1,396 research outputs found
{Chemo-spectrophotometric evolution of spiral galaxies: IV. Star formation efficiency and effective ages of spirals
We study the star formation history of normal spirals by using a large and
homogeneous data sample of local galaxies. For our analysis we utilise detailed
models of chemical and spectrophotometric galactic evolution, calibrated on the
Milky Way disc. We find that star formation efficiency is independent of
galactic mass, while massive discs have, on average, lower gas fractions and
are redder than their low mass counterparts; put together, these findings
convincingly suggest that massive spirals are older than low mass ones. We
evaluate the effective ages of the galaxies of our sample and we find that
massive spirals must be several Gyr older than low mass ones. We also show that
these galaxies (having rotational velocities in the 80-400 km/s range) cannot
have suffered extensive mass losses, i.e. they cannot have lost during their
lifetime an amount of mass much larger than their current content of gas+stars.Comment: 11 pages, 8 figures, MNRAS in pres
The GALEX Ultraviolet Virgo Cluster Survey (GUViCS). II. Constraints on star formation in ram-pressure stripped gas
Context: Several galaxies in the Virgo cluster are known to have large HI gas
tails related to a recent ram-pressure stripping event. The Virgo cluster has
been extensively observed at 1539 A in the far-ultraviolet for the GALEX
Ultraviolet Virgo Cluster Survey (GUViCS), and in the optical for the Next
Generation Virgo Survey (NGVS), allowing a study of the stellar emission
potentially associated with the gas tails of 8 cluster members. On the
theoretical side, models of ram-pressure stripping events have started to
include the physics of star formation. Aim: We aim to provide quantitative
constraints on the amount of star formation taking place in the ram-pressure
stripped gas, mainly on the basis of the far-UV emission found in the GUViCS
images in relation with the gas content of the tails. Methods: We have
performed three comparisons of the young stars emission with the gas column
density: visual, pixel-by-pixel and global. We have compared our results to
other observational and theoretical studies. Results: We find that the level of
star formation taking place in the gas stripped from galaxies by ram-pressure
is low with respect to the available amount of gas. Star formation is lower by
at least a factor 10 compared to the predictions of the Schmidt Law as
determined in regular spiral galaxy disks. It is also lower than measured in
dwarfs galaxies and the outer regions of spirals, and than predicted by some
numerical simulations. We provide constraints on the star formation efficiency
in the ram-pressure stripped gas tails, and compare these with current models.Comment: Accepted in A&A, 17 pages (including the appendix and "on-line"
figures of the paper
The effect of the environment on the HI scaling relations
We use a volume-, magnitude-limited sample of nearby galaxies to investigate
the effect of the environment on the HI scaling relations. We confirm that the
HI-to-stellar mass ratio anti correlates with stellar mass, stellar mass
surface density and NUV-r colour across the whole range of parameters covered
by our sample (10^9 <M*<10^11 Msol, 7.5 <mu*<9.5 Msol kpc^-2, 2<NUV-r<6 mag).
These scaling relations are also followed by galaxies in the Virgo cluster,
although they are significantly offset towards lower gas content.
Interestingly, the difference between field and cluster galaxies gradually
decreases moving towards massive, bulge-dominated systems. By comparing our
data with the predictions of chemo-spectrophotometric models of galaxy
evolution, we show that starvation alone cannot explain the low gas content of
Virgo spirals and that only ram-pressure stripping is able to reproduce our
findings. Finally, motivated by previous studies, we investigate the use of a
plane obtained from the relations between the HI-to-stellar mass ratio, stellar
mass surface density and NUV-r colour as a proxy for the HI deficiency
parameter. We show that the distance from the `HI gas fraction plane' can be
used as an alternative estimate for the HI deficiency, but only if carefully
calibrated on pre-defined samples of `unperturbed' systems.Comment: Accepted for publication on MNRAS main journal. 11 pages, 6 figures,
1 tabl
The origin of the mu_e - M_B and Kormendy relations in dwarf elliptical galaxies
The present work is aimed at studying the distribution of galaxies of
different types and luminosities along different structural scaling relations
to see whether massive and dwarf ellipticals have been shaped by the same
formation process. This exercise is here done by comparing the distribution of
Virgo cluster massive and dwarf ellipticals and star forming galaxies along the
B band effective surface brightness and effective radius vs. absolute magnitude
relations and the Kormendy relation to the predictions of models tracing the
effects of ram-pressure stripping on disc galaxies entering the cluster
environment and galaxy harassment. Dwarf ellipticals might have been formed
from low luminosity, late-type spirals that recently entered into the cluster
and lost their gas because of a ram-pressure stripping event, stopping their
activity of star formation. The perturbations induced by the abrupt decrease of
the star formation activity are sufficient to modify the structural properties
of disc galaxies into those of dwarf ellipticals. Galaxy harassment induce a
truncation of the disc and generally an increase of the effective surface
brightness of the perturbed galaxies. The lack of dynamical simulations of
perturbed galaxies spanning a wide range in luminosity prevents us to drive any
firm conclusion on a possible harassment-induced origin of the low surface
brightness dwarf elliptical galaxy population inhabiting the Virgo cluster.
