197 research outputs found
The stellar host in blue compact dwarf galaxies: the need for a two-dimensional fit
The structural properties of the low surface brightness stellar host in blue
compact dwarf galaxies are often studied by fitting r^{1/n} models to the outer
regions of their radial profiles. The limitations imposed by the presence of a
large starburst emission overlapping the underlying component makes this kind
of analysis a difficult task. We propose a two-dimensional fitting methodology
in order to improve the extraction of the structural parameters of the LSB
host. We discuss its advantages and weaknesses by using a set of simulated
galaxies and compare the results for a sample of eight objects with those
already obtained using a one-dimensional technique. We fit a PSF convolved
Sersic model to synthetic galaxies, and to real galaxy images in the B, V, R
filters. We restrict the fit to the stellar host by masking out the starburst
region and take special care to minimize the sky-subtraction uncertainties. In
order to test the robustness and flexibility of the method, we carry out a set
of fits with synthetic galaxies. Furthermore consistency checks are performed
to assess the reliability and accuracy of the derived structural parameters.
The more accurate isolation of the starburst emission is the most important
advantage and strength of the method. Thus, we fit the host galaxy in a range
of surface brightness and in a portion of area larger than in previous
published 1D fits with the same dataset. We obtain robust fits for all the
sample galaxies, all of which, except one, show Sersic indices n very close to
1, with good agreement in the three bands. These findings suggest that the
stellar hosts in BCDs have near-exponential profiles, a result that will help
us to understand the mechanisms that form and shape BCD galaxies, and how they
relate to the other dwarf galaxy classes.Comment: 22 pages, 15 figures (low resolution), accepted for publication in
A&A. A higher resolution version of the figures can be provided upon reques
The star formation history of mass-selected galaxies in the COSMOS field
We explore the evolution of the specific star formation rate (SSFR) for
3.6um-selected galaxies of different M_* in the COSMOS field. The average SFR
for sub-sets of these galaxies is estimated with stacked 1.4GHz radio continuum
emission. We separately consider the total sample and a subset of galaxies (SF)
that shows evidence for substantive recent star formation in the rest-frame
optical SED. At 0.2<z<3 both populations show a strong and M_*-independent
decrease in their SSFR towards z=0.2, best described by a power- law (1+z)^n,
where n~4.3 for all galaxies and n~3.5 for SF sources. The decrease appears to
have started at z>2, at least above 4x10^10M_Sun where our conclusions are most
robust. We find a tight correlation with power-law dependence, SSFR (M_*)^beta,
between SSFR and M_* at all z. It tends to flatten below ~10^10M_Sun if
quiescent galaxies are included; if they are excluded a shallow index beta_SFG
-0.4 fits the correlation. On average, higher M_* objects always have lower
SSFRs, also among SF galaxies. At z>1.5 there is tentative evidence for an
upper SSFR-limit that an average galaxy cannot exceed. It is suggested by a
flattening of the SSFR-M_* relation (also for SF sources), but affects massive
(>10^10M_Sun) galaxies only at the highest z. Below z=1.5 there thus is no
direct evidence that galaxies of higher M_* experience a more rapid waning of
their SSFR than lower M_* SF systems. In this sense, the data rule out any
strong 'downsizing'. We combine our results with recent measurements of the
galaxy (stellar) mass function in order to determine the characteristic mass of
a SF galaxy (M_*=10^(10.6\pm0.4)M_Sun). In this sense, too, there is no
'downsizing'. Our analysis constitutes the most extensive SFR density
determination with a single technique to z=3. Recent Herschel results are
consistent with our results, but rely on far smaller samples.Comment: 37 pages, 14 figures, 7 tables; accepted for publication in the
Astrophysical Journal; High resolution versions of all figures available at
