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Search Results: 1 - 10 of 194552 matches for " Susan G. Neff "
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The Complex North Transition Region of Centaurus A: A Galactic Wind
Susan G. Neff,Jean A. Eilek,Frazer N. Owen
Physics , 2015, DOI: 10.1088/0004-637X/802/2/88
Abstract: We present deep GALEX images of NGC 5128, the parent galaxy of Centaurus A. We detect a striking "weather ribbon" of Far-UV and H$\alpha$ emission, which extends more than 35 kpc northeast of the galaxy. The ribbon is associated with a knotty ridge of radio/X-ray emission, and is an extension of the previously known string of optical emission-line filaments. Many phenomena in the region are too short-lived to have survived transit out from the inner galaxy; something must be driving them locally. We also detect Far-UV emission from the galaxy's central dust lane. Combining this with previous radio and Far-IR measurements, we infer an active starburst in the central galaxy, which is currently forming stars at $\sim 2 M_{sun}$yr$^{-1}$, and has been doing so for 50-100Myr. If the wind from this starburst is enhanced by energy and mass driven out from the AGN, the powerful augmented wind can be the driver needed for the northern weather system. We argue that both the diverse weather system, and the enhanced radio emission in the same region, result from the wind's encounter with cool gas left by one of the recent merger/encounter events in the history of NGC 5128.
The Complex North Transition Region of Centaurus A: Radio Structure
Susan G. Neff,Jean A. Eilek,Frazer N. Owen
Physics , 2015, DOI: 10.1088/0004-637X/802/2/87
Abstract: We present deep radio images of the inner 50 kpc of Centaurus A, taken with the Karl G. Jansky Very Large Array (VLA) at 90cm. We focus on the Transition Regions between the inner galaxy - including the active nucleus, inner radio lobes, and star-forming disk - and the outer radio lobes. We detect previously unknown extended emission around the Inner Lobes, including radio emission from the star-forming disk. We find that the radio-loud part of the North Transition Region, known as the North Middle Lobe, is significantly overpressured relative to the surrounding ISM. We see no evidence for a collimated flow from the Active Galactic Nucleus (AGN) through this region. Our images show that the structure identified by Morganti et al. (1999) as a possible large-scale jet appears to be part of a narrow ridge of emission within the broader, diffuse, radio-loud region. This knotty radio ridge is coincident with other striking phenomena: compact X-ray knots, ionized gas filaments, and streams of young stars. Several short-lived phenomena in the North Transition Region, as well as the frequent re-energization required by the Outer Lobes, suggest that energy must be flowing through both Transition Regioins at the present epoch. We suggest that the energy flow is in the form of a galactic wind.
The Luminous Starburst Ring in NGC 7771: Sequential Star Formation?
Denise A. Smith,Terry Herter,Martha P. Haynes,Susan G. Neff
Physics , 1998, DOI: 10.1086/306605
Abstract: Only two of the twenty highly luminous starburst galaxies analyzed by Smith et al. exhibit circumnuclear rings of star formation. These galaxies provide a link between 10^11 L_sun systems and classical, less-luminous ringed systems. We report the discovery of a near-infrared counterpart to the nuclear ring of radio emission in NGC 7771. A displacement between the ~10 radio bright clumps and the ~10 near-infrared bright clumps indicates the presence of multiple generations of star formation. The estimated thermal emission from each radio source is equivalent to that of ~35000 O6 stars. Each near-infrared bright knot contains ~5000 red supergiants, on average. The stellar mass of each knot is estimated to be ~10^7 M_sun. The implied time-averaged star formation rate is \~40 M_sun per yr. Several similarities are found between the properties of this system and other ringed and non-ringed starbursts. Morphological differences between NGC 7771 and the starburst + Seyfert 1 galaxy NGC 7469 suggest that NGC 7771 may not be old enough to fuel an AGN, or may not be capable of fueling an AGN. Alternatively, the differences may be unrelated to the presence or absence of an AGN and may simply reflect the possibility that star formation in rings is episodic.
