Liese van Zee

THE ARECIBO LEGACY FAST ALFA SURVEY. I. SCIENCE GOALS, SURVEY DESIGN, AND STRATEGY

The recently initiated Arecibo Legacy Fast ALFA (ALFALFA) survey aims to map ~7000 deg2 of the high Galactic latitude sky visible from Arecibo, providing a H I line spectral database covering the redshift range between -1600 and 18,000 km s-1 with ~5 km s-1 resolution. Exploiting Arecibos large collecting area and small beam size, ALFALFA is specifically designed to probe the faint end of the H I mass function in the local universe and will provide a census of H I in the surveyed sky area to faint flux limits, making it especially useful in synergy with wide-area surveys conducted at other wavelengths. ALFALFA will also provide the basis for studies of the dynamics of galaxies within the Local Supercluster and nearby superclusters, allow measurement of the H I diameter function, and enable a first wide-area blind search for local H I tidal features, H I absorbers at z < 0.06, and OH megamasers in the redshift range 0.16 < z < 0.25. Although completion of the survey will require some 5 years, public access to the ALFALFA data and data products will be provided in a timely manner, thus allowing its application for studies beyond those targeted by the ALFALFA collaboration. ALFALFA adopts a two-pass, minimum intrusion, drift scan observing technique that samples the same region of sky at two separate epochs to aid in the discrimination of cosmic signals from noise and terrestrial interference. Survey simulations, which take into account large-scale structure in the mass distribution and incorporate experience with the ALFA system gained from tests conducted during its commissioning phase, suggest that ALFALFA will detect on the order of 20,000 extragalactic H I line sources out to z ~ 0.06, including several hundred with H I masses M < 107.5 M⊙.

Spectroscopy of Outlying H II Regions in Spiral Galaxies: Abundances and Radial Gradients

We present the results of low-dispersion optical spectroscopy of 186 H II regions spanning a range of radius in 13 spiral galaxies. Abundances for several elements (oxygen, nitrogen, neon, sulfur, and argon) were determined for 185 of the H II regions. As expected, low metallicities were found for the outlying H II regions of these spiral galaxies. Radial abundance gradients were derived for the 11 primary galaxies; similar to results for other spiral galaxies, the derived abundance gradients are typically -0.04 to -0.07 dex kpc-1.

COMPARISON OF Hα AND UV STAR FORMATION RATES IN THE LOCAL VOLUME: SYSTEMATIC DISCREPANCIES FOR DWARF GALAXIES

Using a complete sample of ~300 star-forming galaxies within 11 Mpc of the Milky Way, we evaluate the consistency between star formation rates (SFRs) inferred from the far ultraviolet (FUV) non-ionizing continuum and Hα nebular emission, assuming standard conversion recipes in which the SFR scales linearly with luminosity at a given wavelength. Our analysis probes SFRs over 5 orders of magnitude, down to ultra-low activities on the order of ~10^–4 M_☉ yr^–1. The data are drawn from the 11 Mpc Hα and Ultraviolet Galaxy Survey (11HUGS), which has obtained Hα fluxes from ground-based narrowband imaging, and UV fluxes from imaging with GALEX. For normal spiral galaxies (SFR ~ 1 M_☉ yr^–1), our results are consistent with previous work which has shown that FUV SFRs tend to be lower than Hα SFRs before accounting for internal dust attenuation, but that there is relative consistency between the two tracers after proper corrections are applied. However, a puzzle is encountered at the faint end of the luminosity function. As lower luminosity dwarf galaxies, roughly less active than the Small Magellanic Cloud, are examined, Hα tends to increasingly underpredict the total SFR relative to the FUV. The trend is evident prior to corrections for dust attenuation, which affects the FUV more than the nebular Hα emission, so this general conclusion is robust to the effects of dust. Although past studies have suggested similar trends, this is the first time this effect is probed with a statistical sample for galaxies with SFR ≤0.1 M_☉ yr^–1. By SFR ~ 0.003 M_☉ yr–1, the average Hα-to-FUV flux ratio is lower than expected by a factor of two, and at the lowest SFRs probed, the ratio exhibits an order of magnitude discrepancy for the handful of galaxies that remain in the sample. A range of standard explanations does not appear to be able to fully account for the magnitude of the systematic. Some recent work has argued for a stellar initial mass function which is deficient in high-mass stars in dwarf and low surface brightness galaxies, and we also consider this scenario. Under the assumption that the FUV traces the SFR in dwarf galaxies more robustly, the prescription relating Hα luminosity to SFR is re-calibrated for use in the low SFR regime when FUV data are not available.

