137 research outputs found
Galaxy chemical evolution models: The role of molecular gas formation
In our classical grid of multiphase chemical evolution models, star formation in the disc occurs in two steps: first, molecular gas forms, and then stars are created by cloud-cloud collisions or interactions of massive stars with the surrounding molecular clouds. The formation of both molecular clouds and stars are treated through the use of free parameters we refer to as efficiencies. In this work, we modify the formation of molecular clouds based on several new prescriptions existing in the literature, and we compare the results obtained for a chemical evolution model of theMilkyWay Galaxy regarding the evolution of the Solar region, the radial structure of the Galactic disc and the ratio between the diffuse and molecular components, H I /H 2 . Our results show that the six prescriptions we have tested reproduce fairly consistent most of the observed trends, differing mostly in their predictions for the (poorly constrained) outskirts of the Milky Way and the evolution in time of its radial structure. Among them, the model proposed by Ascasibar et al. (in preparation), where the conversion of diffuse gas into molecular clouds depends on the local stellar and gas densities as well as on the gas metallicity, seems to provide the best overall match to the observed data
Physical properties of circumnuclear ionising clusters. II. NGC 7469
Circumnuclear star forming regions (CNSFR) are massive clusters found close
to galactic nuclei. These entities give us an excellent opportunity to study
star formation in environments with high metallicity and to relate it with
active galactic nuclei. Our principal aim is to derive the physical properties
and dynamical masses of the CNSFRs in the two rings of the spiral NGC 7469,
categorized as a Luminous Infrared Galaxy (ULIRG) and hosting a Seyfert 1
nucleus. We have used archival data obtained with the MUSE spectrograph
attached to one of the ESO VLT telescopes and we have applied the techniques
shown in the first paper of the series. Regions in the studied galaxy show
large sizes which can be explained by the stellar winds produced by WR stars.
The inner ring regions seem to be more compact than the outer ones, showing
higher electron densities and filling factors. The young stellar population of
the clusters has contributions of ionising populations with ages around 5 Ma
and its masses constitute less than a 1\% of the total mass of each cluster.
The inner ring regions which are close to the active galactic nucleus probably
are the only ones that have enough mass to survive the action of the AGN. They
constitute the 90 \% of the total inner ring mass.Comment: 22 pages, 25 figures, Submitted to Astronomy & Astrophysic
High resolution spectroscopy of the BCD galaxy Haro 15:I. Internal kinematics
Using echelle spectroscopy, obtained at Las Campanas Observatory, we present
a detailed study of the internal kinematics of the nebular material in multiple
knots of the blue compact dwarf galaxy Haro 15. A detailed analysis of the
complex emission line profiles show the presence of an underlying broad
component in almost all knots, and the brightest star-forming region shows
unmistakable signs for the presence of two distinct narrow kinematical
components. We also study the information that our analysis provides regarding
the motion of the individual knots in the Haro 15 galaxy potential, confirming
that they follow galactic rotation. Finally, we examine the relation between
their velocity dispersion and luminosity, finding that almost all knots follow
the relation for virialised systems. This holds for the strong narrow
components identified in complex fits and for single profile fits, although the
latter show a flatter slope. In agreement with previous findings, in this paper
we show that the existence of multiple kinematical components among massive
starbursts cannot be overlooked, as it has a noticeable effect on any
subsequent analysis that relies on basic parameters.Comment: 12 pages, 21 figure
The role of gas infall in the evolution of disc galaxies
Spiral galaxies are thought to acquire their gas through a protracted infall
phase resulting in the inside-out growth of their associated discs. For field
spirals, this infall occurs in the lower density environments of the cosmic
web. The overall infall rate, as well as the galactocentric radius at which
this infall is incorporated into the star-forming disc, plays a pivotal role in
shaping the characteristics observed today. Indeed, characterising the
functional form of this spatio-temporal infall in-situ is exceedingly
difficult, and one is forced to constrain these forms using the present day
state of galaxies with model or simulation predictions. We present the infall
rates used as input to a grid of chemical evolution models spanning the mass
spectrum of discs observed today. We provide a systematic comparison with
alternate analytical infall schemes in the literature, including a first
comparison with cosmological simulations. Identifying the degeneracies
associated with the adopted infall rate prescriptions in galaxy models is an
important step in the development of a consistent picture of disc galaxy
formation and evolution.Comment: 12 pages, 12 figures, MNRAS, accepte
Nature or nurture? Clues from the distribution of specific star formation rates in SDSS galaxies
This work investigates the main mechanism(s) that regulate the specific star
formation rate (SSFR) in nearby galaxies, cross-correlating two proxies of this
quantity -- the equivalent width of the \Ha\ line and the colour --
with other physical properties (mass, metallicity, environment, morphology, and
the presence of close companions) in a sample of galaxies extracted
from the Sloan Digital Sky Survey (SDSS). The existence of a relatively tight
`ageing sequence' in the colour-equivalent width plane favours a scenario where
the secular conversion of gas into stars (i.e. `nature') is the main physical
driver of the instantaneous SSFR and the gradual transition from a `chemically
primitive' (metal-poor and intensely star-forming) state to a `chemically
evolved' (metal-rich and passively evolving) system. Nevertheless,
environmental factors (i.e. `nurture') are also important. In the field,
galaxies may be temporarily affected by discrete `quenching' and `rejuvenation'
episodes, but such events show little statistical significance in a
probabilistic sense, and we find no evidence that galaxy interactions are, on
average, a dominant driver of star formation. Although visually classified
mergers tend to display systematically higher EW(H) and bluer
colours for a given luminosity, most galaxies with high SSFR have uncertain
morphologies, which could be due to either internal or external processes.
