Juan M. Uson

Observatoire de Paris, Lutetia Parisorum, Île-de-France, France

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Publications (19)46.07 Total impact

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    ABSTRACT: We have used the VIRUS-P integral-field spectrometer on the University of Texas McDonald Observatory 2.7m telescope to observe the edge of the superthin spiral galaxy UGC7321. We detect faint Hα emission as expected from the exposure of the peripheral neutral Hydrogen gas traced by its 21cm radio emission to the metagalactic UV background. Observations of the intensity of the UV background and its redshift evolution are important to the theory and simulations of the evolution of large scale structure in the Universe as the UV background controls the cooling and collapse of small halos and is itself determined by the global histories of quasar and star formation. We have used dithered expositions with three pointings that fill in the gaps in the VIRUS-P detector for essentially full spatial coverage over a field of view of 1.6‧ x 1.6‧ and a spectral resolution of R = 3860 from 6040 Å to 6740 Å that allows us to resolve bright OH sky lines and geocoronal Hα from our target wavelength of 6574 Å. The Hα layer appears rather thin, with a peak surface brightness of Σ = 1.4 x 10-19 erg s-1 cm-2 arcsec-2 Å-1 for spectra smoothed with a 15″ spatial kernel. This leads to a measurement of the cosmic UV background induced HI photoionization rate Γ = 3.4 x 10-14 s-1 ( 5σ, preliminary absolute calibration). Contrary to past observational attempts, our measurements covered a large, two-dimensional on-sky area. We reach flux limits that are 50 times fainter than the sky background with significant smoothing over spatial elements and applying a sky background model that accounts for variations in the spectral resolution of our instrument. At this writing, we are continuing with the analysis of the data. Final results will be announced at the meeting.
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    Juan M. Uson, William D. Cotton
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    ABSTRACT: Radio astronomical imaging using aperture synthesis telescopes requires deconvolution of the point spread function as well as calibration of the instrumental characteristics (primary beam) and foreground (ionospheric/atmospheric) effects. These effects vary in time and also across the field of view, resulting in directionally-dependent (DD), time-varying gains. The primary beam will deviate from the theoretical estimate in real cases at levels that will limit the dynamic range of images if left uncorrected. Ionospheric electron density variations cause time and position variable refraction of sources. At low frequencies and sufficiently high dynamic range this will also defocus the images producing error patterns that vary with position and also with frequency due to the chromatic aberration of synthesis telescopes. Superposition of such residual sidelobes can lead to spurious spectral signals. Field-based ionospheric calibration as well as "peeling" calibration of strong sources leads to images with higher dynamic range and lower spurious signals but will be limited by sensitivity on the necessary short-time scales. The results are improved images although some artifacts remain.
    Comptes Rendus Physique 11/2011; 13(1). DOI:10.1016/j.crhy.2011.10.016 · 1.64 Impact Factor
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    ABSTRACT: We present new integral-field spectroscopy in the outskirts of two nearby, edge-on, late-type galaxies to search for the Hα emission that is expected from the exposure of their hydrogen gas to the metagalactic ultraviolet background (UVB). Despite the sensitivity of the VIRUS-P spectrograph on the McDonald 2.7 m telescope to low surface brightness emission and the large field of view, we do not detect Hα to 5σ upper limits of 6.4 × 10–19 erg s–1 cm–2 arcsec–2 in UGC 7321 and of 25 × 10–19 erg s–1 cm–2 arcsec–2 in UGC 1281 in each of the hundreds of independent spatial elements (fibers). We fit gas distribution models from overlapping 21 cm data of H I, extrapolate one scale length beyond the H I data, and estimate predicted Hα surface brightness maps. We analyze three types of limits from the data with stacks formed from increasingly large spatial regions and compare to the model predictions: (1) single fibers, (2) convolution of the fiber grid with a Gaussian, circular kernel (10'' full width at half-maximum), and (3) the co-added spectra from a few hundred fibers over the brightest model regions. None of these methods produce a significant detection (>5σ) with the most stringent constraints on the H I photoionization rate of Γ(z = 0) < 1.7 × 10–14 s–1 in UGC 7321 and Γ(z = 0) < 14 × 10–14 s–1 in UGC 1281. The UGC 7321 limit is below previous measurement limits and also below current theoretical models. Restricting the analysis to the fibers bound by the H I data leads to a comparable limit; the limit is Γ(z = 0) < 2.3 × 10–14 s–1 in UGC 7321. We discuss how a low Lyman limit escape fraction in z ~ 0 redshift star-forming galaxies might explain this lower than predicted UVB strength and the prospects of deeper data to make a direct detection.
