Browsing by Author "Walter, Fabian"

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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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    Physical Properties of Molecular Clouds at 2 pc Resolution in the Low-metallicity Dwarf Galaxy NGC 6822 and the Milky Way
    (2017) Leroy, Adam K; Bigiel, Frank; Bolatto, Alberto D; Tacconi, Linda; Dishoeck, Ewine F van; Walter, Fabian
    We present the Atacama Large Millimeter/submillimeter Array survey of CO(2-1) emission from the 1/5 solar metallicity, Local Group dwarf galaxy NGC 6822. We achieve high (0buildrel{primeprime}over{.} 9≈ 2 pc) spatial resolution while covering a large area: four 250 pc × 250 pc regions that encompass ˜ 2/3 of NGC 6822's star formation. In these regions, we resolve ˜ 150 compact CO clumps that have small radii (˜2-3 pc), narrow line width (˜ 1 km s-1), and low filling factor across the galaxy. This is consistent with other recent studies of low-metallicity galaxies, but here shown with a 15× larger sample. At parsec scales, CO emission correlates with 8 μ {{m}} emission better than with 24 μ {{m}} emission and anticorrelates with Hα, so that polycyclic aromatic hydrocarbon emission may be an effective tracer of molecular gas at low metallicity. The properties of the CO clumps resemble those of similar-size structures in Galactic clouds except of slightly lower surface brightness and with CO-to-H2 ratio ˜1-2× the Galactic value. The clumps exist inside larger atomic-molecular complexes with masses typical for giant molecular clouds. Using dust to trace H2 for the entire complex, we find the CO-to-H2 ratio to be ˜ 20{--}25× the Galactic value, but with strong dependence on spatial scale and variations between complexes that may track their evolutionary state. The H2-to-H I ratio is low globally and only mildly above unity within the complexes. The ratio of star formation rate to H2 is ˜ 3{--}5× higher in the complexes than in massive disk galaxies, but after accounting for the bias from targeting star-forming regions, we conclude that the global molecular gas depletion time may be as long as in massive disk galaxies.
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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 impact of the gas distribution on the determination of dynamical masses of galaxies using unresolved observations
    (2014) de Blok, W J G; Walter, Fabian
    Dynamical mass (M dyn) is a key property of any galaxy, yet a determination of M dyn is not straightforward if spatially resolved measurements are not available. This situation occurs in single-dish H I observations of the local universe, but also frequently in high-redshift observations. M dyn measurements in high-redshift galaxies are commonly obtained through observations of the CO line, the most abundant tracer of the molecular medium. Even though in most cases the CO line width can be determined with reasonable accuracy, a measurement of the size of the emitting region is typically challenging given current facilities. We show how the integrated spectra ("global profiles") of a variety of galaxy models depend on the spatial distribution of the tracer gas as well as its velocity dispersion. We demonstrate that the choice of tracer emission line (e.g., H I tracing extended, "flat," emission versus CO tracing more compact, "exponential," emission) significantly affects the shape of the global profiles. In particular, in the case of high (~50 km s–1) velocity dispersions, compact tracers (such as CO) result in Gaussian-like (non-double-horned) profiles, as is indeed frequently seen in high-redshift observations. This leads to significantly different determinations of M dyn if different distributions of the tracer material ("flat" versus "exponential") are considered. We determine at which radii the rotation curve reaches the rotation velocity corresponding to the velocity width, and find that for each tracer this happens at a well-defined radius: H I velocity widths typically originate at ~5 optical scale lengths, while CO velocity widths trace the rotation velocity at ~2 scale lengths. We additionally explore other distributions to take into account that CO distributions at high redshift likely differ from those at low redshift. Our models, while not trying to reproduce individual galaxies, define characteristic radii that can be used in conjunction with the measured velocity widths in order to define dynamical masses consistent with the assumed gas distribution.