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Search Results: 1 - 10 of 231435 matches for " William C. Keel "
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Unveiling Galaxy Interactions: Watching the Tides Roll
William C. Keel
Physics , 2009,
Abstract: I set the stage for discussion of the stellar populations in interacting galaxies by looking back over the slow development of our understanding of these systems. From early anecdotal collections, to systematic cataloging, and finally to increasingly sophisticated n-body calculations, we have seen how gravity in distributed systems can produce the stunning variety of structures we see. At the same time, measures across the spectrum have made it clear that galaxy interactions are linked to star formation, albeit with the physical mechanisms much less clear. Improved data sets, including HST imaging, deep IR data, and large samples with well-defined selection criteria, have started to reveal correlations with dynamical parameters pointing to detailed histories of starbirth during collisions. The merger hypothesis for elliptical galaxies has broadened into seeing interactions and mergers as important parts of the overall evolution of galaxies. The connection becomes more important as we look to higher redshift, where more frequent interactions can drive the evolution of galaxies in multiple ways. Links between the properties of central black holes and surrounding galaxies makes it important likewise to understand the connections between AGN and interactions, which has remained more ambiguous due to the strong role of sample selection. Finally, contemporary data reach deep enough to show that most galaxies have interacted in the observable past; we must consider these events to be a normal part of galaxy history.
Massive star clusters in ongoing galaxy interactions -- clues to cluster formation
William C. Keel,Kirk D. Borne
Physics , 2003, DOI: 10.1086/377482
Abstract: We present HST WFPC2 observations, supplemented by ground-based H-alpha data, of the star-cluster populations in two pairs of interacting galaxies, selected for being in very different kinds of encounters seen at different stages. Dynamical information and n-body simulations provide the details of encounter geometry, mass ratio, and timing. In NGC 5752/4, we see a weak encounter well past closest approach, after about 2.5x10^8 years. The large spiral NGC 5754 has a normal population of disk clusters, while the companion NGC 5752 exhibits a rich population of luminous clusters with a flatter luminosity function. The strong, ongoing encounter in NGC 6621/2, seen about 10^8 years past closest approach between roughly equal-mass galaxies, has produced a rich population of luminous clusters, particularly young and bright in a small region between the nuclei. This region is dynamically interesting, so strongly perturbed that the rotation curve reverses sign. Cluster formation requires a threshold level of perturbation, with stage of the interaction less important. The location of the most active star formation in NGC 6621/2 draws attention to a possible role for the Toomre stability threshold in shaping star formation in interacting galaxies. The rich cluster populations in NGC 5752 and 6621 show that direct contact between gas-rich galaxy disks is not a requirement to form luminous clusters, and that they can be triggered by processes happening within a single galaxy disk (albeit triggered by external perturbations). (Abridged)
Seeing Galaxies Through Thick & Thin. III. HST Imaging of the Dust in Backlit Spiral Galaxies
William C. Keel,Raymond E. White III
Physics , 2000, DOI: 10.1086/319386
Abstract: We present analysis of WFPC2 imaging of two spiral galaxies partially backlit by E/S0 systems in the pairs AM1316-241 and AM0500-620, and the spiral foreground system in NGC 1275. Images in B and I are used to determine the reddening curve of in these systems. The spiral component of AM1316-241 shows dust strongly concentrated in discrete arms, with a reddening law very close to the Milky Way mean. The dust distribution is scale-free between about 100 pc and the arm scale. The spiral in AM0500-620 shows dust concentrated in arms and interarm spurs, with measurable interarm extinction as well. Although its dust properties are less well-determined, we find evidence for a steeper extinction law here. The shape of the reddening law suggests that, at least in AM1316-241, we have resolved most of the dust structure. In AM0500-620, the slope of the fractal perimeter-scale relation steepens systematically from low to high extinction. In AM1316-241, we cannot determine a unique fractal dimension from the defining area-perimeter relation, so the projected dust distribution is best defined as fractal-like. In neither galaxy do we see regions even on single-pixel scales in spiral arms with AB > 2.5. The measurements in NGC 1275 are compromised by our lack of independent knowledge of the foreground system's light distribution, but masked sampling of the absorption suggests an effective reddening curve much flatter than the Milky Way mean (perhaps indicating that the foreground system has been affected by immersion in the hot intracluster gas).
