arXiv subjects

J. H. van Gorkom

Publications and source records attributed to J. H. van Gorkom.

At least 37 records · Page 2Linked to original sources

arXiv2016

The Void Galaxy Survey: Star Formation Properties

We study the star formation properties of 59 void galaxies as part of the Void Galaxy Survey (VGS). Current star formation rates are derived from $\rm{Hα}$ and recent star formation rates from near-UV imaging. In addition, infrared 3.4 $\rm{μm}$, 4.6 $\rm{μm}$, 12 $\rm{μm}$ and 22 $\rm{μm}$ WISE emission is used as star formation and mass indicator. Infrared and optical colours show that the VGS sample displays a wide range of dust and metallicity properties. We combine these measurements with stellar and HI masses to measure the specific SFRs ($\rm{SFR/M_{*}}$) and star formation efficiencies ($\rm{SFR/M_{HI}}$). We compare the star formation properties of our sample with galaxies in the more moderate density regions of the cosmic web, 'the field'. We find that specific SFRs of the VGS galaxies as a function of stellar and HI mass are similar to those of the galaxies in these field regions. Their $\rm{SFRα}$ is slightly elevated than the galaxies in the field for a given total HI mass. In the global star formation picture presented by Kennicutt-Schmidt, VGS galaxies fall into the regime of low average star formation and correspondingly low HI surface density. Their mean $\rm{SFRα/M_{HI}}$ and $\rm{SFRα/M_{*}}$ are of the order of $\rm{10^{-9.9}}$ $\rm{yr^{-1}}$. We conclude that while the large scale underdense environment must play some role in galaxy formation and growth through accretion, we find that even with respect to other galaxies in the more mildly underdense regions, the increase in star formation rate is only marginal.

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arXiv2015

The Zurich Environmental Study (ZENS) of Galaxies in Groups along the Cosmic Web. V. Properties and Frequency of Merging Satellites and Centrals in Different Environments

We use the Zurich ENvironmental Study (ZENS) database to investigate the environmental dependence of the merger fraction $Γ$ and merging galaxy properties in a sample of ~1300 group galaxies with $M>10^{9.2}M_\odot$ and 0.05 10^{13.5} M_\odot$ relative to less massive systems, indicating a suppression of merger activity in large potential wells. In the fiducial case of relaxed groups only, we measure a variation $ΔΓ/Δ\log (M_{HALO}) \sim - 0.07$ dex$^{-1}$, which is almost independent of galaxy mass and merger stage. At galaxy masses $>10^{10.2} M_\odot$, most mergers are dry accretions of quenched satellites onto quenched centrals, leading to a strong increase of $Γ$ with decreasing group-centric distance at these mass scales.Both satellite and central galaxies in these high mass mergers do not differ in color and structural properties from a control sample of nonmerging galaxies of equal mass and rank. At galaxy masses $<10^{10.2} M_\odot$, where we mostly probe satellite-satellite pairs and mergers between star-forming systems, close pairs (projected distance $<10-20$ kpc) show instead $\sim2\times$ enhanced (specific) star formation rates and $\sim1.5\times$ larger sizes than similar mass, nonmerging satellites. The increase in both size and SFR leads to similar surface star-formation densities in the merging and control-sample satellite populations.

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arXiv2015

The Void Galaxy Survey: Morphology and Star Formation Properties of Void Galaxies

We present the structural and star formation properties of 59 void galaxies as part of the Void Galaxy Survey (VGS). Our aim is to study in detail the physical properties of these void galaxies and study the effect of the void environment on galaxy properties. We use Spitzer 3.6 $\rm{μm}$ and B-band imaging to study the morphology and color of the VGS galaxies. For their star formation properties, we use Halpha and GALEX near-UV imaging. We compare our results to a range of galaxies of different morphologies in higher density environments. We find that the VGS galaxies are in general disk dominated and star forming galaxies. Their star formation rates are, however, often less than 1 $\rm{M_{\odot}}$ $\rm{yr^{-1}}$. There are two early-type galaxies in our sample as well. In $\rm{r_{e}}$ versus $\rm{M_{B}}$ parameter space, VGS galaxies occupy the same space as dwarf irregulars and spirals.

