Publications (11)14.7 Total impact
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Article: Polar Ring Galaxies in the Galaxy Zoo
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ABSTRACT: We report observations of 16 candidate polar ring galaxies (PRGs) identified by the Galaxy Zoo project in the Sloan Digital Sky Survey (SDSS) database. Deep images of five galaxies are available in the SDSS Stripe82 database, while to reach similar depth we observed the remaining galaxies with the 1.8-m Vatican Advanced Technology Telescope. We derive integrated magnitudes and u-r colours for the host and ring components and show continuum-subtracted H\alpha+[NII] images for seven objects. We present a basic morphological and environmental analysis of the galaxies and discuss their properties in comparison with other types of early-type galaxies. Follow-up photometric and spectroscopic observations will allow a kinematic confirmation of the nature of these systems and a more detailed analysis of their stellar populations.02/2012; -
Article: Dust and Ionized Gas Association in E/S0 Galaxies with Dust Lanes: Clues to their Origin
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ABSTRACT: We present results from an on-going programme to study the dust and ionized gas in E/S0 galaxies with dust lanes. Our data, together with results from previous studies of E/S0 galaxies, are used to demonstrate the tight relationship between these two components. This relationship is discussed in light of our current understanding of the nature and origin of the interstellar medium (ISM), and in particular in the context of the interplay between the different multi-temperature components. We show that focusing on dust obscured regions as tracers of the ISM, and on their properties, serves as independent evidence for the external origin of the dust and ionized gas.02/2012; -
Article: Continuum removal in H\alpha\ extragalactic measurements
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ABSTRACT: We point out an important source of error in measurements of extragalactic H-alpha emission and suggest ways to reduce it. The H-alpha line, used for estimating star formation rates, is commonly measured by imaging in a narrow band and a wide band, both which include the line. The image analysis relies on the accurate removal of the underlying continuum. We discuss in detail the derivation of the emission line's equivalent width and flux for extragalactic extended sources, and the required photometric calibrations. We describe commonly used continuum-subtraction procedures, and discuss the uncertainties that they introduce. Specifically, we analyse errors introduced by colour effects. We show that the errors in the measured H-alpha equivalent width induced by colour effects can lead to underestimates as large as 40% and overestimates as large as 10%, depending on the underlying galaxy's stellar population and the continuum-subtraction procedure used. We also show that these errors may lead to biases in results of surveys, and to the underestimation of the cosmic star formation rate at low redshifts (the low z points in the Madau plot). We suggest a method to significantly reduce these errors using a single colour measurement.10/2011; -
Article: Hoag’s Object: evidence for cold accretion on to an elliptical galaxy
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ABSTRACT: We present new photometric and spectroscopic observations of the famous Hoag’s Object, a peculiar ring galaxy with a central roundish core. The nature of Hoag’s Object is still under controversy. Previous studies demonstrated that a major accretion event that took place at least 2–3 Gyr ago can account for the observational evidence. However, the role of internal non-linear mechanisms in forming the outer ring was not yet completely ruled out.The observations reported here consist of WFPC2 optical data retrieved from the Hubble Space Telescope archive as well as long-slit and 3D spectroscopic data obtained at the Russian BTA 6-m telescope. These new data, together with H i and optical information from the literature, are used to demonstrate that Hoag’s Object is a relatively isolated system surrounded by a luminous quasi-spiral pattern and a massive, low-density H i disc. The main stellar body is an old, mildly triaxial elliptical galaxy with very high angular momentum.We review previous formation scenarios of Hoag’s Object in light of the new data and conclude that the peculiar morphology could not represent a late phase in barred early-type galaxies evolution. In addition, no observational evidence supports late merging events in the evolution of the galaxy, although further tests are required before safely dismissing this idea. Combining all the information, we propose a new scenario where the elliptical core formed in the early Universe with the H i disc forming shortly after the core by prolonged ‘cold’ accretion of primordial gas from the intergalactic medium.The low gas density does not allow intense star formation to occur everywhere in the disc, but only along a tightly wound spiral pattern of enhanced density induced by the triaxial gravitational potential. According to this view, the physical mechanism that forms rings in Hoag-like galaxies is closely linked with that in some non-barred disc galaxies, although the formation and evolution of both classes of galaxies are clearly distinct. Whether or not this unique evolutionary track is related to the galaxy residing in an underdensed environment remains to be solved. A detailed H i mapping of Hoag’s Object and its environment is required to test our hypothesis and to examine the nature of the H i disc.Monthly Notices of the Royal Astronomical Society 09/2011; 418(3):1834 - 1849. · 4.90 Impact Factor -
Article: Hoag's Object: Evidence for Cold Accretion onto an Elliptical Galaxy
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ABSTRACT: (Abridged) We present new photometric and spectroscopic observations of the famous Hoag's Object, a peculiar ring galaxy with a central roundish core. The nature of Hoag's Object is still under controversial discussion. Previous studies demonstrated that a major accretion event that took place at least 2-3 Gyr ago can account for the observational evidence. However, the role of internal nonlinear mechanisms in forming the outer ring was not yet completely ruled out. These new data, together with HI and optical information from the literature, are used to demonstrate that Hoag's Object is a relatively isolated system surrounded by a luminous quasi-spiral pattern and a massive, low-density HI disc. The main stellar body is an old, mildly triaxial elliptical galaxy with very high angular momentum. We review previous formation scenarios of Hoag's Object in light of the new data and conclude that the peculiar morphology could not represent a late phase in barred early-type galaxies evolution. In addition, no observational evidence supports late merging events in the evolution of the galaxy, although further tests are required before safely dismissing this idea. We propose a new scenario where the elliptical core formed in the early Universe with the HI disc forming shortly after the core by prolonged "cold" accretion of primordial gas from the intergalactic medium. The low gas density does not allow intense star formation to occur everywhere in the disc, but only along a tightly wound spiral pattern of enhanced density induced by the triaxial gravitational potential.08/2011; -
Article: UGC 4599: A Photometric Study of the Nearest Hoag-Type Ring Galaxy
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ABSTRACT: We present a photometric study of UGC 4599, a low-luminosity galaxy superficially resembling Hoag's Object in that on sky survey images it appears to be a complete ring surrounding a roundish core. The nature of the outer ring of Hoag-type galaxies is still debated and may be related either to slow secular evolution or to environmental processes, such as galaxy-galaxy interactions. we show that in UGC 4599 (a) the nearly round central body follows well an r^1/4 light profile almost all the way to the centre, (b) the isophotes are strongly twisted with a sharp 45 deg transition at a radius of r~6 arcsec, (c) the blue ring seems to have reached near-equilibrium configuration with the central body, (d) the ring is actually composed of a one-and-a-half turn spiral feature, and (e) one side of the spiral shows conspicuous star formation in the form of at least nine HII regions, revealed by their H_alpha emission. Based on the photometric data, together with HI information from the literature, we characterize UGC 4599 as an elliptical-like object surrounded by a luminous ring and a massive, extremely extended HI disc. Given its observed properties, we rule out UGC 4599 as representing a late phase in barred early-type galaxies evolution. We discuss the origin of UGC 4599 and conclude that this galaxy could be the result of a major interaction between two gas-rich spiral galaxies that took place at least 5 Gyr ago. However, deep optical imaging and a detailed stellar population analysis are required to determine whether the large gas reservoir could have been accreted directly from the intergalactic medium onto a pre-existing elliptical galaxy in the early Universe. A detailed kinematical study will shed light on the exact nature of the central body and the ring of UGC 4599.02/2011; -
Article: A candidate polar-ring galaxy in the Subaru Deep Field
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ABSTRACT: We discuss the properties of an object in the Subaru Deep Field (SDF) classified as a galaxy in on-line data bases and revealed on the Subaru images as a genuine polar-ring galaxy (PRG) candidate. We analyse available photometric data and conclude that this object consists of a >5 Gyr old early-type central body surrounded by a faint, narrow inner ring tilted at a ~25 deg angle relative to the polar axis of the host galaxy. The halo surrounding the main stellar body exhibits a diversity of spatially extended stellar features of low surface brightness, including a faint asymmetric stellar cloud and two prominent loops. These faint features, together with the unperturbed morphology of the central host, are clear signs of a recent coalescence of two highly unequal mass galaxies, most likely a pre-existing early-type galaxy and a close-by gas-rich dwarf galaxy. The presumed stellar remnants observed near the edges of the ring, including possibly the surviving captured companion itself, indicate that the merger is still taking place. Comment: 6 pages, 5 figures, 1 table. Accepted for publication in MNRAS12/2010; -
Article: Determining the extragalactic extinction law with SALT – II. Additional sample★
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ABSTRACT: We present new results from an ongoing programme to study the dust extragalactic extinction law in E/S0 galaxies with dust lanes with the Southern African Large Telescope (SALT) during its performance verification phase. The wavelength dependence of the dust extinction for seven galaxies is derived in six spectral bands ranging from the near-ultraviolet atmospheric cut-off to the near-infrared. The derivation of an extinction law is performed by fitting model galaxies to the unextinguished parts of the image in each spectral band, and subtracting from these the actual images. We compare our results with the derived extinction law in the Galaxy and find them to run parallel to the Galactic extinction curve with a mean total-to-selective extinction value of RV= 2.71 ± 0.43. We use total optical extinction values to estimate the dust mass for each galaxy, compare these with dust masses derived from IRAS measurements, and find them to range from 104 to 107 M⊙. We study the case of the well-known dust-lane galaxy NGC 2685 for which Hubble Space Telescope/Wide Field Planetary Camera 2 (HST/WFPC2) data are available to test the dust distribution on different scales. Our results imply a scale-free dust distribution across the dust lanes, at least within ∼1 arcsec (∼60 pc) regions.Monthly Notices of the Royal Astronomical Society 11/2010; 409(2):727 - 736. · 4.90 Impact Factor -
Article: Ionized gas in E/S0 galaxies with dust lanes
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ABSTRACT: We report the results of multicolour observations of 30 E/S0 galaxies with dust lanes. For each galaxy we obtained broad-band images and narrow-band images using interference filters isolating the H\alpha +[NII] emission lines to derive the amount and morphology of dust and ionized gas. To improve the wavelength coverage we retrieved data from the SDSS and 2MASS and combined these with our data. Ionized gas is detected in 25 galaxies and shows in most cases a smooth morphology, although knots and filamentary structure are also observed in some objects. The extended gas distribution closely follows the dust structure, with a clear correlation between the mass of both components. An extinction law by the extragalactic dust in the dark lanes is derived and is used to estimate the dust content of the galaxies. The derived extinction law is used to correct the measured colours for intrinsic dust extinction and the data are fitted with a stellar population synthesis model. We find that the H-alpha emission and colours of most objects are consistent with the presence of an "old" stellar population (~10 Gyr) and a small fraction of a "young" population (~10-100 Myr). To check this we closely examine NGC5363, for which archival Spitzer/IRAC and GALEX data are available, as a representative dust-lane E/S0 galaxy of the sample. Comment: 28 pages, 15 figures and 10 tables. Minor corrections included to match published MNRAS version. R-band contour maps are low-resolution versions of those used in the MNRAS version05/2010; -
Article: Determining the extragalactic extinction law with SALT★
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ABSTRACT: We present CCD imaging observations of early-type galaxies with dark lanes obtained with the Southern African Large Telescope (SALT) during its performance-verification phase. The observations were performed in six spectral bands that span the spectral range from the near-ultraviolet atmospheric cut-off to the near-infrared. We derive the extinction law by the extragalactic dust in the dark lanes in the spectral range 1.11 < λ−1 < 2.94 μm−1 by fitting model galaxies to the unextinguished parts of the image, and subtracting from these the actual images. This procedure allows the derivation, with reasonably high signal-to-noise ratio, of the extinction in each spectral band we used for each resolution element of the image. We also introduce an alternative method to derive the extinction values by comparing various colour-index maps under the assumption of negligible intrinsic colour gradients in these galaxies. We than compare the results obtained using these two methods.We compare the total-to-selective extinction derived for these galaxies with previously obtained results and with similar extinction values of Milky Way dust to derive conclusions about the properties of extragalactic dust in different objects and conditions.We find that the extinction curves run parallel to the Galactic extinction curve, which implies that the properties of dust in the extragalactic environment are similar to those of the Milky Way, despite our original expectations. The ratio of the total V-band extinction to the selective extinction between the V and B bands is derived for each galaxy with an average of 2.82 ± 0.38, compared to a canonical value of 3.1 for the Milky Way. The similar values imply that galaxies with well-defined dark lanes have characteristic dust grain sizes similar to those of Galactic dust. We use total optical extinction values to estimate the dust mass for each galaxy, compare these with dust masses derived from IRAS measurements, and find them in the range 104–107 M⊙.Monthly Notices of the Royal Astronomical Society 10/2008; 390(3):969 - 984. · 4.90 Impact Factor -
Article: Determining the extragalactic extinction law with SALT
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ABSTRACT: We present CCD imaging observations of early-type galaxies with dark lanes obtained with the Southern African Large Telescope (SALT) during its performance-verification phase. We derive the extinction law by the extragalactic dust in the dark lanes in the spectral range 1.11mu m^{-1} < lambda^{-1} < 2.94 mu m^{-1} by fitting model galaxies to the unextinguished parts of the image, and subtracting from these the actual images. We find that the extinction curves run parallel to the Galactic extinction curve, which implies that the properties of dust in the extragalactic enviroment are similar to those of the Milky Way. The ratio of the total V band extinction to the selective extinction between the V and B bands is derived for each galaxy with an average of 2.82+-0.38, compared to a canonical value of 3.1 for the Milky Way. The similar values imply that galaxies with well-defined dark lanes have characteristic dust grain sizes similar to those of Galactic dust. Comment: 20 pages, 15 figures and 4 tables. Accepted for publication in MNRAS08/2008;
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Institutions
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2008–2011
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Tel-Aviv University
- Faculty of Exact Sciences
Tel Aviv, Tel Aviv, Israel
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