144 research outputs found
Outlying HII Regions in HI-Selected Galaxies
We present results from the first systematic search for outlying HII regions,
as part of a sample of 96 emission-line point sources (referred to as ELdots -
emission-line dots) derived from the NOAO Survey for Ionization in Neutral Gas
Galaxies (SINGG). Our automated ELdot-finder searches SINGG narrow-band and
continuum images for high equivalent width point sources outside the optical
radius of the target galaxy (> 2 X r25 in the R-band). Follow-up longslit
spectroscopy and deep GALEX images (exposure time > 1000 s) distinguish
outlying HII regions from background galaxies whose strong emission lines
([OIII], Hbeta or [OII]) have been redshifted into the SINGG bandpass. We find
that these deep GALEX images can serve as a substitute for spectroscopic
follow-up because outlying HII regions separate cleanly from background
galaxies in color-color space. We identify seven SINGG systems with outlying
massive star formation that span a large range in Halpha luminosities
corresponding to a few O stars in the most nearby cases, and unresolved dwarf
satellite companion galaxies in the most distant cases. Six of these seven
systems feature galaxies with nearby companions or interacting galaxies.
Furthermore, our results indicate that some outlying HII regions are linked to
the extended-UV disks discovered by GALEX, representing emission from the most
massive O stars among a more abundant population of lower mass (or older) star
clusters. The overall frequency of outlying HII regions in this sample of
gas-rich galaxies is 8 - 11% when we correct for background emission-line
galaxy contamination (~75% of ELdots).Comment: 20 pages, 14 Figures, Accepted by A
A Comprehensive Investigation of Metals in the Circumgalactic Medium of Nearby Dwarf Galaxies
Dwarf galaxies are found to have lost most of their metals via feedback
processes; however, there still lacks consistent assessment on the retention
rate of metals in their circumgalactic medium (CGM). Here we investigate the
metal content in the CGM of 45 isolated dwarf galaxies with
() using {\it
HST}/COS. While H I (Ly) is ubiquitously detected () within the
CGM, we find low detection rates () in C II, C IV, Si II, Si
III, and Si IV, largely consistent with literature values. Assuming these ions
form in the cool ( K) CGM with photoionization equilibrium, the
observed H I and metal column density profiles can be best explained by an
empirical model with low gas density and high volume filling factor. For a
typical galaxy with (median of the sample),
our model predicts a cool gas mass of ,
corresponding to of the galaxy's baryonic budget. Assuming a
metallicity of , we estimate that the dwarf galaxy's cool CGM
likely harbors of the metals ever produced, with the rest either in
more ionized states in the CGM or transported to the intergalactic medium. We
further examine the EAGLE simulation and show that H I and low ions may arise
from a dense cool medium, while C IV arises from a diffuse warmer medium. Our
work provides the community with a uniform dataset on dwarf galaxies' CGM that
combines our recent observations, additional archival data and literature
compilation, which can be used to test various theoretical models of dwarf
galaxies.Comment: Finalized version. Accepted for publication in Ap
Control of cambium initiation and activity in Arabidopsis by the transcriptional regulator AHL15
Plant secondary growth, which is the basis of wood formation, includes the production of secondary xylem, which is derived from meristematic cambium cells embedded in vascular tissue. Here, we identified an important role for the Arabidopsis thaliana (Arabidopsis) AT-HOOK MOTIF CONTAINING NUCLEAR LOCALIZED 15 (AHL15) transcriptional regulator in controlling vascular cambium activity. The limited secondary xylem development in inflorescence stems of herbaceous Arabidopsis plants was significantly reduced in ahl15 loss-of-function mutants, whereas constitutive or vascular meristem-specific AHL15 overexpression produced woody inflorescence stems. AHL15 was required for enhanced secondary xylem formation in the woody suppressor of overexpression of constans 1 (soc1) fruitfull (ful) double loss-of-function mutant. Moreover, we found that AHL15 induces vascular cambium activity downstream of the repressing SOC1 and FUL transcription factors, most likely similar to how it enhances lateral branching by promoting biosynthesis of the hormone cytokinin. Our results uncover a novel pathway driving cambium development, in which AHL15 expression levels act in parallel to and are dependent on the well-established TDIF-PXY-WOX pathway to differentiate between herbaceous and woody stem growth.Plant science
Observational Diagnostics of Gas Flows: Insights from Cosmological Simulations
Galactic accretion interacts in complex ways with gaseous halos, including
galactic winds. As a result, observational diagnostics typically probe a range
of intertwined physical phenomena. Because of this complexity, cosmological
hydrodynamic simulations have played a key role in developing observational
diagnostics of galactic accretion. In this chapter, we review the status of
different observational diagnostics of circumgalactic gas flows, in both
absorption (galaxy pair and down-the-barrel observations in neutral hydrogen
and metals; kinematic and azimuthal angle diagnostics; the cosmological column
density distribution; and metallicity) and emission (Lya; UV metal lines; and
diffuse X-rays). We conclude that there is no simple and robust way to identify
galactic accretion in individual measurements. Rather, progress in testing
galactic accretion models is likely to come from systematic, statistical
comparisons of simulation predictions with observations. We discuss specific
areas where progress is likely to be particularly fruitful over the next few
years.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics
and Space Science Library, eds. A. J. Fox & R. Dave, to be published by
Springer. Typos correcte
Gas Accretion and Star Formation Rates
Cosmological numerical simulations of galaxy evolution show that accretion of
metal-poor gas from the cosmic web drives the star formation in galaxy disks.