    The Astrophysical Journal 01/2011; 728(2):107. DOI:10.1088/0004-637X/728/2/107 · 6.28 Impact Factor
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    ABSTRACT: We present new integral-field spectroscopy in the outskirts of two nearby, edge-on, late-type galaxies to search for the H-alpha emission that is expected from the exposure of their hydrogen gas to the metagalactic ultraviolet background (UVB). Despite the sensitivity of the VIRUS-P spectrograph on the McDonald 2.7m telescope to low surface brightness emission and the large field-of-view, we do not detect H-alpha to 5 sigma upper limits of 6.4 x 10^(-19) erg/s/cm^2/arcsec^2 in UGC 7321 and of 25 x 10^(-19) erg/s/cm^2/arcsec^2 in UGC 1281 in each of the hundreds of independent spatial elements (fibers). We fit gas distribution models from overlapping 21 cm data of HI, extrapolate one scale length beyond the HI data, and estimate predicted H-alpha surface brightness maps. We analyze three types of limits from the data with stacks formed from increasingly large spatial regions and compare to the model predictions: 1) single fibers, 2) convolution of the fiber grid with a Gaussian, circular kernel (10\arcsec\ full width half maximum), and 3) the coadded spectra from a few hundred fibers over the brightest model regions. None of these methods produce a significant detection (>5 sigma) with the most stringent constraints on the HI photoionization rate of Gamma(z=0)<1.7 x 10^(-14) s^(-1) in UGC 7321 and Gamma(z=0)<14 x 10^(-14) s^(-1) in UGC 1281. The UGC 7321 limit is below previous measurement limits and also below current theoretical models. Restricting the analysis to the fibers bound by the HI data leads to a comparable limit; the limit is Gamma(z=0)<2.3 x 10^(-14) s^(-1) in UGC 7321. We discuss how a low Lyman limit escape fraction in z~0 redshift star forming galaxies might explain this lower than predicted UVB strength and the prospects of deeper data to make a direct detection. Comment: accepted to ApJ; 16 pages, 6 figures, 2 tables
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    P. Aguirre, J. M. Uson, L. D. Matthews
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    ABSTRACT: UGC 10043 is an edge-on spiral galaxy with some peculiar morphological features that relate it to unsual objects with orthogonally rotating components like polar-ring galaxies and orthogonally decoupled bulge systems, which are the result of peculiar evolutionary processes that probably include mergers, acretions or other ``second events''. We observed UGC0043 with the VLA in HI spectral imaging mode in order to further investigate its structure, kinematics and evolutionary history, and find that UGC 10043 is forming part of a small group environment and most important, it is presently interacting with its neighbour galaxy MCG+04-37-035The discovery of this interaction helps us understand some of the peculiarities seen in UGC 10043, and shows us that its morphology and kinematics have been strongly affected by its local environment.
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    L. D. Matthews, Juan M. Uson
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    ABSTRACT: We recently reported the discovery of a regular corrugation pattern in the H I disk of the isolated, edge-on spiral galaxy IC 2233. Here we present measurements of the vertical structure of this galaxy at several additional wavelengths, ranging from the far-ultraviolet to the far-infrared. We find that undular patterns with amplitude 5'' ( 250 pc) are visible in a variety of Population I tracers in IC 2233, including the young to intermediate-age stars, the H II regions, and the dust. However, the vertical excursions become less pronounced in the older stellar populations traced by the mid-infrared light. This suggests that the process leading to the vertical displacements may be linked with the regulation of star formation in the galaxy. We have also identified a relationship between the locations of the density corrugations and small-amplitude ( 5 km s−1) velocity undulations in the H I rotation curve. We are able to exclude several possible mechanisms for the origin of the observed corrugations, including tidal interaction from a companion, Parker instabilities, or a galactic bore. Global gravitational instabilities appear to be the most likely explanation, although local perturbations may also be important.
