643 research outputs found
Fence methods for mixed model selection
Many model search strategies involve trading off model fit with model
complexity in a penalized goodness of fit measure. Asymptotic properties for
these types of procedures in settings like linear regression and ARMA time
series have been studied, but these do not naturally extend to nonstandard
situations such as mixed effects models, where simple definition of the sample
size is not meaningful. This paper introduces a new class of strategies, known
as fence methods, for mixed model selection, which includes linear and
generalized linear mixed models. The idea involves a procedure to isolate a
subgroup of what are known as correct models (of which the optimal model is a
member). This is accomplished by constructing a statistical fence, or barrier,
to carefully eliminate incorrect models. Once the fence is constructed, the
optimal model is selected from among those within the fence according to a
criterion which can be made flexible. In addition, we propose two variations of
the fence. The first is a stepwise procedure to handle situations of many
predictors; the second is an adaptive approach for choosing a tuning constant.
We give sufficient conditions for consistency of fence and its variations, a
desirable property for a good model selection procedure. The methods are
illustrated through simulation studies and real data analysis.Comment: Published in at http://dx.doi.org/10.1214/07-AOS517 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Two extremely metal-poor emission-line galaxies in the Sloan Digital Sky Survey
We present spectroscopic observations with the 3.6m ESO telescope of two
emission-line galaxies, J2104-0035 and J0113+0052, selected from the Data
Release 4 (DR4) of the Sloan Digital Sky Survey (SDSS). From our data we
determine the oxygen abundance of these systems to be respectively 12+logO/H =
7.26+/-0.03 and 7.17+/-0.09, making them the two most metal-deficient galaxies
found thus far in the SDSS and placing them among the five most metal-deficient
emission-line galaxies ever discovered. Their oxygen abundances are close to
those of the two most metal-deficient emission-line galaxies known,
SBS0335-052W with 12+logO/H = 7.12+/-0.03 and I Zw 18 with 12+logO/H =
7.17+/-0.01.Comment: 5 pages, 3 figures. Accepted for publication in Astronomy and
Astrophysic
The HI content of extremely metal-deficient blue compact dwarf galaxies
We have obtained new HI observations with the 100m Green Bank Telescope (GBT)
for a sample of 29 extremely metal-deficient star-forming Blue Compact Dwarf
(BCD) galaxies, selected from the Sloan Digital Sky Survey spectral data base
to be extremely metal-deficient (12+logO/H<7.6). Neutral hydrogen was detected
in 28 galaxies, a 97% detection rate. Combining the HI data with SDSS optical
spectra for the BCD sample and adding complementary galaxy samples from the
literature to extend the metallicity and mass ranges, we have studied how the
HI content of a galaxy varies with various global galaxian properties. There is
a clear trend of increasing gas mass fraction with decreasing metallicity, mass
and luminosity. We obtain the relation M(HI)/L(g)~L(g)^{-0.3}, in agreement
with previous studies based on samples with a smaller luminosity range. The
median gas mass fraction f(gas) for the GBT sample is equal to 0.94 while the
mean gas mass fraction is 0.90+/-0.15, with a lower limit of ~0.65. The HI
depletion time is independent of metallicity, with a large scatter around the
median value of 3.4 Gyr. The ratio of the baryonic mass to the dynamical mass
of the metal-deficient BCDs varies from 0.05 to 0.80, with a median value of
~0.2. About 65% of the BCDs in our sample have an effective yield larger than
the true yield, implying that the neutral gas envelope in BCDs is more
metal-deficient by a factor of 1.5-20, as compared to the ionized gas.Comment: 21 pages, 13 figures, accepted for publication in MNRA
SBS 0335-052W: The Lowest-Metallicity Star-Forming Galaxy Known
We present 4-meter Kitt Peak telescope and 6.5-meter MMT spectrophotometry of
the extremely low-metallicity galaxy SBS 0335-052W, the western companion of
the blue compact dwarf galaxy SBS 0335-052E. These observations have been
combined with published 10-meter Keck data to derive for the brightest region
of SBS 0335-052W an oxygen abundance 12+logO/H=7.12+/-0.03. This makes SBS
0335-052W the lowest metallicity star-forming galaxy known in the local
universe. Using a Monte Carlo technique, we fit the spectral energy
distribution of SBS 0335-052W to derive the age of the oldest stars
contributing to its optical light. We find that star formation in SBS 0335-052W
began less than 500 Myr ago, making it a likely nearby young dwarf galaxy.Comment: 13 pages, 3 figures, accepted for publication in the Astrophysical
Journa
FUSE observations of the HI interstellar gas of IZw18
We present the analysis of FUSE observations of the metal-deficient dwarf
galaxy IZw18. We measured column densities of HI, NI, OI, ArI, SiII, and FeII.
