559 research outputs found
A Link to the Past: Using Markov Chain Monte Carlo Fitting to Constrain Fundamental Parameters of High-Redshift Galaxies
We have a developed a new method for fitting spectral energy distributions
(SEDs) to identify and constrain the physical properties of high-redshift (4 <
z < 8) galaxies. Our approach uses an implementation of Bayesian based Markov
Chain Monte Carlo (PiMC^2) that allows us to compare observations to
arbitrarily complex models and to compute 95% credible intervals that provide
robust constraints for the model parameters. The work is presented in 2
sections. In the first, we test PiMC^2 using simulated SEDs to not only confirm
the recovery of the known inputs but to assess the limitations of the method
and identify potential hazards of SED fitting when applied specifically to high
redshift (z>4) galaxies. Our tests reveal five critical results: 1) the ability
to confidently constrain metallicity, population ages, and Av all require
photometric accuracy better than what is currently achievable (i.e. less than a
few percent); 2) the ability to confidently constrain stellar masses (within a
factor of two) can be achieved without the need for high-precision photometry;
3) the addition of IRAC photometry does not guarantee that tighter constraints
of the stellar masses and ages can be defined; 4) different assumptions about
the star formation history can lead to significant biases in mass and age
estimates; and 5) we are able to constrain stellar age and Av of objects that
are both young and relatively dust free. In the second part of the paper we
apply PiMC^2 to 17 4<z<8 objects, including the GRAPES Ly alpha sample (4<z<6),
supplemented by HST/WFC3 near-IR observations, and several broad band selected
z>6 galaxies. Using PiMC^2, we are able to constrain the stellar mass of these
objects and in some cases their stellar age and find no evidence that any of
these sources formed at a redshift much larger than z_f=8, a time when the
Universe was ~ 0.6 Gyr old.Comment: Submitted to ApJ (Full abstract, 47 pages, 17 figures, 7 tables
Spectroscopic Confirmation of Faint Lyman Break Galaxies at Redshifts Four and Five in the Hubble Ultra Deep Field
We present the faintest spectroscopically confirmed sample of redshift four
and five Lyman break galaxies to date. The sample is based on slitless grism
spectra of the Hubble Ultra Deep Field region from the GRAPES (Grism ACS
Program for Extragalactic Science) and PEARS (Probing Evolution and
Reionization Spectroscopically) projects, using the G800L grism on the HST
Advanced Camera for Surveys. We report here confirmations of 39 galaxies,
pre-selected as candidate Lyman break galaxies using photometric selection
criteria. We compare a "traditional" V-dropout selection to a more liberal one
(with V-i > 0.9), and find that the traditional criteria are about 64% complete
and 81% reliable. We also study the Lyman alpha emission properties of our
sample. We find that Lyman alpha emission is detected in about 1/4 of the
sample, and that our broad-band color selected sample includes 55% of
previously published line-selected Lyman alpha sources. Finally, we examine our
stacked 2D spectra. We demonstrate that strong, spatially extended (arcsecond
scale) Lyman alpha emission is not a generic property of these Lyman break
galaxies, but that a modest extension of the Lyman alpha photosphere (compared
to the starlight) may be present in those galaxies with prominent Lyman alpha
emission.Comment: Submitted to The Astrophysical Journal. Reduced spectra from both
GRAPES and PEARS are available from STScI, at
http://www.stsci.edu/science/grapes/ and at
http://archive.stsci.edu/prepds/pears
Emission-Line Galaxies from the Hubble Space Telescope Probing Evolution and Reionization Spectroscopically (PEARS) Grism Survey. II: The Complete Sample
We present a full analysis of the Probing Evolution And Reionization
Spectroscopically (PEARS) slitess grism spectroscopic data obtained with the
Advanced Camera for Surveys on HST. PEARS covers fields within both the Great
Observatories Origins Deep Survey (GOODS) North and South fields, making it
ideal as a random survey of galaxies, as well as the availability of a wide
variety of ancillary observations to support the spectroscopic results. Using
the PEARS data we are able to identify star forming galaxies within the
redshift volume 0< z<1.5. Star forming regions in the PEARS survey are
pinpointed independently of the host galaxy. This method allows us to detect
the presence of multiple emission line regions (ELRs) within a single galaxy.
