157 research outputs found
POX 186: the ultracompact Blue Compact Dwarf Galaxy reveals its nature
High resolution, ground based R and I band observations of the ultra compact
dwarf galaxy POX 186 are presented. The data, obtained with the ESO New
Technology Telescope (NTT), are analyzed using a new deconvolution algorithm
which allows one to resolve the innermost regions of this stellar-like object
into three Super-Star Clusters (SSC). Upper limits to both masses (M\sim 10^5
M_{\odot}) and the physical sizes (\le 60pc) of the SSCs are set. In addition,
and maybe most importantly, extended light emission underlying the compact
star-forming region is clearly detected in both bands. The R-I color rules out
nebular H\alpha contamination and is consistent with an old stellar population.
This casts doubt on the hypothesis that Blue Compact Dwarf Galaxies (BCDG) are
young galaxies.Comment: 4 figures postscript, 2 tables, to appear in A&A main journa
3D Spectroscopic Observations of Star-Forming Dwarf Galaxies
We give an introduction into the observational technique of integral field or
3D spectroscopy. We discuss advantages and drawbacks of this type of
observations and highlight a few science projects enabled by this method. In
the second part we describe our 3D spectroscopic survey of Blue Compact Dwarf
Galaxies. We show preliminary results from data taken with the VIMOS integral
field unit and give an outlook on how automated spectral analysis and
forthcoming instruments can provide a new view on star formation and associated
processes in dwarf galaxies.Comment: To appear in the proceedings of the JENAM 2010 Symposium "Dwarf
Galaxies: Keys to Galaxy Formation and Evolution" (Lisbon, 9-10 September
2010), P. Papaderos, S. Recchi, G. Hensler (eds.), Springer Verlag (2011), in
pres
The effects of star formation on the low-metallicity ISM: NGC4214 mapped with Herschel/PACS spectroscopy
We present Herschel/PACS spectroscopic maps of the dwarf galaxy NC4214
observed in 6 far infrared fine-structure lines: [C II] 158mu, [O III] 88mu, [O
I] 63mu, [O I] 146mu, [N II] 122mu, and [N II] 205mu. The maps are sampled to
the full telescope spatial resolution and reveal unprecedented detail on ~ 150
pc size scales. We detect [C II] emission over the whole mapped area, [O III]
being the most luminous FIR line. The ratio of [O III]/[C II] peaks at about 2
toward the sites of massive star formation, higher than ratios seen in dusty
starburst galaxies. The [C II]/CO ratios are 20 000 to 70 000 toward the 2
massive clusters, which are at least an order of magnitude larger than spiral
or dusty starbursts, and cannot be reconciled with single-slab PDR models.
Toward the 2 massive star-forming regions, we find that L[CII] is 0.5 to 0.8%
of the LTIR . All of the lines together contribute up to 2% of LTIR . These
extreme findings are a consequence of the lower metallicity and young,
massive-star formation commonly found in dwarf galaxies. These conditions
promote large-scale photodissociation into the molecular reservoir, which is
evident in the FIR line ratios. This illustrates the necessity to move to
multiphase models applicable to star-forming clusters or galaxies as a whole.Comment: Accepted for publication in the A&A Herschel Special Issu
Linking dust emission to fundamental properties in galaxies: The low-metallicity picture
In this work, we aim at providing a consistent analysis of the dust
properties from metal-poor to metal-rich environments by linking them to
fundamental galactic parameters. We consider two samples of galaxies: the Dwarf
Galaxy Survey (DGS) and KINGFISH, totalling 109 galaxies, spanning almost 2 dex
in metallicity. We collect infrared (IR) to submillimetre (submm) data for both
samples and present the complete data set for the DGS sample. We model the
observed spectral energy distributions (SED) with a physically-motivated dust
model to access the dust properties. Using a different SED model (modified
blackbody), dust composition (amorphous carbon), or wavelength coverage at
submm wavelengths results in differences in the dust mass estimate of a factor
two to three, showing that this parameter is subject to non-negligible
systematic modelling uncertainties. For eight galaxies in our sample, we find a
rather small excess at 500 microns (< 1.5 sigma). We find that the dust SED of
low-metallicity galaxies is broader and peaks at shorter wavelengths compared
to more metal-rich systems, a sign of a clumpier medium in dwarf galaxies. The
PAH mass fraction and the dust temperature distribution are found to be driven
mostly by the specific star-formation rate, SSFR, with secondary effects from
metallicity. The correlations between metallicity and dust mass or total-IR
luminosity are direct consequences of the stellar mass-metallicity relation.