Oxygen and nitrogen in isolated dwarf irregular galaxies

We present long-slit optical spectroscopy of 67 H II regions in 21 dwarf irregular galaxies to investigate the enrichment of oxygen, nitrogen, neon, sulfur, and argon in low-mass galaxies. Oxygen abundances are obtained via direct detection of the temperature-sensitive emission lines for 25 H II regions; for the remainder of the sample, oxygen abundances are estimated from strong-line calibrations. The direct abundance determinations are compared to the strong-line abundance calibrations of both McGaugh and Pilyugin. While the McGaugh calibration yields a statistical offset of 0.07 dex, the photoionization model grid traces the appropriate isometallicity contour shape in the R23-O32 diagnostic diagram. In contrast, while the Pilyugin calibration yields a negligible statistical offset, the residuals in this strong-line calibration method are correlated with ionization parameter. Thus, these observations indicate that oxygen abundances will be overestimated by the p-method for H II regions with low-ionization parameters. Global oxygen and nitrogen abundances for this sample of dwarf irregular galaxies are examined in the context of open- and closed-box chemical evolution models. While several galaxies are consistent with closed-box chemical evolution, the majority of this sample has an effective yield ~ of the expected yield for a constant star formation rate and Salpeter IMF, indicating that either outflow of enriched gas or inflow of pristine gas has occurred. The effective yield strongly correlates with MH/LB in the sense that gas-rich galaxies are more likely to be closed systems. However, the effective yield does not appear to correlate with other global parameters such as dynamical mass, absolute magnitude, star formation rate, or surface brightness. In addition, open and closed systems are not identified easily in other global abundance measures; for example, the observed correlation between luminosity and metallicity is consistent with other recent results in the literature. A correlation is found between the observed nitrogen-to-oxygen ratio and the color of the underlying stellar population; redder dwarf irregular galaxies have higher N/O ratios than blue dwarf irregular galaxies. The relative abundance ratios are interpreted in the context of delayed release of nitrogen and varied star formation histories.

Neutral gas distributions and kinematics of five blue compact dwarf galaxies

?????We present the results of high spatial resolution H I observations of five intrinsically compact dwarf galaxies that are currently experiencing a strong burst of star formation. The H I maps indicate that these systems have a complex and clumpy interstellar medium. Unlike typical dwarf irregular galaxies, these blue compact dwarf galaxies (BCDs) exhibit strong central concentrations in their neutral gas distributions that may provide a clue to the origin of their strong starburst activity. Furthermore, while all of the systems appear to be rotating, based on observed velocity gradients, the kinematics are complex. All systems have nonordered kinematic structure at some level; some of the extended gas is not necessarily kinematically connected to the main system. The observed gas distributions and kinematics place constraints on evolutionary scenarios for BCDs. Evolutionary links between BCDs, dwarf irregular galaxies, and dwarf elliptical galaxies have been postulated to explain their high star formation rates and low-luminosity, low-metallicity nature. The BCDs appear to have higher central mass concentrations in both gas and stellar content than the dwarf irregulars, indicating that evolutionary scenarios connecting these two classes will require mass redistribution. In addition, the fact that BCDs are rotationally supported systems indicates that they are unlikely to evolve into dwarf ellipticals without substantial loss of angular momentum. Thus, while such evolutionary scenarios may still be possible with the aid of mergers or tidal interactions, the isolated nature of BCDs suggests that the majority of BCDs will not fade to become objects similar to the present-day dwarf ellipticals.

A GALEX ULTRAVIOLET IMAGING SURVEY OF GALAXIES IN THE LOCAL VOLUME

We present results from a GALEX ultraviolet (UV) survey of a complete sample of 390 galaxies within ~11 Mpc of the Milky Way. The UV data are a key component of the composite Local Volume Legacy, an ultraviolet-to-infrared imaging program designed to provide an inventory of dust and star formation in nearby spiral and irregular galaxies. The ensemble data set is an especially valuable resource for studying star formation in dwarf galaxies, which comprise over 80% of the sample. We describe the GALEX survey programs that obtained the data and provide a catalog of far-UV (~1500 A) and near-UV (~2200 A) integrated photometry. General UV properties of the sample are briefly discussed. We compute two measures of the global star formation efficiency, the star formation rate (SFR) per unit H I gas mass, and the SFR per unit stellar mass, to illustrate the significant differences that can arise in our understanding of dwarf galaxies when the FUV is used to measure the SFR instead of Hα. We find that dwarf galaxies may not be as drastically inefficient in converting gas into stars as suggested by prior Hα studies. In this context, we also examine the UV properties of late-type dwarf galaxies that appear to be devoid of star formation because they were not detected in previous Hα narrowband observations. Nearly all such galaxies in our sample are detected in the FUV and have FUV SFRs that fall below the limit where the Hα flux is robust to Poisson fluctuations in the formation of massive stars. Otherwise, the UV colors and star formation efficiencies of Hα-undetected, UV-bright dwarf irregulars appear to be relatively unremarkable with respect to those exhibited by the general population of star-forming galaxies.

Abundances in Spiral Galaxies: Evidence for Primary Nitrogen Production

We present the results of nitrogen and oxygen abundance measurements for 185 H II regions spanning a range of radii in 13 spiral galaxies. As expected, the nitrogen-to-oxygen ratio increases linearly with the oxygen abundance for high-metallicity H II regions, indicating that nitrogen is predominantly a secondary element. However, the nitrogen-to-oxygen ratio plateaus for oxygen abundances less than 1/3 solar [12 + log (O/H) < 8.45], as is also seen in low-metallicity dwarf galaxies. This result suggests that the observed trend in dwarf galaxies is not due to the outflow of enriched material in a shallow gravitational potential. While the effects of the infall of pristine material and delayed nitrogen delivery are still unconstrained, nitrogen does appear to have both a primary and a secondary component at low metallicities in all types of galaxies.

