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Browsing Faculty by Author "Crowl, Hugh H."

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    Dense Cloud Ablation and Ram Pressure Stripping of the Virgo Spiral NGC 4402
    (The American Astronomical Society, 2005-07) Crowl, Hugh H.; Kenney, Jeffrey D. P.; van Gorkom, J. H.; Vollmer, Bernd
    We present optical, H i, and radio continuum observations of the highly inclined Virgo Cluster Sc galaxy NGC 4402, which show evidence for ram pressure stripping and dense cloud ablation. Very Large Array H i and radio continuum maps show a truncated gas disk and emission to the northwest of the main disk emission. In particular, the radio continuum emission is asymmetrically extended to the north and skewed to the west. The H image shows numerous H ii complexes along the southern edge of the gas disk, possibly indicating star formation triggered by the intracluster medium (ICM) pressure. Our BVR images at 0B5 resolution obtained with the WIYN Tip-Tilt Imager show a remarkable dust lane morphology: at half the optical radius, the dust lane of the galaxy curves up and out of the disk, matching the H i morphology. Large dust plumes extend upward for 1.5 kpc from luminous young star clusters at the southeast edge of the truncated gas disk. These star clusters are very blue, indicating very little dust reddening, which suggests dust blown away by an ICM wind at the leading edge of the interaction. To the south of the main ridge of interstellar material, where the galaxy is relatively clean of gas and dust, we have discovered 1 kpc long linear dust filaments with a position angle that matches the extraplanar radio continuum tail; we interpret this angle as the projected ICM wind direction. One of the observed dust filaments has an H ii region at its head.We interpret these dust filaments as large, dense clouds that were initially left behind as the low-density interstellar medium was stripped but were then ablated by the ICM wind. These results provide striking new evidence on the fate of molecular clouds in stripped cluster galaxies.
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    The Line-of-Sight Depth of Populous Clusters in the Small Magellanic Cloud
    (The American Astronomical Society, 2001-07) Crowl, Hugh H.; Sarajedini, Ata; Piatti, Andrés E.; Geisler, Doug; Bica, Eduardo; Clariá, Juan J.; João, F. C. Santos, Jr.
    We present an analysis of age, metal abundance, and positional data on populous clusters in the Small Magellanic Cloud (SMC) with the ultimate aim of determining the line-of-sight (LOS) depth of the SMC by using these clusters as proxies. Our data set contains 12 objects and is limited to clusters with the highest-quality data for which the ages and abundances are best known and can be placed on an internally consistent scale. We have analyzed the variation of the clustersÏ properties with position on the sky and with line-of-sight depth. Based on this analysis, we draw the following conclusions : (1) The observational data indicate that the eastern side of the SMC (facing the Large Magellanic Cloud) contains younger and more metal-rich clusters as compared with the western side. This is not a strong correlation because our data set of clusters is necessarily limited, but it is suggestive and warrants further study. (2) Depending on how the reddening is computed to our clusters, we find a mean distance modulus that ranges from (m-M)^0 = 18.71± 0.06 to 18.82 ± 0.05. (3) The intrinsic ±1 σ LOS depth of the SMC populous clusters in our study is between ~ 6 and ~ 12 kpc, depending primarily on whether we adopt the Burstein & Heiles reddenings or those from Schlegel et al. (4) Viewing the SMC as a triaxial galaxy with declination, right ascension, and LOS depth as the three axes, we find axial ratios of approximately 1:2:4. Taken together, these conclusions largely agree with those of previous investigators and underscore the utility of populous star clusters as probes of the structure of the Small Magellanic Cloud.
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    A Spectacular Hα Complex in Virgo: Evidence for a Collision between M86 and NGC 4438 and Implications for the Collisional ISM Heating of Ellipticals
    (The American Astronomical Society, 2008-10) Kenney, Jeffrey D. P.; Tal, Tomer; Crowl, Hugh H.; Feldmeier, John; Jacoby, George H.