The OI/HI ratio (log(OI/HI)=-4.7^{+0.8}_{-0.6}) is consistent with the O/H
ratio observed in the HII regions (all uncertainties are 2-sigma). If the
oxygen is depleted in the HI region compared to the HII regions, the depletion
is at most 0.5dex. This is also consistent with the log(O/H) ratios ~-5
measured with FUSE in the HI regions of other blue compact dwarf galaxies. With
log(NI/OI)=-2.4^{+0.6}_{-0.8}, the measured NI/OI ratio is lower than expected
for primary nitrogen. The determination of the NII column density is needed to
discriminate between a large ionization of NI or a possible nitrogen
deficiency. The neutral argon is also apparently underabundant, indicating that
ionization into ArII is likely important. The column densities of the other
alpha-chain elements SiII and ArI favor the lower edge of the permitted range
of OI column density, log(N(OI))~16.3.Comment: Accepted for publication in A&
Deep Hubble Space Telescope/ACS Observations of I Zw 18: a Young Galaxy in Formation
We present V and I photometry of the resolved stars in the most
metal-deficient blue compact dwarf galaxy known, I Zw 18 (Zsun/50), using
Hubble Space Telescope/Advanced Camera for Surveys (ACS) images, the deepest
ones ever obtained for this galaxy. The resulting I vs. V-I color-magnitude
diagram (CMD) reaches limiting magnitudes V=I=29 mag. It reveals a young
stellar population of blue main-sequence (MS) stars (age <30 Myr) and blue and
red supergiants (10 Myr<age<100 Myr), but also an older evolved population of
asymptotic giant branch (AGB) stars (100 Myr<age<500 Myr). We derive a distance
to I Zw 18 in the range 12.6 Mpc - 15 Mpc from the brightness of its AGB stars,
with preferred values in the higher range. The red giant branch (RGB) stars are
conspicuous by their absence, although, for a distance of I Zw 18 <15 Mpc, our
imaging data go ~ 1-2 mag below the tip of the RGB. Thus, the most evolved
stars in the galaxy are not older than 500 Myr and I Zw 18 is a bona fide young
galaxy. Several star formation episodes can be inferred from the CMDs of the
main body and the C component. There have been respectively three and two
episodes in these two parts, separated by periods of ~ 100-200 Myr. In the main
body, the younger MS and massive post-MS stars are distributed over a larger
area than the older AGB stars, suggesting that I Zw 18 is still forming from
the inside out. In the C component, different star formation episodes are
spatially distinct, with stellar population ages decreasing from the northwest
to the southeast, also suggesting the ongoing build-up of a young galaxy.Comment: 29 pages, 13 Postscript figures, accepted for publication in the
Astrophysical Journa
Intense CIII] 1907,1909 emission from a strong Lyman continuum emitting galaxy
We have obtained the first complete ultraviolet (UV) spectrum of a strong
Lyman continuum(LyC) emitter at low redshift -- the compact, low-metallicity,
star-forming galaxy J1154+2443 -- with a Lyman continuum escape fraction of 46%
discovered recently. The Space Telescope Imaging Spectrograph spectrum shows
strong Lya and CIII] 1909 emission, as well as OIII] 1666. Our observations
show that strong LyC emitters can have UV emission lines with a high equivalent
width (e.g. EW(CIII]) rest-frame), although their equivalent
widths should be reduced due to the loss of ionizing photons. The intrinsic
ionizing photon production efficiency of J1154+2443 is high, erg Hz, comparable to that of other recently discovered
LyC emitters. Combining our measurements and earlier
determinations from the literature, we find a trend of increasing with increasing CIII] 1909 equivalent width, which can be understood by
a combination of decreasing stellar population age and metallicity. Simple
ionization and density-bounded photoionization models can explain the main
observational features including the UV spectrum of J1154+2443.Comment: 5 pages, 4 figures. Accepted for publication in A&A Letter
- âŠ