1,748 research outputs found
Non-equilibrium dissociating nitrogen flow over a wedge
Experimental results for dissociating nitrogen flow over a wedge, obtained in a free-piston shock tunnel, are described. Interferograms of the flow show clearly the curvature of the shock wave and the rise in fringe shift after the shock associated with the dissociation. It is shown that the shock curvature at the tip of the wedge can be used to calculate the initial dissociation rate and that it is a more sensitive indication of the rate than can be obtained from fringe shift measurements under the prevailing experimental conditions. Because the freestream dissociation fraction can be adjusted in the shock tunnel, the dependence on atomic nitrogen concentration of the dissociation rate can be determined by the shock curvature method. A detailed calculation of the flow field by an inverse method, starting from the measured shock shape, shows good agreement with experiments
The Mass-Metallicity and Luminosity-Metallicity Relation from DEEP2 at z ~ 0.8
We present the mass-metallicity (MZ) and luminosity-metallicity (LZ)
relations at z ~ 0.8 from ~1350 galaxies in the Deep Extragalactic Evolutionary
Probe 2 (DEEP2) survey. We determine stellar masses by fitting the spectral
energy distribution inferred from photometry with current stellar population
synthesis models. This work raises the number of galaxies with metallicities at
z ~ 0.8 by more than an order of magnitude. We investigate the evolution in the
MZ and LZ relations in comparison with local MZ and LZ relations determined in
a consistent manner using ~21,000 galaxies in the Sloan Digital Sky Survey. We
show that high stellar mass galaxies (log(M/M_solar)~10.6) at z ~ 0.8 have
attained the chemical enrichment seen in the local universe, while lower
stellar mass galaxies (log(M/M_solar)~9.2) at z ~ 0.8 have lower metallicities
(Delta log(O/H)~0.15 dex) than galaxies at the same stellar mass in the local
universe. We find that the LZ relation evolves in both metallicity and B-band
luminosity between z ~ 0.8 and z~ 0, with the B-band luminosity evolving as a
function of stellar mass. We emphasize that the B-band luminosity should not be
used as a proxy for stellar mass in chemical evolution studies of star-forming
galaxies. Our study shows that both the metallicity evolution and the B-band
luminosity evolution for emission-line galaxies between the epochs are a
function of stellar mass, consistent with the cosmic downsizing scenario of
galaxy evolution.Comment: Accepted Version: 18 pages, 13 figure
A universal, turbulence-regulated star formation law: from Milky Way clouds to high-redshift disk and starburst galaxies
Whilst the star formation rate (SFR) of molecular clouds and galaxies is key
in understanding galaxy evolution, the physical processes which determine the
SFR remain unclear. This uncertainty about the underlying physics has resulted
in various different star formation laws, all having substantial intrinsic
scatter. Extending upon previous works that define the column density of star
formation (Sigma_SFR) by the gas column density (Sigma_gas), we develop a new
universal star formation (SF) law based on the multi-freefall prescription of
gas. This new SF law relies predominantly on the probability density function
(PDF) and on the sonic Mach number of the turbulence in the star-forming
clouds. By doing so we derive a relation where the star formation rate (SFR)
correlates with the molecular gas mass per multi-freefall time, whereas
previous models had used the average, single-freefall time. We define a new
quantity called maximum (multi-freefall) gas consumption rate (MGCR) and show
that the actual SFR is only about 0.4% of this maximum possible SFR, confirming
the observed low efficiency of star formation. We show that placing
observations in this new framework (Sigma_SFR vs. MGCR) yields a significantly
improved correlation with 3-4 times reduced scatter compared to previous SF
laws and a goodness-of-fit parameter R^2=0.97. By inverting our new
relationship, we provide sonic Mach number predictions for kpc-scale
observations of Local Group galaxies as well as unresolved observations of
local and high-redshift disk and starburst galaxies that do not have
independent, reliable estimates for the turbulent cloud Mach number.Comment: 6 pages, 2 figures, Accepted for publication in ApJ Letters, Movie
available here:
http://www.mso.anu.edu.au/~chfeder/pubs/universal_sf_law/universal_sf_law.htm
The Role of Starburst-AGN composites in Luminous Infrared Galaxy Mergers: Insights from the New Optical Classification Scheme
We investigate the fraction of starbursts, starburst-AGN composites,
Seyferts, and LINERs as a function of infrared luminosity (L_IR) and merger
progress for ~500 infrared-selected galaxies. Using the new optical
classifications afforded by the extremely large data set of the Sloan Digital
Sky Survey, we find that the fraction of LINERs in IR-selected samples is rare
(< 5%) compared with other spectral types. The lack of strong infrared emission
in LINERs is consistent with recent optical studies suggesting that LINERs
contain AGN with lower accretion rates than in Seyfert galaxies. Most
previously classified infrared-luminous LINERs are classified as starburst-AGN
composite galaxies in the new scheme. Starburst-AGN composites appear to
"bridge" the spectral evolution from starburst to AGN in ULIRGs. The relative
strength of the AGN versus starburst activity shows a significant increase at
high infrared luminosity. In ULIRGs (L_IR >10^12 L_odot), starburst-AGN
composite galaxies dominate at early--intermediate stages of the merger, and
AGN galaxies dominate during the final merger stages. Our results are
consistent with models for IR-luminous galaxies where mergers of gas-rich
spirals fuel both starburst and AGN, and where the AGN becomes increasingly
dominant during the final merger stages of the most luminous infrared objects.Comment: 30 pages, 19 figures, 10 tables, ApJ accepte
The A2667 Giant Arc at z=1.03: Evidence for Large-scale Shocks at High Redshift
We present the spatially resolved emission line ratio properties of a ~10^10
M_sun star-forming galaxy at redshift z=1.03. This galaxy is gravitationally
lensed as a triple-image giant arc behind the massive lensing cluster Abell
2667. The main image of the galaxy has magnification factors of 14+/-2.1 in
flux and ~ 2 by 7 in area, yielding an intrinsic spatial resolution of 115-405
pc after AO correction with OSIRIS at KECK II. The HST morphology shows a
clumpy structure and the H\alpha\ kinematics indicates a large velocity
dispersion with V_{max} sin(i)/\sigma ~ 0.73, consistent with high redshift
disk galaxies of similar masses. From the [NII]/H\alpha\ line ratios, we find
that the central 350 parsec of the galaxy is dominated by star formation. The
[NII]/H\alpha\ line ratios are higher in the outer-disk than in the central
regions. Most noticeably, we find a blue-shifted region of strong
[NII]/H\alpha\ emission in the outer disk. Applying our recent HII region and
slow-shock models, we propose that this elevated [NII]/H\alpha\ ratio region is
contaminated by a significant fraction of shock excitation due to galactic
outflows. Our analysis suggests that shocked regions may mimic flat or inverted
metallicity gradients at high redshift.Comment: 11 pages, 9 figures, ApJ accepte
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