1,222 research outputs found
Molecular hydrogen deficiency in HI-poor galaxies and its implications for star formation
We use a sample of 47 homogeneous and high sensitivity CO images taken from
the Nobeyama and BIMA surveys to demonstrate that, contrary to common belief, a
significant number (~40%) of HI-deficient nearby spiral galaxies are also
depleted in molecular hydrogen. While HI-deficiency by itself is not a
sufficient condition for molecular gas depletion, we find that H2 reduction is
associated with the removal of HI inside the galaxy optical disk. Those
HI-deficient galaxies with normal H2 content have lost HI mainly from outside
their optical disks, where the H2 content is low in all galaxies. This finding
is consistent with theoretical models in which the molecular fraction in a
galaxy is determined primarily by its gas column density. Our result is
supported by indirect evidence that molecular deficient galaxies form stars at
a lower rate or have dimmer far infrared fluxes than gas rich galaxies, as
expected if the star formation rate is determined by the molecular hydrogen
content. Our result is consistent with a scenario in which, when the atomic gas
column density is lowered inside the optical disk below the critical value
required to form molecular hydrogen and stars, spirals become quiescent and
passive evolving systems. We speculate that this process would act on the
time-scale set by the gas depletion rate and might be a first step for the
transition between the blue and red sequence observed in the color-magnitude
diagram.Comment: 12 pages, 9 figures, accepted for publication in Ap
Kinematic Properties and Dark Matter Fraction of Virgo Dwarf Early-Type Galaxies
What happens to dwarf galaxies as they enter the cluster potential well is
one of the main unknowns in studies of galaxy evolution. Several evidence
suggests that late-type galaxies enter the cluster and are transformed to dwarf
early-type galaxies (dEs). We study the Virgo cluster to understand which
mechanisms are involved in this transformation. We find that the dEs in the
outer parts of Virgo have rotation curves with shapes and amplitudes similar to
late-type galaxies of the same luminosity. These dEs are rotationally
supported, have disky isophotes, and younger ages than those dEs in the center
of Virgo, which are pressure supported, often have boxy isophotes and are
older. Ram pressure stripping, thus, explains the properties of the dEs located
in the outskirts of Virgo. However, the dEs in the central cluster regions,
which have lost their angular momentum, must have suffered a more violent
transformation. A combination of ram pressure stripping and harassment is not
enough to remove the rotation and the spiral/disky structures of these
galaxies. We find that on the the Faber-Jackson and the Fundamental Plane
relations dEs deviate from the trends of massive elliptical galaxies towards
the position of dark matter dominated systems such as the dwarf spheroidal
satellites of the Milky Way and M31. Both, rotationally and pressure supported
dEs, however, populate the same region in these diagrams. This indicates that
dEs have a non-negligible dark matter fraction within their half light radius.Comment: Proceeding of the XXVIII IAU General Assembly, Special Session 3:
Galaxy Evolution through Secular Processes. Edts: R. Buta and D. Pfennige
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
Mid--IR emission of galaxies in the Virgo cluster: II. Integrated properties
We analyse the integrated properties of the Mid-IR emission of a complete,
optically selected sample of galaxies in the Virgo cluster observed with the
ISOCAM instrument on board the ISO satellite. The analysis shows that the
Mid-IR emission up to 15 mic of optically-selected, normal early-type galaxies
(E, S0 and S0a) is dominated by the Rayleigh-Jeans tail of the cold stellar
component. The Mid-IR emission of late-type galaxies is instead dominated by
the thermal emission from dust. The small dust grains emitting in the Mid-IR
have an excess of emission if compared to big grains emitting in the Far-IR.