    Deep wide-field Hα+[N II] imaging around the Virgo Cluster giant elliptical galaxy M86 reveals a highly complex and disturbed ISM/ICM. The most striking feature is a set of Hα filaments which clearly connect M86 with the nearby disturbed spiral NGC 4438 (23' = 120 kpc projected away), providing strong evidence for a previously unrecognized collision between them. Spectroscopy of selected regions shows a fairly smooth velocity gradient between M86 and NGC 4438, consistent with the collision scenario. Such a collision would impart significant energy into the ISM of M86, probably heating the gas and acting to prevent the gas from cooling to form stars. We propose that cool gas stripped from NGC 4438 during the collision and deposited in its wake is heated by shocks, ram pressure drag, or thermal conduction, producing most of the Hα filaments. Some Hα filaments are associated with the well-known ridge of bright X-ray emission to the NW of the nucleus, suggesting that the collision is responsible for peculiarities of M86 previously ascribed to other effects. M86 is radio-quiet; thus AGN heating is unlikely to play a significant role. The M86 system has implications for understanding the role of gravitational interactions in the heating of the ISM in ellipticals, and how collisions in clusters transform galaxies.
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    The Stellar Population of Stripped Cluster Spiral NGC 4522: A Local Analog to K+A Galaxies?
    (The American Astronomical Society, 2006-10) Crowl, Hugh H.; Kenney, Jeffrey D. P.
    We present observations of the stripped Virgo Cluster spiral galaxy NGC 4522, a clear, nearby example of a galaxy currently undergoing interstellar medium-intracluster medium (ISM-ICM) stripping. Utilizing SparsePak integral field spectroscopy on the Wisconsin-Indiana-Yale-NOAO (WIYN) 3.5 m telescope and Galaxy Evolution Explorer (GALEX) UV photometry, we present an analysis of the outer disk (r>3 kpc) stellar population of this galaxy, beyond the H I and Hα truncation radius. We find that the star formation in the gas-stripped outer disk ceased very recently, ~100 Myr ago, in agreement with previous claims that this galaxy is currently being stripped. At the time of this stripping, data and models suggest that the galaxy experienced a modest starburst. The stripping is occurring in a region of the cluster well outside the cluster core, likely because this galaxy is experiencing extreme conditions from a dynamic ICM due to an ongoing subcluster merger. The outer disk has a spectrum of a K+A galaxy, traditionally observed in high-redshift cluster galaxies. In the case of NGC 4522, a K+A spectrum is formed by simple stripping of the interstellar gas by the hot intracluster medium. These data show K+A spectra can be created by cluster processes and that these processes likely extend beyond the cluster core.
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    The Stellar Populations of Stripped Spiral Galaxies in the Virgo Cluster
    (The American Astronomical Society, 2008-10) Crowl, Hugh H.; Kenney, Jeffrey D. P.
    We present an analysis of the stellar populations of the gas-stripped outer disks of ten Virgo cluster spiral galaxies, utilizing SparsePak integral field spectroscopy on the WIYN 3.5 m telescope and Galaxy Evolution Explorer (GALEX) UV photometry. The galaxies in our sample show evidence for being gas-stripped spiral galaxies, with star formation within a truncation radius and a passive population beyond the truncation radius. We find that all of the galaxies with spatially truncated star formation have outer-disk stellar populations consistent with star formation ending within the last 500 Myr. The synthesis of optical spectroscopy and GALEX observations demonstrate that star formation was relatively constant until the quenching time, after which the galaxies passively evolved. Large starbursts at the time of quenching are excluded for all galaxies, but there is evidence of a modest starburst in at least one galaxy. For approximately half of our galaxies, the timescales derived from our observations are consistent with galaxies being stripped in or near the cluster core, where simple ram-pressure estimates can explain the observed stripping. However, the other half of our sample galaxies were clearly stripped outside the cluster core. Such galaxies provide evidence that the intra-cluster medium is not static and smooth. For three of the most recently stripped galaxies, there are estimates for the stripping timescales from detailed gas-stripping simulations. For all three of these galaxies, our stripping timescales agree with those from the gas-stripping simulations, suggesting that star formation is quenched near the time of peak pressure. While the stripping of star-forming gas in the outer disk creates a passive population in our sample of galaxies, there is still normal star formation in the center of these galaxies. It may be that Virgo is not massive enough to completely strip these spiral galaxies and, in a more dynamically active cluster or a cluster with a higher density intracluster medium, such a process would lead to passive spirals and/or S0s.
