Jacqueline H. van Gorkom

Minkowski's Object : A starburst triggered by a radio jet, revisited

We present neutral hydrogen, ultraviolet, optical, and near-infrared imaging, and optical spectroscopy, of Minkowskis Object (MO), a star-forming peculiar galaxy near NGC 541. The observations strengthen evidence that star formation in MO was triggered by the radio jet from NGC 541. Key new results are the discovery of a 4.9 × 10^8 M_⊙ double H I cloud straddling the radio jet downstream from MO, where the jet changes direction and decollimates; strong detections of MO, also showing double structure, in UV and Hα; and numerous H II regions and associated clusters in MO. In UV, MO resembles the radio-aligned, rest-frame UV morphologies in many high-redshift radio galaxies (HzRGs), also thought to be caused by jet-induced star formation. MOs stellar population is dominated by a 7.5 Myr old, 1.9 × 10^7 M_⊙ instantaneous burst, with a current star formation rate of 0.52 M_⊙ yr^(-1) (concentrated upstream from where the H I column density is high). This is unlike the jet-induced star formation in Centaurus A, where the jet interacts with preexisting cold gas; in MO, the H I may have cooled out of a warmer, clumpy intergalactic or interstellar medium as a result of jet interaction, followed by the collapse of the cooling clouds and subsequent star formation (consistent with numerical simulations). Since the radio source that triggered star formation in MO is much less luminous, and therefore more common than powerful HzRGs, and because the environment around MO is not particularly special in terms of abundant dense, cold gas, jet-induced star formation in the early universe might be even more prevalent than previously thought.

THE ZURICH ENVIRONMENTAL STUDY OF GALAXIES IN GROUPS ALONG THE COSMIC WEB. I. WHICH ENVIRONMENT AFFECTS GALAXY EVOLUTION

The Zurich Environmental Study (ZENS) is based on a sample of ~1500 galaxy members of 141 groups in the mass range ~1012.5-14.5 M ☉ within the narrow redshift range 0.05 1010 M ☉, satellite galaxies in relaxed and unrelaxed groups have similar size, color, and (specific) star formation rate distributions; at lower galaxy masses, satellites are marginally redder in relaxed relative to unrelaxed groups, suggesting quenching of star formation in low-mass satellites by physical processes active in relaxed halos. Overall, relaxed and unrelaxed groups show similar stellar mass populations, likely indicating similar stellar mass conversion efficiencies. In the enclosed ZENS catalog, we publish all environmental diagnostics as well as the galaxy structural and photometric measurements described in companion ZENS papers II and III.

A search for H I in E + A galaxies

We present the results of H I line and radio continuum observations of five nearby E+A galaxies. These galaxies have spectra that are dominated by a young stellar component but lack the emission lines characteristic of significant ongoing star formation. They are selected from a unique sample of 21 E+As identified by Zabludoff et al. in their spectroscopic search (published in 1996) for E+A galaxies using the Las Campanas Redshift Survey, in which over 11,000 nearby galaxies were examined. The five E+A galaxies span a range of environments: three are in the field, and two are in clusters. Only one system was detected in H I emission, the field E+A galaxy EA 1, with a total flux of 0.30 ± 0.02 Jy km s-1 and an H I mass of 3.5 ± 0.2 × 109 h-2 M⊙. The H I morphology and kinematics of EA 1 suggest a galaxy-galaxy interaction, with a dynamical age of ~6 × 108 h-1 yr inferred from the H I tail lengths and velocities. This age estimate is consistent with the interpretation drawn from optical spectroscopy that starbursts in E+A galaxies began (and subsequently ended) within the last 109 yr. Our H I detection limits are such that if the other E+As in our sample had the H I properties of EA 1, we would have detected (or marginally detected) them. We conclude that E+A galaxies have a range of H I properties. None of the galaxies were detected in radio continuum emission, with upper limits to the radio power of ~1021 h-2 W Hz-1. Our limits exclude the possibility that these E+As are dust-enshrouded massive starburst galaxies, but they are insufficient to exclude modest star formation rates of less than a few h-2 M⊙ yr-1.

The neutral hydrogen ring, mass-to-light ratio, and dark halo of the elliptical galaxy IC 2006

Observations of the E1 galaxy IC 2006 are presented, and the imaging, photometric, and spectroscopic data are analyzed. The galaxy is found to be in every respect normal for an elliptical galaxy of its luminosity. It is surrounded by an unusually regular ring of 18.9 kps radius, containing (4.8 + or - 0.8) x 10 to the 8th solar masses of H I that rotates in the opposite sense of the stars near the center. The integrated M/L(B) within the radius of the ring indicates the presence of a halo of dark matter with about twice the mass of the luminous matter. Very weak emission lines reveal a central mass of ionized gas extending to about 5 kpc radius. This gas also rotates counter to the stars and is highly turbulent. The dwarf companion Karachentseva 22 lies at a projected distance of 3.1 ring radii and has one-third as much H I as the ring, but is unlikely to be the donor of the ring H I. 73 references.