While the Far-IR emission increases with the intensity of the interstellar
radiation field, their Mid-IR emission is non--linearly related to the UV
radiation field. The spectral energy distributions of the target galaxies
indicate that there is a linear relationship between the UV radiation field and
the Mid-IR emission of galaxies for low or intermediate activities of star
formation, while the emission from the hot dust seems to drop for strong UV
fields. The Mid-IR colour of late-type galaxies is not related to their
activity of star formation. The properties of the dust emission in the Mid-IR
seem more related to the mass than to the morphological type of the target
galaxy. Since the activity of star formation is anticorrelated to the mass of
galaxies, this reflects a relationship between the emission of dust in the
Mid-IR and the UV radiation field: galaxies with the lowest Mid-IR emission for
a given UV field are low mass, dwarf galaxies. These observational evidences
are easily explained if the carriers of the Unidentified Infrared Bands that
dominate the 6.75 mic emission are destroyed by the intense UV radiation field
of dwarf galaxies, although abundance effects can also play a role.Comment: 17 pages, Latex, 7 figures; to be published in Astronomy &
Astrophysics, Main Journal; Figure legend should be corrected in: 1 - 1a; 2 -
1b; 3 - 2; 4 - 3a; 5 - 3b; 6 - 3c; 7 - 3d; 8 - 3e; 9 - 4; 10 - 5; 11 - 6; 12
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Mid-IR emission of galaxies in the Virgo cluster and in the Coma supercluster.IV. The nature of the dust heating sources
We study the relationship between the mid-IR (5-18 m) emission of
late-type galaxies and various other star formation tracers in order to
investigate the nature of the dust heating sources in this spectral domain. The
analysis is carried out using a sample of 123 normal, late-type, nearby
galaxies with available data at several frequencies. The mid-IR luminosity
(normalized to the H-band luminosity) correlates better with the far-IR
luminosity than with more direct tracers of the young stellar population such
as the H and the UV luminosity. The comparison of resolved images
reveals a remarkable similarity in the H and mid-IR morphologies, with
prominent HII regions at both frequencies. The mid-IR images, however, show in
addition a diffuse emission not associated with HII regions nor with the
diffuse H emission. This evidence indicates that the stellar population
responsible for the heating of dust emitting in the mid-IR is similar to that
heating big grains emitting in the far-IR, including relatively evolved stars
responsible for the non-ionizing radiation. The scatter in the mid-IR vs.
H, UV and far-IR luminosity relation is mostly due to metallicity
effects, with metal-poor objects having a lower mid-IR emission per unit star
formation rate than metal-rich galaxies. Our analysis indicates that the mid-IR
luminosity is not an optimal star formation tracer in normal, late-type
galaxies.Comment: accepted for publication on A&
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
Multiple merging in the Abell cluster 1367
We present a dynamical analysis of the central ~1.3 square degrees of the
cluster of galaxies Abell 1367, based on 273 redshift measurements (of which
119 are news). From the analysis of the 146 confirmed cluster members we derive
a significantly non-Gaussian velocity distribution, with a mean location C_{BI}
= 6484+/-81 km/s and a scale S_{BI} = 891+/-58 km/s. The cluster appears
elongated from the North-West to the South-East with two main density peaks
associated with two substructures. The North-West subcluster is probably in the
early phase of merging into the South-East substructure (~ 0.2 Gyr before core
crossing). A dynamical study of the two subclouds points out the existence of a
group of star-forming galaxies infalling into the core of the South-East
subcloud and suggests that two other groups are infalling into the NW and SE
subclusters respectively. These three subgroups contain a higher fraction of
star-forming galaxies than the cluster core, as expected during merging events.
Abell 1367 appears as a young cluster currently forming at the intersection of
two filaments.Comment: 15 pages, 13 figures, 7 tables. Accepted for publication on A&A. High
resolution figures at http://goldmine.mib.infn.it/papers/a1367.htm
Molecular gas in late-type galaxies
We present CO(J=1--0) line observations of 22 low-luminosity spiral
galaxies in the Virgo cluster. These data, together with 244 others available
in the literature, allow us to build a large sample that we use to study the
molecular gas properties of galaxies spanning a large range of morphological
types and luminosities and belonging to different environments (clusters -
field). The molecular gas content of the target galaxies is estimated using a
luminosity-dependent X = conversion factor that has been
calibrated on a sample of nearby galaxies. spans from 10 mol
cm (K km s in giant spirals to 10 mol
cm (K km s in dwarf irregulars. The value of the
conversion factor is found consistent with a value derived independently from
dust masses estimated from FIR fluxes, with a metallicity-dependent dust to gas
ratio. The relationships between X and the UV radiation field (as traced by the
), the metallicity and the H band luminosity are analysed.
We show that the molecular gas contained in molecular clouds or complexes is of
the order of 15% of the total gas on average whatever the luminosity or the
Hubble type of the galaxies. We discuss the relation between the star formation
rate and the molecular gas content and estimate the average star formation
efficiency of late-type galaxies.Comment: accepted for publication on Astronomy and Astrophysic
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
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