Scientific Objectives for UV/Visible Astrophysics Investigations: A Summary of Responses by the Community (2012)
Paul A. Scowen,Mario R. Perez,Susan G. Neff,Dominic J. Benford
Physics , 2013, DOI: 10.1007/s10686-013-9363-0
Abstract: Following several recommendations presented by the Astrophysics Decadal Survey 2010 centered around the need to define "a future ultraviolet-optical space capability," on 2012 May 25, NASA issued a Request for Information (RFI) seeking persuasive ultraviolet (UV) and visible wavelength astrophysics science investigations. The goal was to develop a cohesive and compelling set of science objectives that motivate and support the development of the next generation of ultraviolet/visible space astrophysics missions. Responses were due on 10 August 2012 when 34 submissions were received addressing a number of potential science drivers. A UV/visible Mission RFI Workshop was held on 2012 September 20 where each of these submissions was summarized and discussed in the context of each other. We present a scientific analysis of these submissions and presentations and the pursuant measurement capability needs, which could influence ultraviolet/visible technology development plans for the rest of this decade. We also describe the process and requirements leading to the inception of this community RFI, subsequent workshop and the expected evolution of these ideas and concepts for the remainder of this decade.
Ultraviolet Imaging of the Irregular Galaxy NGC 4449 with UIT: Photometry and Recent Star-Formation History
Robert S. Hill,Michael N. Fanelli,Denise A. Smith,Ralph C. Bohlin,Susan G. Neff,Robert W. O'Connell,Morton S. Roberts,Andrew M. Smith,Theodore P. Stecher
Physics , 1998, DOI: 10.1086/306302
Abstract: The bright Magellanic irregular galaxy NGC 4449 was observed during the Astro-2 Space Shuttle mission by the Ultraviolet Imaging Telescope (UIT), which obtained images of a ~40 arcmin field centered on the galaxy in two broad far-ultraviolet (FUV) bands centered at 1520 A and 1620 A, with 3 arcsec - 5 arcsec spatial resolution. Together with H-alpha and H-beta fluxes from ground-based Fabry-Perot images, these data are analyzed in order to explore the recent star formation history of NGC 4449. Maps of the flux ratios H-alpha/FUV and FUV/blue continuum are presented and interpreted using evolutionary synthesis models. Photometry is presented both for 22 apertures containing large OB complexes and for 57 small apertures containing compact FUV-emitting knots. The OB complexes along the northern edge of the visible system have high H-alpha/FUV ratios, and thus appear to be more dominated by the current generation of stars than are other parts of the galaxy. However, young sources do exist elsewhere and are particularly conspicuous along the bar. The small aperture analysis shows three candidate regions for sequential star formation. Surface brightness profiles are consistent with an exponential disk in both the FUV and the optical continuum.
Ultraviolet Signatures of Tidal Interaction in the Giant Spiral Galaxy, M101
William H. Waller,Ralph C. Bohlin,Robert H. Cornett,Michael N. Fanelli,Wendy L. Freedman,Jesse K. Hill,Barry F. Madore,Susan G. Neff,Joel D. Offenberg,Robert W. O'Connell,Morton S. Roberts,Andrew M. Smith,Theodore P. Stecher
Physics , 1996, DOI: 10.1086/304057
Abstract: We present new evidence for tidal interactions having occurred in the disk of M101 in the last 10^8 - 10^9 years. Recent imaging of the far-ultraviolet emission from M101 by the Shuttle-borne Ultraviolet Imaging Telescope (UIT) reveals with unprecedented clarity a disk-wide pattern of multiple linear arm segments (``crooked arms''). The deep FUV image also shows a faint outer spiral arm with a (``curly tail'') feature that appears to loop around the supergiant HII region NGC 5471 - linking this outlying starburst with the rest of the galaxy. These FUV-bright features most likely trace hot O & B-type stars along with scattered light from associated nebular dust. Counterparts of the outermost ``crooked arms'' are evident in maps at visible wavelengths and in the 21-cm line of HI. The inner-disk FUV arms are most closely associated with H$\alpha$ knots and the outer (downstream) sides of CO arms. Comparisons of the ``crooked arm'' and ``curly tail'' morphologies with dynamical simulations yield the greatest similitude, when the non- axisymmetric forcing comes from a combination of ``external interactions'' with one or more companion galaxies and ``internal perturbations'' from massive objects orbiting within the disk. We speculate that NGC 5471 represents one of these ``massive disturbers'' within the disk, whose formation followed from a tidal interaction between M101 and a smaller galaxy.
