1,039 research outputs found
HII Galaxies as deep cosmological probes
We re-investigated the use of the Hubble diagram to measure the cosmological
constant () and the mass density of the Universe (). We find
an important focusing effect in for redshifts about 3. This effect
implies that the apparent magnitude of a standard candle at redshifts z=2-3 has
almost no dependence on for . This means that
can be measured independently of by targeting the
redshift range according to an estimate of the value of .
We explore the evidence in support of the suggestion that extreme starburst
galaxies also known as HII galaxies can be used as distance estimators over a
wide range of redshifts and reaching very high values. We have compiled
literature data of HII galaxies up to and found a good correlation
between their luminosity and velocity dispersion measured from their strong
emission lines, thus confirming the correlation already known to exist for HII
galaxies in the nearby Universe. Several systematic effects such as age,
extinction, kinematics, and metallicity are discussed as well as the effects of
different cosmologies.Comment: To appear in MNRAS, 7 pages, 6 figure
An Active Instance-based Machine Learning method for Stellar Population Studies
We have developed a method for fast and accurate stellar population
parameters determination in order to apply it to high resolution galaxy
spectra. The method is based on an optimization technique that combines active
learning with an instance-based machine learning algorithm. We tested the
method with the retrieval of the star-formation history and dust content in
"synthetic" galaxies with a wide range of S/N ratios. The "synthetic" galaxies
where constructed using two different grids of high resolution theoretical
population synthesis models. The results of our controlled experiment shows
that our method can estimate with good speed and accuracy the parameters of the
stellar populations that make up the galaxy even for very low S/N input. For a
spectrum with S/N=5 the typical average deviation between the input and fitted
spectrum is less than 10**{-5}. Additional improvements are achieved using
prior knowledge.Comment: 14 pages, 25 figures, accepted by Monthly Notice
A photometric comprehensive study of circumnuclear star forming rings: the sample
We present photometry in U, B, V, R and I continuum bands and in H
and H emission lines for a sample of 336 circumnuclear star forming
regions (CNSFR) located in early type spiral galaxies with different levels of
activity in their nuclei. They are nearby galaxies, with distances less than
100 Mpc, 60\% of which are considered as interacting objects.
This survey of 20 nuclear rings aims to provide insight into their star
formation properties as age, stellar population and star formation rate.
Extinction corrected H luminosities range from to
, with most of the regions showing values between
39.5 40, which implies masses for the ionizing
clusters higher than .
H and H images have allowed us to obtain an accurate measure
of extinction. We have found an average value of A = 1.85 magnitudes. (U-B)
colour follows a two maximum distribution around (U-B) -0.7, and -0.3;
(R-I) also presents a bimodal behaviour, with maximum values of 0.6 and 0.9.
Reddest (U-B) and (R-I) regions appear in non-interacting galaxies. Reddest
(R-I) regions lie in strongly barred galaxies.
For a significant number of HII regions the observed colours and equivalent
widths are not well reproduced by single burst evolutionary theoretical models.Comment: Accepted for publication in MNRAS. Accepted 15 May 2015. Received, 21
April 201
Stellar indices and kinematics in Seyfert 1 nuclei
We present spectra of 6 type 1 Seyfert galaxies, 2 Seyfert 2, a starburst
galaxy and a compact narrow line radiogalaxy, taken in two spectral ranges
centered around the near--IR CaII triplet (CaT) (at ~8600 Angstroms), and the
Mgb stellar feature at 5180 Angstroms. We measured the equivalent width (EWs)
of these features and the Fe52 and Fe53 spectral indices. We found that the
strength of the CaT in type 1 Seyfert galaxies with prominent central point
sources, is larger than what would be expected from the observed strength of
the blue indices. This could be explained by the presence of red supergiants in
the nuclei of Seyfert 1 galaxies. On the other hand, the blue indices of these
galaxies could also be diluted by the strong FeII multiplets that can be seen
in their spectra.
We have also measured the stellar and gas velocity dispersions of the
galaxies in the sample. The stellar velocity dispersions were measured using
both, the Mgb and CaT stellar features. The velocity dispersion of the gas in
the narrow line region (NLR) was measured using the strong emission lines
[OIII] 5007, 4959 and [SIII] 9069. We compare the gas and star velocity
dispersions and find that both magnitudes are correlated in Seyfert galaxies.
Most of the Seyfert 1 we observe have stellar velocity dispersion somehow
greater than that of the gas in the NLR.Comment: To appear in MNRAS, 18 pages, 9 figure
Properties of the ionized gas of circumnuclear star-forming regions in early type spirals
(Abbr.) A study of cicumnuclear star-forming regions (CNSFRs) in several
early type spirals has been made in order to investigate their main properties:
stellar and gas kinematics, dynamical masses, ionising stellar masses, chemical
abundances and other properties of the ionised gas. Both high resolution (R20000) and moderate resolution (R ~ 5000) have been used.
In some cases these regions, about 100 to 150 pc in size, are seen to be
composed of several individual star clusters with sizes between 1.5 and 4.9 pc
estimated from Hubble Space Telescope (HST) images. Stellar and gas velocity
dispersions are found to differ by about 20 to 30 km/s with the H
emission lines being narrower than both the stellar lines and the [OIII]
5007 \AA lines. The twice ionized oxygen, on the other hand, shows
velocity dispersions comparable to those shown by stars. We have applied the
virial theorem to estimate dynamical masses of the clusters, assuming that
systems are gravitationally bounded and spherically symmetric, and using
previously measured sizes. The measured values of the stellar velocity
dispersions yield dynamical masses of the order of 10 to 10 solar
masses for the whole CNSFRs.
...Comment: 7 pages, 8 figures, proceeding of the meeting "Young massive star
clusters - Initial conditions and environments", Granada, Spain, 200
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