Seeing Galaxies though Thick and Thin. IV. The Superimposed Spiral Galaxies of NGC 3314
William C. Keel,Raymond E. White III
Physics , 2001, DOI: 10.1086/322117
Abstract: The superimposed pair of spiral galaxies NGC 3314 offers a unique opportunity to trace the dust properties in a spiral galaxy. We analyze multicolor HST imaging, supported by ground-based near-IR imaging and fiber-array spectroscopy to measure dust extinction in the foreground Sc galaxy NGC 3314A, which is backlit by the Sb system NGC 3314B. We can measure extinctions over a wide range of galactocentric radii in the foreground galaxy, from 0.4-4.5 kpc. In the outer disk, the extinction is strongly localized in discrete dust lanes. These dust features show an extinction curve with a slope close to the Galactic mean (R = 3.5+/-0.3) from 1.6 to 3.8 kpc, with no radial trend. Using the I-K color of the background nucleus, we derive an extinction A(I) = 3.3 through the disk at a projected distance 400 pc from the nucleus of NGC 3314A. The extinction in even the inner disk of NGC 3314A is quite patchy, since background H-alpha emission is detected from all parts of the system. Local anticorrelations between foreground and background line emission demonstrate that the dust is concentrated to star-forming regions, as has been found for the blue light in several systems. Colors of dust lanes in NGC 3314A which are projected only partially against the background disk indicate that the dust scale height in the foreground disk is substantially smaller than that of the stars.
An Unusual Radio Galaxy in Abell 428: A Large, Powerful FR I Source in a Disk-Dominated Host
Michael J. Ledlow,Frazer N. Owen,William C. Keel
Physics , 1997, DOI: 10.1086/305251
Abstract: We report the discovery of a powerful (~10^{24} Watts/Hz) FR I radio source in a highly flattened disk-dominated galaxy. Half of the radio flux from this source is concentrated within the host galaxy, with the remainder in a pair of nearly symmetrical lobes with total extent ~200kpc nearly perpendicular to the disk. The traditional wisdom maintains that powerful, extended radio sources are found only in ellipticals or recent merger events. We report B,R,J, and K imaging, optical spectroscopy, a rotation curve, an IRAS detection, and a VLA 20cm image for this galaxy, 0313-192. The optical and NIR images clearly show a disk. We detect apparent spiral arms and a dust-lane from B band imaging. The reddened nucleus is consistent with extinction by a similar dust-lane. The optical spectrum suggests a central AGN and some evidence of a starburst, with both the AGN and central starlight appearing substantially reddened. From analysis of the extended line emission in [OIII] and H-alpha we derive a rotation curve consistent with an early- type, dusty spiral seen edge-on. From the IRAS detection at 60 and 100 microns, we find that the ratio of Far IR to radio flux places this object firmly as a radio galaxy (i.e. the radio emission is not powered by star formation). The radio structure suggests that the radio source in this galaxy is related to the same physical mechanisms present in jet-fed powerful radio sources, and that such powerful, extended sources can (albeit extremely rarely) occur in a disk-dominated host.
Seeing Galaxies Through Thick and Thin: I. Optical Opacity Measures in Overlapping Galaxies
Raymond E. White III,William C. Keel,Christopher J. Conselice
Physics , 1996,
Abstract: We describe the use of partially overlapping galaxies to provide direct measurements of the effective absorption in galaxy disks, independent of assumptions about internal disk structure. The non-overlapping parts of the galaxies and symmetry considerations are used to reconstruct, via differential photometry, how much background galaxy light is lost in passing through the foreground disks. Extensive catalog searches yield ~15-25 nearby galaxy pairs suitable for varying degrees of our analysis; ten of the best such examples are presented here. From these pairs, we find that interarm extinction is modest, declining from A_B ~1 magnitude at 0.3 R_25^B to essentially zero by R_25^B; the interarm dust has a scale length consistent with that of the disk starlight. In contrast, dust in spiral arms and resonance rings may be optically thick (A_B > 2) at virtually any radius. Some disks have flatter extinction curves than the Galaxy, with A_B/A_I = 1.6; this is probably the signature of clumpy dust distributions. Even though typical spirals are not optically thick throughout their disks, where they {\it are} optically thick is correlated with where they are most luminous: in spiral arms and inner disks. This correlation between absorption and emission regions may account for their apparent surface brightness being only mildly dependent on inclination, erroneously indicating that spirals are generally optically thick. Taken as an ensemble, the opacities of spiral galaxies may be just great enough to significantly affect QSO counts, though not enough to cause their high redshift cutoff.
