The triggering of starbursts in low-mass galaxies

Strong bursts of star formation in galaxies may be triggered either by internal or external mechanisms. We study the distribution and kinematics of the HI gas in the outer regions of 18 nearby starburst dwarf galaxies, that have accurate star-formation histories from HST observations of resolved stellar populations. We find that starburst dwarfs show a variety of HI morphologies, ranging from heavily disturbed HI distributions with major asymmetries, long filaments, and/or HI-stellar offsets, to lopsided HI distributions with minor asymmetries. We quantify the outer HI asymmetry for both our sample and a control sample of typical dwarf irregulars. Starburst dwarfs have more asymmetric outer HI morphologies than typical irregulars, suggesting that some external mechanism triggered the starburst. Moreover, galaxies hosting an old burst (>100 Myr) have more symmetric HI morphologies than galaxies hosting a young one (<100 Myr), indicating that the former ones probably had enough time to regularize their outer HI distribution since the onset of the burst. We also investigate the nearby environment of these starburst dwarfs and find that most of them ($\sim$80$\%$) have at least one potential perturber at a projected distance <200 kpc. Our results suggest that the starburst is triggered either by past interactions/mergers between gas-rich dwarfs or by direct gas infall from the IGM.Comment: 21 pages, 8 figures, 6 tables, accepted for publication in MNRA

The Ursa Major Cluster of Galaxies. IV ; HI synthesis observations

In this data paper we present the results of an extensive 21cm-line synthesis imaging survey of 43 spiral galaxies in the nearby Ursa Major cluster using the Westerbork Synthesis Radio Telescope. Detailed kinematic information in the form of position-velocity diagrams and rotation curves is presented in an atlas together with HI channel maps, 21cm continuum maps, global HI profiles, radial HI surface density profiles, integrated HI column density maps, and HI velocity fields. The relation between the corrected global HI linewidth and the rotational velocities Vmax and Vflat as derived from the rotation curves is investigated. Inclination angles obtained from the optical axis ratios are compared to those derived from the inclined HI disks and the HI velocity fields. The galaxies were not selected on the basis of their HI content but solely on the basis of their cluster membership and inclination which should be suitable for a kinematic analysis. The observed galaxies provide a well-defined, volume limited and equidistant sample, useful to investigate in detail the statistical properties of the Tully-Fisher relation and the dark matter halos around them.Comment: 32 pages, including 2 sample pages of HI atlas. For full atlas (104 pages, 11 MB) see http://www.nrao.edu/library/preprints/00173.ps.gz . Accepted for publication in Astronomy and Astrophysic

Dynamics of Starbursting Dwarf Galaxies. III. A HI study of 18 nearby objects

We investigate the dynamics of starbursting dwarf galaxies, using both new and archival HI observations. We consider 18 nearby galaxies that have been resolved into single stars by HST observations, providing their star formation history and total stellar mass. We find that 9 objects have a regularly-rotating HI disk, 7 have a kinematically disturbed HI disk, and 2 show unsettled HI distributions. Two galaxies (NGC 5253 and UGC 6456) show a velocity gradient along the minor axis of the HI disk, that we interpret as strong radial motions. For galaxies with a regularly rotating disk we derive rotation curves, while for galaxies with a kinematically disturbed disk we estimate the rotation velocities in their outer parts. We derive baryonic fractions within about 3 optical scale lengths and find that, on average, baryons constitute at least 30$\%$ of the total mass. Despite the star formation having injected $\sim$10$^{56}$ ergs in the ISM in the last $\sim$500 Myr, these starbursting dwarfs have both baryonic and gas fractions similar to those of typical dwarf irregulars, suggesting that they did not eject a large amount of gas out of their potential wells.Comment: Published on A&A (23 pages, 9 tables, 12 figures, plus an optical-HI atlas). Typos fixe

WIYN Integral-Field Kinematics of Disk Galaxies

We have completed a new fiber array, SparsePak, optimized for low-surface-brightness studies of extended sources on the WIYN telescope. We are now using this array as a measuring engine of velocity and velocity-dispersion fields of stars and ionized gas in disk galaxies from high to low surface-brightness. Here we present commissioning data on the velocity ellipsoids, surface densities and mass-to-light ratios in two blue, high surface-brightness, yet small disks. If our preliminary results survive further observation and more sophisticated analysis, then NGC 3949 has sigma_z/sigma_R >> 1, implying strong vertical heating, while NGC 3982's disk is substantially sub-maximal. These galaxies are strikingly unlike the Milky Way, and yet would be seen more easily at high redshift.Comment: to appear in Disk Galaxies: Kinematics, Dynamics and Perturbations, eds. E. Athanassoula and A. Bosma, ASP Conference Series; 4 pages with 4 embedded figure

