Stellar Motions in the Polar Ring Galaxy NGC 4650A

We present the first measurement of the stellar kinematics in the polar ring of NGC 4650A. There is well defined rotation, with the stars and gas rotating in the same direction, and with similar amplitude. The gaseous and stellar kinematics suggest an approximately flat rotation curve, providing further support for the hypothesis that the polar material resides in a disk rather than in a ring. The kinematics of the emission line gas at and near the center of the S0 suggests that the polar disk lacks a central hole. We have not detected evidence for two, equal mass, counterrotating stellar polar streams, as is predicted in the resonance levitation model proposed by Tremaine & Yu. A merger seems the most likely explanation for the structure and kinematics of NGC 4650A.Comment: 4 pages, accepted for publication in ApJ Letter

The Stellar and Gas Kinematics of Several Irregular Galaxies

We present long-slit spectra of three irregular galaxies from which we determinethe stellar kinematics in two of the galaxies (NGC 1156 and NGC 4449) and ionized-gas kinematics in all three (including NGC 2366). We compare this to the optical morphology and to the HI kinematics of the galaxies. In the ionized gas, we see a linear velocity gradient in all three galaxies. In NGC 1156 we also detect a weak linear velocity gradient in the stars of (5+/-1/sin i) km/s/kpc to a radius of 1.6 kpc. The stars and gas are rotating about the same axis, but this is different from the major axis of the stellar bar which dominates the optical light of the galaxy. In NGC 4449 we do not detect organized rotation of the stars and place an upper limit of (3/sin i) km/s/kpc to a radius of 1.2 kpc. For NGC 4449, which has signs of a past interaction with another galaxy, we develop a model to fit the observed kinematics of the stars and gas. In this model the stellar component is in a rotating disk seen nearly face-on while the gas is in a tilted disk with orbits whose planes precess in the gravitational potential. This model reproduces the apparent counter-rotation of the inner gas of the galaxy. The peculiar orbits of the gas are presumed due to acquisition of gas in the past interaction.Comment: To be published in ApJ, November 20, 200

The DiskMass Survey. X. Radio synthesis imaging of spiral galaxies

We present results from 21 cm radio synthesis imaging of 28 spiral galaxies from the DiskMass Survey obtained with the VLA, WSRT, and GMRT facilities. We detail the observations and data reduction procedures and present a brief analysis of the radio data. We construct 21 cm continuum images, global HI emission-line profiles, column-density maps, velocity fields, and position-velocity diagrams. From these we determine star formation rates (SFRs), HI line widths, total HI masses, rotation curves, and azimuthally-averaged radial HI column-density profiles. All galaxies have an HI disk that extends beyond the readily observable stellar disk, with an average ratio and scatter of R_{HI}/R_{25}=1.35+/-0.22, and a majority of the galaxies appear to have a warped HI disk. A tight correlation exists between total HI mass and HI diameter, with the largest disks having a slightly lower average column density. Galaxies with relatively large HI disks tend to exhibit an enhanced stellar velocity dispersion at larger radii, suggesting the influence of the gas disk on the stellar dynamics in the outer regions of disk galaxies. We find a striking similarity among the radial HI surface density profiles, where the average, normalized radial profile of the late-type spirals is described surprisingly well with a Gaussian profile. These results can be used to estimate HI surface density profiles in galaxies that only have a total HI flux measurement. We compare our 21 cm radio continuum luminosities with 60 micron luminosities from IRAS observations for a subsample of 15 galaxies and find that these follow a tight radio-infrared relation, with a hint of a deviation from this relation at low luminosities. We also find a strong correlation between the average SFR surface density and the K-band surface brightness of the stellar disk.Comment: 22 pages + Appendix, 16 figures + Atlas, 5 tables. Accepted for publication in Astronomy & Astrophysic

The stellar velocity dispersion in the inner 1.3 disk scale-lengths of the irregular galaxy NGC 4449

We present measurements of the stellar velocity dispersion in the inner 1 arcmin radius (1.3 disk scale-lengths) of the irregular galaxy NGC 4449 determined from long-slit absorption-line spectra. The average observed dispersion is 29 +/-2 km/s, the same as predicted from NGC 4449's luminosity. No significant rotation in the stars is detected. If we assume a maximum rotation speed of the stars from the model determined from the gas kinematics of Hunter et al. (2002), the ratio V_max/sigma_z measured globally is 3. This ratio is comparable to values measured in spiral galaxies, and implies that the stellar disk in NGC 4449 is kinematically relatively cold. The intrinsic minor-to-major axis ratio (b/a)_0 is predicted to be in the range 0.3-0.6, similar to values derived from the distribution of observed b/a of Im galaxies. However, V/sigma_z measured locally is 0.5-1.1, and so the circular velocity of NGC 4449 is comparable or less than the velocity of the stars within the central 1.3 disk scale-lengths of the galaxy.Comment: To be published in ApJ, Nov 200

SparsePak: A Formatted Fiber Field-Unit for The WIYN Telescope Bench Spectrograph. II. On-Sky Performance