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arXiv2014

Discovery of a Small Central Disk of CO and HI in the Merger Remnant NGC 34

We present CO(1-0) and HI(21-cm) observations of the central region of the wet merger remnant NGC 34. The Combined Array for Research in Millimeter-wave Astronomy (CARMA) observations detect a regularly rotating disk in CO with a diameter of 2.1 kpc and a total molecular hydrogen mass of ($2.1 \pm 0.2) \times10^9~M_\odot$. The rotation curve of this gas disk rises steeply, reaching maximum velocities at 1" (410 pc) from the center. Interestingly, HI observations done with the Karl G. Jansky Very Large Array show that the absorption against the central continuum has the exact same velocity range as the CO in emission. This strongly suggests that the absorbing HI also lies within 1" from the center, is mixed in and corotates with the molecular gas. A comparison of HI absorption profiles taken at different resolutions (5"-45") shows that the spectra at lower resolutions are less deep at the systemic velocity. This provides evidence for HI emission in the larger beams, covering the region from 1 kpc to 9 kpc from the center. The central rapidly rotating disk was likely formed either during the merger or from fall-back material. Lastly, the radio continuum flux of the central source at mm wavelengths ($5.4\pm1.8$ mJy) is significantly higher than expected from an extrapolation of the synchrotron spectrum, indicating the contribution of thermal free-free emission from the central starburst.

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arXiv2013

The Zurich Environmental Study of Galaxies in Groups along the Cosmic Web. III. Galaxy Photometric Measurements and the Spatially-Resolved Color Properties of Early- and Late-Type Satellites in Diverse Environments

We present photometric measurements for the galaxies - and when possible their bulges and disks - in the 0.05 10^10 Msun, these satellites have shallower color gradients in higher mass groups and in the cores of groups compared with lower mass groups and the outskirts of groups. Stellar population analyses and semi-analytic models suggest that disk-dominated satellites undergo quenching of star formation in their outer disks, on timescales ~2 Gyr, as they progressively move inside the group potential.

astro-ph.CO↗

arXiv2013

The Zurich Environmental Study (ZENS) of Galaxies in Groups along the Cosmic Web. II. Galaxy Structural Measurements and the Concentration of Morphologically Classified Satellites in Diverse Environments

We present structural measurements for the galaxies in the 0.05 10^10 M_sun, the concentration of disk satellites increases with increasing stellar mass, separately within each morphological bin of B/T. The known increase in concentration with stellar mass for disk satellites is thus due, at least in part, to an increase in galaxy central stellar density at constant B/T. The correlation between concentration and galaxy stellar mass becomes progressively steeper for later morphological types. The concentration of disk satellites shows a barely significant dependence on delta_LSS or R/R_200. The strongest environmental effect is found with group mass for M>10^10 M_sun disk-dominated satellites, which are ~10% more concentrated in high mass groups than in lower mass groups.

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arXiv2013

A Pilot for a VLA HI Deep Field

High-resolution 21-cm HI deep fields provide spatially and kinematically resolved neutral gas maps at different redshifts, which are key to understanding galaxy evolution across cosmic time and testing predictions of cosmological simulations. Here we present results from a pilot for the COSMOS HI Large Extragalactic Survey (CHILES) done with the Karl G. Jansky Very Large Array (VLA). We take advantage of the newly expanded capabilities of the telescope to probe the redshift interval 0<z<0.193 in one observation. We observe the COSMOS field for 50 hours, which contains 413 galaxies with optical spectroscopic redshifts in the imaged field of view of 34' x 34' and the observed redshift interval. We have detected neutral hydrogen gas in 33 galaxies in different environments spanning the probed redshift range, including three without a previously known spectroscopic redshift. The detections have a range of HI and stellar masses, indicating the diversity of galaxies we are probing. We discuss the observations, data reduction, results and highlight interesting detections. We find that the VLA's B-array is the ideal configuration for HI deep fields since its long spacings mitigate RFI. This pilot shows that the VLA is ready to carry out such a survey, and serves as a test for future HI deep fields planned with other SKA pathfinders.

