46 research outputs found
Gas-Phase Oxygen Gradients in Strongly Interacting Galaxies: I. Early-Stage Interactions
A consensus is emerging that interacting galaxies show depressed nuclear gas
metallicities compared to isolated star-forming galaxies. Simulations suggest
that this nuclear underabundance is caused by interaction-induced inflow of
metal-poor gas, and that this inflow concurrently flattens the radial
metallicity gradients in strongly interacting galaxies. We present
metallicities of over 300 HII regions in a sample of 16 spirals that are
members of strongly interacting galaxy pairs with mass ratio near unity. The
deprojected radial gradients in these galaxies are about half of those in a
control sample of isolated, late-type spirals. Detailed comparison of the
gradients with simulations show remarkable agreement in gradient distributions,
the relationship between gradients and nuclear underabundances, and the shape
of profile deviations from a straight line. Taken together, this evidence
conclusively demonstrates that strongly interacting galaxies at the present day
undergo nuclear metal dilution due to gas inflow, as well as significant
flattening of their gas-phase metallicity gradients, and that current
simulations can robustly reproduce this behavior at a statistical level.Comment: Accepted for publication in Ap
THE DISTANCE TO THE MASSIVE GALACTIC CLUSTER WESTERLUND 2 FROM A SPECTROSCOPIC AND HST PHOTOMETRIC STUDY , ,
We present a spectroscopic and photometric determination of the distance to the young Galactic open cluster Westerlund 2 using WFPC2 imaging from the Hubble Space Telescope (HST) and ground-based optical spectroscopy. HST imaging in the F336W, F439W, F555W, and F814W filters resolved many sources previously undetected in ground-based observations and yielded photometry for 1136 stars. We identified 15 new O-type stars, along with two probable binary systems, including MSP 188 (O3 + O5.5). We fit reddened spectral energy distributions based on the Padova isochrones to the photometric data to determine individual reddening parameters RV and AV for O-type stars in Wd2. We find average values RV = 3.77 ± 0.09 and AV = 6.51 ± 0.38 mag, which result in a smaller distance than most other spectroscopic and photometric studies. After a statistical distance correction accounting for close unresolved binaries (factor of 1.08), our spectroscopic and photometric data on 29 O-type stars yield that Westerlund 2 has a distance d = 4.16 ± 0.07 (random) +0.26 (systematic) kpc. The cluster's age remains poorly constrained, with an upper limit of 3 Myr. Finally, we report evidence of a faint mid-IR polycyclic aromatic hydrocarbon ring surrounding the well-known binary candidate MSP 18, which appears to lie at the center of a secondary stellar grouping within Westerlund 2
The DEEP Groth Strip Survey XII: The Metallicity of Field Galaxies at 0.26<z<0.82 and the Evolution of the Luminosity-Metallicity Relation
Using spectroscopic data from the Deep Extragalactic Evolutionary Probe
(DEEP) Groth Strip survey (DGSS), we analyze the gas-phase oxygen abundances
for 56 emission-line field galaxies in the redshift range 0.26<z<0.82. Oxygen
abundances relative to hydrogen range between 8.4<12+log(O/H)<9.0 with typical
uncertainties of 0.17 dex. The 56 DGSS galaxies collectively exhibit a
correlation between B-band luminosity and metallicity, i.e., an L-Z relation.
Subsets of DGSS galaxies binned by redshift also exhibit L-Z correlations but
with different zero points. Galaxies in the highest redshift bin (z=0.6-0.82)
are brighter by ~1 mag compared to the lowest redshift bin (z=0.26-0.40) and
brighter by ~1-2 mag compared to local (z<0.1) field galaxies. This offset from
the local L-Z relation is greatest for objects at the low-luminosity (M_B ~
-19) end of the sample, and vanishingly small for objects at the
high-luminosity end of the sample (M_B ~ -22). Thus, both the slope and zero
point of the L-Z relation appear to evolve. Either the least-luminous DGSS
field galaxies have faded by 1--2 mag due to decreasing levels of star
formation, or they have experienced an increase in the mean metallicity of the
interstellar medium by factors of 1.3--2 (0.1-0.3 dex). The relatively greater
degree of luminosity and metallicity evolution seen among the lower luminosity
(sub L*) galaxies in the last 8 Gyr implies either a more protracted assembly
process, or a more recent formation epoch compared to more luminous L*
galaxies. (abridged)Comment: Submitted to ApJ, comments welcome; 43 pages, 14 figures Version with
full figures available at http://physics.uwyo.edu/~chip/Pubs/Grot
The DEEP Groth Strip Survey VI. Spectroscopic, Variability, and X-ray Detection of AGN
We identify active galactic nuclei (AGN) in the Groth-Westphal Survey Strip
(GSS) using the independent and complementary selection techniques of optical
spectroscopy and photometric variability. We discuss the X-ray properties of
these AGN using Chandra/XMM data for this region. From a sample of 576 galaxies
with high quality spectra we identify 31 galaxies with AGN signatures. Seven of
these have broad emission lines (Type 1 AGNs). We also identify 26 galaxies
displaying nuclear variability in HST WFPC2 images of the GSS separated by ~7
years. The primary overlap of the two selected AGN samples is the set of
broad-line AGNs, of which 80% appear as variable. Only a few narrow-line AGNs
approach the variability threshold. The broad-line AGNs have an average
redshift of z~1.1 while the other spectroscopic AGNs have redshifts closer to
the mean of the general galaxy population (z~0.7). Eighty percent of the
identified broad-line AGNs are detected in X-rays and these are among the most
luminous X-ray sources in the GSS. Only one narrow-line AGN is X-ray detected.
