Galaxy Evolution from Emission Linewidths

The major thrust of the Tully-Fisher (TF) surveys of distant galaxies is the measurement of linewidths rather than mere redshifts or colors. Linewidths are a measure of galaxy mass and should therefore be a more stable indicator of size than galaxy brightness, which can be badly affected by luminosity evolution. Masses may provide the best way to relate galaxies at different epochs, but for such a program to work, we must control systematic effects that could bias linewidth measurements at high redshift and skew comparisons with local Tully-Fisher calibrations. Potential sources of confusion in TF studies of galaxy structure and evolution include central or extended star bursts, infalling gas, turbulence and outflows, dust extinction, calibration of emission linewidths, and improper application of local TF calibrations to high redshift galaxies.Comment: 7 pages, 3 figures; Proceedings contribution in The Mass of Galaxies at Low and High Redshift, eds. R. Bender and A. Renzini (ESO Astrophysics Symposia

On the Uncertainties of Stellar Mass Estimates via Colour Measurements

Mass-to-light versus colour relations (MLCRs), derived from stellar population synthesis models, are widely used to estimate galaxy stellar masses (M$_*$) yet a detailed investigation of their inherent biases and limitations is still lacking. We quantify several potential sources of uncertainty, using optical and near-infrared (NIR) photometry for a representative sample of nearby galaxies from the Virgo cluster. Our method for combining multi-band photometry with MLCRs yields robust stellar masses, while errors in M$_*$ decrease as more bands are simultaneously considered. The prior assumptions in one's stellar population modelling dominate the error budget, creating a colour-dependent bias of up to 0.6 dex if NIR fluxes are used (0.3 dex otherwise). This matches the systematic errors associated with the method of spectral energy distribution (SED) fitting, indicating that MLCRs do not suffer from much additional bias. Moreover, MLCRs and SED fitting yield similar degrees of random error ($\sim$0.1-0.14 dex) when applied to mock galaxies and, on average, equivalent masses for real galaxies with M$_* \sim$ 10$^{8-11}$ M$_{\odot}$. The use of integrated photometry introduces additional uncertainty in M$_*$ measurements, at the level of 0.05-0.07 dex. We argue that using MLCRs, instead of time-consuming SED fits, is justified in cases with complex model parameter spaces (involving, for instance, multi-parameter star formation histories) and/or for large datasets. Spatially-resolved methods for measuring M$_*$ should be applied for small sample sizes and/or when accuracies less than 0.1 dex are required. An Appendix provides our MLCR transformations for ten colour permutations of the $grizH$ filter set.Comment: Accepted to MNRAS. 43 pages, 12 figures, 3 table

Stripping of the Hot Gas Halos in Member Galaxies of Abell 1795

The nearby cluster Abell 1795 is used as a testbed to examine whether hot gas in cluster galaxies is stripped by the ram pressure of the intracluster medium (ICM). The expected X-ray emission in and around Abell 1795 galaxies is likely dominated by the ICM, low-mass X-ray binaries, active galactic nuclei, and hot gas halos. In order to constrain these components, we use archival Chandra X-ray Observatory and Sloan Digital Sky Survey (SDSS) observations of Abell 1795 and identify 58 massive (M_star>10^10 M_sun) spectroscopic cluster members within 5 arcmin of the Chandra optical axis. X-ray images at 0.5-1.5 keV and 4-8 keV were created for each cluster member and then stacked into two clustercentric radius bins: inner (0.25<R/R_500<1) and outer (1<R/R_500<2.5). Surface brightness profiles of inner and outer cluster members are fit using Markov chain Monte Carlo sampling in order to generate model parameters and measure the 0.5-1.5 keV luminosities of each model component. Leveraging effective total Chandra exposure times of 3.4 and 1.7 Msec for inner and outer cluster members, respectively, we report the detection of hot gas halos, in a statistical sense, around outer cluster members. Outer members have 0.5-1.5 keV hot halo luminosities (L_X = 8.1(-3.5/+5)x10^39 erg/s) that are six times larger than the upper limit for inner cluster members (L_X < 1.3x10^39 erg/s). This result suggests that the ICM is removing hot gas from the halos of Abell 1795 members as they fall into the cluster.Comment: 15 pages, nine figures, accepted for publication in The Astrophysical Journa

Stellar Populations in Spiral Galaxies

We report preliminary results of the characterization of bulge and inner disk stellar populations for 8 nearby spiral galaxies using Gemini/GMOS. The long-slit spectra extend out to 1-2 disk scale lengths with S/N/Ang > 50. Two different model fitting techniques, absorption-line indices and full spectral synthesis, are found to weigh age, metallicity, and abundance ratios differently, but with careful attention to the data/model matching (resolution and flux calibration), we are able constrain real signatures of age and metallicity gradients in star-forming galaxies.Comment: 4 pages, 3 figures. To appear in the proceedings for IAUS 241 "Stellar Populations as Building Blocks of Galaxies", Eds. R.F. Peletier and A. Vazdeki