Rotationally Supported Virgo Cluster Dwarf Elliptical Galaxies: Stripped Dwarf Irregular Galaxies?

New observations of 16 dwarf elliptical galaxies (dEs) in the Virgo Cluster indicate that at least seven dEs have significant velocity gradients along their optical major axis, with typical rotation amplitudes of 20–30 km s-1. Of the remaining nine galaxies in this sample, six have velocity gradients of less than 20 km s-1 kpc-1, while the other three observations had too low a signal-to-noise ratio to determine an accurate velocity gradient. Typical velocity dispersions for these galaxies are ~44 ± 5 km s-1, indicating that rotation can be a significant component of the stellar dynamics of Virgo dEs. When corrected for the limited spatial extent of the spectral data, the rotation amplitudes of the rotating dEs are comparable to those of similar-brightness dwarf irregular galaxies (dIs). Evidence of a relationship between the rotation amplitude and galaxy luminosity is found and, in fact, agrees well with the Tully-Fisher relation. The similarity in the scaling relations of dIs and dEs implies that it is unlikely that dEs evolve from significantly more luminous galaxies. These observations reaffirm the possibility that some cluster dEs may be formed when the neutral gaseous medium is stripped from dIs in the cluster environment. We hypothesize that several different mechanisms are involved in the creation of the overall population of dEs and that stripping of infalling dIs may be the dominant process in the creation of dEs in clusters like Virgo.

Stellar populations of dwarf elliptical galaxies: UBVRI photometry of dwarf elliptical galaxies in the Virgo Cluster

We present UBVRI surface photometry for 16 dwarf elliptical galaxies in the Virgo Cluster with previously measured kinematic properties. The global optical colors are red, with median values for the sample of 0.24 ? 0.03 in U - B, 0.77 ? 0.02 in B - V, and 1.02 ? 0.03 in V - I. We recover the well-known color-magnitude relation for cluster galaxies but find no significant difference in dominant stellar population between rotating and nonrotating dwarf elliptical galaxies; the average age of the dominant stellar population is 5?7 Gyr in all 16 galaxies in this sample. Analysis of optical spectra confirm these age estimates and indicate Fe and Mg abundances in the range of 1/20 to one-third of solar, as expected for low-luminosity galaxies. Based on Lick indices and simple stellar population models, the derived [?/Fe] ratios are subsolar to solar, indicating a more gradual chemical enrichment history for dEs as compared with giant elliptical galaxies in the Virgo Cluster. These observations confirm the marked difference in stellar population and stellar distribution between dwarf and giant elliptical galaxies and further substantiate the need for alternative evolutionary scenarios for the lowest mass cluster galaxies. We argue that it is likely that several different physical mechanisms played a significant role in the production of the Virgo Cluster dE galaxies including in situ formation, infall of dEs that were once part of Local Group analogs, and transformation of dwarf irregular galaxies by the cluster environment. The observations support the hypothesis that a large fraction of the Virgo Cluster dEs are formed by ram pressure stripping of gas from infalling dIs.

Kinematic Constraints on Evolutionary Scenarios for Blue Compact Dwarf Galaxies. I. Neutral Gas Dynamics

We present the results of high spatial resolution, H I synthesis observations of six blue compact dwarf (BCD) galaxies. Optically, the selected galaxies have smooth, symmetric isophotes and thus are the most likely of the BCD class to fade into an object morphologically similar to a dwarf elliptical when the current starburst ends. The neutral gas in all six galaxies appears to be rotationally supported, however, indicating that true morphological transformation from a BCD to a dE will require signi—cant loss of angular momentum. Based on the observed neutral gas dynamics of these and other BCDs, it is unlikely that present-day BCDs will evolve directly into dwarf ellipticals after a starburst phase. We discuss alternative evolutionary scenarios for BCDs and place them within the larger context of galaxy formation and evolution models. In general, BCDs appear to have steeper rotation curves than similar luminosity, low surface brightness dwarf galaxies. BCDs have centrally concentrated mass distributions (stars, gas, and dark matter) and have lower speci—c angular momenta. Based on disk instability analyses, steeply rising rotation curves result in higher threshold densities for the onset of star formation. These results suggest that angular momentum may play a crucial role in the morphological evolution of low-mass galaxies: galaxies with low angular momenta will be able to collapse into small, compact galaxies, while galaxies with high angular momenta will be more diUuse systems. Furthermore, because the star formation threshold density is higher in low angular momenta systems, star formation will be delayed until an extremely high surface density is reached. Thus, angular momentum may be the fundamental parameter that determines whether a low-mass galaxy will have centrally concentrated stellar and gaseous distributions and be more susceptible to a burst mode of star formation.