    The Astrophysical Journal 12/2008; 688(1):237. DOI:10.1086/592086 · 6.28 Impact Factor
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    ABSTRACT: We have detected an X-ray absorption feature against the core of the galaxy cluster A2029 (z = 0.0767) that we identify with the foreground galaxy UZC J151054.6+054313 (z = 0.0221). Optical observations (B, V, R, and I) indicate that it is an Scd galaxy seen nearly edge-on at an inclination of 87° ± 3°. H I observations give a rotation velocity of 108 km s-1 and an atomic hydrogen mass of M = 3.1 × 109d M☉, where d90 is the distance to the galaxy in units of 90 Mpc. X-ray spectral fits to the Chandra absorption feature yield a hydrogen column density of (2.0 ± 0.4) × 1021 cm-2, assuming solar abundances. If the absorber is uniformly distributed over the disk of the galaxy, the implied hydrogen mass is MH = (6.2 ± 1.2) × 108d M☉. Since the absorbing gas in the galaxy is probably concentrated toward the center of the galaxy and the middle of the disk, this is a lower limit to the total hydrogen mass. On the other hand, the absorption measurements imply that the dark matter in UZC J151054.6+054313 is not distributed in a relatively uniform diffuse gas.
    The Astrophysical Journal 12/2008; 601(2):798. DOI:10.1086/380819 · 6.28 Impact Factor
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    W. D. Cotton, Juan M. Uson
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    ABSTRACT: This study investigates some of the consequences of representing the sky by a rectangular grid of pixels on the dynamic range of images derived from radio interferometric measurements. In particular, the effects of image pixelization coupled to the CLEAN deconvolution representation of the sky as a set of discrete delta functions can limit the dynamic range obtained when representing bright emission not confined to pixels on the grid. Sky curvature effects on non-coplanar arrays will limit the dynamic range even if strong sources are centered on a pixel in a "fly's eye" representation when such pixel is not located at the corresponding facet's tangent point. Uncertainties in the response function of the individual antennas as well as in the calibration of actual data due to ionospheric, atmospheric or other effects will limit the dynamic range even when using grid-less subtraction (i.e. in the visibility domain) of strong sources located within the field of view of the observation. A technique to reduce these effects is described and examples from an implementation in the Obit package are given. Application of this technique leads to significantly superior results without a significant increase in the computing time. Comment: 8 pages, 4 figures (one double, one triple), to appear in Astronomy and Astrophysics (accepted: September 5, 2008)
    Astronomy and Astrophysics 09/2008; DOI:10.1051/0004-6361:20079104 · 4.48 Impact Factor
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    Juan M. Uson, W. D. Cotton
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    ABSTRACT: Radio telescopes with off-axis feeds, such as the (E)VLA, suffer from "beam squint" in which the two orthogonal circular polarizations sampled have different pointing centers on the sky. Its effects are weak near the beam center but become increasingly important towards the edge of the antenna power pattern where gains in the two polarizations at a given sky position are significantly different. This effect has limited VLA measurements of circular polarization (Stokes V) and introduced dynamic range limiting, wide-field artifacts in images made in Stokes I. We present an adaptation of the visibility-based deconvolution CLEAN method that can correct this defect "on the fly" while imaging, correcting as well the associated self-calibration. We present two examples of this technique using the procedure "Squint" within the Obit package which allows wide-field imaging in Stokes V and reduced artifacts in Stokes I. We discuss the residual errors in these examples as well as a scheme for future correction of some of these errors. This technique can be generalized to implement temporally- and spatially-variable corrections, such as pointing and cross-polarization leakage errors.