www.mpia-hd.mpg.de/homes/karim/research.htm
Stellar Mass and Velocity Functions of Galaxies: Backward evolution and the fate of Milky Way siblings
We attempt in this paper to check the consistency of the observed Stellar
Mass Function (SMF), SFR functions and the cosmic star formation rate density
with simple backward evolutionary models. Starting from observed SMF for
star-forming galaxies, we use backwards models to predict the evolution of a
number of quantities, such as the SFR function, the cosmic SFR density and the
Velocity Function. The velocity being a parameter attached to a galaxy during
its history (contrary to the stellar mass), this approach allows us to quantify
the number density evolution of galaxies of a given velocity, e.g. of the Milky
Way siblings. Observations suggest that the SMF of star forming galaxies is
constant between redshift 0 and 1. In order to reproduce this result, we must
quench star formation in a number of star forming galaxies. The SMF of these
quenched galaxies is consistent with available data concerning the increase in
the population of quiescent galaxies in the same redshift interval. The SMF of
quiescent galaxies is then mainly determined by the distribution of active
galaxies that must stop star formation, with a modest mass redistribution
during mergers. The cosmic SFR density, and the evolution of the SFR functions
are relatively well recovered, although they provide some clue for a small
evolution of the SMF of star forming galaxies at the lowest redshifts. We thus
consider that we have obtained in a simple way a relatively consistent picture
of the evolution of galaxies at intermediate redshifts. We note that if this
picture is correct, 50 percent of the Milky-Way sisters (galaxies with the same
velocity as our Galaxy, i.e. 220 km/s) have quenched their star formation since
redshift 1 (and an even larger fraction for larger velocities). We discuss the
processes that might be responsible for this transformation.Comment: 12 pages, 14 figures, accepted in Astronomy and Astrophysic
AEGIS: The Diversity of Bright Near-IR Selected Distant Red Galaxies
We use deep and wide near infrared (NIR) imaging from the Palomar telescope combined with DEEP2 spectroscopy and Hubble Space Telescope (HST) and Chandra Space Telescope imaging to investigate the nature of galaxies that are red in NIR colors. We locate these `distant red galaxies' (DRGs) through the color cut (J-K)_{vega} > 2.3 over 0.7 deg^{2}, where we find 1010 DRG candidates down to K_s = 20.5. We combine 95 high quality spectroscopic redshifts with photometric redshifts from BRIJK photometry to determine the redshift and stellar mass distributions for these systems, and morphological/structural and X-ray properties for 107 DRGs in the Extended Groth Strip. We find that many bright (J-K)_{vega}>2.3 galaxies with K_s2 systems massive with M_*>10^{11} M_solar. HST imaging shows that the structural properties and morphologies of DRGs are also diverse, with the majority elliptical/compact (57%), and the remainder edge-on spirals (7%), and peculiar galaxies (29%). The DRGs at z < 1.4 with high quality spectroscopic redshifts are generally compact, with small half light radii, and span a range in rest-frame optical properties. The spectral energy distributions for these objects differ from higher redshift DRGs: they are bluer by one magnitude in observed (I-J) color. A pure IR color selection of high redshift populations is not sufficient to identify unique populations, and other colors, or spectroscopic redshifts are needed to produce homogeneous samples
Neutral Hydrogen and Star Formation in the Irregular Galaxy NGC 2366
We present UBVJHKHalpha and HI data of the irregular galaxy NGC 2366. It is a
normal boxy-shaped disk seen at high inclination angle. We do not see any
unambiguous observational signature of a bar. There is an asymmetrical
extension of stars along one end of the major axis of the galaxy, and this is
where the furthest star-forming regions are found, at 1.3R_Holmberg. The HI is
normal in many respects but shows some anomalies: 1) The integrated HI shows
two ridges running parallel to the major axis that deproject to a large ring.
