438 research outputs found
Comparison of the Halpha equivalent width of HII regions in a flocculent and a grand design galaxy: possible evidences for IMF variations
We present here a study of the Halpha equivalent widths of the flocculent
galaxy NGC 4395 and the grand design galaxy NGC 5457. A difference between the
mean values of the Halpha equivalent widths for the two galaxies has been
found. Several hypotheses are presented in order to explain this difference:
differences in age, metallicity, star formation rate, photon leakage and
initial mass function. Various tests and Monte Carlo models are used to find
out the most probable cause of this difference. The resultsshow that the
possible cause for the difference could be a variation in the initial mass
function. This difference is such that it seems to favor a fraction of more
massive stars in the grand design galaxy when compared with the flocculent
galaxy. This could be due to a change of the environmental conditions due to a
density wave.Comment: 29 pages, 19 figures, accepted for publication in Ap
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
Density waves and star formation in grand design spirals
HII regions in the arms of spiral galaxies are indicators of recent
star-forming processes. They may have been caused by the passage of the density
wave or simply created by other means near the arms. The study of these regions
may give us clues to clarifying the controversy over the existence of a
triggering scenario, as proposed in the density wave theory. Using H
direct imaging, we characterize the HII regions from a sample of three grand
design galaxies: NGC5457, NGC628 and NGC6946. Broad band images in R and I were
used to determine the position of the arms. The HII regions found to be
associated with arms were selected for the study. The age and the star
formation rate of these HII regions was obtained using measures on the
H line. The distance between the current position of the selected HII
regions and the position they would have if they had been created in the centre
of the arm is calculated. A parameter, T, which measures whether a region was
created in the arm or in the disc, is defined. With the help of the T parameter
we determine that the majority of regions were formed some time after the
passage of the density wave, with the regions located `behind the arm' (in the
direction of the rotation of the galaxy) the zone they should have occupied had
they been formed in the centre of the arm. The presence of the large number of
regions created after the passage of the arm may be explained by the effect of
the density wave, which helps to create the star-forming regions after its
passage. There is clear evidence of triggering for NGC5457 and a co-rotation
radius is proposed. A more modest triggering seems to exist for NGC628 and non
significant evidence of triggering are found for NGC6946.Comment: 10 pages, 20 figures, accepted for publication in A&
The oxygen abundance gradient in M101: the reliability of the P method
We present the oxygen abundance determination for 90 HII regions in the inner
parts of the grand design galaxy M101. The abundances were derived employing
the P method (Pilyugin 2001a). A comparison is made with previous
determinations using another calibration and direct measurements of electron
temperature to derive the oxygen abundance. The results show agreement with the
abundances derived from the electron temperature method and also show that the
older calibration is not as accurate as the P method.Comment: 4 pages, 3 figures. Accepted for publication in A&
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