Ultraviolet Imaging Telescope (UIT) Observations of the Small Magellanic Cloud
Robert H. Cornett,Michael R. Greason,Jesse K. Hill,Joel Wm. Parker,William H. Waller,Ralph C. Bohlin,Kwang-Peng Cheng,Susan G. Neff,Robert W. O'Connell,Morton S. Roberts,Andrew M. Smith,Theodore P. Stecher
Physics , 1996,
Abstract: A mosaic of four UIT far-UV (FUV; 1620A) images, which covers most of the SMC bar, is presented, with derived stellar and HII region photometry. The UV morphology of the Bar shows that recent star formation there has left striking features including: a) four concentrations of UV-bright stars spread from northeast to southwest at nearly equal (~30 arcmin=0.5 kpc) spacings; b) a well-defined 8-arcmin ring of UV-bright stars surrounded by a larger H-alpha ring, suggesting sequential star formation. FUV PSF photometry is obtained for 11,306 stars, and a FUV luminosity function is derived. A (FUV-V,V) color-magnitude diagram for 195 identified supergiants, with derived extinctions; the bluest (least-reddened) stars of each spectral type have FUV-V colors consistent with models. FUV photometry is obtained for 42 H-alpha-selected HII regions, both for the stars and for the total emission within the apertures defined by Kennicutt & Hodge. The flux-weighted average ratio of total to stellar FUV flux is 2.15; the stellar FUV luminosity function indicates that most of the excess total flux is due to scattered FUV radiation, rather than faint stars. Both stellar and total emission are well correlated with H-alpha fluxes, and yield FUV/H-alpha ratios that are consistent with models of single-burst clusters with SMC metallicity, ages from 1-5 Myr, and moderate (E(B-V)=0.0-0.1 mag) internal SMC extinction.
Luminous Thermal Flares from Quiescent Supermassive Black Holes
Suvi Gezari,Tim Heckman,S. Bradley Cenko,Michael Eracleous,Karl Forster,Thiago S. Goncalves,D. Chris Martin,Patrick Morrissey,Susan G. Neff,Mark Seibert,David Schiminovich,Ted K. Wyder
Physics , 2009, DOI: 10.1088/0004-637X/698/2/1367
Abstract: A dormant supermassive black hole lurking in the center of a galaxy will be revealed when a star passes close enough to be torn apart by tidal forces, and a flare of electromagnetic radiation is emitted when the bound fraction of the stellar debris falls back onto the black hole and is accreted. Here we present the third candidate tidal disruption event discovered in the GALEX Deep Imaging Survey: a 1.6x10^{43} erg s^{-1} UV/optical flare from a star-forming galaxy at z=0.1855. The UV/optical SED during the peak of the flare measured by GALEX and Palomar LFC imaging can be modeled as a single temperature blackbody with T_{bb}=1.7x10^{5} K and a bolometric luminosity of 3x10^{45} erg s^{-1}, assuming an internal extinction with E(B-V)_{gas}=0.3. The Chandra upper limit on the X-ray luminosity during the peak of the flare, L_{X}(2-10 keV)< 10^{41} erg s^{-1}, is 2 orders of magnitude fainter than expected from the ratios of UV to X-ray flux density observed in active galaxies. We compare the light curves and broadband properties of all three tidal disruption candidates discovered by GALEX, and find that (1) the light curves are well fitted by the power-law decline expected for the fallback of debris from a tidally disrupted solar-type star, and (2) the UV/optical SEDs can be attributed to thermal emission from an envelope of debris located at roughly 10 times the tidal disruption radius of a ~10^{7} M_sun central black hole. We use the observed peak absolute optical magnitudes of the flares (-17.5 > M_{g} > -18.9) to predict the detection capabilities of upcoming optical synoptic surveys. (Abridged)
