254 research outputs found
Gas inflows, star formation and metallicity evolution in galaxy pairs
It has been known since many decades that galaxy interactions can induce star
formation (hereafter SF) enhancements and that one of the driving mechanisms of
this enhancement is related to gas inflows into the central galaxy regions,
induced by asymmetries in the stellar component, like bars. In the last years
many evidences have been accumulating, showing that interacting pairs have
central gas-phase metallicities lower than those of field galaxies, by {\sim}
0.2-0.3 dex on average. These diluted ISM metallicities have been explained as
the result of inflows of metal-poor gas from the outer disk to the galaxy
central regions. A number of questions arises: What's the timing and the
duration of this dilution? How and when does the SF induced by the gas inflow
enrich the circumnuclear gas with re-processed material? Is there any
correlation between the timing and strength of the dilution and the timing and
intensity of the SF? By means of Tree-SPH simulations of galaxy major
interactions, we have studied the effect that gas inflows have on the ISM
dilution, and the effect that the induced SF has, subsequently, in re-enriching
the nuclear gas. In this contribution, we present the main results of this
study.Comment: Proceedings of the IAU Symposium 277 "Tracing the Ancestry of
Galaxies", 4 pages, 2 figure
Feedback in the local LBG Analog Haro 11 as probed by far-UV and X-ray observations
We have re-analyzed FUSE data and obtained new Chandra observations of Haro
11, a local (D_L=88 Mpc) UV luminous galaxy. Haro 11 has a similar far-UV
luminosity (10^10.3 L_\odot), UV surface brightness (10^9.4 L_\odot kpc^-2),
SFR, and metallicity to that observed in Lyman Break Galaxies (LBGs). We show
that Haro 11 has extended, soft thermal (kT~0.68 keV) X-ray emission with a
luminosity and size which scales with the physical properties (e.g. SFR,
stellar mass) of the host galaxy. An enhanced alpha/Fe, ratio of ~4 relative to
solar abundance suggests significant supernovae enrichment. These results are
consistent with the X-ray emission being produced in a shock between a
supernovae driven outflow and the ambient material. The FUV spectra show strong
absorption lines similar to those observed in LBG spectra. A blueshifted
absorption component is identified as a wind outflowing at ~200-280 km/s.
OVI\lambda\lambda1032,1038 emission, the dominant cooling mechanism for coronal
gas at T~10^5.5 K is also observed. If associated with the outflow, the
luminosity of the OVI emission suggests that <20% of the total mechanical
energy from the supernovae and solar winds is being radiated away. This implies
that radiative cooling through OVI is not significantly inhibiting the growth
of the outflowing gas. In contradiction to the findings of Bergvall et al 2006,
we find no convincing evidence of Lyman continuum leakage in Haro 11. We
conclude that the wind has not created a `tunnel' allowing the escape of a
significant fraction of Lyman continuum photons and place a limit on the escape
fraction of f_{esc}<2%. Overall, both Haro 11 and a previously observed LBG
analogue VV 114, provide an invaluable insight into the X-ray and FUV
properties of high redshift LBGs.Comment: Accepted for publication in ApJ, 40 pages, 17 figure
The starburst phenomenon from the optical/near-IR perspective
The optical/near-IR stellar continuum carries unique information about the
stellar population in a galaxy, its mass function and star-formation history.
