3,537 research outputs found
Density wave triggered star formation in grand design spirals
In normal spiral galaxies the arms are the main sites for star formation. This is the cause of their optical contrast compared with the rest of the disc. The spiral structure can be observed as a higher concentration of H2 regions, neutral gas (both atomic and molecular via CO), dust and stars than in the interarm disc. It seens generally accepted that, at least in grand design spirals, there are density waves in the discs. However, several questions are not clear yet and still under discussion. An important question could be termed the triggering dilemma (by analogy with the 'winding dilemma' raised in the forties): Is the enhanced star formation in the spiral arms triggered by the passage of a system of density waves or is it simply due to the presence of a higher column density of gas there? In the present work, we use triggering in the same sense as the moderate to strong triggering defined by Elmegreen (1992), that is to say that star formation in the arms occurs at a rate faster than that in the interarm zone, relative to the available placental gas. Our group has designed several tests to elucidate whether or not star formation is triggered in the arms with respect to the interarm region and we summarize one of them, that of the ratio of the star formation efficiency in the arms divided by that of the interarm zone at the same galactocentric distance which we may call the relative massive star formation efficiency, where the efficiency is defined using the ratio of the mass of stars (evaluated via the H alpha flux) to the mass of neutral gas, atomic plus molecular (which must be measured with the adequate angular resolution). If the relative efficiency is of order unity, the star formation is proportional to the mass of gas, if some kind of induced star formation is present, the relative efficiency should be considerably larger than unity
A Morphological Method to Determine Co-Rotation Radii in Spiral Galaxies
Shock induced star formation in a stellar density wave scenario produces an
azimuthal gradient of ages across the spiral arms which has opposite signs on
either side of the corotation resonance (CR). We present a method based on the
Fourier analysis of azimuthal profiles, to locate the CR and determine the arm
character (trailing or leading) in spiral galaxies. Basically, we compare the
behavior of the phase angle of the two-armed spiral in blue and infrared colors
which pick out respectively young and older disk stellar population. We
illustrate the method using theoretical leading and trailing, spirals. We have
also applied the method to the spiral galaxies NGC 7479, for which we confirm
the reported leading arms, and NGC 1832. In these galaxies we find two and
three CRs respectively.Comment: 9 pages, accepted for publication in ApJL, figures 4 and 6 avaliables
at ftp://ftp.inaoep.mx/pub/salida/puerari, full paper also avaliable at
http://www.inaoep.mx/~puerar
Mapping atomic and diffuse interstellar band absorption across the Magellanic Clouds and the Milky Way
Diffuse interstellar bands (DIBs) trace warm neutral and weakly-ionized
diffuse interstellar medium (ISM). Here we present a dedicated, high
signal-to-noise spectroscopic study of two of the strongest DIBs, at 5780 and
5797 \AA, in optical spectra of 666 early-type stars in the Small and Large
Magellanic Clouds, along with measurements of the atomic Na\,{\sc i}\,D and
Ca\,{\sc ii}\,K lines. The resulting maps show for the first time the
distribution of DIB carriers across large swathes of galaxies, as well as the
foreground Milky Way ISM. We confirm the association of the 5797 \AA\ DIB with
neutral gas, and the 5780 \AA\ DIB with more translucent gas, generally tracing
the star-forming regions within the Magellanic Clouds. Likewise, the Na\,{\sc
i}\,D line traces the denser ISM whereas the Ca\,{\sc ii}\,K line traces the
more diffuse, warmer gas. The Ca\,{\sc ii}\,K line has an additional component
at --220 km s seen towards both Magellanic Clouds; this may be
associated with a pan-Magellanic halo. Both the atomic lines and DIBs show
sub-pc-scale structure in the Galactic foreground absorption; the 5780 and 5797
\AA\ DIBs show very little correlation on these small scales, as do the
Ca\,{\sc ii}\,K and Na\,{\sc i}\,D lines. This suggests that good correlations
between the 5780 and 5797 \AA\ DIBs, or between Ca\,{\sc ii}\,K and Na\,{\sc
i}\,D, arise from the superposition of multiple interstellar structures.
Similarity in behaviour between DIBs and Na\,{\sc i} in the SMC, LMC and Milky
Way suggests the abundance of DIB carriers scales in proportion to metallicity.Comment: Accepted for publication in MNRA
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