Intergalactic and Extended Atomic Gas

Observational results are discussed on the properties of the diffuse atomic gas in the outer parts of galaxies and between galaxies. In the local universe, no H I in emission is found at column densities less than about 1019 cm-2, possibly because the lower column density gas is mostly ionized. The presence of large gas reservoirs around galaxies strongly depends on the environment that they are in. Galaxies with low-surfacebrightness, huge H I disks are found in very low density environments. In small groups, merging and interacting galaxies are often surrounded by large H I envelopes and many gas-rich dwarfs. Results of unbiased searches directly in the H I line are presented, both at z = 0 and z = 3.3. At low redshift, H I rich dwarf galaxies follow the structure as defined by the bright galaxies. The data leave no room for a substantial population of H I dwarfs or gas-rich, low surface brightness giants that could have been missed by optical catalogs. Few, if any, large neutral structures seem to have survived at redshifts z ≈ 3.3, although a tentative detection of a possible pancake is reported. H I imaging results of quasar — galaxy pairs shows that at least the nearby metal absorbers arise in the outskirts of tidally disturbed galaxies. A sensitive search for H I emission from the vicinity of nearby Lya clouds failed to detect any H I.

The Effect of Cluster Environment on Galaxy Evolution in the Pegasus I Cluster

We present neutral hydrogen observations of 54 galaxies in the Pegasus Cluster. The observations include single-dish H I measurements, obtained with the Arecibo telescope for all 54 galaxies in the sample, as well as H I images obtained with the Very Large Array (VLA) for 10 of these. The Arecibo profiles reveal an overall H I deficiency in the cluster, with ~40% of the galaxies in the core of the cluster showing modest deficiencies of typically a factor of 2-3. The H I morphology of some galaxies shows that the H I disk is smaller than the optical disk and slightly offset from the stars. We find a correlation between H I deficiency and the ratio of the H I disk size to optical disk size. More H I-deficient galaxies have relatively smaller H I disks, a configuration that is usually attributed to an interaction between the interstellar medium (ISM) of the galaxy and the hot intracluster medium (ICM). Such a result is surprising, since the Pegasus Cluster has a low level of X-ray emission and a low velocity dispersion. The low velocity dispersion, coupled with the lack of a dense hot ICM, indicate that ram pressure stripping should not play a significant role in this environment. In addition, two of the galaxies, NGC 7604 and NGC 7648, are morphologically peculiar. Their peculiarities indicate contradictory scenarios of what is triggering their unusual star formation. Hα imaging, along with long-slit spectroscopy of NGC 7648, reveal morphological features that point to a recent tidal interaction. On the other hand, Hα imaging of NGC 7604 reveals a strong episode of star formation concentrated into an asymmetric arc, preferentially located on one side of the galaxy. VLA H I mapping shows that H I is also highly concentrated into that region, suggestive of a ram pressure event. Our data indicate that ISM-ICM interactions may play a role in a wider variety of environments than suggested by simple ram pressure arguments.

BUDHIES I: characterizing the environments in and around two clusters at z⋍0.2

We present the optical spectroscopy for the Blind Ultra Deep H I Environmental Survey (BUD-HIES). With the Westerbork Synthesis Radio Telescope, BUDHIES has detected H I in over 150 galaxies in and around two Abell clusters at z similar or equal to 0.2. With the aim of characterizing the environments of the H I-detected galaxies, we obtained multifibre spectroscopy with the William Herschel Telescope. In this paper, we describe the spectroscopic observations, report redshifts and EW[O II] measurements for similar to 600 galaxies, and perform an environmental analysis. In particular, we present cluster velocity dispersion measurements for five clusters and groups in the BUDHIES volume, as well as a detailed substructure analysis.

Early Science with the Large Millimeter Telescope: COOL BUDHIES I - a pilot study of molecular and atomic gas at z ≃ 0.2

An understanding of the mass build-up in galaxies over time necessitates tracing the evolution of cold gas (molecular and atomic) in galaxies. To that end, we have conducted a pilot study called CO Observations with the LMT of the Blind Ultra-Deep H I Environment Survey (COOL BUDHIES). We have observed 23 galaxies in and around the two clusters Abell 2192 (z = 0.188) and Abell 963 (z = 0.206), where 12 are cluster members and 11 are slightly in the foreground or background, using about 28 total hours on the Redshift Search Receiver on the Large Millimeter Telescope (LMT) to measure the 12CO J = 1 → 0 emission line and obtain molecular gas masses. These new observations provide a unique opportunity to probe both the molecular and atomic components of galaxies as a function of environment beyond the local Universe. For our sample of 23 galaxies, nine have reliable detections (S/N ≥ 3.6) of the 12CO line, and another six have marginal detections (2.0 CO line, and another six have marginal detections (2.0 CO line, and another six have marginal detections (2.0 9 and 1010 M⊙. Comparing our results to other studies of molecular gas, we find that our sample is significantly more abundant in molecular gas overall, when compared to the stellar and the atomic gas component, and our median molecular gas fraction lies about 1σ above the upper limits of proposed redshift evolution in earlier studies. We discuss possible reasons for this discrepancy, with the most likely conclusion being target selection and Eddington bias.

The effect of ram pressure on the molecular gas of galaxies: three case studies in the Virgo cluster

We present 12CO (2-1) data of three Virgo spirals - NGC 4330, NGC 4402 and NGC 4522 obtained using the Submillimeter Array. These three galaxies show clear evidence of ram pressure stripping due to the cluster medium as found in previous HI imaging studies. Using high-resolution CO data, we investigate how the properties of the inner molecular gas disc change while a galaxy is undergoing HI stripping in the cluster. At given sensitivity limits, we do not find any clear signs of molecular gas stripping. However, both its morphology and kinematics appear to be quite disturbed as those of HI. Morphological peculiarities present in the molecular and atomic gas are closely related with each other, suggesting that molecular gas can be also affected by strong ICM pressure even if it is not stripped. CO is found to be modestly enhanced along the upstream sides in these galaxies, which may change the local star formation activity in the disc. Indeed, the distribution of H

H I Imaging of Clusters

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