Hydrogen 21-Centimeter Emission from a Galaxy at Cosmological Distance

We have detected the neutral atomic hydrogen (HI) emission line at a cosmologically significant distance (z=0.18) in the rich galaxy cluster Abell 2218 with the Westerbork Synthesis Radio Telescope. The HI emission originates in a spiral galaxy 2.0 megaparsecs from the cluster core. No other significant detections have been made in the cluster, suggesting that the mechanisms that remove neutral gas from cluster galaxies are efficient. We infer that fewer than three gas-rich galaxies were accreted by Abell 2218 over the past 10^9 years. This low accretion rate is qualitatively consistent with low-density cosmological models in which clusters are largely assembled at z>1.Comment: 7 pages, 2 figures. Science 2001 September 7; 293, 1800 (in reports). Also available from http://www.sciencemag.org/cgi/content/abstract/293/5536/180

The latest on Apertif

We describe a Phased Array Feed (PAF) system, called Apertif, which will be installed in the Westerbork Synthesis Radio Telescope (WSRT). The aim of Apertif is, at frequencies from 1.0 to 1.7 GHz, to increase the instantaneous field of view of the WSRT 8 deg^2 and its observing bandwidth to 300 MHz with high spectral resolution. This system will turn the WSRT into an effective survey telescope with scientific applications ranging from deep surveys of the northern sky of HI and OH emission and polarised continuum to efficient searches for pulsars and transients. We present results obtained with a prototype PAF installed in one of the WSRT dishes. These results demonstrate that at decimetre wavelengths PAFs have excellent performance and that even for a single beam on the sky they outperform single feed radio dishes. PAFs turn radio telescopes into very effective survey instruments. Apertif is now fully funded and the community is invited to express their interest in using Apertif (http://www.astron.nl/radio-observatory/call-expressions-interest-apertif-surveys )Comment: Talk presented at 'A New Golden Age for Radio Astronomy', International SKA Forum 2010, 10-14 June 2010, Hof van Saksen, N

Growth and Destruction of Disks: Combined HI and HII View

How large disk galaxies have evolved in, and out of, the blue cloud of actively star-forming galaxies as a function of environment and time is an outstanding question. Some of the largest disks become systems like M31, M33 and the Milky Way today. In denser environments, it appears they transform onto the red sequence. Tracking disk systems since z<0.5 as a function HI mass, dynamical mass, and environment should be possible in the coming decade. HI and optical data combined can sample outer and inner disk dynamics to connect halo properties with regions of most intense star-formation, and the gas reservoir to the consumption rate. We describe existing and future IFUs on 4-10m telescopes that complement upcoming HI surveys for studying disks at z<0.5. Multiple units, deployable over large fields-of-view, and with logarithmic sampling will yield kinematic and star-formation maps and properties of the stellar populations, resolving the core but retaining sensitivity to disk outskirts.Comment: 6 pages, 3 figures; to appear in The Evolution of Galaxies through the Neutral Hydrogen Window, eds. R. Minchin and E. Momjian, AIPC, in pres

The Multi-Wavelength Tully-Fisher relation with spatially resolved HI kinematics

In this paper we investigate the statistical properties of the Tully-Fisher relation for a sample of 32 galaxies with measured distances from the Cepheid period-luminosity relation and/or TRGB stars. We take advantage of panchromatic photometry in 12 bands (from FUV to 4.5 $\mu$m) and of spatially resolved HI kinematics. We use these data together with three kinematic measures ($W^{i}_{50}$, $V_{max}$ and $V_{flat}$) extracted from the global HI profiles or HI rotation curves, so as to construct 36 correlations allowing us to select the one with the least scatter. We introduce a tightness parameter $\sigma_{\perp}$ of the TFr, in order to obtain a slope-independent measure of the goodness of fit. We find that the tightest correlation occurs when we select the 3.6 $\mu$m photometric band together with the $V_{flat}$ parameter extracted from the HI rotation curve.Comment: 16 pages, 16 figures, accepted for publication in MNRAS, minor changes due to proof correction

Evolution of dwarf galaxies:A dynamical perspective

For a rotating galaxy, the inner circular-velocity gradient dRV(0) provides a direct estimate of the central dynamical mass density, including gas, stars, and dark matter. We consider 60 low-mass galaxies with high-quality H I and/or stellar rotation curves (including starbursting dwarfs, irregulars, and spheroidals), and estimate dRV(0) as VRd/Rd, where Rd is the galaxy scale length. For gas-rich dwarfs, we find that VRd/Rd correlates with the central surface brightness μ0, the mean atomic gas surface density Σgas, and the star formation rate surface density ΣSFR. Starbursting galaxies, such as blue compact dwarfs (BCDs), generally have higher values of VRd/Rd than dwarf irregulars, suggesting that the starburst is closely related to the inner shape of the potential well. There are, however, some "compact" irregulars with values of VRd/Rd similar to BCDs. Unless a redistribution of mass takes place, BCDs must evolve into compact irregulars. Rotating spheroidals in the Virgo cluster follow the same correlation between VRd/Rd and μ0 as gas-rich dwarfs. They have values of VRd/Rd comparable to those of BCDs and compact irregulars, pointing to evolutionary links between these types of dwarfs. Finally, we find that, as for spiral galaxies and massive starbursts, the star-formation activity in dwarfs can be parametrized as ΣSFR = ɛ Σgas/τorb, where τorb is the orbital time and ɛ ≃ 0.02. Appendices are available in electronic form at http://www.aanda.or