Although the observed scaling relations are consistent with the idea that the
distribution of elliptical galaxies along the mentioned scaling relation is
just due to a gradual variation with luminosity of the Sersic index n, the
comparison with models indicates that dwarf ellipticals might have been formed
by a totally different process than giant ellipticalsComment: Accepted for publication on A&
The Star Formation Rate in disk galaxies: thresholds and dependence on gas amount
We reassess the applicability of the Toomre criterion in galactic disks and
we study the local star formation law in 16 disk galaxies for which abundance
gradients are published. The data we use consists of stellar light profiles,
atomic and molecular gas (deduced from CO with a metallicity-dependent
conversion factor), star formation rates (from H-alpha emissivities),
metallicities, dispersion velocities and rotation curves. We show that the
Toomre criterion applies successfully to the case of the Milky Way disk, but it
has limited success with the data of our sample; depending on whether the
stellar component is included or not in the stability analysis, we find average
values for the threshold ratio of the gas surface density to the critical
surface density in the range 0.5 to 0.7. We also test various star formation
laws proposed in the literature, i.e. either the simple Schmidt law or
modifications of it, that take into account dynamical factors. We find only
small differences among them as far as the overall fit to our data is
concerned; in particular, we find that all three SF laws (with parameters
derived from the fits to our data) match particularly well observations in the
Milky Way disk. In all cases we find that the exponent n of our best fit SFR
has slightly higher values than in other recent works and we suggest several
reasons that may cause that discrepancy.Comment: 18 pages, 12 figures, accepted in MNRA
Higgs decay into four charged leptons in the presence of dimension-six operators
We study the indirect effects of New Physics in the Higgs decay into four
charged leptons, using an Effective Field Theory (EFT) approach to Higgs
interactions. We evaluate the deviations induced by the EFT dimension-six
operators in observables like partial decay width and various kinematic
distributions, including angular observables, and compare them with the
contribution of the full SM electroweak corrections. The calculation is
implemented in an improved version of the event generator Hto4l, which can
provide predictions in terms of different EFT-bases and is available for data
analysis at the LHC. We also perform a phenomenological study in order to
assess the benefits coming from the inclusion of differential information in
the future analyses of very precise data which will be collected during the
high luminosity phase of the LHC.Comment: 31 pages, 8 figures, 1 table. Version to appear on JHEP, expanded
phenomenological section including an analysis for HL-LH
1.65mic (H-band) surface photometry of galaxies. VI: The history of star formation in normal late-type galaxies
We have collected a large body of NIR (H band), UV (2000 A) and Halpha
measurements of late-type galaxies. These are used, jointly with spectral
evolutionary synthesis models, to study the initial mass function (IMF) in the
mass range m > 2 Mo. For spirals (Sa-Sd), Magellanic irregulars (Im) and blue
compact dwarfs (BCD), our determination is consistent with a Salpeter IMF with
an upper mass cutoff M_up = 80 Mo. The history of star formation and the amount
of total gas (per unit mass) of galaxies are found to depend primarily on their
total masses (as traced by the H band luminosities) and only secondarily on
morphological type. The present star formation activity of massive spirals is
up to 100 times smaller than that average over their lifetime, while in low
mass galaxies it is comparable to or higher than that at earlier epochs. Dwarf
galaxies have presently larger gas reservoirs per unit mass than massive
spirals. The efficiency in transforming gas into stars and the time scale for
gas depletion (10 Gyrs) are independent of the luminosity and/or of the
morphological type. These evidences are consistent with the idea that galaxies
are coeval systems,that they evolved as closed-boxes forming stars following a
simple, universal star formation law whose characteristic time scale is small
(1 Gyr) in massive spirals and large (10 Gyr) in low mass galaxies. A similar
conclusion was drawn by Gavazzi and Scodeggio (1996) to explain the
colour-magnitude relation of late-type galaxies. The consequences of this
interpretation on the evolution of the star formation rate and of the gas
density per comoving volume of the Universe with look-back time are discussed.Comment: LaTex, 24 pages, 12 figures, accepted for publication on Astronomical
Journa
High mass star formation in normal late-type galaxies: observational constraints to the IMF
We use Halpha and FUV GALEX data for a large sample of nearby objects to
study the high mass star formation activity of normal late-type galaxies. The
data are corrected for dust attenuation using the most accurate techniques at
present available, namely the Balmer decrement and the total far-infrared to
FUV flux ratio. The sample shows a highly dispersed distribution in the Halpha
to FUV flux ratio indicating that two of the most commonly used star formation
tracers give star formation rates with uncertainties up to a factor of 2-3. The
high dispersion is due to the presence of AGN, where the UV and the Halpha
emission can be contaminated by nuclear activity, highly inclined galaxies, for
which the applied extinction corrections are probably inaccurate, or starburst
galaxies, where the stationarity in the star formation history required for
transforming Halpha and UV luminosities into star formation rates is not
satisfied. Excluding these objects we reach an uncertainty of ~50% on the SFR.
