John E. Hibbard

THE 2014 ALMA LONG BASELINE CAMPAIGN: FIRST RESULTS FROM HIGH ANGULAR RESOLUTION OBSERVATIONS TOWARD THE HL TAU REGION

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 cir cumstellar disk surrounding the young solar analogue 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 ◦ ). We obtain a high-fidelity image of the 1.0 mm spectral index (�), which ranges from � � 2.0 in the optically-thick central peak and two brightest ring s, increasing to 2.3-3.0 in the dark rings. The dark rings are not devoid of emission, and we estimate a grain emissivity index of 0.8 for the innermost dark ring and lower for subsequent dark rings, consistent with some degree of grain growth and evolution. Additional clues that the rings arise from planet formation incl ude an increase in their central offsets with radius and the presence of numerous orbital resonances. At a resolution of 35 AU, we resolve the molecular component of the disk in HCO + (1-0) which exhibits a pattern over LSR velocities from 2-12 km s -1 consistent with Keplerian motion around a �1.3M⊙ star, although complicated by absorption at low blue-shifted velocities. We also serendipitously detect and resolve the nearby protost ars XZ Tau (A/B) and LkH�358 at 2.9 mm. Subject headings: stars: individual (HL Tau, XZ Tau, LkH�358) — protoplanetary disks — stars: formation — submillimeter: planetary systems — techniques: interferometric

Dynamical Modeling of NGC 7252 and the Return of Tidal Material

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.

Ubvr and hubble space telescope mid-ultraviolet and near-infrared surface photometry and radial color gradients of late-type, irregular, and peculiar galaxies

We introduce a data set of 142 mostly late-type spiral, irregular, and peculiar (interacting or merging) nearby galaxies observed in UBVR at the Vatican Advanced Technology Telescope (VATT), and we present an analysis of their radial color gradients. We confirm that nearby elliptical and early- to mid-type spiral galaxies show either no or only small color gradients, becoming slightly bluer with radius. In contrast, we find that late-type spiral, irregular, peculiar, and merging galaxies become on average redder with increasing distance from the center. The scatter in radial color gradient trends increases toward later Hubble type. As a preliminary analysis of a larger data set obtained with the Hubble Space Telescope (HST), we also analyze the color gradients of six nearby galaxies observed with NICMOS in the near-IR (H) and with WFPC2 in the mid-UV (F300W) and red (F814W). We discuss the possible implications of these results on galaxy formation and compare our nearby galaxy color gradients to those at high redshift. We present examples of images and UBVR radial surface brightness and color profiles, as well as of the tables of measurements; the full atlas and tables are published in the electronic edition only.

From globular clusters to tidal dwarfs: Structure formation in the tidal tails of merging galaxies

Using V and I images obtained with the Wide Field Planetary Camera 2 (WFPC2) of the Hubble Space Telescope, 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 on 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. NGC 3921 and NGC 7252 have small populations of clusters along their tails. 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 H I 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.

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

We present VLA H I observations and University of Hawaii 88 inch (2.2 m) deep optical B- and R-band observations of the IR-luminous merger Arp 299 (= NGC 3690 + IC 694). These data reveal a gas-rich (MH I = 3.3 × 109 M⊙) optically faint (μB 27 mag arcsec-2, μR 26 mag arcsec-2) 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 starburst (20 Myr). The observations reveal a most remarkable structure within the tidal tail: it appears to be composed of two parallel filaments separated by approximately 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 H I 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 will merge into a single object in roughly 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.

The H I Kinematics and Distribution of Four Blue Compact Dwarf Galaxies

We present VLA H I observations of the blue compact dwarf (BCD) galaxies NGC 2366, NGC 4861, VII Zw 403, and Haro 2. These galaxies span a range of BCD morphological types. The cometary-like BCDs NGC 2366 and NGC 4861 have regular rotational kinematics with a V/σ of 8.7 and 6.4, respectively. On the other hand, the velocity fields of the iE BCD VII Zw 403 and of the nE BCD Haro 2 lack regularity, and their rotational motion is around the major, not the minor, axis. The H I distribution is centrally peaked in VII Zw 403 and Haro 2, a general feature of all iE and nE-type BCDs, the most common ones. In contrast, cometary-type BCDs have multiple H I peaks that are scattered over the disk. The active regions of star formation are associated with regions of high H I column densities, with slight displacements between the H I and stellar peaks. NGC 2366 shows many H I minima, resulting from the disruptive influence of massive star formation and supernovae on the interstellar medium (ISM). In NGC 2366 and NGC 4861, there is a tendency for H I gas with a higher velocity dispersion to be associated with regions of lower H I column density. This anticorrelation can be understood in the context of a two-phase model of the ISM. In all BCDs, the radio continuum emission is associated with the star-forming regions and is predominantly thermal in nature. H I clouds with no optical counterparts have been found in the vicinity of NGC 4861 and Haro 2.