Of the variable nuclei galaxies within the X-ray survey, 27% are X-ray
detected. We find that 1.9+/-0.6% of GSS galaxies to V=24 are broad-line AGNs,
1.4+/-0.5% are narrow-line AGNs, and 4.5+/-1.4% contain variable nuclei. The
fraction of spectroscopically identified BLAGNs and NLAGNs at z~1 reveals a
marginally significant increase of 1.3+/-0.9% when compared to the local
population.Comment: 29 pages, 8 figures, accepted for publication in ApJ
The Balloon-Borne Large Aperture Submillimeter Telescope (BLAST) 2005: A 10 deg^2 Survey of Star Formation in Cygnus X
We present Cygnus X in a new multi-wavelength perspective based on an
unbiased BLAST survey at 250, 350, and 500 micron, combined with rich datasets
for this well-studied region. Our primary goal is to investigate the early
stages of high mass star formation. We have detected 184 compact sources in
various stages of evolution across all three BLAST bands. From their
well-constrained spectral energy distributions, we obtain the physical
properties mass, surface density, bolometric luminosity, and dust temperature.
Some of the bright sources reaching 40 K contain well-known compact H II
regions. We relate these to other sources at earlier stages of evolution via
the energetics as deduced from their position in the luminosity-mass (L-M)
diagram. The BLAST spectral coverage, near the peak of the spectral energy
distribution of the dust, reveals fainter sources too cool (~ 10 K) to be seen
by earlier shorter-wavelength surveys like IRAS. We detect thermal emission
from infrared dark clouds and investigate the phenomenon of cold ``starless
cores" more generally. Spitzer images of these cold sources often show stellar
nurseries, but these potential sites for massive star formation are ``starless"
in the sense that to date there is no massive protostar in a vigorous accretion
phase. We discuss evolution in the context of the L-M diagram. Theory raises
some interesting possibilities: some cold massive compact sources might never
form a cluster containing massive stars; and clusters with massive stars might
not have an identifiable compact cold massive precursor.Comment: 42 pages, 31 Figures, 6 table
Little Things
We present LITTLE THINGS (Local Irregulars That Trace Luminosity Extremes,
The HI Nearby Galaxy Survey) that is aimed at determining what drives star
formation in dwarf galaxies. This is a multi-wavelength survey of 37 Dwarf
Irregular and 4 Blue Compact Dwarf galaxies that is centered around HI-line
data obtained with the National Radio Astronomy Observatory (NRAO) Very Large
Array (VLA). The HI-line data are characterized by high sensitivity (less than
1.1 mJy/beam per channel), high spectral resolution (less than or equal to 2.6
km/s), and high angular resolution (~6 arcseconds. The LITTLE THINGS sample
contains dwarf galaxies that are relatively nearby (less than or equal to 10.3
Mpc; 6 arcseconds is less than or equal to 300 pc), that were known to contain
atomic hydrogen, the fuel for star formation, and that cover a large range in
dwarf galactic properties. We describe our VLA data acquisition, calibration,
and mapping procedures, as well as HI map characteristics, and show channel
maps, moment maps, velocity-flux profiles, and surface gas density profiles. In
addition to the HI data we have GALEX UV and ground-based UBV and Halpha images
for most of the galaxies, and JHK images for some. Spitzer mid-IR images are
available for many of the galaxies as well. These data sets are available
on-line.Comment: In press in A
The DEEP2 Galaxy Redshift Survey: Design, Observations, Data Reduction, and Redshifts
We describe the design and data sample from the DEEP2 Galaxy Redshift Survey,
the densest and largest precision-redshift survey of galaxies at z ~ 1
completed to date. The survey has conducted a comprehensive census of massive
galaxies, their properties, environments, and large-scale structure down to
absolute magnitude M_B = -20 at z ~ 1 via ~90 nights of observation on the
DEIMOS spectrograph at Keck Observatory. DEEP2 covers an area of 2.8 deg^2
divided into four separate fields, observed to a limiting apparent magnitude of
R_AB=24.1. Objects with z < 0.7 are rejected based on BRI photometry in three
of the four DEEP2 fields, allowing galaxies with z > 0.7 to be targeted ~2.5
times more efficiently than in a purely magnitude-limited sample. Approximately
sixty percent of eligible targets are chosen for spectroscopy, yielding nearly
53,000 spectra and more than 38,000 reliable redshift measurements. Most of the
targets which fail to yield secure redshifts are blue objects that lie beyond z
~ 1.45. The DEIMOS 1200-line/mm grating used for the survey delivers high
spectral resolution (R~6000), accurate and secure redshifts, and unique
internal kinematic information. Extensive ancillary data are available in the
DEEP2 fields, particularly in the Extended Groth Strip, which has evolved into
one of the richest multiwavelength regions on the sky. DEEP2 surpasses other
deep precision-redshift surveys at z ~ 1 in terms of galaxy numbers, redshift
accuracy, sample number density, and amount of spectral information. We also
provide an overview of the scientific highlights of the DEEP2 survey thus far.