The dust-to-stellar mass ratios of metal-rich sources follow the well-studied
trend of decreasing ratio for decreasing SSFR. The relation is more complex for
highly star-forming low-metallicity galaxies and depends on the chemical
evolutionary stage of the source (i.e., gas-to-dust mass ratio). Dust growth
processes in the ISM play a key role in the dust mass build-up with respect to
the stellar content at high SSFR and low metallicity. (abridged)Comment: 44 pages (20 pages main body plus 5 Appendices), 11 figures, 9
tables, accepted for publication in A&
Gas-to-Dust mass ratios in local galaxies over a 2 dex metallicity range
This paper analyses the behaviour of the gas-to-dust mass ratio (G/D) of
local Universe galaxies over a large metallicity range. We combine three
samples: the Dwarf Galaxy Survey, the KINGFISH survey and a subsample from
Galametz et al. (2011) totalling 126 galaxies, covering a 2 dex metallicity
range, with 30% of the sample with 12+log(O/H) < 8.0. The dust masses are
homogeneously determined with a semi-empirical dust model, including submm
constraints. The atomic and molecular gas masses are compiled from the
literature. Two XCO are used to estimate molecular gas masses: the Galactic
XCO, and a XCO depending on the metallicity (as Z^{-2}). Correlations with
morphological types, stellar masses, star formation rates and specific star
formation rates are discussed. The trend between G/D and metallicity is
empirically modelled using power-laws (slope of -1 and free) and a broken
power-law. We compare the evolution of the G/D with predictions from chemical
evolution models. We find that out of the five tested galactic parameters,
metallicity is the galactic property driving the observed G/D. The G/D versus
metallicity relation cannot be represented by a power-law with a slope of -1
over the whole metallicity range. The observed trend is steeper for
metallicities lower than ~ 8.0. A large scatter is observed in the G/D for a
given metallicity, with a dispersion of 0.37 dex in metallicity bins of ~0.1
dex. The broken power-law reproduces best the observed G/D and provides
estimates of the G/D that are accurate to a factor of 1.6. The good agreement
of the G/D and its scatter with the three tested chemical evolution models
shows that the scatter is intrinsic to galactic properties, reflecting the
different star formation histories, dust destruction efficiencies, dust grain
size distributions and chemical compositions across the sample. (abriged)Comment: 23 pages, 12 figures, accepted in Astronomy & Astrophysic
A Compact Population of Red Giants in the Blue Compact Dwarf Galaxy UGCA 290
We present HST/WFPC2 single-star photometry for the blue dwarf galaxy UGCA
290, whose morphology is intermediate between classic iE Blue Compact Dwarfs
and blue dwarfs which exhibit no red background sheet of older stars. The
color-magnitude diagram of this galaxy in V and I, extending over six
magnitudes, is remarkably similar to that of the star-forming region in the iE
Blue Compact Dwarf VII Zw 403. There is no evidence for gaps in its
star-formation history over the last billion years, and the color of its red
giant branch indicates a very metal-poor stellar population. From the magnitude
of the tip of the red giant branch, we derive a distance of 6.7 Mpc, more than
twice the distance estimated from the brightest blue supergiants.Comment: 10 pages, 3 color figures, LaTeX2e. Accepted for publication in ApJ
Letter
ISM enrichment and local pollution in dwarf galaxies
The fate of metals after they are released in starburst episodes is still
unclear. What phases of the interstellar medium are involved, in which
timescales? Evidence has grown over the past few years that the neutral phase
of blue compact dwarf (BCD) galaxies may be metal- deficient as compared to the
ionized gas of their HII regions. These results have strong implications for
our understanding of the chemical evolution of galaxies. We review here the
main results and the main caveats in the abundance determination from far-UV
