can decompose this disconnect into two distinct components through a Principal Component Analysis of the H-band normalized SED of the pixels: one mainly correlated with variations in the IR, the other correlated with variations in the optical. By mapping these two components it is clear they arise from distinct spatial regions. The IR dominated component is strongly associated with the specific star-formation rate, while the optical-dominated component is broadly associated with the stellar mass density. However, when the pixels of all galaxies are compared, the well known optical-IR colour correlations return, demonstrating that the variance observed within galaxies is around a mean which follows the well-known trend. We also examine the extremely strong correlations between the IRAC-NIR colours and demonstrate that they are tight enough to use a single IRAC-NIR colour (i.e. 8mum-H) to determine the fluxes in the other IRAC bands. These correlations arise from the differing contribution of stellar light and dust to the IRAC bands, enabling us to determine pure stellar colours for these bands, but still demonstrating the need for dust (or stellar) corrections in these bands when being used as stellar (dust) tracers.
We have measured the near-infrared colors and the fluxes of individual pixels in 68 galaxies common to the Spitzer Infrared Nearby Galaxies Survey and the Large Galaxy Atlas Survey. Each galaxy was separated into regions of increasingly red near-infr
ared colors. In the absence of dust extinction and other non-stellar emission, stellar populations are shown to have relatively constant NIR colors, independent of age. In regions of high star formation, the average intensity of pixels in red-excess regions (at 1.25, 3.6, 4.5, 5.6, 8.0 and 24 micron) scales linearly with the intrinsic intensity of Halpha emission, and thus with the star-formation rate within the pixel. This suggests that most NIR-excess regions are not red because their light is being depleted by absorption. Instead, they are red because additional infrared light is being contributed by a process linked to star-formation. This is surprising because the shorter wavelength bands in our study (1.25 micron-5.6 micron) do not probe emission from cold (10-20 K) and warm (50-100 K) dust associated with star-formation in molecular clouds. However, emission from hot dust (700-1000 K) and/or Polycyclic Aromatic Hydrocarbon molecules can explain the additional emission seen at the shorter wavelengths in our study. The contribution from hot dust and/or PAH emission at 2-5micron and PAH emission at 5.6 and 8.0 micron scales linearly with warm dust emission at 24 micron and the intrinsic Halpha emission. Since both are tied to the star-formation rate, our analysis shows that the NIR excess continuum emission and PAH emission at ~1-8 micron can be added to spectral energy distribution models in a very straight-forward way, by simply adding an additional component to the models that scales linearly with star-formation rate.
Physical processes influencing the properties of galaxies can be traced by the dependence and evolution of galaxy properties on their environment. A detailed understanding of this dependence can only be gained through comparison of observations with
models, with an appropriate quantification of the rich parameter space describing the environment of the galaxy. We present a new, multiscale parameterization of galaxy environment which retains an observationally motivated simplicity whilst utilizing the information present on different scales. We examine how the distribution of galaxy (u-r) colours in the Sloan Digital Sky Survey (SDSS), parameterized using a double gaussian (red plus blue peak) fit, depends upon multiscale density. This allows us to probe the detailed dependence of galaxy properties on environment in a way which is independent of the halo model. Nonetheless, cross-correlation with the group catalogue constructed by Yang et al, 2007 shows that galaxy properties trace environment on different scales in a way which mimics that expected within the halo model. This provides independent support for the existence of virialized haloes, and important additional clues to the role played by environment in the evolution of the galaxy population. This work is described in full by Wilman et al., 2010, MNRAS, accepted
We present a multiscale approach to measurements of galaxy density, applied to a volume-limited sample constructed from SDSS DR5. We populate a rich parameter space by obtaining independent measurements of density on different scales for each galaxy,
avoiding the implicit assumptions involved, e.g., in the construction of group catalogues. As the first application of this method, we study how the bimodality in galaxy colour distribution (u-r) depends on multiscale density. The u-r galaxy colour distribution is described as the sum of two gaussians (red and blue) with five parameters: the fraction of red galaxies (f_r) and the position and width of the red and blue peaks (mu_r, mu_b, sigma_r and sigma_b). Galaxies mostly react to their smallest scale (< 0.5 Mpc) environments: in denser environments red galaxies are more common (larger f_r), redder (larger mu_r) and with a narrower distribution (smaller sigma_r), while blue galaxies are redder (larger mu_b) but with a broader distribution (larger sigma_b). There are residual correlations of f_r and mu_b with 0.5 - 1 Mpc scale density, which imply that total or partial truncation of star formation can relate to a galaxys environment on these scales. Beyond 1 Mpc (0.5 Mpc for mu_r) there are no positive correlations with density. However f_r (mu_r) anti-correlates with density on >2 (1) Mpc scales at fixed density on smaller scales. We examine these trends qualitatively in the context of the halo model, utilizing the properties of haloes within which the galaxies are embedded, derived by Yang et al, 2007 and applied to a group catalogue. This yields an excellent description of the trends with multiscale density, including the anti-correlations on large scales, which map the region of accretion onto massive haloes. Thus we conclude that galaxies become red only once they have been accreted onto haloes of a certain mass.
The presence of a diffuse stellar component in galaxy clusters has been established by a number of observational works in recent years. In this contribution I summarize our results (Zibetti et al. 2005) obtained by stacking SDSS images of 683 cluster
s, selected with the maxBCG algorithm at 0.2< z <0.3. Thanks to our large sample and the advantages of image stacking applied to SDSS images, we are able to measure the systematic properties of the intracluster light (ICL) with very high accuracy. We find that the average surface brightness of the ICL ranges between 26 and 32 mag/arcsec^2, and constantly declines from 70 kpc cluster-centric distance (i.e. distance from the BCG) to 700 kpc. The fraction of diffuse light over the total light (including galaxies), monotonically declines from ~50 to <~5% over the same range of distances, thus showing that the ICL is more easily produced close to the bottom of a clusters potential well. Clusters lacking a bright BCG hardly build up a large amount of intracluster stellar component. The link between the growth of the BCG and the ICL is also suggested by the strong degree of alignment between these two components which is observed in clusters where the BCG displays a significant elongation. With the additional fact that the colors of the ICL are consistent with those of galaxies, all this appears to be evidence for IC stars being stripped from galaxies that suffer very strong tidal interactions in the center of clusters and eventually merge into the BCG. Our measurements also show that IC stars are a minor component of a clusters baryonic budget, representing only ~10% of the total optical emission within 500 kpc. Finally, we discuss some open issues that emerge from a comparison of the present results with other observations and recent theoretical modeling.