This paper is intended as a handbook for users of the DEEP2 Data Release 4,
which includes all DEEP2 spectra and redshifts, as well as for the
publicly-available DEEP2 DEIMOS data reduction pipelines. [Abridged]Comment: submitted to ApJS; data products available for download at
http://deep.berkeley.edu/DR4
Ubiquitous outflows in DEEP2 spectra of star-forming galaxies at z=1.4
Galactic winds are a prime suspect for the metal enrichment of the
intergalactic medium and may have a strong influence on the chemical evolution
of galaxies and the nature of QSO absorption line systems. We use a sample of
1406 galaxy spectra at z~1.4 from the DEEP2 redshift survey to show that
blueshifted Mg II 2796, 2803 A absorption is ubiquitous in starforming galaxies
at this epoch. This is the first detection of frequent outflowing galactic
winds at z~1. The presence and depth of absorption are independent of AGN
spectral signatures or galaxy morphology; major mergers are not a prerequisite
for driving a galactic wind from massive galaxies. Outflows are found in
coadded spectra of galaxies spanning a range of 30x in stellar mass and 10x in
star formation rate (SFR), calibrated from K-band and from MIPS IR fluxes. The
outflows have column densities of order N_H ~ 10^20 cm^-2 and characteristic
velocities of ~ 300-500 km/sec, with absorption seen out to 1000 km/sec in the
most massive, highest SFR galaxies. The velocities suggest that the outflowing
gas can escape into the IGM and that massive galaxies can produce
cosmologically and chemically significant outflows. Both the Mg II equivalent
width and the outflow velocity are larger for galaxies of higher stellar mass
and SFR, with V_wind ~ SFR^0.3, similar to the scaling in low redshift
IR-luminous galaxies. The high frequency of outflows in the star-forming galaxy
population at z~1 indicates that galactic winds occur in the progenitors of
massive spirals as well as those of ellipticals. The increase of outflow
velocity with mass and SFR constrains theoretical models of galaxy evolution
that include feedback from galactic winds, and may favor momentum-driven models
for the wind physics.Comment: Accepted by ApJ. 25 pages, 17 figures. Revised to add discussions of
intervening absorbers and AGN-driven outflows; conclusions unchange
The True Durations of Starbursts: HST Observations of Three Nearby Dwarf Starburst Galaxies
The duration of a starburst is a fundamental parameter affecting the
evolution of galaxies yet, to date, observational constraints on the durations
of starbursts are not well established. Here we study the recent star formation
histories (SFHs) of three nearby dwarf galaxies to rigorously quantify the
duration of their starburst events using a uniform and consistent approach. We
find that the bursts range from ~200 - ~400 Myr in duration resolving the
tension between the shorter timescales often derived observationally with the
longer timescales derived from dynamical arguments. If these three starbursts
are typical of starbursts in dwarf galaxies, then the short timescales (3 - 10
Myr) associated with starbursts in previous studies are best understood as
"flickering" events which are simply small components of the larger starburst.
In this sample of three nearby dwarfs, the bursts are not localized events. All
three systems show bursting levels of star formation in regions of both high
and low stellar density. The enhanced star formation moves around the galaxy
during the bursts and covers a large fraction of the area of the galaxy. These
massive, long duration bursts can significantly affect the structure, dynamics,
and chemical evolution of the host galaxy and can be the progenitors of
"superwinds" that drive much of the recently chemically enriched material from
the galaxy into the intergalactic medium.Comment: 41 pages, 14 figures, ApJ, Accepte