We present a performance analysis of SparsePak and the WIYN Bench Spectrograph for precision studies of stellar and ionized gas kinematics of external galaxies. We focus on spectrograph configurations with echelle and low-order gratings yielding spectral resolutions of ~10000 between 500-900nm. These configurations are of general relevance to the spectrograph performance. Benchmarks include spectral resolution, sampling, vignetting, scattered light, and an estimate of the system absolute throughput. Comparisons are made to other, existing, fiber feeds on the WIYN Bench Spectrograph. Vignetting and relative throughput are found to agree with a geometric model of the optical system. An aperture-correction protocol for spectrophotometric standard-star calibrations has been established using independent WIYN imaging data and the unique capabilities of the SparsePak fiber array. The WIYN point-spread-function is well-fit by a Moffat profile with a constant power-law outer slope of index -4.4. We use SparsePak commissioning data to debunk a long-standing myth concerning sky-subtraction with fibers: By properly treating the multi-fiber data as a ``long-slit'' it is possible to achieve precision sky subtraction with a signal-to-noise performance as good or better than conventional long-slit spectroscopy. No beam-switching is required, and hence the method is efficient. Finally, we give several examples of science measurements which SparsePak now makes routine. These include H$\alpha$ velocity fields of low surface-brightness disks, gas and stellar velocity-fields of nearly face-on disks, and stellar absorption-line profiles of galaxy disks at spectral resolutions of ~24,000.Comment: To appear in ApJSupp (Feb 2005); 19 pages text; 7 tables; 27 figures (embedded); high-resolution version at http://www.astro.wisc.edu/~mab/publications/spkII_pre.pd

The DiskMass Survey. II. Error Budget

We present a performance analysis of the DiskMass Survey. The survey uses collisionless tracers in the form of disk stars to measure the surface-density of spiral disks, to provide an absolute calibration of the stellar mass-to-light ratio, and to yield robust estimates of the dark-matter halo density profile in the inner regions of galaxies. We find a disk inclination range of 25-35 degrees is optimal for our measurements, consistent with our survey design to select nearly face-on galaxies. Uncertainties in disk scale-heights are significant, but can be estimated from radial scale-lengths to 25% now, and more precisely in the future. We detail the spectroscopic analysis used to derive line-of-sight velocity dispersions, precise at low surface-brightness, and accurate in the presence of composite stellar populations. Our methods take full advantage of large-grasp integral-field spectroscopy and an extensive library of observed stars. We show that the baryon-to-total mass fraction (F_b) is not a well-defined observational quantity because it is coupled to the halo mass model. This remains true even when the disk mass is known and spatially-extended rotation curves are available. In contrast, the fraction of the rotation speed supplied by the disk at 2.2 scale lengths (disk maximality) is a robust observational indicator of the baryonic disk contribution to the potential. We construct the error-budget for the key quantities: dynamical disk mass surface-density, disk stellar mass-to-light ratio, and disk maximality (V_disk / V_circular). Random and systematic errors in these quantities for individual galaxies will be ~25%, while survey precision for sample quartiles are reduced to 10%, largely devoid of systematic errors outside of distance uncertainties.Comment: To appear in ApJ; 88 pages, 4 tables, 18 figures. High-resolution version available at http://www.astro.wisc.edu/~mab/publications/DMS_II_preprint.pd

The DiskMass Survey. I. Overview

We present a survey of the mass surface-density of spiral disks, motivated by outstanding uncertainties in rotation-curve decompositions. Our method exploits integral-field spectroscopy to measure stellar and gas kinematics in nearly face-on galaxies sampled at 515, 660, and 860 nm, using the custom-built SparsePak and PPak instruments. A two-tiered sample, selected from the UGC, includes 146 nearly face-on galaxies, with B<14.7 and disk scale-lengths between 10 and 20 arcsec, for which we have obtained H-alpha velocity-fields; and a representative 46-galaxy subset for which we have obtained stellar velocities and velocity dispersions. Based on re-calibration of extant photometric and spectroscopic data, we show these galaxies span factors of 100 in L(K) (0.03 < L/L(K)* < 3), 8 in L(B)/L(K), 10 in R-band disk central surface-brightness, with distances between 15 and 200 Mpc. The survey is augmented by 4-70 micron Spitzer IRAC and MIPS photometry, ground-based UBVRIJHK photometry, and HI aperture-synthesis imaging. We outline the spectroscopic analysis protocol for deriving precise and accurate line-of-sight stellar velocity dispersions. Our key measurement is the dynamical disk-mass surface-density. Star-formation rates and kinematic and photometric regularity of galaxy disks are also central products of the study. The survey is designed to yield random and systematic errors small enough (i) to confirm or disprove the maximum-disk hypothesis for intermediate-type disk galaxies, (ii) to provide an absolute calibration of the stellar mass-to-light ratio well below uncertainties in present-day stellar-population synthesis models, and (iii) to make significant progress in defining the shape of dark halos in the inner regions of disk galaxies.Comment: To appear in ApJ; 72 pages, 3 tables, 18 figures. High-resolution version available at http://www.astro.wisc.edu/~mab/publications/DMS_I_preprint.pd