arXiv subjects

J. E. Hibbard

Publications and source records attributed to J. E. Hibbard.

35 records · Page 2Linked to original sources

arXiv2004

UV Morphology and Star Formation in the Tidal Tails of NGC 4038/39

We present GALEX FUV (1530 A) and NUV (2310 A) observations of the archetypal merging system NGC 4038/39, ``The Antennae". Both tails are relatively bright in the UV, especially in the vicinity of the Tidal Dwarf Galaxy candidates at the end of the southern tail. The UV light generally falls within the optically delineated tails, although the UV light is considerably more structured, with a remarkably similar morphology to the tidal HI. The UV colors suggest that there has been continuing star formation within the tidal tails, even outside the previously studied Tidal Dwarf regions. Within the inner disk regions, there are interesting UV features which appear to be related to the extended soft X-ray loops and halo recently discovered by CHANDRA.

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arXiv2003

From Globular Clusters to Tidal Dwarfs: Structure Formation in the Tidal Tails of Merging Galaxies

Using V and I images obtained with WFPC2/HST, we investigate compact stellar structures within tidal tails. Six regions of tidal debris in the four classic ``Toomre Sequence'' mergers: NGC 4038/39 (``Antennae''), NGC 3256, NGC 3921, and NGC 7252 (``Atoms for Peace'') have been studied in order to explore how the star formation depends upon the local and global physical conditions. These mergers sample a range of stages in the evolutionary sequence and tails with and without embedded tidal dwarf galaxies. The six tails are found to contain a variety of stellar structures, with sizes ranging from those of globular clusters up to those of dwarf galaxies. From V and I WFPC2 images, we measure the luminosities and colors of the star clusters. NGC 3256 is found to have a large population of blue clusters (0.2 < V-I < 0.9), particularly in its Western tail, similar to those found in the inner region of the merger. In contrast, NGC 4038/39 has no clusters in the observed region of the tail, only less luminous point sources likely to be individual stars. A significant cluster population is clearly associated with the prominent tidal dwarf candidates in the eastern and western tails of NGC 7252. The cluster-rich Western tail of NGC 3256 is not distinguished from the others by its dynamical age or by its total HI mass. However, the mergers that have few clusters in the tail all have tidal dwarf galaxies, while NGC 3256 does not have prominent tidal dwarfs. We speculate that star formation in tidal tails may manifest itself either in small structures like clusters along the tail or in large structures such as dwarf galaxies, but not in both. Also, NGC 3256 has the highest star formation rate of the four mergers studied, which may contribute to the high number of star clusters in its tidal tails.

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arXiv2002

A Search for HI in Five Ellipticals with Fine Structure

We report on VLA HI spectral line observations of five early-type galaxies classified as optically peculiar due to the presence of jets, ripples or other optical "fine structure". We detect HI within the primary beam (30' HPBW) in four of the five systems. However, in only one case is this gas associated with the targeted elliptical galaxy. In the other cases the HI is associated with a nearby gas-rich disk or dwarf galaxy. The one HI detection is for NGC 7626, where we tentatively detect an HI cloud lying between 20 and 40 kpc southwest of the galaxy center. Its origin is unclear. Our failure to detect obvious tidal HI features suggests that if these fine-structure ellipticals are remnants of disk galaxy mergers, either the progenitors were gas poor, or they are well evolved and any gaseous tidal features have dispersed and/or been converted into other phases. Our targeted systems all reside in groups or clusters, and it seems likely that tidal HI is shorter lived in these environments than suggested by studies of more isolated merger remnants.