Star-forming regions display rich emission-line spectra from which we can
derive the dust and gas distribution, map velocity fields, metallicities and
young massive stars and locate shocks and stellar winds. All this information
is very useful in the dissection of the starburst phenomenon. We discuss a few
of the advantages and limitations of observations in the optical/near-IR region
and focus on some results. Special attention is given to the role of
interactions and mergers and observations of the relatively dust-free starburst
dwarfs. In the future we expect new and refined diagnostic tools to provide us
with more detailed information about the IMF, strength and duration of the
burst and its triggering mechanisms.Comment: 6 pages, 3 figures, to appear in "Starbursts: from 30 Doradus to
Lyman Break Galaxies" 2005, eds. R. de Grijs and R. M. Gonzalez Delgado
(Kluwer
Efficiency of the dynamical mechanism
The most extreme starbursts occur in galaxy mergers, and it is now
acknowledged that dynamical triggering has a primary importance in star
formation. This triggering is due partly to the enhanced velocity dispersion
provided by gravitational instabilities, such as density waves and bars, but
mainly to the radial gas flows they drive, allowing large amounts of gas to
condense towards nuclear regions in a small time scale. Numerical simulations
with several gas phases, taking into account the feedback to regulate star
formation, have explored the various processes, using recipes like the Schmidt
law, moderated by the gas instability criterion. May be the most fundamental
parameter in starbursts is the availability of gas: this sheds light on the
amount of external gas accretion in galaxy evolution. The detailed mechanisms
governing gas infall in the inner parts of galaxy disks are discussed.Comment: 6 pages, 3 figures, to be published in "Starbursts - From 30 Doradus
to Lyman break galaxies", ed. R. de Grijs and R. Gonzalez-Delgad
An Exploration of the Tully-Fisher Relation for Extreme Late-Type Spiral Galaxies
This paper explores the adherence of 47 extreme late-type galaxies to the B-
and V-band Tully-Fisher relations defined by a sample of local calibrators. In
both bands we find the mean luminosity at a given line width for extreme
late-type spirals to lie below that predicted by standard Tully-Fisher
relations. While many of the extreme late-type spirals do follow the
Tully-Fisher relation to within our observational uncertainties, most of these
galaxies lie below the normal, linear Tully-Fisher relation, and some are
underluminous by more than 2 sigma (i.e. >1.16 magnitudes in V). This suggests
a possible downward curvature of the Tully-Fisher relation for some of the
smallest and faintest rotationally supported disk galaxies. This may be a
consequence of the increasing prevalence of dark matter in these systems. We
find the deviation from the Tully-Fisher relation to increase with decreasing
luminosity and decreasing optical linear size in our sample, implying that the
physically smallest and faintest spirals may be a structurally and
kinematically distinct class of objects.Comment: 32 pages, 13 figures; to appear in the November A
On the Detectability of Lyman-alpha Emission in Star-forming Galaxies: The Role of Dust
Lyman-alpha is now widely used to investigate the galaxy formation and
evolution in the high redshift universe. However, without a rigorous
understanding of the processes which regulate the Lya escape fraction, physical
interpretations of high-z observations remain questionable. We examine six
nearby star-forming galaxies to disentangle the role of the dust from other
parameters such as gas kinematics, geometry and ISM morphology in the
obscuration of Ly-alpha. Thereby we aim to understand the Ly-a escape physics
and infer the implications for high-redshift studies. We use HST/ACS to produce
continuum-subtracted Lya maps, and ground-based observations (ESO/NTT and NOT)
to map the Halpha emission and the extinction E(B-V) in the gas phase derived
from the Balmer decrement Halpha/Hbeta. When large outflows are present, the
Lya emission appears not to correlate with the dust content, confirming the
role of the HI kinematics in the escape of Lya photons. In the case of a dense,
static HI covering, we observe a damped absorption with a declining
relationship between Lya and E(B-V). We found that the Lya escape fraction does
not exceed 10% in all our galaxies and is mostly about 3% or below. Finally,
because of the radiative transfer complexity of the Lya line, star formation
rate based on Lya luminosity is underestimated with respect to that derived
from UV luminosity. The failure of simple dust correction to recover the
intrinsic Lya/Ha ratio or the total star formation rate should prompt us to be
more cautious when interpreting high-z observations and related properties,
such as SFRs based on Lya alone. To this end we propose a more realistic
calibration for SFR(Lya) which accounts for dust attenuation and resonant
scattering effects via the Lya escape fraction.Comment: Accepted for publication in A&A, 19 pages, 15 figure
A search for Low Surface Brightness galaxies in the near-infrared I. Selection of the sample
A sample of about 3,800 Low Surface Brightness (LSB) galaxies was selected
using the all-sky near-infrared (J, H and K_s-band) 2MASS survey. The selected
objects have a mean central surface brightness within a 5 arcsec radius around
their centre fainter than 18 mag/sq.arcsec in the K_s band, making them the
lowest surface brightness galaxies detected by 2MASS. A description is given of
the relevant properties of the 2MASS survey and the LSB galaxy selection
procedure, as well as of basic photometric properties of the selected objects.