    Astronomy and Astrophysics 07/2008; DOI:10.1051/0004-6361:200809509 · 4.48 Impact Factor
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    ABSTRACT: Astronomical imaging using aperture synthesis telescopes requires deconvolution of the point spread function as well as calibration of instrumental and atmospheric effects. In general, such effects are time-variable and vary across the field of view as well, resulting in direction-dependent (DD), time-varying gains. Most existing imaging and calibration algorithms assume that the corruptions are direction independent, preventing even moderate dynamic range full-beam, full-Stokes imaging. We present a general framework for imaging algorithms which incorporate DD errors. We describe as well an iterative deconvolution algorithm that corrects known DD errors due to the antenna power patterns and pointing errors for high dynamic range full-beam polarimetric imaging. Using simulations we demonstrate that errors due to realistic primary beams as well as antenna pointing errors will limit the dynamic range of upcoming higher sensitivity instruments and that our new algorithm can be used to correct for such errors. We have applied this algorithm to VLA 1.4 GHz observations of a field that contains two ``4C'' sources and have obtained Stokes-I and -V images with systematic errors that are one order of magnitude lower than those obtained with conventional imaging tools. Our simulations show that on data with no other calibration errors, the algorithm corrects pointing errors as well as errors due to known asymmetries in the antenna pattern. Comment: submitted to A&A; some clarifications added in the text; accepted for publication in A&A
    Astronomy and Astrophysics 05/2008; DOI:10.1051/0004-6361:20079284 · 4.48 Impact Factor
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    Juan M. Uson, L. D. Matthews
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    ABSTRACT: We have used the Very Large Array to image the isolated "superthin" galaxy UGC 7321 in the H I line with a spatial resolution of 16'' and a spectral resolution of 24 kHz (5.2 km s-1). We have reached a sensitivity of (0.36–0.40) mJy beam-1 channel-1, which correspond to a column density of (8–9) × 1018 atoms cm-2 (1 σ). UGC 7321 has a gas-rich disk, with MH I = (1.06 ± 0.01) × 109 d M and MH I/LB = 1.0 (d10 is the distance to UGC 7321 in units of 10 Mpc, the value adopted in this paper), and no detectable radio continuum emission (FCONT = 0.41 ± 0.25 mJy). The global H I distribution of UGC 7321 is rather symmetric and extends to ~1.5 times the optical radius (DH I = 865 ± 015 at nH I = 3 × 1019 atoms cm-2). An "integral sign" warp is observed in the H I disk, commencing near the edge of the stellar distribution and twisting back toward the equatorial plane in the outermost regions. In addition, the position-velocity diagram suggests the presence of a bar or inner arm within ~40'' from the center. The rotation curve of UGC 7321 is slowly rising; it reaches its asymptotic velocity of ~110 km s-1 at ~25 from the center (about 0.9 optical radii) and declines near the edge of the H I disk. The ratio of the inferred dynamical mass to the mass in gas and stars is ~12d, implying that UGC 7321 is a highly dark-matter–dominated galaxy.
    The Astronomical Journal 12/2007; 125(5):2455. DOI:10.1086/374627 · 4.05 Impact Factor
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    Lynn D. Matthews, Juan M. Uson
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    ABSTRACT: We have used the Very Large Array to image the H I 21 cm line emission in the edge-on Sd galaxy IC 2233 and the blue compact dwarf NGC 2537. We also present new optical B, R, and Hα imaging of IC 2233 obtained with the WIYN telescope. Despite evidence of localized massive star formation in the form of prominent H II regions and shells, supergiant stars, and a blue integrated color, IC 2233 is a low surface brightness system with a very low global star formation rate (0.05 M☉ yr−1), and we detect no significant 21 cm radio continuum emission from the galaxy. The H I and ionized gas disks of IC 2233 are clumpy and vertically distended, with scale heights comparable to that of the young stellar disk. Both the stellar and H I disks of IC 2233 appear flared, and we also find a vertically extended, rotationally anomalous component of H I extending to ~ 2.4d10 kpc from the midplane. The H I disk exhibits a mild lopsidedness as well as a global corrugation pattern with a period of ~7d10 kpc and an amplitude of ~150d10 pc. To our knowledge, this is the first time corrugations of the gas disk have been reported in an external galaxy; these undulations may be linked to bending instabilities or to underlying spiral structure and suggest that the disk is largely self-gravitating. Lying at a projected distance of from IC 2233, NGC 2537 has an H I disk with a bright, tilted inner ring and a flocculent, dynamically cold outer region that extends to ~3.5 times the extent of the stellar light (D25). Although NGC 2537 is rotationally-dominated, we measure H I velocity dispersions as high as km s−1 near its center, indicative of significant turbulent motions. The inner rotation curve rises steeply, implying a strong central mass concentration. Our data indicate that IC 2233 and NGC 2537 do not constitute a bound pair and most likely lie at different distances. We also find no compelling evidence of a recent minor merger in either IC 2233 or NGC 2537, suggesting that both are examples of small disk galaxies evolving in relative isolation.