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arXiv2002

An HST Survey of the mid-UV Morphology of Nearby Galaxies

(Abbreviated) We present an imaging survey of 37 nearby galaxies observed with HST/WFPC2 in the mid-UV F300W filter and in F814W. 11 galaxies were also imaged in F255W. These galaxies were selected to be detectable with WFPC2 in one orbit, and cover a wide range of Hubble types and inclinations. The mid-UV spans the gap between our groundbased optical/NIR images and far-UV images available from the Astro/UIT missions. Our first qualitative results are: (1) Early-type galaxies show a significant decrease in surface brightness going from the red to the mid-UV, and in some cases the presence of dust lanes. Some galaxies would be classified different when viewed in the mid-UV, some become dominated by a blue nuclear feature or point source. (2) Half of the mid-type spiral and star-forming galaxies appear as a later morphological type in the mid-UV, as Astro/UIT also found in the far-UV. Some- times these differences are dramatic. The mid-UV images show a considerable range in the scale and surface brightness of individual star-forming regions. Almost all mid-type spirals have their small bulges bi-sected by a dust-lane. (3) Most of the heterogeneous subset of late-type, irregular, peculiar, and merging galaxies display F300W morphologies that are similar to those seen in F814W, but with differences due to recognizable dust features absorbing the bluer light, and due to UV-bright hot stars, star-clusters, and star-forming ridges. In the rest-frame mid-UV, early- to mid-type galaxies are more likely to be misclassified as later types than vice versa. This morphological K-correction explains only part of the excess faint blue galaxies seen in deep HST fields.

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arXiv2001

An HI Rogues Gallery

We have begun a compilation of HI maps of peculiar galaxies kindly contributed by individual investigators, many as part of the ``Gas & Galaxy Evolution'' conference. In this gallery we present images of the first 181 objects, which includes over 400 individually cataloged galaxies. The images consist of a greyscale representation of the optical morphology and an accompanying optical image with HI contours superimposed. A web-based ``living Gallery'' is being maintained on the NRAO homepage (presently at http://www.nrao.edu/astrores/HIrogues/).

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arXiv2001

High-resolution HI Mapping of NGC 4038/9 ("The Antennae") and its Tidal Dwarf Galaxy Candidates

We present new VLA C+D-array HI observations and optical and NIR imaging of the well known interacting system NGC 4038/9, ``The Antennae''. The radio data reveal a wealth of gaseous sub-structure both within the main bodies of the galaxies and along the tidal tails. In agreement with previous HI studies, we find that the northern tail has HI along its outer length, but none along its base. We suggest that the HI at the base of this tail has been ionized by massive stars in the disk of NGC 4038. The gas in the southern tail has a bifurcated structure, with one filament lying along the optical tail and another running parallel to it but with no optical counterpart. The two filaments join just before the location of several star forming regions near the end of the tail. The HI velocity field at the end of the tail is dominated by strong velocity gradients which suggest that at this location the tail is bending away from us. We delineate and examine two regions within the tail previously identified as possible sites of a so-called ``tidal dwarf galaxy'' condensing out of the expanding tidal material. The tail velocity gradients mask any clear kinematic signature of a self-gravitating condensation in this region. A dynamical analysis suggest that there is not enough mass in gas alone for either of these regions to be self-gravitating. Conversely, if they are bound they require a significant contribution to their dynamical mass from evolved stars or dark matter. (Abridged)

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arXiv2001

HST Observations of the Nuclear Regions of the Toomre Sequence of Merging Galaxies

We present first results from an HST WFPC2 imaging and STIS spectroscopy program to investigate the structural and star forming properties in the nuclear regions of the Toomre Sequence of merging galaxies. Here we discuss V-band, I-band and H-alpha images of the nuclei. We comment briefly on the connection between the nuclear morphology of the ionized gas and the merger stage.

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arXiv2000

Molecular Gas in Optically Selected Mergers

We have mapped the 2.6 mm CO (1-0) emission in three optically selected "Toomre Sequence" mergers (NGC 520, NGC 3921, NGC 4676). The molecular gas distribution is well resolved by the observations. For NGC 520 and NGC 4676A, the nuclear gas concentrations form a disk- or a ring-like structure, and the gas kinematics are regular and are consistent with simple rotation. Discrete molecular gas complexes are found along the stellar bar in NGC 4676B, and the gas kinematics is consistent with the disk rotation traced in H alpha. The molecular gas distribution in NGC 3921 is asymmetric about the stellar remnant, and both the distribution and kinematics suggest that the molecular gas has not settled into the center of the remnant. Molecular gas clouds are detected outside the central regions of NGC 3921 and NGC 4676, and they may be associated with the tidal tails and bridges mapped in HI. Departures from the canonical scenario for a merger involving two large spiral galaxies are found in all three Toomre Sequence mergers studied. Our data suggest that one of the progentor disks in NGC 520 and NGC 3921 were relatively gas-poor. A detailed comparison of these optically selected mergers and more luminous IR selected mergers is deferred to a companion paper (Paper II).