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arXiv2013

An Interacting Galaxy System Along a Filament in a Void

Cosmological voids provide a unique environment for the study of galaxy formation and evolution. The galaxy population in their interior have significantly different properties than average field galaxies. As part of our Void Galaxy Survey (VGS), we have found a system of three interacting galaxies (VGS_31) inside a large void. VGS_31 is a small elongated group whose members are embedded in a common HI envelope. The HI picture suggests a filamentary structure with accretion of intergalactic cold gas from the filament onto the galaxies. We present deep optical and narrow band H_alpha data, optical spectroscopy, near-UV and far-UV GALEX and CO(1-0) data. We find that one of the galaxies, a Markarian object, has a ring-like structure and a tail evident both in optical and HI. While all three galaxies form stars in their central parts, the tail and the ring of the Markarian object are devoid of star formation. We discuss these findings in terms of a gravitational interaction and ongoing growth of galaxies out of a filament. VGS_31 is one of the first observed examples of a filamentary structure in a void. It is an important prototype for understanding the formation of substructure in a void. This system also shows that the galaxy evolution in voids can be as dynamic as in high density environments.

astro-ph.CO↗

arXiv2012

The Void Galaxy Survey: Optical Properties and H I Morphology and Kinematics

We have carefully selected a sample of 60 galaxies that reside in the deepest underdensities of geometrically identified voids within the SDSS. HI imaging of 55 galaxies with the WSRT reveals morphological and kinematic signatures of ongoing interactions and gas accretion. We probe a total volume of 485 Mpc^3 within the voids, with an angular resolution of 8 kpc at an average distance of 85 Mpc. We reach column density sensitivities of 5 x 10^19 cm^-2, corresponding to an HI mass limit of 3 x 10^8 M_sun. We detect HI in 41 galaxies, with total masses ranging from 1.7 x 10^8 to 5.5 x 10^9 M_sun. The upper limits on the 14 non-detections are not inconsistent with their luminosities, given their expected HI mass to light ratios. We find that the void galaxies are generally gas rich, low luminosity, blue disk galaxies, with optical and HI properties that are not unusual for their luminosity and morphology. The sample spans a range of absolute magnitudes (-16.1 > M_r > -20.4) and colors (0.06 < g-r < 0.87), and includes disk and irregular galaxies. We also identify three as early type galaxies, all of which are not detected in HI. All galaxies have stellar masses less than 3 x 10^10 M_sun, and many have kinematic and morphological signs of ongoing gas accretion, suggesting that the void galaxy population is still in the process of assembling. The small scale clustering in the void, within 600 kpc and 200 km/s, is similar to that in higher density regions, and we identify 18 HI rich neighboring galaxies in the voids. Most are within 100 kpc and 100 km/s of the targeted galaxy, and we find no significant population of HI rich low luminosity galaxies filling the voids, contrary to what is predicted by simulations.

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arXiv2012

The Hi Environment of Counter-rotating Gas Hosts: Gas Accretion from Cold Gas Blobs