    The Astronomical Journal 12/2007; 135(1):291. DOI:10.1088/0004-6256/135/1/291 · 4.05 Impact Factor
  • Juan M. Uson, L. D. Matthews
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    ABSTRACT: We have obtained new observations of UGC3697, the "Integral-Sign" galaxy. Spectral synthesis observations of neutral hydrogen (HI) with the VLA at a resolution of 12" show that the galaxy is located in a small group in which we detect hydrogen emission from eight distinct galaxies. We have obtained as well VRI wide-field observations with the Palomar Observatory 60" telescope and B, R, and H-alpha imaging with the WIYN telescope. UGC3697 is a low-mass, Sd spiral galaxy whose stellar disk exhibits a "superthin" yet highly warped structure with no spheroidal component and no visible dust lane. The warp is shared by the neutral and ionized gaseous components. The kinematic properties that follow from the HI observations show the remnants of a recent merger that provides a straightforward explanation for the properties of this galaxy, especially the contrast between its rather thin disk and the pronounced warp. We believe that the present state must be rather short-lived and the galaxy will soon undergo significant structural changes.
  • Lynn D. Matthews, J. M. Uson
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    ABSTRACT: The HI gas and other Population I tracers in the disk of the Milky Way are known to exhibit vertical deviations from the mean plane in the form of a regular pattern of displacements or "corrugations." The origin of these corrugations has been a longstanding puzzle, and little has been known about the presence of this phenomenon in external galaxies. Using data from the Very Large Array, we have discovered the first example of a corrugated structure in the HI disk of an external galaxy: the isolated, edge-on Sd spiral IC2233. The undulations in the HI disk of IC2233 are visible as a remarkably regular pattern of positive and negative displacements with a wavelength of ˜150" (7 kpc) and an amplitude that increases with distance from the center of the galaxy, reaching a maximum of ˜3" (? pc). Using data from Spitzer and GALEX, we confirm that these corrugations also affect the dust and young stellar populations of the galaxy, but are not as readily traceable in the old stars. The isolated nature of IC2233 suggests an internal rather than external origin for the corrugations, and we will evaluate the two most likely triggers: bending instabilities and spiral arms.
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    Lynn D. Matthews, Juan M. Uson
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    ABSTRACT: We have used the VLA to image the HI 21-cm line emission in the edge-on Sd galaxy IC2233 and the blue compact dwarf NGC2537. We also present new optical B,R, and H alpha imaging of IC2233 obtained with the WIYN telescope. Despite evidence of localized massive star formation, IC2233 has a low surface brightness disk with a low global star formation rate (~0.05 M_sun/yr), and no significant 21-cm radio continuum emission. The HI and ionized gas disks of IC2233 are clumpy and vertically distended, with scale heights comparable to the young stars. Both the stellar and HI disks of IC2233 appear flared, and we also find a vertically extended, rotationally anomalous HI component extending to z~2.4 kpc. The HI disk exhibits a mild lopsidedness as well as a global corrugation pattern with a period of ~7 kpc and an amplitude of ~150 pc. To our knowledge, this is the first time corrugations of the gas disk have been reported in an external galaxy; these undulations may be linked to bending instabilities or to underlying spiral structure and suggest that the disk is largely self-gravitating. Lying at a projected distance of 16.7' from IC2233, NGC2537 has an HI disk with a bright, tilted inner ring and a flocculent, dynamically cold outer region that extends to ~3.5D_25. Although NGC2537 is rotationally-dominated, it shows significant turbulence near its center. The inner rotation curve rises steeply, implying a strong central mass concentration. Our data indicate that IC2233 and NGC2537 do not constitute a bound pair and most likely lie at different distances. We also find no compelling evidence of a recent minor merger in either galaxy, suggesting that both are examples of small disk galaxies evolving in relative isolation. (Abridged)
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    ABSTRACT: UGC10043 is an edge-on Sbc spiral galaxy with several peculiar morphological characteristics that distinguish it from typical Sb-Sc galaxies, suggesting a complex formation history and evolution. Despite its thin, dynamically cold stellar disk, it shows a vertically elongated, triaxial bulge that is bisected on its SW quadrant by a dust lane perpendicular to its major axis. It also exhibits a large-scale galactic wind powered by a faint central starburst. Such features relate UGC10043 to families of relatively rare objects such as polar ring galaxies, orthogonally decoupled bulge systems, and minor-axis dust-lane ellipticals, suggesting that its evolution must have included a significant merger or accretion event. New HI spectral imaging observations carried out with the VLA in its C and D configurations have provided us with new insight into the complex evolutionary history of UGC10043. Our observations reveal that the galaxy is located in a small group, and we detect HI emission from four of its galaxies. We also have uncovered a kinematically continuous HI bridge parallel to the major axis of UGC10043 that connects it to a previously unknown companion, MGC+04-37-035, which is being dynamically heated by the interaction. We will discuss how the current interaction has likely affected the present morphological and kinematic peculiarities of UGC10043 and more generally how studies of this system can offer important insights into how the group environment affects the structure and evolution of typical disk galaxies.