UIT Detection of Hot Stars in the Globular Cluster NGC362
Ben Dorman,Ronak Y. Shah,Robert W. O'Connell,Wayne B. Landsman,Robert T. Rood,Ralph C. Bohlin,Susan G. Neff,Morton S. Roberts,Andrew M. Smith,Theodore P. Stecher
Physics , 1997, DOI: 10.1086/310613
Abstract: We used the Ultraviolet Imaging Telescope during the March 1995 Astro-2 mission to obtain a deep far-UV image of the globular cluster NGC 362, which was formerly thought to have an almost entirely red horizontal branch (HB). 84 hot (T_eff > 8500 K) stars were detected within a radius of 8'.25 of the cluster center. Of these, 43 have FUV magnitudes consistent with HB stars in NGC 362, and at least 34 are cluster members. The number of cluster members is made uncertain by background contamination from blue stars in the Small Magellanic Cloud (SMC). There are six candidate supra-HB stars which have probably evolved from the HB. We discuss the implications of these results for the production of hot blue stars in stellar populations.
Ultraviolet Signposts of Resonant Dynamics in the Starburst-Ringed Sab Galaxy, M94 (NGC 4736)
William H. Waller,Michael N. Fanelli,William C. Keel,Ralph Bohlin,Nicholas R. Collins,Barry F. Madore,Pamela M. Marcum,Susan G. Neff,Robert W. O'Connell,Joel D. Offenberg,Morton S. Roberts,Andrew M. Smith,Theodore P. Stecher
Physics , 2000, DOI: 10.1086/319384
Abstract: M94 (NGC 4736) is investigated using images from the Ultraviolet Imaging Telescope (FUV-band), Hubble Space Telescope (NUV-band), Kitt Peak 0.9-m telescope (H-alpha, R, and I bands), and Palomar 5-m telescope (B-band), along with spectra from the International Ultraviolet Explorer and Lick 1-m telescopes. The wide-field UIT image shows FUV emission from (a) an elongated nucleus, (b) a diffuse inner disk, where H-alpha is observed in absorption, (c) a bright inner ring of H II regions at the perimeter of the inner disk (R = 48 arcsec. = 1.1 kpc), and (d) two 500-pc size knots of hot stars exterior to the ring on diametrically opposite sides of the nucleus (R= 130 arcsec. = 2.9 kpc). The HST/FOC image resolves the NUV emission from the nuclear region into a bright core and a faint 20 arcsec. long ``mini-bar'' at a position angle of 30 deg. Optical and IUE spectroscopy of the nucleus and diffuse inner disk indicates an approximately 10^7 or 10^8 yr-old stellar population from low-level starbirth activity blended with some LINER activity. Analysis of the H-alpha, FUV, NUV, B, R, and I-band emission along with other observed tracers of stars and gas in M94 indicates that most of the star formation is being orchestrated via ring-bar dynamics involving the nuclear mini-bar, inner ring, oval disk, and outer ring. The inner starburst ring and bi-symmetric knots at intermediate radius, in particular, argue for bar-mediated resonances as the primary drivers of evolution in M94 at the present epoch. Similar processes may be governing the evolution of the ``core-dominated'' galaxies that have been observed at high redshift. The gravitationally-lensed ``Pretzel Galaxy'' (0024+1654) at a redshift of approximately 1.5 provides an important precedent in this regard.
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