1162 Ha, [OIII] and/or [OII] emission lines have been identified in the PEARS
sample of ~906 galaxies down to a limiting flux of ~1e-18 erg/s/cm^2. The ELRs
have also been compared to the properties of the host galaxy, including
morphology, luminosity, and mass. From this analysis we find three key results:
1) The computed line luminosities show evidence of a flattening in the
luminosity function with increasing redshift; 2) The star forming systems show
evidence of disturbed morphologies, with star formation occurring predominantly
within one effective (half-light) radius. However, the morphologies show no
correlation with host stellar mass; and 3) The number density of star forming
galaxies with M_* > 1e9} M_sun decreases by an order of magnitude at z<0.5
relative to the number at 0.5<z<0.9 in support of the argument for galaxy
downsizing.Comment: Submitted. 48 pages. 19 figures. Accepted to Ap
A Link to the Past: Using Markov Chain Monte Carlo Fitting to Constrain Fundamental Parameters of High-Redshift Galaxies
We have a developed a new method for fitting spectral energy distributions
(SEDs) to identify and constrain the physical properties of high-redshift (4 <
z < 8) galaxies. Our approach uses an implementation of Bayesian based Markov
Chain Monte Carlo (PiMC^2) that allows us to compare observations to
arbitrarily complex models and to compute 95% credible intervals that provide
robust constraints for the model parameters. The work is presented in 2
sections. In the first, we test PiMC^2 using simulated SEDs to not only confirm
the recovery of the known inputs but to assess the limitations of the method
and identify potential hazards of SED fitting when applied specifically to high
redshift (z>4) galaxies. Our tests reveal five critical results: 1) the ability
to confidently constrain metallicity, population ages, and Av all require
photometric accuracy better than what is currently achievable (i.e. less than a
few percent); 2) the ability to confidently constrain stellar masses (within a
factor of two) can be achieved without the need for high-precision photometry;
3) the addition of IRAC photometry does not guarantee that tighter constraints
of the stellar masses and ages can be defined; 4) different assumptions about
the star formation history can lead to significant biases in mass and age
estimates; and 5) we are able to constrain stellar age and Av of objects that
are both young and relatively dust free. In the second part of the paper we
apply PiMC^2 to 17 4<z<8 objects, including the GRAPES Ly alpha sample (4<z<6),
supplemented by HST/WFC3 near-IR observations, and several broad band selected
z>6 galaxies. Using PiMC^2, we are able to constrain the stellar mass of these
objects and in some cases their stellar age and find no evidence that any of
these sources formed at a redshift much larger than z_f=8, a time when the
Universe was ~ 0.6 Gyr old.Comment: Submitted to ApJ (Full abstract, 47 pages, 17 figures, 7 tables
The Slitless Spectroscopy Data Extraction Software aXe
The methods and techniques for the slitless spectroscopy software aXe, which
was designed to reduce data from the various slitless spectroscopy modes of
Hubble Space Telescope instruments, are described. aXe can treat slitless
spectra from different instruments such as ACS, NICMOS and WFC3 through the use
of a configuration file which contains all the instrument dependent parameters.
The basis of the spectral extraction within aXe are the position, morphology
and photometry of the objects on a companion direct image. Several aspects of
slitless spectroscopy, such as the overlap of spectra, an extraction dependent
on object shape and the provision of flat-field cubes, motivate a dedicated
software package, and the solutions offered within aXe are discussed in detail.
The effect of the mutual contamination of spectra can be quantitatively
assessed in aXe, using spectral and morphological information from the
companion direct image(s). A new method named 'aXedrizzle' for 2D rebinning and
co-adding spectral data, taken with small shifts or dithers, is described. The
extraction of slitless spectra with optimal weighting is outlined and the
correction of spectra for detector fringing for the ACS CCD's is presented.
Auxiliary software for simulating slitless data and for visualizing the results
of an aXe extraction is outlined.Comment: 18 pages, 10 figures, accepted for publication in PASP. A high
resolution version is available at
http://www.stecf.org/software/slitless_software/axe/axe_PASP.pd
Quantified HI Morphology VII: star-formation and tidal influence on local dwarf HI morphology
Scale-invariant morphology parameters applied to atomic hydrogen maps (HI) of
galaxies can be used to quantify the effects of tidal interaction or
star-formation on the ISM. Here we apply these parameters, Concentration,
Asymmetry, Smoothness, Gini, M20, and the GM parameter, to two public surveys
of nearby dwarf galaxies, the VLA-ANGST and LITTLE-THINGS survey, to explore
whether tidal interaction or the ongoing or past star-formation is a dominant
force shaping the HI disk of these dwarfs.
