The 2X-HI disks of spiral galaxies

The outskirts of galaxies - especially the very extended HI disks of galaxies - are strongly affected by their local environment. I highlight the giant 2X-HI disks of nearby galaxies (M 83, NGC 3621, and NGC 1512), studied as part of the Local Volume HI Survey (LVHIS), their kinematics and relation to XUV disks, signatures of tidal interactions and accretion events, the MHI - DHI relation as well as the formation of tidal dwarf galaxies. - Using multi-wavelength data, I create 3D visualisations of the gas and stars in galaxies, with the shape of their warped disks obtained through kinematic modelling of their HI velocity fields.Comment: 3 pages, incl. one page of colour figures; Proceedings of the IAU Symposium No. 321, 2016, Formation and Evolution of Galaxy Outskirts, eds. A. Gil de Paz, J. Knapen, & J. Le

ATCA HI Observations of the NGC 6845 Galaxy Group

We present the results of ATCA HI line and 20-cm radio continuum observations of the galaxy Quartet NGC 6845. The HI emission extends over all four galaxies but can only be associated clearly with the two spiral galaxies, NGC 6845A and B, which show signs of strong tidal interaction. We derive a total HI mass of at least 1.8 x 10^10 Msun, most of which is associated with NGC 6845A, the largest galaxy of the group. We investigate the tidal interaction between NGC 6845A and B by studying the kinematics of distinct HI components and their relation to the known HII regions. No HI emission is detected from the two lenticular galaxies, NGC 6845C and D. A previously uncatalogued dwarf galaxy, ATCA J2001-4659, was detected 4.4' NE from NGC 6845B and has an HI mass of about 5 x 10^8 Msun. No HI bridge is visible between the group and its newly detected companion. Extended 20-cm radio continuum emission is detected in NGC 6845A and B as well as in the tidal bridge between the two galaxies. We derive star formation rates of 15 to 40 Msun/yr.Comment: accepted, to be published in MNRAS, for more details and the full ps-file see http://www.atnf.csiro.au/people/bkoribal/ngc684

Discovery of Pulsed OH Maser Emission Stimulated by a Pulsar

Stimulated emission of radiation has not been directly observed in astrophysical situations up to this time. Here we demonstrate that photons from pulsar B1641-45 stimulate pulses of excess 1720 MHz line emission in an interstellar OH cloud. As this stimulated emission is driven by the pulsar, it varies on a few millisecond timescale, orders of magnitude shorter than the quickest OH maser variations previously detected. Our 1612 MHz spectra are inverted copies of the 1720 MHz spectra. This "conjugate line" phenomenon enables us to constrain the properties of the interstellar OH line-producing gas. We also show that pulsar signals suffer significantly deeper OH absorption than do other background sources; confirming earlier tentative findings that OH clouds are clumpier on small scales than neutral hydrogen clouds.Comment: Accepted by Scienc

The Minimum Amount of Stars a Galaxy Will Form

We present an analysis of the atomic hydrogen and stellar properties of 38 late-type galaxies in the local Universe covering a wide range of HI mass-to-light ratios (M_HI/L_B), stellar luminosities, and surface brightnesses. From these data we have identified an upper envelope for the M_HI/L_B as a function of galaxy luminosity. This implies an empirical relation between the minimum amount of stars a galaxy will form and its initial baryonic mass. While the stellar mass of a galaxy seems to be only loosely connected to its baryonic mass, the latter quantity is strongly linked to the galaxy's dynamical mass as it is observed in the baryonic Tully-Fisher relation. We find that dwarf irregular galaxies with generally high M_HI/L_B-ratios follow the same trend as defined by lower M_HI/L_B giant galaxies, but are underluminous for their rotation velocity to follow the trend in a stellar mass Tully-Fisher relation, suggesting that the baryonic mass of the dwarf galaxies is normal but they have failed to produced a sufficient amount of stars. Finally, we present a three dimensional equivalent to the morphology-density relation which shows that high M_HI/L_B galaxies preferentially evolve and/or survive in low-density environments. We conclude that an isolated galaxy with a shallow dark matter potential can retain a large portion of its baryonic matter in the form of gas, only producing a minimum quantity of stars necessary to maintain a stable gas disk.Comment: 37 Pages, 7 Figures. Accepted for publication in A