637 research outputs found
The evolution of H{\sc ii} galaxies: Testing the bursting scenario through the use of self-consistent models
We have computed a series of realistic and self-consistent models of the
emitted spectra of H{\sc ii} galaxies. Our models combine different codes of
chemical evolution, evolutionary population synthesis and photoionization. The
emitted spectrum of H{\sc ii} galaxies is reproduced by means of the
photoionization code CLOUDY, using as ionizing spectrum the spectral energy
distribution of the modelled H{\sc ii} galaxy, which in turn is calculated
according to a Star Formation History (SFH) and a metallicity evolution given
by a chemical evolution model that follows the abundances of 15 different
elements. The contribution of emission lines to the broad-band colours is
explicitly taken into account.
The results of our code are compared with photometric and spectroscopic data
of H{\sc ii} galaxies. Our technique reproduces observed diagnostic diagrams,
abundances, equivalent width-colour and equivalent width-metallicity relations
for local H{\sc ii} galaxies.Comment: 13 figures and 2 tables, accepted for publication in MNRAS Main
Journa
Emission-Line Galaxy Surveys as Probes of the Spatial Distribution of Dwarf Galaxies. I. The University of Michigan Survey
Objective-prism surveys which select galaxies on the basis of line-emission
are extremely effective at detecting low-luminosity galaxies and constitute
some of the deepest available samples of dwarfs. In this study, we confirm that
emission-line galaxies (ELGs) in the University of Michigan (UM)
objective-prism survey (MacAlpine et al. 1977-1981) are reliable tracers of
large-scale structure, and utilize the depth of the samples to examine the
spatial distribution of low-luminosity (M -18.0) dwarfs relative to
higher luminosity giant galaxies (M -18.0) in the Updated Zwicky
Catalogue (Falco et al. 1999). New spectroscopic data are presented for 26 UM
survey objects. We analyze the relative clustering properties of the overall
starbursting ELG and normal galaxy populations, using nearest neighbor and
correlation function statistics. This allows us to determine whether the
activity in ELGs is primarily caused by gravitational interactions. We conclude
that galaxy-galaxy encounters are not the sole cause of activity in ELGs since
ELGs tend to be more isolated and are more often found in the voids when
compared to their normal galaxy counterparts. Furthermore, statistical analyses
performed on low-luminosity dwarf ELGs show that the dwarfs are less clustered
when compared to their non-active giant neighbors. The UM dwarf samples have
greater percentages of nearest neighbor separations at large values and lower
correlation function amplitudes relative to the UZC giant galaxy samples. These
results are consistent with the expectations of galaxy biasing.Comment: 17 pages, 4 tables, 10 figures. Accepted for publication in the Ap
On the feedback from super stellar clusters. I. The structure of giant HII regions and HII galaxies
We review the structural properties of giant extragalactic HII regions and
HII galaxies based on 2D hydrodynamic calculations, and propose an evolutionary
sequence that accounts for their observed detailed structure. The model assumes
a massive and young stellar cluster surrounded by a large collection of clouds.
These are thus exposed to the most important star-formation feedback
mechanisms: photoionization and the cluster wind. The models show how the two
feedback mechanisms compete in the disruption of clouds and lead to two
different hydrodynamic solutions: The storage of clouds into a long lasting
ragged shell that inhibits the expansion of the thermalized wind, and the
steady filtering of the shocked wind gas through channels carved within the
cloud stratum. Both solutions are claimed to be concurrently at work in giant
HII regions and HII galaxies, causing their detailed inner structure. This
includes multiple large-scale shells, filled with an X-ray emitting gas, that
evolve to finally merge with each other, giving the appearance of shells within
shells. The models also show how the inner filamentary structure of the giant
superbubbles is largely enhanced with matter ablated from clouds and how cloud
ablation proceeds within the original cloud stratum. The calculations point at
the initial contrast density between the cloud and the intercloud media as the
factor that defines which of the two feedback mechanisms becomes dominant
throughout the evolution. Animated version of the models can be found at
http://www.iaa.csic.es/\~{}eperez/ssc/ssc.html.Comment: 28 pages, 10 figures, accepted for publication in the ApJ. Animated
version of the models can be found at
http://www.iaa.csic.es/\~{}eperez/ssc/ssc.htm
Heavy element abundances in blue compact galaxies
We present high-quality ground-based spectroscopic observations of 54
supergiant H II regions in 50 low-metallicity blue compact galaxies with oxygen
abundances 12 + log O/H between 7.1 and 8.3. We use the data to determine
abundances for the elements N, O, Ne, S, Ar and Fe. We also analyze Hubble
Space Telescope (HST) Faint Object Spectrograph archival spectra of 10
supergiant H II regions to derive C and Si abundances in a subsample of 7 BCGs.