Previously, HI morphological criteria were identified for ongoing
spiral-spiral interactions. When we apply these to the Irregular dwarf
population, they either select almost all or none of the population. We find
that only the Asymmetry-based criteria can be used to identify very isolated
dwarfs (i.e., these have a low tidal indication). Otherwise, there is little or
no relation between the level of tidal interaction and the HI morphology. We
compare the HI morphology to three star-formation rates based on either Halpha,
FUV or the resolved stellar population, probing different star-formation
time-scales.
The HI morphology parameters that trace the inequality of the distribution,
the Gini, GM, and M20 parameters, correlate weakly with all these
star-formation rates. This is in line with the picture that local physics
dominates the ISM appearance and not tidal effects. Finally, we compare the
SDSS measures of star-formation and stellar mass to the HI morphological
parameters for all four HI surveys. In the two lower-resolution HI surveys
(12"), there is no relation between star-formation measures and HI morphology.
The morphology of the two high-resolution HI surveys (6"), the Asymmetry,
Smoothness, Gini, M20, and GM, do show a link to the total star-formation, but
a weak one.Comment: 26 figures, 4 tables, two appendices. Third appendix (HI maps of all
galaxies) omitted. Accepted by MNRA
Freshly ionized matter around the final Helium shell flash object V4334 Sgr (Sakurai's object)
We report on the discovery of recently ionized hydrogen-deficient gas in the
immediate circumstellar environment of the final helium shell flash star V4334
Sgr (Sakurai's object). On spectra obtained with FORS2 multi-object
spectroscopy we have found spatially extended (about 2") emission from [N II],
[O I], [O II] and very faint Halpha and [S II]. In the [N II] (ll6548,83) lines
we have identified two components located at velocities -350 +/-50 and +200
+/-50 km/s, relative to V4334 Sgr itself. The full width of the [N II] l6583
feature at zero intensity corresponds to a velocity spread of about 1500 km/s.
Based on the available data it is not possible to conclusively determine the
mechanism of ionization. Both photo-ionization, from a rapidly evolving central
star, and shock excitation, as the result of the collision of the fast ouflows
with slower circumstellar matter, could account for the observed lines. The
central star is still hidden behind strong dust absorption, since only a faint
highly reddened continuum is apparent in the spectra. Theory states that it
will become hotter and will retrace its post-asymptotic giant branch evolution
towards the planetary nebula domain. Our detection of the ionized ejecta from
the very late helium shell flash marks the beginning of a new phase in this
star's amazingly rapid evolution.Comment: 11 pages, 2 figures. Accepted by ApJ
Spectrophotometrically Identified stars in the PEARS-N and PEARS-S fields
Deep ACS slitless grism observations and identification of stellar sources
are presented within the Great Observatories Origins Deep Survey (GOODS) North
and South fields which were obtained in the Probing Evolution And Reionization
Spectroscopically (PEARS) program. It is demonstrated that even low resolution
spectra can be a very powerful means to identify stars in the field, especially
low mass stars with stellar types M0 and later. The PEARS fields lay within the
larger GOODS fields, and we used new, deeper images to further refine the
selection of stars in the PEARS field, down to a magnitude of mz = 25 using a
newly developed stellarity parameter. The total number of stars with reliable
spectroscopic and morphological identification was 95 and 108 in the north and
south fields respectively. The sample of spectroscopically identified stars
allows constraints to be set on the thickness of the Galactic thin disk as well
as contributions from a thick disk and a halo component. We derive a thin disk
scale height, as traced by the population of M4 to M9 dwarfs along two
independent lines of sight, of h_thin = 370 +60/-65 pc. When including the more
massive M0 to M4 dwarf population, we derive h_thin = 300 +/- 70pc. In both
cases, we observe that we must include a combination of thick and halo
components in our models in order to account for the observed numbers of faint
dwarfs. The required thick disk scale height is typically h_thick=1000 pc and
the acceptable relative stellar densities of the thin disk to thick disk and
the thin disk to halo components are in the range of 0.00025<f_halo<0.0005 and
0.05<f_thick<0.08 and are somewhat dependent on whether the more massive M0 to
M4 dwarfs are included in our sample
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