2) The velocity field exhibits several large-scale anomalies superposed on a
rotating disk. 3) The inclination and position angles derived from the
kinematics differ from those dervied from the optical and HI mor- phology. 4)
There are regions in the HI of unusually high velocity dispersion that
correlate with deficits of HI emission in a manner suggestive of long-range,
turbulent pressure equilibrium. Star-forming regions are found where the gas
densities locally exceed 6 Msolar/pc^2. NGC 2366, like other irregulars, has
low gas densities relative to the critical gas densities of gravitational
instability models. Because of the lack of shear in the optical galaxy, there
is little competition to the slow gravitational contraction that follows energy
dissipation. However, the peak gas densities in the star-forming regions are
equal to the local tidal densities for gravitational self-binding of a rotating
cloud. Evidently the large scale gas concentrations are marginally bound
against background galactic tidal forces. This condition for self-binding may
be more fundamental than the instability condition because it is local,
three-dimensional, and does not involve spiral arm generation as an
intermediate step toward star formation.Comment: To be published in ApJ; better figures available ftp.lowell.edu, cd
pub/dah/n2366pape
The COSMOS-WIRCam near-infrared imaging survey: I: BzK selected passive and star forming galaxy candidates at z>1.4
(abridged) We present a new near-infrared survey covering the 2 deg sq COSMOS
field. Combining our survey with Subaru B and z images we construct a deep,
wide-field optical-infrared catalogue. At Ks<23 (AB magnitudes) our survey
completeness is greater than 90% and 70% for stars and galaxies respectively
and contains 143,466 galaxies and 13,254 stars. At z~2 our catalogues contain
3931 quiescent and 25,757 star-forming BzK-selected galaxies representing the
largest and most secure sample of these objects to date. Our counts of
quiescent galaxies turns over at Ks~22 an effect which we demonstrate cannot be
due to sample incompleteness. In our survey both the number of faint and bright
quiescent objects exceeds the predictions of a semi-analytic galaxy formation
model, indicating potentially the need for further refinements in the amount of
merging and AGN feedback at z~2 in these models. We measure the angular
correlation function for each sample and find that at small scales the
correlation function for passive BzK galaxies exceeds the clustering of dark
matter. We use 30-band photometric redshifts to derive the spatial correlation
length and the redshift distributions for each object class. At Ks<22 we find
r_0^{\gamma/1.8}=7.0 +/-0.5h^{-1} Mpc for the passive BzK candidates and
4.7+/-0.8h^{-1} Mpc for the star-forming BzK galaxies. Our pBzK galaxies have
an average photometric redshift of z_p~1.4, in approximate agreement with the
limited spectroscopic information currently available. The stacked Ks image
will be made publicly available from IRSA.Comment: Accepted for publication in Astrophysical Journal. 17 pages, 17
figures, minor revisions to match published version available at
http://adsabs.harvard.edu/abs/2010ApJ...708..202
Integral field spectroscopy with SINFONI of VVDS galaxies. I. Galaxy dynamics and mass assembly at 1.2 < z < 1.6
Context. Identifying the main processes of galaxy assembly at high redshifts
is still a major issue to understand galaxy formation and evolution at early
epochs in the history of the Universe. Aims. This work aims to provide a first
insight into the dynamics and mass assembly of galaxies at redshifts 1.2<z<1.6,
the early epoch just before the sharp decrease of the cosmic star formation
rate. Methods. We use the near-infrared integral field spectrograph SINFONI on
the ESO-VLT under 0.65 seeing to obtain spatially resolved spectroscopy on nine
emission line galaxies with 1.2<z<1.6 from the VIMOS VLT Deep Survey. We derive
the velocity fields and velocity dispersions on kpc scales using the Halpha
emission line. Results. Out of the nine star-forming galaxies, we find that
galaxies distribute in three groups: two galaxies can be well reproduced by a
rotating disk, three systems can be classified as major mergers and four
galaxies show disturbed dynamics and high velocity dispersion. We argue that
there is evidence for hierarchical mass assembly from major merger, with most
massive galaxies with M>10^11Msun subject to at least one major merger over a 3
Gyr period as well as for continuous accretion feeding strong star formation.
Conclusions. These results point towards a galaxy formation and assembly
scenario which involves several processes, possibly acting in parallel, with
major mergers and continuous gas accretion playing a major role. Well
controlled samples representative of the bulk of the galaxy population at this
key cosmic time are necessary to make further progress.Comment: 23 pages, 22 figures, accepted for publication in A&
Mapping Luminous Blue Compact Galaxies with VIRUS-P: morphology, line ratios and kinematics
[abridged] We carry out an integral field spectroscopy (IFS) study of a
sample of luminous BCGs, with the aim to probe the morphology, kinematics, dust
extinction and excitation mechanisms of their warm interstellar medium (ISM).
IFS data for five luminous BCGs were obtained using VIRUS-P, the prototype
instrument for the Visible Integral Field Replicable Unit Spectrograph,
attached to the 2.7m Harlan J. Smith Telescope at the McDonald Observatory.
VIRUS-P consists of a square array of 247 optical fibers, which covers a
109"x109" field of view, with a spatial sampling of 4.2" and a 0.3 filling
factor. We observed in the 3550-5850 Angstrom spectral range, with a resolution
of 5 A FWHM. From these data we built two-dimensional maps of the continuum and
the most prominent emission-lines ([OII]3727, Hgamma, Hbeta and [OIII]5007),
and investigate the morphology of diagnostic emission-line ratios and the
extinction patterns in the ISM as well as stellar and gas kinematics.