absorption-lines. We also discuss possible scenarios concerning the journey of
metals into the interstellar medium, or even their ejection from the galaxy
into the intergalactic medium.Comment: Long version of a proceeding for the conference "A Universe of Dwarf
Galaxies: Observations, Theories, Simulations" held in Lyon, France
(14th-18th, June 2010
An imaging and spectroscopic study of the very metal-deficient blue compact dwarf galaxy Tol 1214--277
We present a spectrophotometric study based on VLT/FORS I observations of one
of the most metal-deficient blue compact dwarf (BCD) galaxies known, Tol
1214-277 (Z ~ Zsun/25). The data show that roughly half of the total luminosity
of the BCD originates from a bright and compact starburst region located at the
northeastern tip of a faint dwarf galaxy with cometary appearance. The
starburst has ignited less than 4 Myr ago and its emission is powered by
several thousands O7V stars and ~ 170 late-type nitrogen Wolf-Rayet stars
located within a compact region with < 500 pc in diameter. For the first time
in a BCD, a relatively strong [Fe V] 4227 emission line is seen which together
with intense He II 4686 emission indicates the presence of a very hard
radiation field in Tol 1214-277. We argue that this extraordinarily hard
radiation originates from both Wolf--Rayet stars and radiative shocks in the
starburst region. The structural properties of the low-surface-brightness (LSB)
component underlying the starburst have been investigated by means of surface
photometry down to 28 B mag/sq.arcsec. We find that, for a surface brightness
level fainter than ~ 24.5 B mag/sq.arcsec, an exponential fitting law provides
an adequate approximation to its radial intensity distribution. The broad-band
colors in the outskirts of the LSB component of Tol 1214-277 are nearly
constant and are consistent with an age below one Gyr. This conclusion is
supported by the comparison of the observed spectral energy distribution (SED)
of the LSB host with theoretical SEDs.Comment: 17 pages, 11 Postscript figures, uses emulateapj.sty, to appear in
Astronomical Journa
POX 186: A Dwarf Galaxy in the Process of Formation?
We present deep U, V and I band images of the "ultracompact" blue dwarf
galaxy POX 186 obtained with the Planetary Camera 2 of the Hubble Space
Telescope. We have also obtained a near-ultraviolet spectrum of the object with
the Space Telescope Imaging Spectrograph, and combine this with a new
ground-based optical spectrum. The images confirm the galaxy to be extremely
small, with a maximum extent of only 300 pc, a luminosity ~ 10^-4 L*, and an
estimated mass ~ 10^7 M(sun). Its morphology is highly asymmetric, with a tail
of material on its western side that may be tidal in origin. The U-band image
shows this tail to be part of a stream of material in which stars have recently
formed. Most of the star formation in the galaxy is however concentrated in a
central, compact (d ~ 10 - 15 pc) star cluster. The outer regions of the galaxy
are significantly redder than the cluster, with V - I colors consistent with a
population dominated by K and M stars. While these results rule out earlier
speculation that POX 186 is a protogalaxy, its morphology, mass and active star
formation suggest that it represents a recent (within ~ 10^8 yr) collision
between two clumps of stars of sub-galactic size (~ 100 pc). POX 186 may thus
be a very small dwarf galaxy that, dynamically speaking, is still in the
process of formation. This interpretation is supported by the fact that it
resides in a void, so its morphology cannot be explained as the result of an
encounter with a more massive galaxy. Clumps of stars this small may represent
the building blocks required by hierarchical models of galaxy formation, and
these results also support the recent "downsizing" picture of galaxy formation
in which the least massive objects are the last to form.Comment: accepted for publication in ApJ; 23 pages, 5 figure
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