The latter properties are compared to those of other samples of galaxies, of
both LSBs and `classical' high surface brightness (HSB) objects, which were
selected in the optical. The 2MASS LSBs have a (B_T_c)-(K_T) colour which is on
average 0.9 mag bluer than that of HSBs from the NGC. The 2MASS sample does not
appear to contain a significant population of red objects.Comment: accepted for publication in Astronomy and Astrophysics on 24/2/2003;
62 page
A Comparison of the Intrinsic Shapes of Two Different Types of Dwarf Galaxies: Blues Compact Dwarfs and Dwarf Ellipticals
We measure the apparent shapes for a sample of 62 blue compact dwarf galaxies
(BCDs), and compare them with the apparent shapes for a sample of 80 dwarf
elliptical galaxies (dEs). The BCDs are flatter, on average, than the dEs, but
the difference is only marginally significant. We then use both non-parametric
and parametric techniques to determine possible distributions of intrinsic
shapes for the BCDs. The hypothesis that BCDs are oblate spheroids can be ruled
out with a high confidence level (), but the hypothesis that they are
prolate spheroids cannot be excluded. The apparent shapes of BCDs are totally
consistent with the hypothesis that they are triaxial ellipsoids. If the
intrinsic axis ratios, and , are distributed according to a
Gaussian with means and and standard deviation ,
we find the best-fitting distribution for BCDs has , while that for dEs has . Our results are consistent with the hypothesis that BCDs
have a close evolutionary relation with dEs.Comment: total 23 pages, 9 figures, and 1 Table, submitted to ApJ on Sep 19
1997. Email addresses: [email protected],
[email protected], [email protected],
[email protected], [email protected]
The First Deep ACS Lyman alpha Images of Local Starburst Galaxies
We report the first results from a deep Lya imaging program of local
starburst galaxies with the Advanced Camera for Surveys (ACS) of the Hubble
Space Telescope. The two observed galaxies ESO 350-IG038 and SBS 0335-052 have
luminosities similar to those of the Magellanic Clouds but differ in their
chemical composition. ESO 350-IG038 has an oxygen abundance of 1/8 solar,
whereas SBS 0335-052 is known to have one of the lowest abundances among blue
galaxies (~1/30). The ACS imaging reveals a complex Lya morphology, with
sometimes strong offsets between the emission of Lya and the location of
stellar light, ionized gas traced by Halpha, and the neutral gas. Overall, more
Lya photons escape from the more metal- and dust-rich galaxy ESO 350-IG038. The
absence of clear SBS 0335-052 Lya emission over all observed knots, whatever
their dust content or/and color indices, contradicts model expectations of a
lower escape fraction from dust-rich gas due to destruction of Lya photons by
dust grains. Rather, the results are in qualitative agreement with models
suggesting the kinematic properties of the gas as the dominant Lya escape
regulator. If the properties of the two observed galaxies are representative
for starburst galaxies in general, Lya will be difficult to interpret as a
star-formation indicator, in particular if based on Lya imaging at low spatial
resolution.Comment: 6 pages, 2 figures (1 in color) New version of Fig
Infrared Properties of Star Forming Dwarf Galaxies: Blue Compact Dwarfs in the Virgo Cluster
A sample of 16 blue compact dwarf galaxies (BCDs) in the Virgo Cluster has
been imaged in the near-infrared (NIR) in and on the 2.1m telescope
at OAN-SPM in Mexico. Isophotes as faint as = 24 mag arcsec and
= 23 mag arcsec have been reached in most of the targets.
Surface brightness profiles can be fitted across the whole range of radii by
the sum of two components: a hyperbolic secant (sech) function, which is known
to fit the light profiles of dIs, and a Gaussian component, which quantifies
the starburst near the centre. Isophotal and total fitted NIR magnitudes have
been calculated, along with semimajor axes at = 23 mag arcsec
and = 22 mag arcsec. The diffuse underlying component and
the young starburst have been quantified using the profile fitting. Most color
profiles show a constant color, between =0.7 to 0.9 mag. The diffuse
component represents the overwhelming majority of the NIR light for most BCDs,
the starburst enhancing the flux by less than about 0.3 mag. Linear
correlations were found between the sech scale length and the sech magnitude,
and between the sech semimajor axis and the sech magnitude. Overall, galaxies
with more luminous diffuse components are larger and brighter in the centre.
The central burst correlates with the diffuse component, with brighter BCDs
having stronger star-bursts, suggesting that more massive objects are forming
stars more efficiently. BCDs lie on the ``fundamental plane'' defined by dwarf
irregulars (dISs) in Paper I, following the same relation between sech absolute
magnitude, sech central surface brightness, and the hydrogen line-width
, although the scatter is larger than for the dIs. [one sentence cut]Comment: Accepted in A
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