We probe the HI properties and the gas environments of three early-type barred galaxies harbouring counter-rotating ionized gas, NGC 128, NGC 3203 and NGC 7332. Each system has one or more optically-identified galaxy, at a similar or as yet unknown redshift within a 50 kpc projected radius. Using HI synthesis imaging data, we investigate the hypothesis that the counter-rotating gas in these galaxies has been accreted from their neighbours. In NGC 128 and NGC 3203, we find 9.6e7 and 2.3e8 Msun of HI, respectively, covering almost the entire stellar bodies of dwarf companions that appear physically connected. Both the HI morphology and kinematics are suggestive of tidal interactions. In NGC 7332, we do not find any directly-associated HI. Instead, NGC 7339, a neighbour of a comparable size at about 10 kpc, is found with 8.9e8 Msun of HI gas. More recently in a single dish observation, however, another group discovered a large HI structure which seems to be an extension of NGC 7339's HI disc and also covers NGC 7332. All these observations thus suggest that HI gas is being accreted in these three galaxies from their companions, which is likely responsible for the kinematically-decoupled gas component present in their central region. Considering the incompleteness of existing studies of the faint dwarf galaxy population both in the optical and in HI, accretion from cold gas blobs, presumably gas-rich dwarfs, is expected to occur even more frequently than what is inferred from such cases that have been observed to date.

astro-ph.CO↗

arXiv2012

Hot and Cold Galactic Gas in the NGC 2563 Galaxy Group

The role of environmentally induced gas stripping in driving galaxy evolution in groups remains poorly understood. Here we present extensive Chandra and Very Large Array mosaic observations of the hot and cold interstellar medium within the members of the nearby, X-ray bright NGC 2563 group, a prime target for studies of the role of gas stripping and interactions in relatively small host halos. Our observations cover nearly all group members within a projected radius of 1.15 Mpc (~1.4 R_vir) of the group center, down to a limiting X-ray luminosity and HI mass of 3e39 erg/s and 2e8 M_sun, respectively. The X-ray data are consistent with efficient ram pressure stripping of the hot gas halos of early-type galaxies near the group core, but no X-ray tails are seen and the limited statistics preclude strong conclusions. The HI results suggest moderate HI mass loss from the group members when compared to similar field galaxies. Six of the 20 HI-detected group members show HI evidence of ongoing interactions with other galaxies or with the intragroup medium. Suggestive evidence is further seen for galaxies with close neighbors in position-velocity space to show relatively low HI content, consistent with tidal removal of HI. The results thus indicate removal of both hot and cold gas from the group members via a combination of ram pressure stripping and tidal interactions. We also find that 16 of the 20 HI detections occur on one side of the group, reflecting an unusual morphological segregation whose origin remains unclear.

astro-ph.CO↗

arXiv2011

Ram pressure stripping of the multiphase ISM and star formation in the Virgo spiral galaxy NGC 4330

It has been shown that the Virgo spiral galaxy NGC 4330 shows signs of ongoing ram pressure stripping in multiple wavelengths: at the leading edge of the interaction, the Halpha and dust extinction curve sharply out of the disk; on the trailing side, a long Halpha/UV tail has been found which is located upwind of a long HI tail. We complete the multiwavelength study with IRAM 30m HERA CO(2-1) and VLA 6 cm radio continuum observations of NGC 4330. The data are interpreted with the help of a dynamical model including ram pressure and, for the first time, star formation. Our best-fit model reproduces qualitatively the observed projected position, radial velocity of the galaxy, the molecular and atomic gas distribution and velocity field, and the UV distribution in the region where a gas tail is present. However, the observed red UV color on the windward side is currently not reproduced by the model. Based on our model, the galaxy moves to the north and still approaches the cluster center with the closest approach occurring in ~100 Myr. In contrast to other Virgo spiral galaxies affected by ram pressure stripping, NGC 4330 does not show an asymmetric ridge of polarized radio continuum emission. We suggest that this is due to the relatively slow compression of the ISM and the particular projection of NGC 4330. The observed offset between the HI and UV tails is well reproduced by our model. Since collapsing and starforming gas clouds decouple from the ram pressure wind, the UV-emitting young stars have the angular momentum of the gas at the time of their creation. On the other hand, the gas is constantly pushed by ram pressure. The reaction (phase change, star formation) of the multiphase ISM (molecular, atomic, ionized) to ram pressure is discussed in the framework of our dynamical model.