The Halpha to FUV flux ratio increases with their total stellar mass. If
limited to normal star forming galaxies, however, this relationship reduces to
a weak trend that might be totally removed using different extinction
correction recipes. In these objects the Halpha to FUV flux ratio seems also
barely related with the FUV-H colour, the H band effective surface brightness,
the total star formation activity and the gas fraction. The data are consistent
with a Kroupa and Salpeter initial mass function in the high mass stellar range
and imply, for a Salpeter IMF, that the variations of the slope cannot exceed
0.25, from g=2.35 for massive galaxies to g=2.60 in low luminosity systems. We
show however that these observed trends, if real, can be due to the different
micro history of star formation in massive galaxies with respect to dwarf.Comment: Accepted for publication on Ap
Non-linear Dependence of L(B) on L(FIR) and M(H2) among Spiral Galaxies and Effects of Tidal Interaction
Through the study of a carefully selected sample of isolated spiral galaxies,
we have established that two important global physical quantities for tracing
star forming activities, L(FIR) and M(H2), have non-linear dependence on
another commonly cited global quantity L(B). Furthermore we show that simple
power law relations can effectively describe these non-linear relations for
spiral galaxies spanning four orders of magnitude in FIR and M(H2) and nearly
three orders of magnitude in L(B). While the existence of non-linear dependence
of M(H2) (assuming a constant CO-to-H2 conversion) and L(FIR) on optical
luminosity L(B) has been previously noted in the literature, an improper
normalization of simple scaling by L(B) has been commonly used in many previous
studies to claim enhanced molecular gas content and induced activities among
tidally interacting and other types of galaxies. We remove these non-linear
effects using the template relations derived from the isolated galaxy sample
and conclude that strongly interacting galaxies do not have enhanced molecular
gas content, contrary to previous claims. With these non-linear relations among
L(B), L(FIR) and M(H2) properly taken into account, we confirm again that the
FIR emission and the star formation efficiency L(FIR)/M(H2) are indeed enhanced
by tidal interactions. Virgo galaxies show the same level of M(H2) and L(FIR)
as isolated galaxies. We do not find any evidence for enhanced star forming
activity among barred galaxies.Comment: 19 pages and 5 figures, requires AAS style files, ApJ, accepte
Ultraviolet to infrared emission of z>1 galaxies: Can we derive reliable star formation rates and stellar masses?
We seek to derive star formation rates (SFR) and stellar masses (M_star) in
distant galaxies and to quantify the main uncertainties affecting their
measurement. We explore the impact of the assumptions made in their derivation
with standard calibrations or through a fitting process, as well as the impact
of the available data, focusing on the role of IR emission originating from
dust. We build a sample of galaxies with z>1, all observed from the UV to the
IR (rest frame). The data are fitted with the code CIGALE, which is also used
to build and analyse a catalogue of mock galaxies. Models with different SFHs
are introduced. We define different set of data, with or without a good
sampling of the UV range, NIR, and thermal IR data. The impact of these
different cases on the determination of M_star and SFR are analysed.
Exponentially decreasing models with a redshift formation of the stellar
population z ~8 cannot fit the data correctly. The other models fit the data
correctly at the price of unrealistically young ages when the age of the single
stellar population is taken to be a free parameter. The best fits are obtained
with two stellar populations. As long as one measurement of the dust emission
continuum is available, SFR are robustly estimated whatever the chosen model
is, including standard recipes. M_star measurement is more subject to
uncertainty, depending on the chosen model and the presence of NIR data, with
an impact on the SFR-M_star scatter plot. Conversely, when thermal IR data from
dust emission are missing, the uncertainty on SFR measurements largely exceeds
that of stellar mass. Among all physical properties investigated here, the
stellar ages are found to be the most difficult to constrain and this
uncertainty acts as a second parameter in SFR measurements and as the most
important parameter for M_star measurements.Comment: 14 pages, 14 figures, accepted for publication A&
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