Distribution and Content of Dust in Overlapping Galaxy Systems
Raymond E. White III,William C. Keel,Christopher J. Conselice
Physics , 1996,
Abstract: Partially overlapping galaxies are used to directly determine the effective absorption in spiral galaxy disks. The non-overlapping parts of the galaxies and symmetry considerations are used to reconstruct, via differential photometry, how much background galaxy light is lost in passing through the foreground disks.
Deep HST/PC Images of a Young Radio Galaxy at z=2.390
Rogier A. Windhorst,William C. Keel,Sebastian M. Pascarelle
Physics , 1997,
Abstract: We present deep 63-orbit HST/PC images at ~0.06" FWHM resolution in the filters B_450, V_606 & I_814 - as well as in redshifted Lya - of the radio source LBDS 53W002, a compact narrow-line galaxy at z=2.390. These images allow us to distinguish several morphological components: (1) an unresolved nuclear point source (<=500 pc at z=2.390 for Ho=75, qo=0), likely the central AGN which contains <=20-25% of the total light in BVI; (2) a compact continuum core (re~0.05"); (3) a more extended envelope with an r^1/4-like light-profile and re~0.25" (~2 kpc); (4) two blue ''clouds'' roughly colinear across the nucleus aligned with the radio source axis and contained well within the size of the radio source. (B-I) color maps may suggest a narrow dust lane crossing between the nucleus and the smaller blue cloud. The radio source is not smaller than the distance between the blue continuum clouds, and coincides with a bright Lya ''arc'' in the western cloud, suggesting that jet-induced star-formation could cause both blue clouds, except the outer parts of the western cloud. The shape of this larger blue cloud suggests reflected AGN continuum-light shining through a cone (plus re-radiated Lya in emission). The OVRO interferometric CO-detection (Scoville et al. 1997) on both sides of 53W002 - and in the same direction as the continuum clouds and the radio jet - also suggest a star-bursting region induced by its radio jet, at least in the inner parts. Even at radio powers ~1.5 dex fainter than the 3CR sources, we thus find many of the same aligned features and complex morphology, although at much smaller angular scales and lower optical-UV luminosities. We discuss the consequences for 53W002's formation in the context of the 16 sub-galactic objects at z~2.40 around 53W002 (Pascarelle et al. 1996).
Compact Lyman-alpha Emitting Candidates at z~2.4 in Deep Medium-band HST WFPC2 Images
Sebastian M. Pascarelle,Rogier A. Windhorst,William C. Keel
Physics , 1998, DOI: 10.1086/300634
Abstract: Medium-band imaging with HST/WFPC2 in the F410M filter has previously revealed a population of compact Lyman-alpha emission objects around the radio galaxy 53W002 at z~2.4. We report detections of similar objects at z~2.4 in random, high-latitude HST parallel observations of three additional fields, lending support to the idea that they constitute a widespread population at these redshifts. The three new fields contain 18 Lyman-alpha candidates, in contrast to the 17 detected in the deeper exposure of the single WFPC2 field around 53W002. We find substantial differences in the number of candidates from field to field, suggesting that significant large-scale structure is already present in the galaxy distribution at this cosmic epoch. The likely existence of z~2.4 sub-galactic clumps in several random fields shows that these objects may have been common in the early universe and strengthens the argument that such objects may be responsible for the formation of a fraction of the luminous present-day galaxies through hierarchical merging.
Seeing Galaxies Through Thick and Thin: II. Direct Measures of Extinction in Spiral Disks Through Spectroscopy of Overlapping Galaxies
Donovan L. Domingue,William C. Keel,Raymond E. White III
Physics , 2000, DOI: 10.1086/317767
Abstract: We use slit spectroscopy of overlapping pairs of galaxies to directly determine the extinction in disks of foreground spiral galaxies. The Doppler shifts of pair members are determined via cross-correlation and their relative correlation amplitudes are used to separate their contributions to the combined spectra in regions of overlap. This spectroscopic approach is less subject to stringent symmetry constraints than our previous purely photometric analyses. Extinctions of foreground members were obtained for 6 of the candidates in our sample of 18 mostly spiral/spiral pairs, when the signal to noise and velocity difference were suitable. In agreement with our previous imaging results, we find that the extinction in interarm regions is very modest, typically A_B=0.1 mag (corrected to face on), while spiral arms exhibit higher extinctions of 0.3 mag.
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