astro-ph.CO↗

arXiv2011

KK246, a dwarf galaxy with extended H I disk in the Local Void

We have found that KK 246, the only confirmed galaxy located within the nearby Tully Void, is a dwarf galaxy with an extremely extended H I disk and signs of an H I cloud with anomalous velocity. It also exhibits clear misalignment between the kinematical major and minor axes, indicative of an oval distortion, and a general misalignment between the H I and optical major axes. We measure a H I mass of 1.05 +- 0.08 x 10^8 M_sun, and a H I extent 5 times that of the stellar disk, one of the most extended H I disks known. We estimate a dynamical mass of 4.1 x 10^9 M_sun, making this also one of the darkest galaxies known, with a mass-to-light ratio of 89. The relative isolation and extreme underdense environment make this an interesting case for examining the role of gas accretion in galaxy evolution.

astro-ph.CO↗

arXiv2011

Caught in the Act: Strong, Active Ram Pressure Stripping in Virgo Cluster Spiral NGC 4330

We present a multi-wavelength study of NGC 4330, a highly-inclined spiral galaxy in the Virgo Cluster which is a clear example of strong, ongoing ICM-ISM ram pressure stripping. The HI has been removed from well within the undisturbed old stellar disk, to 50% - 65% of R_25. Multi-wavelength data (WIYN BVR and H-alpha, VLA 21-cm HI and radio continuum, and GALEX NUV and FUV) reveal several one-sided extraplanar features likely caused by ram pressure at an intermediate disk-wind angle. At the leading edge of the interaction, the H-alpha and dust extinction curve sharply out of the disk in a remarkable and distinctive "upturn" feature that may be generally useful as a diagnostic indicator of active ram pressure. On the trailing side, the ISM is stretched out in a long tail which contains 10% of the galaxy's total HI emission, 6 - 9% of its NUV-FUV emission, but only 2% of the H-alpha. The centroid of the HI tail is downwind of the UV/H-alpha tail, suggesting that the ICM wind has shifted most of the ISM downwind over the course of the past 10 - 300 Myr. Along the major axis, the disk is highly asymmetric in the UV, but more symmetric in H-alpha and HI, also implying recent changes in the distributions of gas and star formation. The UV-optical colors indicate very different star formation histories for the leading and trailing sides of the galaxy. On the leading side, a strong gradient in the UV-optical colors of the gas-stripped disk suggests that it has taken 200-400 Myr to strip the gas from a radius of >8 to 5 kpc, but on the trailing side there is no age gradient. All our data suggest a scenario in which NGC 4330 is falling into cluster center for first time and has experienced a significant increase in ram pressure over the last 200-400 Myr.

astro-ph.CO↗

arXiv2011

The Void Galaxy Survey

The Void Galaxy Survey (VGS) is a multi-wavelength program to study $\sim$60 void galaxies. Each has been selected from the deepest interior regions of identified voids in the SDSS redshift survey on the basis of a unique geometric technique, with no a prior selection of intrinsic properties of the void galaxies. The project intends to study in detail the gas content, star formation history and stellar content, as well as kinematics and dynamics of void galaxies and their companions in a broad sample of void environments. It involves the HI imaging of the gas distribution in each of the VGS galaxies. Amongst its most tantalizing findings is the possible evidence for cold gas accretion in some of the most interesting objects, amongst which are a polar ring galaxy and a filamentary configuration of void galaxies. Here we shortly describe the scope of the VGS and the results of the full analysis of the pilot sample of 15 void galaxies.

astro-ph.CO↗

arXiv2010

Characterizing Ultraviolet Excesses in the Outskirts of a Local Early-Type Sample