The Stellar Content of the Southern Tail of NGC 4038/4039 and a Revised Distance*

We have used the Hubble Space Telescope and Wide Field Planetary Camera 2 to image the putative tidal dwarf galaxy located at the tip of the Southern tidal tail of NGC 4038/4039, the Antennae. We resolve individual stars and identify two stellar populations. Hundreds of massive stars are present, concentrated into tight OB associations on scales of 200 pc, with ages ranging from 2 to 100 Myr. An older stellar population is distributed roughly following the outer contours of the neutral hydrogen in the tidal tail; we associate these stars with material ejected from the outer disks of the two spirals. The older stellar population has a red giant branch tip at I = 26.5 ± 0.2 from which we derive a distance modulus (m - M)0 = 30.7 ± 0.25. The implied distance of 13.8 ± 1.7 Mpc is significantly smaller than commonly quoted distances for NGC 4038/4039. In contrast to the previously studied core of the merger, we find no super–star clusters (SSCs). One might conclude that SSCs require the higher pressures found in the central regions in order to form, while spontaneous star formation in the tail produces the kind of OB star associations seen in dwarf irregular galaxies. The youngest population in the putative tidal dwarf has a total stellar mass of ≈2 × 105 M⊙, while the old population has a stellar mass of ≈7 × 107 M⊙. If our smaller distance modulus is correct, it has far-reaching consequences for this prototypical merger. Specifically, the luminous to dynamical mass limits for the tidal dwarf candidates are significantly less than 1, the central SSCs have sizes typical of Galactic globular clusters, rather than being 1.5 times as large, and the unusually luminous X-ray population becomes both less luminous and less populous.

Molecular Gas in Optically Selected Mergers

We have mapped the 2.6 mm CO J \ 1 ] 0 emission in three optically selected ii Toomre sequence ˇˇ 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 disklike or a ringlike structure, and the gas kinematics are regular and 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 Ha. 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 H I. 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 progenitor disks in NGC 520 and NGC 3921 was relatively gas-poor. A detailed comparison of these optically selected mergers and more luminous IR-selected mergers is deferred to a separate paper. Subject headings: galaxies: individual (NGC 520, NGC 3921, NGC 4676) ¨ galaxies: interactions ¨ + + + +

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 Σ. H I mapping observations from the literature are used to track the cold gas content, and archival ROSAT Position Sensitive Proportional Counter 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 Σ. 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 timescale of several gigayears. This is consistent with the expected mass loss from stars.

MID-INFRARED EVIDENCE FOR ACCELERATED EVOLUTION IN COMPACT GROUP GALAXIES

Compact galaxy groups are at the extremes of the group environment, with high number densities and low velocity dispersions that likely affect member galaxy evolution. To explore the impact of this environment in detail, we examine the distribution in the mid-infrared (MIR) 3.6-8.0 micron colorspace of 42 galaxies from 12 Hickson compact groups in comparison with several control samples, including the LVL+SINGS galaxies, interacting galaxies, and galaxies from the Coma Cluster. We find that the HCG galaxies are strongly bimodal, with statistically significant evidence for a gap in their distribution. In contrast, none of the other samples show such a marked gap, and only galaxies in the Coma infall region have a distribution that is statistically consistent with the HCGs in this parameter space. To further investigate the cause of the HCG gap, we compare the galaxy morphologies of the HCG and LVL+SINGS galaxies, and also probe the specific star formation rate (SSFR) of the HCG galaxies. While galaxy morphology in HCG galaxies is strongly linked to position with MIR colorspace, the more fundamental property appears to be the SSFR, or star formation rate normalized by stellar mass. We conclude that the unusual MIR color distribution of HCG galaxies is a direct product of their environment, which is most similar to that of the Coma infall region. In both cases, galaxy densities are high, but gas has not been fully processed or stripped. We speculate that the compact group environment fosters accelerated evolution of galaxies from star-forming and neutral gas-rich to quiescent and neutral gas-poor, leaving few members in the MIR gap at any time.