Additionally, from integrated spectra we infer total line fluxes and
luminosity-weighted extinction coefficients and gas-phase metallicities. All
galaxies exhibit an overall regular morphology in the stellar continuum, while
their warm ISM morphology is more complex: in II Zw 33 and Mrk 314, the
star-forming regions are aligned along a chain-structure; Haro 1, NGC 4670 and
III Zw 102 display several salient features, such as extended gaseous filaments
and bubbles. A significant intrinsic absorption by dust is present in all
galaxies, the most extreme case being III Zw 102. Our data reveal a manifold of
kinematical patterns, from overall regular gas and stellar rotation to complex
velocity fields produced by structurally and kinematically distinct components.Comment: Accepted for publication in A&A. 16 pages, 10 figure
Study of star-forming galaxies in SDSS up to redshift 0.4 II. Evolution from the fundamental parameters: mass, metallicity & SFR
To understand the formation and evolution of galaxies, it is important to
have a full comprehension of the role played by the metallicity, star formation
rate (SFR), morphology, and color. The interplay of these parameters at
different redshifts will substantially affect the evolution of galaxies and, as
a consequence, the evolution of them will provide important clues and
constraints on the galaxy evolution models. In this work we focus on the
evolution of the SFR, metallicity of the gas, and morphology of galaxies at low
redshift in search of signs of evolution. We use the S2N2 diagnostic diagram as
a tool to classify star--forming, composite, and AGN galaxies. We analyzed the
evolution of the three principal BPT diagrams, estimating the SFR and specific
SFR (SSFR) for our samples of galaxies, studying the luminosity and
mass-metallicity relations, and analyzing the morphology of our sample of
galaxies through the g-r color, concentration index, and SSFR. We found that
the S2N2 is a reliable diagram to classify star--forming, composite, and AGNs
galaxies. We demonstrate that the three principal BPT diagrams show an
evolution toward higher values of [OIII]5007/Hb due to a metallicity decrement.
We found an evolution in the mass-metallicity relation of ~ 0.2 dex for the
redshift range 0.3 < z < 0.4 compared to our local one. From the analysis of
the evolution of the SFR and SSFR as a function of the stellar mass and
metallicity, we discovered a group of galaxies with higher SFR and SSFR at all
redshift samples, whose morphology is consistent with those of late-type
galaxies. Finally, the comparison of our local (0.04<z<0.1) with our higher
redshift sample (0.3<z<0.4), show that the metallicity, the SFR and morphology,
evolve toward lower values of metallicity, higher SFRs, and late--type
morphologies for the redshift range 0.3<z<0.4Comment: 16 pages, 15 figures. Accepted for publication in A&
Decomposing Star Formation and Active Galactic Nucleus with Spitzer Mid-Infrared Spectra: Luminosity Functions and Co-Evolution
We present Spitzer 7-38um spectra for a 24um flux limited sample of galaxies
at z~0.7 in the COSMOS field. The detailed high-quality spectra allow us to
cleanly separate star formation (SF) and active galactic nucleus (AGN) in
individual galaxies. We first decompose mid-infrared Luminosity Functions
(LFs). We find that the SF 8um and 15um LFs are well described by Schechter
functions. AGNs dominate the space density at high luminosities, which leads to
the shallow bright-end slope of the overall mid-infrared LFs. The total
infrared (8-1000um) LF from 70um selected galaxies shows a shallower bright-end
slope than the bolometrically corrected SF 15um LF, owing to the intrinsic
dispersion in the mid-to-far-infrared spectral energy distributions. We then
study the contemporary growth of galaxies and their supermassive black holes
(BHs). Seven of the 31 Luminous Infrared Galaxies with Spitzer spectra host
luminous AGNs, implying an AGN duty cycle of 23+/-9%. The time-averaged ratio
of BH accretion rate and SF rate matches the local M_BH-M_bulge relation and
the M_BH-M_host relation at z ~ 1. These results favor co-evolution scenarios
in which BH growth and intense SF happen in the same event but the former spans
a shorter lifetime than the latter. Finally, we compare our mid-infrared
spectroscopic selection with other AGN identification methods and discuss
candidate Compton-thick AGNs in the sample. While only half of the mid-infrared
spectroscopically selected AGNs are detected in X-ray, ~90% of them can be
identified with their near-infrared spectral indices.Comment: ApJ Accepted. emulateapj style. 16 pages, 9 figures, 4 table
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