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arXiv2000

The Cold and Hot Gas Content of Fine-Structure E and S0 Galaxies

We investigate trends of the cold and hot gas content of early-type galaxies with the presence of optical morphological peculiarities, as measured by the fine-structure index (Sigma). HI mapping observations from the literature are used to track the cold-gas content, and archival ROSAT PSPC data are used to quantify the hot-gas content. We find that E and S0 galaxies with a high incidence of optical peculiarities are exclusively X-ray underluminous and, therefore, deficient in hot gas. In contrast, more relaxed galaxies with little or no signs of optical peculiarities span a wide range of X-ray luminosities. That is, the X-ray excess anticorrelates with Sigma. There appears to be no similar trend of cold-gas content with either fine-structure index or X-ray content. The fact that only apparently relaxed E and S0 galaxies are strong X-ray emitters is consistent with the hypothesis that after strong disturbances such as a merger hot-gas halos build up over a time scale of several gigayears. This is consistent with the expected mass loss from stars.

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arXiv1999

The Neutral Hydrogen Distribution in Merging Galaxies: Differences between Stellar and Gaseous Tidal Morphologies

We have mapped the neutral atomic gas (HI) in the three disk-disk merger systems NGC 520, Arp 220, and Arp 299. These systems differ from the majority of the mergers mapped in HI, in that their stellar and gaseous tidal features do not coincide. In particular, they exhibit large stellar tidal features with little if any accompanying neutral gas and large gas-rich tidal features with little if any accompanying starlight. On smaller scales, there are striking anti-correlations where the gaseous and stellar tidal features appear to cross. We explore several possible causes for these differences, including dust obscuration, ram pressure stripping, and ionization effects. No single explanation can account for all of the observed differences. The fact that each of these systems shows evidence for a starburst driven superwind expanding in the direction of the most striking anti-correlations leads us to suggest that the superwind is primarily responsible for the observed differences, either by sweeping the features clear of gas via ram pressure, or by excavating a clear sightline towards the starburst and allowing UV photons to ionize regions of the tails.

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arXiv1999

Mergers of Galaxies from an HI Perspective

HI spectral line mapping studies are a unique probe of morphologically peculiar systems. Not only does HI imaging often reveal peculiarities that are totally unsuspected at optical wavelengths, but it opens a kinematic window into the outer dynamics of these systems that is unmatched at any other wavelength. In this review I attempt to summarize what we have learned from such studies, and what we may hope to learn from them in the future.

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arXiv1999

Luminosity Profiles of Merger Remnants

Using published luminosity and molecular gas profiles of the late-stage mergers NGC 3921, NGC 7252 and Arp 220, we examine the expected luminosity profiles of the evolved merger remnants, especially in light of the massive CO complexes that are observed in their nuclei. For NGC 3921 and NGC 7252 we predict that the resulting luminosity profiles will be characterized by an r^{1/4} law. In view of previous optical work on these systems, it seems likely that they will evolve into normal ellipticals as regards their optical properties. Due to a much higher central molecular column density, Arp 220 might not evolve such a ``seamless'' light profile. We conclude that ultraluminous infrared mergers such as Arp 220 either evolve into ellipticals with anomalous luminosity profiles, or do not produce many low-mass stars out of their molecular gas complexes.

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arXiv1999

A 180 Kpc Tidal Tail in the Luminous Infrared Merger Arp 299

We present VLA HI observations and UH88 deep optical B- and R-band observations of the IR luminous merger Arp 299 (= NGC 3690 + IC 694). These data reveal a gas-rich, optically faint tidal tail with a length of over 180 kpc. The size of this tidal feature necessitates an old interaction age for the merger (~750 Myr since first periapse), which is currently experiencing a very young star burst (~20 Myr). The observations reveal a most remarkable structure within the tidal tail: it appears to be composed of two parallel filaments separated by ~20 kpc. One of the filaments is gas rich with little if any starlight, while the other is gas poor. We believe that this bifurcation results from a warped disk in one of the progenitors. The quantities and kinematics of the tidal HI suggest that Arp 299 results from the collision of a retrograde Sab-Sb galaxy (IC 694) and a prograde Sbc-Sc galaxy (NGC 3690) that occurred 750 Myr ago and which will merge into a single object in ~60 Myr. We suggest that the present IR luminous phase in this system is due in part to the retrograde spin of IC 694. Finally, we discuss the apparent lack of tidal dwarf galaxies within the tail.