Unfortunately, the observational support for this theoretical prediction is
still indirect, and modeling and analysis are required to identify hints as
actual signs of star-formation feeding from metal-poor gas accretion. Thus, a
meticulous interpretation of the observations is crucial, and this
observational review begins with a simple theoretical description of the
physical process and the key ingredients it involves, including the properties
of the accreted gas and of the star-formation that it induces. A number of
observations pointing out the connection between metal-poor gas accretion and
star-formation are analyzed, specifically, the short gas consumption time-scale
compared to the age of the stellar populations, the fundamental metallicity
relationship, the relationship between disk morphology and gas metallicity, the
existence of metallicity drops in starbursts of star-forming galaxies, the
so-called G dwarf problem, the existence of a minimum metallicity for the
star-forming gas in the local universe, the origin of the alpha-enhanced gas
forming stars in the local universe, the metallicity of the quiescent BCDs, and
the direct measurements of gas accretion onto galaxies. A final section
discusses intrinsic difficulties to obtain direct observational evidence, and
points out alternative observational pathways to further consolidate the
current ideas.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics
and Space Science Library, eds. A. J. Fox & R. Dav\'e, to be published by
Springe
The Disruption and Fueling of M33
The disruption of the M33 galaxy is evident from its extended gaseous
structure. We present new data from the Galactic Arecibo L-Band Feed Array HI
(GALFA-HI) Survey that show the full extent and detailed spatial and kinematic
structure of M33's neutral hydrogen. Over 18% of the HI mass of M33 (M_HI(tot)
=1.4 x 10^9 Msun) is found beyond the star forming disk as traced in the
far-ultraviolet (FUV). The most distinct features are extended warps, an arc
from the northern warp to the disk, diffuse gas surrounding the galaxy, and a
southern cloud with a filament back to the galaxy. The features extend out to
22 kpc from the galaxy center (18 kpc from the edge of the FUV disk) and the
gas is directly connected to M33's disk. Only five discrete clouds (i.e., gas
not directly connected to M33 in position-velocity space) are catalogued in the
vicinity of M33, and these clouds show similar properties to Galactic and M31
halo clouds. M33's gaseous features most likely originate from the tidal
disruption of M33 by M31 1-3 Gyr ago as shown from an orbit analysis which
results in a tidal radius < 15 kpc in the majority of M33's possible orbits.
M33 is now beyond the disruptive gravitational influence of M31 and the gas
appears to be returning to M33's disk and redistributing its star formation
fuel. M33's high mean velocity dispersion in the disk (~18.5 km/s) may also be
consistent with the previous interaction and high rate of star formation. M33
will either exhaust its star formation fuel in the next few Gyrs or eventually
become star formation fuel for M31. The latter represents the accretion of a
large gaseous satellite by a spiral galaxy, similar to the Magellanic Clouds'
relationship to the Galaxy.Comment: published in ApJ (13 figures, several with color), see published
version for full resolution figures; data available at
http://sites.google.com/site/galfahi/dat
MASSIV: Mass Assembly Survey with SINFONI in VVDS. III. Evidence for positive metallicity gradients in z~1.2 star-forming galaxies
A key open issue for galaxy evolution and formation models is the
understanding of the different mechanisms of galaxy assembly at various cosmic
epochs. The aim of this study is to derive the global and spatially-resolved
metal content in high-redshift galaxies. Using VLT/SINFONI IFU spectroscopy of
a first sample of 50 galaxies at z~1.2 in the MASSIV survey, we are able to
measure the Ha and [NII]6584 emission lines. Using the N2 ratio as a proxy for
oxygen abundance in the interstellar medium, we measure the metallicity of the
sample galaxies. We develop a tool to extract spectra in annular regions of
these galaxies, leading to a spatially-resolved estimate of the oxygen
abundance in each galaxy. We derive a metallicity gradient for 26 galaxies in
our sample and discover a significant fraction of galaxies with a "positive"
gradient. Using a simple chemical evolution model, we derive infall rates of
pristine gas onto the disks. Seven galaxies display a positive gradient at a
high confidence level. Four out of these are interacting and one is a chain
galaxy. We suggest that interactions might be responsible for shallowing and
even inverting the abundance gradient. We also identify two interesting
correlations in our sample: a) galaxies with higher gas velocity dispersion
have shallower/positive gradients; and b) metal-poor galaxies tend to show a
positive gradient whereas metal-rich ones tend to show a negative one. This
last observation can be explained by the infall of metal-poor gas into the
center of the disks. We address the question of the origin of this infall under
the influence of gas flows triggered by interactions and/or cold gas accretion.Comment: 13 pages, 9 figures. Accepted for publication in A&
NGC922 - A new drop-through ring galaxy
We have found the peculiar galaxy NGC922 to be a new drop-through ring galaxy
using multi-wavelength (UV-radio) imaging and spectroscopic observations. Its
`C'-shaped morphology and tidal plume indicate a recent strong interaction with
its companion which was identified with these observations. Using numerical
simulations we demonstrate that the main properties of the system can be
generated by a high-speed off-axis drop-through collision of a small galaxy
with a larger disk system, thus making NGC922 one of the nearest known
collisional ring galaxies. While these systems are rare in the local Universe,
recent deep HST images suggest they were more common in the early Universe.Comment: 5 pages, accepted for publication in MNRAS Letter
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