The main result of the present study is that none of the heavy
element-to-oxygen abundance ratios studied here (C/O, N/O, Ne/O, Si/O, S/O,
Ar/O, Fe/O) depend on oxygen abundance for BCGs with 12 + log O/H < 7.6 (Z <
Zsun/20). This constancy implies that all these heavy elements have a primary
origin and are produced by the same massive (M > 10Msun) stars responsible for
O production. The dispersion of the C/O and N/O ratios in these galaxies is
found to be remarkably small, being only +/-0.03 dex and +/-0.02 dex
respectively. This very small dispersion is strong evidence against any
time-delayed production of C and primary N in the lowest-metallicity BCGs
(secondary N production is negligible at these low metallicities). The absence
of a time-delayed production of C and N is consistent with the scenario that
galaxies with 12 + log O/H < 7.6 are undergoing now their first burst of star
formation, and that they are therefore young, with ages not exceeding 40 Myr.
If very low metallicities BCGs are indeed young, this would argue against the
commonly held belief that C and N are produced by intermediate-mass (3Msun < M
< 9Msun) stars at very low metallicities, as these stars would not have yet
completed their evolution in these lowest metallicity galaxies.Comment: 37 pages, 5 EPS figures, to appear in ApJ, February 199
Capture Velocity for a Magneto-Optical Trap in a Broad Range of Light Intensity
In a recent paper, we have used the dark-spot Zeeman tuned slowing technique
[Phys. Rev. A 62, 013404-1, (2000)] to measure the capture velocity as a
function of laser intensity for a sodium magneto optical trap. Due to technical
limitation we explored only the low light intensity regime, from 0 to 27
mW/cm^2. Now we complement that work measuring the capture velocity in a
broader range of light intensities (from 0 to 400 mW/cm^2). New features,
observed in this range, are important to understant the escape velocity
behavior, which has been intensively used in the interpretation of cold
collisions. In particular, we show in this brief report that the capture
velocity has a maximum as function of the trap laser intensity, which would
imply a minimum in the trap loss rates.Comment: 2 pages, 2 figure
An imaging and spectroscopic study of the very metal-deficient blue compact dwarf galaxy Tol 1214--277
We present a spectrophotometric study based on VLT/FORS I observations of one
of the most metal-deficient blue compact dwarf (BCD) galaxies known, Tol
1214-277 (Z ~ Zsun/25). The data show that roughly half of the total luminosity
of the BCD originates from a bright and compact starburst region located at the
northeastern tip of a faint dwarf galaxy with cometary appearance. The
starburst has ignited less than 4 Myr ago and its emission is powered by
several thousands O7V stars and ~ 170 late-type nitrogen Wolf-Rayet stars
located within a compact region with < 500 pc in diameter. For the first time
in a BCD, a relatively strong [Fe V] 4227 emission line is seen which together
with intense He II 4686 emission indicates the presence of a very hard
radiation field in Tol 1214-277. We argue that this extraordinarily hard
radiation originates from both Wolf--Rayet stars and radiative shocks in the
starburst region. The structural properties of the low-surface-brightness (LSB)
component underlying the starburst have been investigated by means of surface
photometry down to 28 B mag/sq.arcsec. We find that, for a surface brightness
level fainter than ~ 24.5 B mag/sq.arcsec, an exponential fitting law provides
an adequate approximation to its radial intensity distribution. The broad-band
colors in the outskirts of the LSB component of Tol 1214-277 are nearly
constant and are consistent with an age below one Gyr. This conclusion is
supported by the comparison of the observed spectral energy distribution (SED)
of the LSB host with theoretical SEDs.Comment: 17 pages, 11 Postscript figures, uses emulateapj.sty, to appear in
Astronomical Journa
Structure and large scale environment of galaxy pairs in the S-PLUS DR4
In this paper, we use photometric data from the S-PLUS DR4 survey to identify
isolated galaxy pairs and analyse their characteristics and properties. Our
results align with previous spectroscopic studies, particularly in luminosity
function parameters, suggesting a consistent trait among galaxy systems. Our
findings reveal a high fraction of red galaxies across all samples,
irrespective of projected distance, velocity difference, or luminosity ratio.
We found that the proximity of a neighbour to its central galaxy influences its
colour due to environmental effects. We also found that central and neighbour
have different behaviours: central galaxies maintain a stable red colour
regardless of luminosity, while neighbour colours vary based on luminosity
ratios. When the central is significantly brighter, the neighbour tends to be
less red. According to our division in red, blue and mixed pairs, we found
evidence of galactic conformity. Red pair fractions increase in closer pairs
and in pairs of similar luminosity, indicating shared environments promoting
red galaxy formation. Analysing local density, the expected colour-density
relation is of course recovered, but it is strongly determined by the stellar
mass of the pair. In denser environments, the red pair fractions increase, blue
pairs decrease and for mixed pairs it depends on their stellar mass: more
massive mixed pairs decrease their fraction whereas the lower massive ones
increase it. These results shed light on the intricate relationship between
galaxy pairs, their characteristics, and environmental influences on colour,
providing insights into their evolutionary histories.Comment: 15 pages, 12 figures, accepted for publication in MNRA
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