Spectroscopic observations of a sample of dwarf spiral galaxies. II- Abundance gradients

Source: arXiv

ABSTRACT The oxygen gradient of four dS galaxies has been determined using abundances for several HII regions determined with four different methods. The gradient slopes of the three non-barred galaxies in the sample are quite steep, larger than -0.2 dex/kpc, while the gradient of the barred galaxy is shallower, only -0.1 dex/kpc. Although these gradients are quite steep they are real, following all the galaxies the same trend. Moreover, the results obtained here agree with those marked by the late-type, non-dwarf spirals, particularly the relationship between the gradient and the absolute magnitude and the optical size for non-barred galaxies, and the surface density for barred ones. Comment: 26 pages, 10 figures, resubmitted to AJ on August 28th after minor referee suggestions

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    ABSTRACT: The oxygen abundances of 18 dwarf spiral galaxies are presented. In most of them, the abundance are determined with semi empirical methods. We find that the values are sub-solar for most of the galaxies, similar to those observed in other late-type galaxies, as BCG or dwarf irregular. The abundance gradient can be determined in four of these galaxies, one of them a barred one. The gradients of the three non-barred galaxies in the sample are larger than -0.15 dex/kpc, while the gradient of the barred galaxy is shallower, only -0.1 dex/kpc.
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    ABSTRACT: A growing number of recent observations have revealed that the Galactic globular cluster (GC) omega Cen is not the only GC that shows abundance spread in heavy elements (e.g., Fe). In order to understand the origin of the Galactic GCs with heavy element abundance spread ("HEAS"), we investigate the formation processes of massive GCs (MGCs) with masses larger than 10^6 M_sun in gas-rich dwarf galaxies interacting and merging with the very young Galaxy. We find that massive and compact stellar clumps with masses larger than 10^6 M_sun, which can be regarded as progenitors of MGCs, can form from massive gas clumps that are developed through merging of gaseous regions initially at different regions and thus with different metallicities. Therefore it is inevitable that MGCs formed in dwarfs have HEAS. The abundance spread in each individual MGC depends on the radial metallicity gradient of the host dwarf such that it can be larger for the steeper metallicity gradient. For example, MGCs formed in a dwarf with a central metallicity of [Fe/H]=-1.1 and the radial gradient of -0.2 dex kpc^{-1} can have the abundance spread of Delta [Fe/H] ~.2. The simulated MGCs appear to be significantly flattened owing to their dissipative formation from gas disks of their host dwarfs. Based on these results, we discuss possibly diverse formation mechanisms for the Galactic GCs such as M22, M54, NGC 2419, omega Cen, and Terzan 5.
    Monthly Notices of the Royal Astronomical Society 12/2011; 421(1). · 5.23 Impact Factor
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    ABSTRACT: We propose that the observed stellar halo around the globular cluster (GC) NGC 1851 is evidence for its formation in the central region of its defunct host dwarf galaxy. We numerically investigate the long-term dynamical evolution of a nucleated dwarf galaxy embedded in a massive dark matter halo under the strong tidal field of the Galaxy. The dwarf galaxy is assumed to have a stellar nucleus (or a nuclear star cluster) that could be the progenitor for NGC 1851. We find that although the dark matter halo and the stellar envelope of the host dwarf of NGC 1851 can be almost completely stripped during its orbital evolution around the Galaxy, a minor fraction of stars in the dwarf can remain trapped by the gravitational field of the nucleus. The stripped nucleus can be observed as NGC 1851 with no/little dark matter whereas stars around the nucleus can be observed as a diffuse stellar halo around NGC 1851. The simulated stellar halo has a symmetric distribution with a power-law density slope of ~ -2 and shows no tidal tails within ~200pc from NGC 1851. We show that two GCs can merge with each other to form a new nuclear GC embedded in field stars owing to the low stellar velocity dispersion of the host dwarf. This result makes no assumption on the ages and/or chemical abundances of the two merging GCs. Thus the observed stellar halo and characteristic multiple stellar populations in NGC 1851 suggest that NGC 1851 could have formed initially in the central region of an ancient dwarf galaxy.
    Monthly Notices of the Royal Astronomical Society 09/2011; · 5.23 Impact Factor

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