We present an analysis of ultraviolet (UV) emission in the outer regions of a local, volume-limited sample of 56 early-type galaxies, where Hα emission from massive star formation is typically absent. We find excess faint NUV emission in the environments of our early-type galaxies compared to blank sky measured in the same tiles, indicating that the excesses are not due to background contamination. We do not observe corresponding faint FUV excesses. Faint NUV excesses increase with galaxy luminosity and are not correlated with the presence or absence of HI in the environments of these galaxies. The faint NUV excesses in the outskirts of early-type galaxies can be interpreted as being due to star formation at or above a few \times 10-5 M\odot yr-1 kpc-2; star formation at this rate can create a few percent of the mass of an early-type galaxy in a Gyr. Faint early types (with MB > -21.3) have on average four times as many bright UV sources within 30 kpc compared to bright early types (with MB < -21.3). The peak of the source distribution detected around faint early types is less luminous and slightly bluer than the peak of the sources detected around bright early types, indicating that early types with MB > -21.3 are more actively building up their mass with young stars. The spatial distribution of bright sources around all early types increases approximately linearly out to 20 kpc and subsequently flattens.

astro-ph.CO↗

arXiv2010

A Radio Perspective on the Wet Merger Remnant NGC 34

We present VLA observations of the neutral hydrogen and radio continuum of NGC 34 (= NGC 17 = Mrk 938). This object is an ideal candidate to study the fate of gas in mergers, since, as shown by an optical study done by Schweizer & Seitzer (2007), it is a gas-rich ("wet") merger remnant of two disk galaxies of unequal mass hosting a strong central starburst and a weak AGN. We detect HI emission from both tidal tails and from nearby galaxies, suggesting that NGC 34 is actually part of a gas-rich group and might have recently interacted with one of its companions. The kinematics of the gas suggests this remnant is forming an outer disk of neutral hydrogen from the gas of the northern tail. We also detect broad HI absorption (514 +/- 21 km/s wide) at both negative and positive velocities with respect to the systemic velocity. This absorption could be explained by the motions of the tidal tails or by the presence of a circumnuclear disk. In addition, we present radio-continuum images that show both nuclear (62.4 +/- 0.3 mJy) and extra-nuclear emission (26.5 +/- 3.0 mJy). The extra-nuclear component is very diffuse and in the shape of two radio lobes, spanning 390 kpc overall. This emission could be a signature of an AGN that has turned off, or it could originate from a starburst-driven superwind. We discuss the possible scenarios that explain our observations, and what they tell us about the location of the gas and the future evolution of NGC 34.

astro-ph.CO↗

arXiv2010

Only the Lonely: H I Imaging of Void Galaxies

Void galaxies, residing within the deepest underdensities of the Cosmic Web, present an ideal population for the study of galaxy formation and evolution in an environment undisturbed by the complex processes modifying galaxies in clusters and groups, as well as provide an observational test for theories of cosmological structure formation. We have completed a pilot survey for the HI imaging aspects of a new Void Galaxy Survey (VGS), imaging 15 void galaxies in HI in local (d < 100 Mpc) voids. HI masses range from 3.5 x 10^8 to 3.8 x 10^9 M_sun, with one nondetection with an upper limit of 2.1 x 10^8 M_sun. Our galaxies were selected using a structural and geometric technique to produce a sample that is purely environmentally selected and uniformly represents the void galaxy population. In addition, we use a powerful new backend of the Westerbork Synthesis Radio Telescope that allows us to probe a large volume around each targeted galaxy, simultaneously providing an environmentally constrained sample of fore- and background control sample of galaxies while still resolving individual galaxy kinematics and detecting faint companions in HI. This small sample makes up a surprisingly interesting collection of perturbed and interacting galaxies, all with small stellar disks. Four galaxies have significantly perturbed HI disks, five have previously unidentified companions at distances ranging from 50 to 200 kpc, two are in interacting systems, and one was found to have a polar HI disk. Our initial findings suggest void galaxies are a gas-rich, dynamic population which present evidence of ongoing gas accretion, major and minor interactions, and filamentary alignment despite the surrounding underdense environment.

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