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    Transformation of a Virgo Cluster Dwarf Irregular Galaxy by Ram Pressure Stripping: IC3418 and its Fireballs
    (The American Astronomical Society, 2014-01-10) Kenney, Jeffrey D. P.; Geha, Marla; Jachym, Pavel; Crowl, Hugh H.; Dague, William; Chung, Aeree; van Gorkom, Jacqueline; Vollmer, Bernd
    We present optical imaging and spectroscopy and H I imaging of the Virgo Cluster galaxy IC 3418, which is likely a "smoking gun" example of the transformation of a dwarf irregular into a dwarf elliptical galaxy by ram pressure stripping. IC 3418 has a spectacular 17 kpc length UV-bright tail comprised of knots, head-tail, and linear stellar features. The only Hα emission arises from a few H II regions in the tail, the brightest of which are at the heads of head-tail UV sources whose tails point toward the galaxy ("fireballs"). Several of the elongated tail sources have Hα peaks outwardly offset by ~80-150 pc from the UV peaks, suggesting that gas clumps continue to accelerate through ram pressure, leaving behind streams of newly formed stars which have decoupled from the gas. Absorption line strengths, measured from Keck DEIMOS spectra, together with UV colors, show star formation stopped 300 ± 100 Myr ago in the main body, and a strong starburst occurred prior to quenching. While neither Hα nor H I emission are detected in the main body of the galaxy, we have detected 4 × 10^7 M☉ of H I from the tail with the Very Large Array. The velocities of tail H II regions, measured from Keck LRIS spectra, extend only a small fraction of the way to the cluster velocity, suggesting that star formation does not happen in more distant parts of the tail. Stars in the outer tail have velocities exceeding the escape speed, but some in the inner tail should fall back into the galaxy, forming halo streams.
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    VLA Imaging of Virgo Spirals in Atomic Gas (VIVA).
    (The American Astronomical Society, 2009-12) Chung, Aeree; Gorkom, J. H.; Kenney, Jeffrey D. P.; Crowl, Hugh H.; Vollmer, Bernd
    We present the results of a new VLA H I Imaging survey of Virgo galaxies, the VLA Imaging survey of Virgo galaxies in Atomic gas (VIVA). The survey includes high-resolution H I data of 53 carefully selected late type galaxies (48 spirals and five irregular systems). The goal is to study environmental effects on H I gas properties of cluster galaxies to understand which physical mechanisms affect galaxy evolution in different density regions, and to establish how far out the impact of the cluster reaches. As a dynamically young cluster, Virgo contains examples of galaxies experiencing a variety of environmental effects. Its nearness allows us to study each galaxy in great detail. We have selected Virgo galaxies with a range of star formation properties in low to high density regions (at projected distances from M87, d 87 = 0.3-3.3 Mpc). Contrary to previous studies, more than half of the galaxies in the sample (~60%) are fainter than 12 mag in BT . Overall, the selected galaxies represent the late type Virgo galaxies (S0/a to Sd/Irr) down to mp <~ 14.6 fairly well in morphological type, systemic velocity, subcluster membership, H I mass, and deficiency. The H I observations were done in C short (CS) configuration of the VLA radio telescope, with a typical spatial resolution of 15'' and a column density sensitivity of ≈3-5 × 1019 cm-2 in 3σ per 10 km s-1 channel. The survey was supplemented with data of comparable quality from the NRAO archive, taken in CS or C configuration. In this paper, we present H I channel maps, total intensity maps, velocity fields, velocity dispersions, global/radial profiles, position-velocity diagrams and overlays of H I/1.4 GHz continuum maps on the optical images. We also present H I properties such as total flux (S H I ), H I mass (M H I ), linewidths (W 20 and W 50), velocity (V H I ), deficiency (def H I ), and size (D eff H I and D iso H I ), and describe the H I morphology and kinematics of individual galaxies in detail. The survey has revealed details of H I features that were never seen before. In this paper, we briefly discuss differences in typical H I morphology for galaxies in regions of different galaxy densities. We confirm that galaxies near the cluster core (d 87 <~ 0.5 Mpc) have H I disks that are smaller compared to their stellar disks (D H I /D 25 < 0.5). Most of these galaxies in the core also show gas displaced from the disk, which is either currently being stripped or falling back after a stripping event. At intermediate distances (d 87 ~ 1 Mpc) from the center, we find a remarkable number of galaxies with long one-sided H I tails pointing away from M87. In a previous letter, we argue that these galaxies are recent arrivals, falling into the Virgo core for the first time. In the outskirts, we find many gas-rich galaxies, with gas disks extending far beyond their optical disks. Interestingly, we also find some galaxies with H I disks that are smaller compared to their stellar disks at large clustercentric distances.