12 research outputs found
The redshift evolution of oxygen and nitrogen abundances in emission-line SDSS galaxies
The oxygen and nitrogen abundance evolutions with redshift and galaxy stellar
mass in emission-line SDSS galaxies are investigated. This is the first such
study for nitrogen abundances, and it provides an additional constraint for the
study of the chemical evolution of galaxies. We have devised a criterion to
recognize and exclude from consideration AGNs and star-forming galaxies with
large errors in the line flux measurements. To select star-forming galaxies
with accurate line fluxes measurements, we require that, for each galaxy, the
nitrogen abundances derived with various calibrations based on different
emission lines agree. Using this selection criterion, subsamples of
star-forming galaxies have been extracted from catalogs of the MPA/JHU group.
We found that the galaxies of highest masses, those with masses > 10^11.2
M_sun, have not been enriched in both oxygen and nitrogen over the last 3 Gyr:
they have formed their stars in the so distant past that these have returned
their nucleosynthesis products to the interstellar medium before z=0.25. The
galaxies in the mass range from 10^11.0 M_sun to 10^11.2 M_sun do not show an
appreciable enrichment in oxygen, but do show some enrichment in nitrogen: they
also formed their stars before z=0.25 but later in comparison to the galaxies
of highest masses; these stars have not returned nitrogen to the interstellar
medium before z=0.25 because they have not had enough time to evolve. This
suggests that stars with lifetimes of 2-3 Gyr contribute to the nitrogen
production. Finally, galaxies with masses < 10^11 M_sun show enrichment in both
oxygen and nitrogen during the last 3 Gyr: they have undergone appreciable star
formation and have converted up to 20% of their mass into stars over this
period.Comment: 43 pages, 15 figures, accepted for publication in the Astrophysical
Journa
On the Influence of Minor Mergers on the Radial Abundance Gradient in Disks of Milky Way-like Galaxies
We investigate the influence of stellar migration caused by minor mergers
(mass ratio from 1:70 to 1:8) on the radial distribution of chemical abundances
in the disks of Milky Way-like galaxies during the last four Gyr. A GPU-based
pure N-body tree-code model without hydrodynamics and star formation was used.
We computed a large set of mergers with different initial satellite masses,
positions, and orbital velocities. We find that there is no significant
metallicity change at any radius of the primary galaxy in the case of accretion
of a low-mass satellite of 10 M (mass ratio 1:70) except for the
special case of prograde satellite motion in the disk plane of the host galaxy.
The accretion of a satellite of a mass M (mass
ratio 1:23) results in an appreciable increase of the chemical abundances at
galactocentric distances larger than kpc. The radial abundance
gradient flattens in the range of galactocentric distances from 5 to 15 kpc in
the case of a merger with a satellite with a mass
M. There is no significant change in the abundance gradient slope in
the outer disk (from kpc up to 25 kpc) in any merger while the scatter
in metallicities at a given radius significantly increases for most of the
satellite's initial masses/positions compared to the case of an isolated
galaxy. This argues against attributing the break (flattening) of the abundance
gradient near the optical radius observed in the extended disks of Milky
Way-like galaxies only to merger-induced stellar migration.Comment: 17 pages, 15 figures, accepted for publication in Ap
On the maximum value of the cosmic abundance of oxygen and the oxygen yield
We search for the maximum oxygen abundance in spiral galaxies. Because this
maximum value is expected to occur in the centers of the most luminous
galaxies, we have constructed the luminosity - central metallicity diagram for
spiral galaxies, based on a large compilation of existing data on oxygen
abundances of HII regions in spiral galaxies. We found that this diagram shows
a plateau at high luminosities (-22.3 < M_B < -20.3), with a constant maximum
value of the gas-phase oxygen abundance 12+log(O/H) ~ 8.87. This provides
strong evidence that the oxygen abundance in the centers of the most luminous
metal-rich galaxies reaches the maximum attainable value of oxygen abundance.
Since some fraction of the oxygen (about 0.08 dex) is expected to be locked
into dust grains, the maximum value of the true gas+dust oxygen abundance in
spiral galaxies is 12+log(O/H) ~ 8.95. This value is a factor of ~ 2 higher
than the recently estimated solar value. Based on the derived maximum oxygen
abundance in galaxies, we found the oxygen yield to be about 0.0035, depending
on the fraction of oxygen incorporated into dust grains.Comment: 8 pages, 5 figures, accepted for publication in MNRA
The Oxygen Abundance of Nearby Galaxies from Sloan Digital Sky Survey Spectra
We have derived the oxygen abundance for a sample of nearby galaxies in the
Data Release 5 of the Sloan Digital Sky Survey (SDSS) which possess at least
two independent spectra of one or several HII regions with a detected
[OIII]4363 auroral line. Since, for nearby galaxies, the [OII]3727 nebular line
is out of the observed wavelength range, we propose a method to derive (O/H)_ff
abundances using the classic Te method coupled with the ff relation. (O/H)_7325
abundances have also been determined, based on the [OII]7320,7330 line
intensities, and using a small modification of the standard Te method. The
(O/H)_ff and (O/H)_7325 abundances have been derived with both the one- and
two-dimensional t_2 - t_3 relations. It was found that the (O/H)_ff abundances
derived with the parametric two-dimensional t_2 - t_3 relation are most
reliable. Oxygen abundances have been determined in 29 nearby galaxies, based
on 84 individual abundance determinations in HII regions. Because of our
selection methods, the metallicity of our galaxies lies in the narrow range 8.2
< 12 + log (O/H) < 8.4. The radial distribution of oxygen abundances in the
disk of the spiral galaxy NGC 4490 is determined for the first time.Comment: 39 pages, 10 figures, 4 tables, accepted for publication in the
Astrophysical Journa
Metal-THINGS: On the metallicity and ionization of ULX sources in NGC 925
We present an analysis of the optical properties of three Ultra Luminous
X-ray (ULX) sources identified in NGC 925. We use Integral field unit data from
the George Mitchel spectrograph in the context of the Metal-THINGS survey. The
optical properties for ULX-1 and ULX-3 are presented, while the spaxel
associated with ULX-2 had a low S/N, which prevented its analysis. We also
report the kinematics and dimensions of the optical nebula associated with each
ULX using ancillary data from the PUMA Fabry-Perot spectrograph. A BPT analysis
demonstrates that most spaxels in NGC 925 are dominated by star-forming
regions, including those associated with ULX-1 and ULX-3. Using the resolved
gas-phase metallicities, a negative metallicity gradient is found, consistent
with previous results for spiral galaxies, while the ionization parameter tends
to increase radially throughout the galaxy. Interestingly, ULX-1 shows a very
low gas metallicity for its galactocentric distance, identified by two
independent methods, while exhibiting a typical ionization. We find that such
low gas metallicity is best explained in the context of the high-mass X-ray
binary population, where the low-metallicity environment favours active Roche
lobe overflows that can drive much higher accretion rates. An alternative
scenario invoking accretion of a low-mass galaxy is not supported by the data
in this region. Finally, ULX-3 shows both a high metallicity and ionization
parameter, which is consistent with the progenitor being a highly-accreting
neutron star within an evolved stellar population region.Comment: Accepted by Ap