We present high-quality aperture synthesis observations of the neutral hydrogen distribution in a sample of five galactic systems believed to be involved in progressive stages of merging: Arp 295, NGC 4676, NGC 520, NGC 3921, and NGC 7252. This is the first time that the atomic hydrogen in such a broad range of disk mergers has been imaged. These data are supplemented by wide-field images taken through a narrow band Hα filter, and by deep (µ R ∼ > 26.5 mag arcsec −2 ) R-band surface photometry.We identify several trends along the merging sequence. In the early stages, large amounts of H i still exist within the galactic disks and star formation is widespread. The ionized gas emission often takes on the appearance of plumes and arcs emanating from the nuclear regions, which are presumably the sites of interaction-induced starbursts. In the final stages there is little if any H i within the remnant bodies, and tidal material is seen moving inward. We conclude that as the merger rearranges the light profiles of the progenitor disk galaxies into r 1/4 profiles, it leads to an efficient conversion of the atomic gas into other forms within the main bodies of the merger remnants. This suggests that these remnants will evolve into elliptical galaxies in their atomic gas contents as well as their photometric properties. However the observations of NGC 520 reveal an extensive rotating gaseous disk, suggesting that perhaps some mergers will not destroy the atomic gas disks of the progenitors.Although the observed tidal features contribute only of order 10% of a system's optical luminosity, they contain the majority of the neutral hydrogen. The morphological similarity between the gaseous and stellar tails and the smooth gas kinematics confirm that gravity plays the dominant role in producing them. There are, however, some striking differences between the faint gas and light distributions. H i mapping often reveals gaseous extension not at all apparent optically, and tidal features of different optical morphologies have different gas characteristics: the edge-brightened tails are gas-rich, while the featureless plumes and halos are gas-poor. Some of these features may be explained by the different velocity dispersions of the gas and stars, some by different gas contents in the progenitors, and some remain unexplained. Overall, large quantities of both gas and starlight (M HI > 10 9 h −2 M ⊙ , L R > 10 9 h −2 L ⊙ ) are seen at large radii (r >50 h −1 kpc). Since this material evolves on very long time scales, it may leave observable signatures for many Gyr.

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations from the 2014 Long Baseline Campaign in dust continuum and spectral line emission from the HL Tau region. The continuum images at wavelengths of 2.9, 1.3, and 0.87 mm have unprecedented angular resolutions of 0″. 075 (10 AU) to 0″. 025 (3.5 AU), revealing an astonishing level of detail in the circumstellar disk surrounding the young solar analog HL Tau, with a pattern of bright and dark rings observed at all wavelengths. By fitting ellipses to the most distinct rings, we measure precise values for the disk inclination (46 .72 0 .05 ± • •) and position angle (138 .02 0 .07).

Motivated by recent neutral hydrogen observations with the VLA, we have undertaken an investigation into the interaction that produced the well known merger remnant NGC 7252. Through fully self-consistent N-body simulations, we are able to reproduce the kinematic character of the HI observations quite well, including the velocity reversals observed along each tidal tail. In the simulation these reversals arise from particles which have turned around in their orbit and are moving to smaller radii. The bases of the tails fall back quickly to small pericentric distances, while the more distant regions fall back more slowly to ever increasing pericentric distances. The delayed return of tidally ejected material may extend over many Gyr. The evolution of this merger is followed numerically for 800 h^-1 Myr beyond the best fit time. We find that nearly half of the present tail material, or of order 10^9 h^-2 of neutral hydrogen and 2x10^9 h^-2 of starlight, will return to within 13 h^-1 kpc of the nucleus within this time span. While the collisionless stars of the tails will continue orbiting between their inner and outer turning points, the observations show the HI gas of the tails disappearing upon its return. We discuss this result in light of the lack of central HI in the main body of this merger remnant.Comment: 28 pages of uuencoded, compressed postscript. Accepted to AJ. 9 Postscript figures available upon request from hos@lick.ucsc.edu; postscript text also available from http://ucowww.ucsc.edu/~hos/home.htm

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A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to ∼15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from 2014 September to late November, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C 138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at ∼350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy.