Browsing by Subject "stars"

Now showing 1 - 14 of 14
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    A high-dispersion molecular gas component in nearby galaxies
    (2013) Caldú-Primo, Anahi; Schruba, Andreas; Walter, Fabian; Leroy, Adam; Sandstrom, Karin; de Blok, W J G; Ianjamasimanana, R; Mogotsi, K M
    We present a comprehensive study of the velocity dispersion of the atomic (H I) and molecular (H2) gas components in the disks (R R 25) of a sample of 12 nearby spiral galaxies with moderate inclinations. Our analysis is based on sensitive high-resolution data from the THINGS (atomic gas) and HERACLES (molecular gas) surveys. To obtain reliable measurements of the velocity dispersion, we stack regions several kiloparsecs in size, after accounting for intrinsic velocity shifts due to galactic rotation and large-scale motions. We stack using various parameters: the galactocentric distance, star formation rate surface density, H I surface density, H2 surface density, and total gas surface density. We fit single Gaussian components to the stacked spectra and measure median velocity dispersions for H I of 11.9 ± 3.1 km s–1 and for CO of 12.0 ± 3.9 km s–1. The CO velocity dispersions are thus, surprisingly, very similar to the corresponding ones of H I, with an average ratio of σH I /σCO= 1.0 ± 0.2 irrespective of the stacking parameter. The measured CO velocity dispersions are significantly higher (factor of ~2) than the traditional picture of a cold molecular gas disk associated with star formation. The high dispersion implies an additional thick molecular gas disk (possibly as thick as the H I disk). Our finding is in agreement with recent sensitive measurements in individual edge-on and face-on galaxies and points toward the general existence of a thick disk of molecular gas, in addition to the well-known thin disk in nearby spiral galaxies.
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    A study of the galactic star cluster NGC 2547
    (1955) Fernie, John Donald
    The following thesis has been undertaken primarily as an observational study. Accordingly, theoretical discussion and speculations have been kept to a minimum, formulas in most case being early quoted and references given for their derivations. Since the observational techniques used are nearly all in standard practice, they have not been fully described, while descriptions of their reductions have been relegated to appendices.
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    Bayesian analysis of the backreaction models
    (2010) Kurek, Aleksandra; Bolejko, Krzysztof; Szydłowski, Marek
    We present a Bayesian analysis of four different types of backreaction models, which are based on the Buchert equations. In this approach, one considers a solution to the Einstein equations for a general matter distribution and then an average of various observable quantities is taken. Such an approach became of considerable interest when it was shown that it could lead to agreement with observations without resorting to dark energy. In this paper we compare the {Lambda}CDM model and the backreaction models with type Ia supernovae, baryon acoustic oscillations, and cosmic microwave background data, and find that the former is favored. However, the tested models were based on some particular assumptions about the relation between the average spatial curvature and the backreaction, as well as the relation between the curvature and curvature index. In this paper we modified the latter assumption, leaving the former unchanged. We find that, by varying the relation between the curvature and curvature index, we can obtain a better fit. Therefore, some further work is still needed--in particular, the relation between the backreaction and the curvature should be revisited in order to fully determine the feasibility of the backreaction models to mimic dark energy.
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    Evidence for a clumpy, rotating gas disk in a submillimeter galaxy at z = 4
    (2012) Hodge, J A; Carilli, C L; Walter, F; de Blok, W J G; Riechers, D; Daddi, E; Lentati, L
    We present Karl G. Jansky Very Large Array observations of the CO(2-1) emission in the z = 4.05 submillimeter galaxy (SMG) GN20. These high-resolution data allow us to image the molecular gas at 1.3 kpc resolution just 1.6 Gyr after the big bang. The data reveal a clumpy, extended gas reservoir, 14 {+-} 4 kpc in diameter, in unprecedented detail. A dynamical analysis shows that the data are consistent with a rotating disk of total dynamical mass 5.4 {+-} 2.4 Multiplication-Sign 10{sup 11} M {sub Sun }. We use this dynamical mass estimate to constrain the CO-to-H{sub 2} mass conversion factor ({alpha}{sub CO}), finding {alpha}{sub CO} = 1.1 {+-} 0.6 M {sub Sun }(K km s{sup -1} pc{sup 2}){sup -1}. We identify five distinct molecular gas clumps in the disk of GN20 with masses a few percent of the total gas mass, brightness temperatures of 16-31K, and surface densities of >3200-4500 Multiplication-Sign ({alpha}{sub CO}/0.8) M {sub Sun} pc{sup -2}. Virial mass estimates indicate they could be self-gravitating, and we constrain their CO-to-H{sub 2} mass conversion factor to be <0.2-0.7 M {sub Sun }(K km s{sup -1} pc{sup 2}){sup -1}. A multiwavelength comparison demonstrates that the molecular gas is concentrated in a region of the galaxy that is heavily obscured in the rest-frame UV/optical. We investigate the spatially resolved gas excitation and find that the CO(6-5)/CO(2-1) ratio is constant with radius, consistent with star formation occurring over a large portion of the disk. We discuss the implications of our results in the context of different fueling scenarios for SMGs.
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    FIRST-YEAR SLOAN DIGITAL SKY SURVEY-II (SDSS-II) SUPERNOVA RESULTS: CONSTRAINTS ON NONSTANDARD COSMOLOGICAL MODELS
    (2009) Sollerman, J; Mörtsell, E; Davis, T M; Blomqvist, M; Bassett, B; Becker, A C; Cinabro, D; Filippenko, A V; Foley, R J; Frieman, J; Garnavich, P; Lampeitl, H; Marriner, J; Miquel, R; Nichol, R C; Richmond, M W; Sako, M; Schneider, D P; Smith, M; VanderPlas, J T; Wheeler, J C
    We use the new Type Ia supernovae discovered by the Sloan Digital Sky Survey-II supernova survey, together with additional supernova data sets as well as observations of the cosmic microwave backgr ...
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    From here to the edge of the observable universe
    (2012) Catchpole, Robin
    Lecture series presented by Dr Robin Catchpole, Institute of Astronomy, Cambridge, formerly Senior Astronomer at the Royal Observatory, Greenwich. This series of audio lectures is for people interested in astronomy who wish to learn more about the observable universe.
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    Galactic and extragalactic distance scales: Some South African contributions
    (2005) Feast, Michael
    The work of South African astronomers in establishing and refining methods of measuring the distances of stars and galaxies is reviewed.
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    Implications of the HST/FGS parallaxof SS Cygnion the disc instability model
    (2002) Schreiber, M R; Gänsicke, B T
    We analyse the consequences of the recently measured parallax of SS Cygni (Harrison et al. 1999) on the accretion disc limit cycle model. Using the observed long term light curve of SS Cyg and d = 166 ± 12 pc, we obtain for the mean mass transfer rate M˙ tr = 4.2±1.7×1017 g s−1. In addition, we calculate the vertical structure of the accretion disc taking into account heating of the outer disc by the stream impact. Comparing the mean accretion rate derived from the observations with the calculated critical mass transfer rate, we find that the disc instability model disagrees with the observed long term light curve of SS Cyg as M˙ tr is greater or similar to the critical mass transfer rate. The failure of the model indicated by this result can be confirmed by considering that the accretion rate at the onset of the decline should be exactly equal to the value critical for stability. In contrast to this prediction of the model, we find that the accretion rate required to explain the observed visual magnitude at the onset of the decline must be significantly higher than the critical mass transfer rate. Our results strongly suggest that either the usually assumed temperature dependence of the viscosity parameter α is not a realistic description of the disc viscosity, that the mass transfer rate in
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    PHOTOMETRIC ESTIMATES OF REDSHIFTS AND DISTANCE MODULI FOR TYPE Ia SUPERNOVAE
    (2010) Kessler, Richard; Cinabro, David; Bassett, Bruce; Dilday, Benjamin; Frieman, Joshua A; Garnavich, Peter M; Jha, Saurabh; Marriner, John; Nichol, Robert C; Sako, Masao; Smith, Mathew; Bernstein, Joseph P; Bizyaev, Dmitry; Goobar, Ariel; Kuhlmann, Stephen; Schneider, Donald P; Stritzinger, Maximilian
    Large planned photometric surveys will discover hundreds of thousands of supernovae (SNe), outstripping the resources available for spectroscopic follow-up and necessitating the development of pure ...
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    PHOTOMETRIC TYPE Ia SUPERNOVA CANDIDATES FROM THE THREE-YEAR SDSS-II SN SURVEY DATA
    (2011) Sako, Masao; Bassett, Bruce; Connolly, Brian; Dilday, Benjamin; Cambell, Heather; Frieman, Joshua A; Gladney, Larry; Kessler, Richard; Lampeitl, Hubert; Marriner, John; Miquel, Ramon; Nichol, Robert C; Schneider, Donald P; Smith, Mathew; Sollerman, Jesper
    We analyze the three-year Sloan Digital Sky Survey II (SDSS-II) Supernova (SN) Survey data and identify a sample of 1070 photometric Type Ia supernova (SN Ia) candidates based on their multiband li ...
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    THE CENTRAL SLOPE OF DARK MATTER CORES IN DWARF GALAXIES: SIMULATIONS VERSUS THINGS
    (2011) Oh, Se-Heon; Brook, Chris; Governato, Fabio; Brinks, Elias; Mayer, Lucio; de Blok, W J G; Brooks, Alyson; Walter, Fabian
    We make a direct comparison of the derived dark matter (DM) distributions between hydrodynamical simulations of dwarf galaxies assuming ACDM cosmology and the observed dwarf galaxies sample from the THINGS survey in terms of (1) the rotation curve shape and (2) the logarithmic inner density slope a of mass density profiles. The simulations, which include the effect of baryonic feedback processes, such as gas cooling, star formation, cosmic UV background heating, and most importantly, physically motivated gas outflows driven by supernovae, form bulgeless galaxies with DM cores. We show that the stellar and baryonic mass is similar to that inferred from photometric and kinematic methods for galaxies of similar circular velocity. Analyzing the simulations in exactly the same way as the observational sample allows us to address directly the so-called cusp/core problem in the ACDM model. We show that the rotation curves of the simulated dwarf galaxies rise less steeply than cold dark matter rotation curves and are consistent with those of the THINGS dwarf galaxies. The mean value of the logarithmic inner density slopes alpha of the simulated galaxies' DM density profiles is similar to-0.4 +/- 0.1, which shows good agreement with alpha = -0.29 +/- 0.07 of the THINGS dwarf galaxies. The effect of non-circular motions is not significant enough to affect the results. This confirms that the baryonic feedback processes included in the simulations are efficiently able to make the initial cusps with alpha similar to-1.0 to -1.5 predicted by DM-only simulations shallower and induce DM halos with a central mass distribution similar to that observed in nearby dwarf galaxies.
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    The effect of peculiar velocities on supernova cosmology
    (2011) Davis, Tamara M; Hui, Lam; Frieman, Joshua A; Haugbølle, Troels; Kessler, Richard; Sinclair, Benjamin; Sollerman, Jesper; Bassett, Bruce; Marriner, John; Mörtsell, Edvard; Nichol, Robert C; Richmond, Michael W; Sako, Masao; Schneider, Donald P; Smith, Mathew
    We analyze the effect that peculiar velocities have on the cosmological inferences we make using luminosity distance indicators, such as Type Ia supernovae. In particular we study the corrections r ...
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    The seventh data release of the sloan digital sky survey
    (2009) Abazajian, Kevork N; Adelman‐McCarthy, Jennifer K; AgYeros, Marcel A; Allam, Sahar S; Prieto, Carlos Allende; An, Deokkeun; Anderson, Kurt S J; Anderson, Scott F; Annis, James; Bahcall, Neta A; Bailer‐Jones, C A L; Barentine, J C; Bassett, Bruce A; Becker, Andrew C; Beers, Timothy C; Bell, Eric F; Belokurov, Vasily; Berlind, Andreas A; Berman, Eileen F; Bernardi, Mariangela; Bickerton, Steven J; Bizyaev, Dmitry; Blakeslee, John P; Blanton, Michael R; Bochanski, John J; Boroski, William N; Brewington, Howard J; Brinchmann, Jarle; Brinkmann, J; Brunner, Robert J; Budavári, Tamás; Carey, Larry N
    This paper describes the Seventh Data Release of the Sloan Digital Sky Survey (SDSS), marking the completion of the original goals of the SDSS and the end of the phase known as SDSS-II. It includes 11, 663 deg2 of imaging data, with most of the ~2000 deg2 increment over the previous data release lying in regions of low Galactic latitude. The catalog contains five-band photometry for 357 million distinct objects. The survey also includes repeat photometry on a 120° long, 2fdg5 wide stripe along the celestial equator in the Southern Galactic Cap, with some regions covered by as many as 90 individual imaging runs. We include a co-addition of the best of these data, going roughly 2 mag fainter than the main survey over 250 deg2. The survey has completed spectroscopy over 9380 deg2 the spectroscopy is now complete over a large contiguous area of the Northern Galactic Cap, closing the gap that was present in previous data releases. There are over 1.6 million spectra in total, including 930, 000 galaxies, 120, 000 quasars, and 460, 000 stars. The data release includes improved stellar photometry at low Galactic latitude. The astrometry has all been recalibrated with the second version of the USNO CCD Astrograph Catalog, reducing the rms statistical errors at the bright end to 45 milliarcseconds per coordinate. We further quantify a systematic error in bright galaxy photometry due to poor sky determination ; this problem is less severe than previously reported for the majority of galaxies. Finally, we describe a series of improvements to the spectroscopic reductions, including better flat fielding and improved wavelength calibration at the blue end, better processing of objects with extremely strong narrow emission lines, and an improved determination of stellar metallicities.
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    Type II-P supernovae from the SDSS-II supernova survey and the standardized candle method
    (2010) D’Andrea, Chris B; Sako, Masao; Dilday, Benjamin; Frieman, Joshua A; Holtzman, Jon; Kessler, Richard; Konishi, Kohki; Schneider, D P; Sollerman, Jesper; Wheeler, J Craig; Yasuda, Naoki; Cinabro, David; Jha, Saurabh; Nichol, Robert C; Lampeitl, Hubert; Smith, Mathew; Atlee, David W; Bassett, Bruce; Castander, Francisco J; Goobar, Ariel; Miquel, Ramon; Nordin, Jakob; Östman, Linda; Prieto, José L; Quimby, Robert; Riess, Adam G; Stritzinger, Maximilian
    We apply the Standardized Candle Method (SCM) for Type II Plateau supernovae (SNe II-P), which relates the velocity of the ejecta of a SN to its luminosity during the plateau, to 15 SNe II-P discov ...