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    ABSTRACT: Using the IRAM 30-m telescope, we have obtained 12CO J=1-0 and 2-1 spectral line observations toward the nuclear regions of 15 edge-on, low surface brightness (LSB) spiral galaxies. Our sample comprises extreme late-type LSB spirals with disk-dominated morphologies and rotational velocities V_rot<~120 km/s. We report detections of four galaxies in at least one transition (>5sigma); for the remainder of the sample we provide upper limits on the nuclear CO content. Adopting a standard Galactic I_CO-to-H_2 conversion factor implies molecular gas masses of (3.3-9.8)x10**6 M_sun in the nuclear regions (inner 1.1-1.8 kpc) of the detected galaxies. Combining our new data with samples of late-type spirals from the literature, we find that the CO-detected LSB spirals adhere to the same M_H2-FIR correlation as more luminous and higher surface brightness galaxies. The amount of CO in the central regions of late-type spirals appears to depend more strongly on mass than on central optical surface brightness, and CO detectability declines significantly for moderate-to-low surface brightness spirals with V_rot<~90 km/s; no LSB spirals have so far been detected in CO below this threshold. Metallicity effects alone are unlikely to account for this trend, and we speculate that we are seeing the effects of a decrease in the mean fraction of a galaxy disk able to support giant molecular cloud formation with decreasing galaxy mass. Comment: accepted to AJ
    The Astronomical Journal 01/2005; 129(4). DOI:10.1086/428857 · 4.05 Impact Factor
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    Lynn D. Matthews, Juan M. Uson
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    ABSTRACT: We summarize results from an HI+optical imaging study of the ``Integral Sign'' galaxy, UGC3697. UGC3697 is a low-mass, Sd spiral that exhibits a ``superthin'' disk morphology despite a prounced gasous and stellar warp. Our new observations show evidence for a recent minor merger in this system that could account for its large-scale warp and a number of other properties of this galaxy. We speculate that UGC3697 has been caught in a rather short-lived dynamical state, and may soon undergo significant structural and morphological changes.
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    W. D. Cotton, Juan M. Uson
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    ABSTRACT: Radio telescopes with o{axis feeds such as the (E)VLA suer from \Beam Squint" in which the two orthogonal polarizations sampled have dieren t pointing centers on the sky. The eect of this is small near the beam center but is increasingly important towards the edge of the antenna power pattern where there are signican tly dieren t gains in the two polarizations. This eect has seriously limited the usability of the VLA for measuring circular polarization (Stokes V) and introduces dynamic range limiting wide{eld artifacts in Stokes I. An adaptation of the visibility{ based CLEAN (5) can correct for this eect. Examples are given of this technique being applied using the Obit (http://www.cv.nrao.edu/ bcotton/Obit.html) package which allows wide{eld imaging in Stokes V and reduced artifacts in Stokes I. This technique can be generalized to deal with temporally and spatially variable corrections.

Publication Stats

145 Citations
46.07 Total Impact Points

Institutions

  • 2011
    • Observatoire de Paris
      Lutetia Parisorum, Île-de-France, France
  • 2007–2008
    • National Radio Astronomy Observatory
      Charlottesville, Virginia, United States
  • 2005
    • University of Massachusetts Amherst
      • Department of Astronomy
      Amherst Center, Massachusetts, United States