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arXiv1999

Redshifts of Galaxies Around Arp 220 and Serendipitous Discovery of Three Star-Forming Dwarf Galaxies at Redshift $z \sim$ 0.5

We present redshift measurements of four faint galaxies around the archetypal ultraluminous infrared galaxy Arp 220. These galaxies have significantly higher redshifts ($z \sim$ 0.036 -- 0.091) than that of Arp 220 ($z \simeq 0.018$). Therefore, we conclude that they are background objects not physically related to Arp 220. Three of these faint galaxies located to the south of Arp 220 are a group of galaxies (or the brightest members in a cluster of galaxies) at $z \simeq 0.09$, as suggested by Heckman et al. [1996, ApJ, 457, 616] on the basis of their associated soft X-ray emission. We also report the serendipitous discovery of three additional galaxies at redshift $z \sim 0.5$, found along one of the slit positions. All three galaxies exhibit an [O ii] λ3727 emission line. The spectrum of the brightest galaxy ($m_R \simeq 24.4$) shows other strong emission lines: Mg ii λ2798, H β, [O iii] λ4959, and [O iii] λ5007. The emission-line properties of these galaxies as well as their intrinsically low luminosities ($M_R \geq -18.4$) indicate that they are star-forming dwarf galaxies.

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arXiv1997

The Apparent Morphology of Peculiar Galaxies at Intermediate to High Redshifts

We use rest frame ultraviolet (UV), B, and V band images of five nearby (z<0.02) interacting and/or starbursting galaxies to simulate deep HST observations of peculiar galaxies at medium to high redshifts. In particular, we simulate Hubble Deep Field (HDF) observations in the F606W and F814W filters of starburst galaxies in the redshift range z~0.5---2.5 by explicitly account for the combined effects of band-shifting and surface brightness dimming. We find that extended morphological features remain readily visible in the long exposures typical of the HDF out to redshifts of ~ 1. For systems above z~1.5, the simulated morphologies look remarkably similar to those of the faint objects found in the HDF and other deep HST fields. Peculiar starburst galaxies therefore appear to be the best local analogs to the highest redshift galaxies in terms of morphology, star formation rates, and spectral energy distributions. Nevertheless, photometric measurements of the z>1.5 images fail to recover the true global properties of the underlying systems. This is because the high-z observations are sensitive to the rest-frame UV emission, which is dominated by the most active star forming regions. The extended distribution of starlight from more evolved populations would not be detected. We conclude that imaging observations in the restframe UV alone cannot reveal whether high-z systems (z>1.5) are proto-galaxies, proto-bulges, or starbursts within a pre-existing population. Definitive statements regarding the global properties and dynamical states of these objects require deep imaging observations at longer wavelengths.

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arXiv1997

Mergers, Interactions, and The Fueling of Starbursts

The most active starbursts are found in galaxies with the highest IR luminosities, with peak star formation rates and efficiencies that are over an order of magnitude higher than in normal disk systems. These systems are almost exclusively on-going mergers. In this review I explore the conditions needed for interactions to experience such a phase by comparing two systems at similar stages of merging but quite different IR luminosities: NGC 4038/9 and Arp 299. These observations show that the most intense starbursts occur at the sites with the highest gas densities, which is a general result for IR luminous mergers. Observations and theory both suggest that the strength of the merger induced starburst depends on the internal structure of the progenitors, the amount and distribution of the gas, and the violence of the interaction. In particular, interactions involving progenitors with dense bulges, gas-rich disks, and/or a retrograde spin are expected to preferentially lead to large amounts of gaseous dissipation, although the interplay between these parameters is unknown. A major outstanding question is how the effects of feedback alter these conclusions.

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arXiv1995

H I, H II, and R-Band Observations of a Galactic Merger Sequence

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: Arp295, NGC4676, NGC520, NGC3921, and NGC7252. These data are supplemented by wide-field images taken through a narrow band Halpha filter, and by deep R-band surface photometry. In the early stages, large amounts of HI 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 HI within the remnant bodies, and tidal material is seen moving inward. 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 NGC520 reveal an extensive rotating gaseous disk, suggesting that perhaps some mergers will not destroy the atomic gas disks of the progenitors. Overall, large quantities of both gas and starlight are seen at large radii. Since this material evolves on very long time scales, it may leave observable signatures for many Gyr.

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