Field galaxies of early and late morphological types are consistent with the
gradual `ageing' scenario, with no obvious signatures of a sudden decrease in
their SSFR. In contrast, star formation is significantly reduced and sometimes
completely quenched on a short time scale in dense environments, where many
objects are found on a `quenched sequence' in the colour-equivalent width
plane.Comment: 18 pages, 9 figure
Kinematics of gas and stars in circumnuclear star-forming regions of early type spirals
(Abbr.) We present high resolution (R~20000) spectra in the blue and the far
red of cicumnuclear star-forming regions (CNSFRs) in three early type spirals
(NGC3351, NGC2903 and NGC3310) which have allowed the study of the kinematics
of stars and ionized gas in these structures and, for the first time, the
derivation of their dynamical masses for the first two. 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. The stellar dispersions have been obtained
from the Calcium triplet (CaT) lines at 8494,8542,8662 \AA,
while the gas velocity dispersions have been measured by Gaussian fits to the
H and [OIII] 5007 \AA lines on the high dispersion
spectra. Values of the stellar velocity dispersions are between 30 and 68 km/s.
We apply 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. 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, in some cases, even larger. We have found
indications of the presence of two different kinematical components in the
ionized gas of the regions...Comment: 4 pages, proceeding of the meeting "Young massive star clusters -
Initial conditions and environments", Granada, Spain, 200
The role of gas infall in the evolution of disc galaxies
Spiral galaxies are thought to acquire their gas through a protracted infall phase resulting in the inside-out growth of their associated discs. For field spirals, this infall occurs in the lower density environments of the cosmic web. The overall infall rate, as well as the galactocentric radius at which this infall is incorporated into the star-forming disc, plays a pivotal role in shaping the characteristics observed today. Indeed, characterising the functional form of this spatio-temporal infall in-situ is exceedingly difficult, and one is forced to constrain these forms using the present day state of galaxies with model or simulation predictions. We present the infall rates used as input to a grid of chemical evolution models spanning the mass spectrum of discs observed today. We provide a systematic comparison with alternate analytical infall schemes in the literature, including a first comparison with cosmological simulations. Identifying the degeneracies associated with the adopted infall rate prescriptions in galaxy models is an important step in the development of a consistent picture of disc galaxy formation and evolution
High-resolution spectroscopy of the blue compact dwarf galaxy Haro 15 : I. Internal kinematics
Using echelle spectroscopy, obtained at Las Campanas Observatory, we present a detailed study of the internal kinematics of the nebular material in multiple knots of the blue compact dwarf galaxy Haro15. A detailed analysis of the complex emission-line profiles shows the presence of an underlying broad component in almost all knots, and the brightest star-forming region shows unmistakable signs for the presence of two distinct narrow kinematical components. We also study the information that our analysis provides regarding the motion of the individual knots in the Haro15 galaxy potential, confirming that they follow galactic rotation. Finally, we examine the relation between their velocity dispersion and luminosity, finding that almost all knots follow the relation for virialized systems. This holds for the strong narrow components identified in complex fits and for single profile fits, although the latter show a flatter slope. In agreement with previous findings, in this paper we show that the existence of multiple kinematical components among massive starbursts cannot be overlooked, as it has a noticeable effect on any subsequent analysis that relies on basic parameters.Facultad de Ciencias Astronómicas y Geofísica
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