87,589 research outputs found
NeIII/OII as an oxygen abundance indicator in the HII regions and HII galaxies
To calibrate the relationship between Ne3O2 (Ne3O2 =
log(\neiii/\oii)) and oxygen abundances, we present a
sample of 3000 \hii galaxies from the Sloan Digital Sky Survey (SDSS)
data release four. They are associated with a sample from the literature
intended to enlarge the oxygen abundance region. We calculated the electron
temperatures () of 210 galaxies in the SDSS sample with the direct method,
and of the other 2960 galaxies in SDSS sample calculated with an
empirical method. Then, we use a linear least-square fitting to calibrate the
Ne3O2 oxygen abundance indicator. It is found that the Ne3O2 estimator follows
a linear relation with \zoh\ that holds for the whole abundance range covered
by the sample, from approximately 7.0 to 9.0. The best linear relationship
between the Ne3O2 and the oxygen abundance is calibrated. The dispersion
between oxygen abundance and Ne3O2 index in the metal rich galaxies may come
partly from the moderate depletion of oxygen onto grains. The method
has the virtue of being single-valued and not affected by internal reddening.
As a result, the method can be a good metallicity indicator in the \hii
regions and \hii galaxies, especially in high-redshift galaxies.Comment: 7 pages, 6 figures. A&A accepte
Polar ring galaxies as tests of gravity
Polar ring galaxies are ideal objects with which to study the
three-dimensional shapes of galactic gravitational potentials since two
rotation curves can be measured in two perpendicular planes. Observational
studies have uncovered systematically larger rotation velocities in the
extended polar rings than in the associated host galaxies. In the dark matter
context, this can only be explained through dark halos that are systematically
flattened along the polar rings. Here, we point out that these objects can also
be used as very effective tests of gravity theories, such as those based on
Milgromian dynamics (MOND). We run a set of polar ring models using both
Milgromian and Newtonian dynamics to predict the expected shapes of the
rotation curves in both planes, varying the total mass of the system, the mass
of the ring with respect to the host, as well as the size of the hole at the
center of the ring. We find that Milgromian dynamics not only naturally leads
to rotation velocities being typically higher in the extended polar rings than
in the hosts, as would be the case in Newtonian dynamics without dark matter,
but that it also gets the shape and amplitude of velocities correct. Milgromian
dynamics thus adequately explains this particular property of polar ring
galaxies.Comment: 9 pages, 8 Figures, 1 Table, Accepted for publication by MNRA
- …