417 research outputs found
Metal-poor, Strongly Star-Forming Galaxies in the DEEP2 Survey: The Relationship between Stellar Mass, Temperature-based Metallicity, and Star Formation Rate
We report on the discovery of 28 metal-poor galaxies in DEEP2.
These galaxies were selected for their detection of the weak
[OIII]4363 emission line, which provides a "direct" measure of the
gas-phase metallicity. A primary goal for identifying these rare galaxies is to
examine whether the fundamental metallicity relation (FMR) between stellar
mass, gas metallicity, and star formation rate (SFR) holds for low stellar mass
and high SFR galaxies. The FMR suggests that higher SFR galaxies have lower
metallicity (at fixed stellar mass). To test this trend, we combine
spectroscopic measurements of metallicity and dust-corrected SFRs, with stellar
mass estimates from modeling the optical photometry. We find that these
galaxies are dex above the z~1 stellar mass-SFR relation, and
dex below the local mass-metallicity relation. Relative to the
FMR, the latter offset is reduced to 0.01 dex, but significant dispersion
remains (0.29 dex with 0.16 dex due to measurement uncertainties). This
dispersion suggests that gas accretion, star formation and chemical enrichment
have not reached equilibrium in these galaxies. This is evident by their short
stellar mass doubling timescale of Myr that suggests
stochastic star formation. Combining our sample with other z~1 metal-poor
galaxies, we find a weak positive SFR-metallicity dependence (at fixed stellar
mass) that is significant at 94.4% confidence. We interpret this positive
correlation as recent star formation that has enriched the gas, but has not had
time to drive the metal-enriched gas out with feedback mechanisms.Comment: Resubmitted to ApJ on March 6, 2015. Revised to discuss selection
biases and methodologies, and address the former by including more metal-rich
galaxies with robust non-detections of [OIII]4363. Primary results on FMR
analyses are unchanged. Additional figures are included to illustrate
selection biases; previous figures have been revised to improve presentatio
"Direct" Gas-phase Metallicities, Stellar Properties, and Local Environments of Emission-line Galaxies at Redshift below 0.90
Using deep narrow-band (NB) imaging and optical spectroscopy from the Keck
telescope and MMT, we identify a sample of 20 emission-line galaxies (ELGs) at
z=0.065-0.90 where the weak auroral emission line, [OIII]4363, is detected at
>3\sigma. These detections allow us to determine the gas-phase metallicity
using the "direct'' method. With electron temperature measurements and dust
attenuation corrections from Balmer decrements, we find that 4 of these
low-mass galaxies are extremely metal-poor with 12+log(O/H) <= 7.65 or
one-tenth solar. Our most metal-deficient galaxy has 12+log(O/H) =
7.24^{+0.45}_{-0.30} (95% confidence), similar to some of the lowest
metallicity galaxies identified in the local universe. We find that our
galaxies are all undergoing significant star formation with average specific
star formation rate (SFR) of (100 Myr)^{-1}, and that they have high central
SFR surface densities (average of 0.5 Msun/yr/kpc^2. In addition, more than
two-thirds of our galaxies have between one and four nearby companions within a
projected radius of 100 kpc, which we find is an excess among star-forming
galaxies at z=0.4-0.85. We also find that the gas-phase metallicities for a
given stellar mass and SFR lie systematically below the local M-Z-(SFR)
relation by \approx0.2 dex (2\sigma\ significance). These results are partly
due to selection effects, since galaxies with strong star formation and low
metallicity are more likely to yield [OIII]4363 detections. Finally, the
observed higher ionization parameter and electron density suggest that they are
lower redshift analogs to typical z>1 galaxies.Comment: Accepted for publication in the Astrophysical Journal (15 November
2013). 31 pages in emulateapj format with 16 figures and 7 tables. Revised to
address referee's comments, which include discussion on selection effects,
similarities to green pea galaxies, and nebular continuum contribution.
Modifications were made for some electron temperature and metallicity
measurement
The Discovery of Extended Thermal X-ray Emission from PKS 2152-699: Evidence for a `Jet-cloud' Interaction
A Chandra ACIS-S observation of PKS 2152-699 reveals thermal emission from a
diffuse region around the core and a hotspot located 10" northeast from the
core. This is the first detection of thermal X-ray radiation on kiloparsec
scales from an extragalactic radio source. Two other hotspots located 47"
north-northeast and 26" southwest from the core were also detected. Using a
Raymond-Smith model, the first hotspot can be characterized with a thermal
plasma temperature of 2.6 K and an electron number density of 0.17
cm. These values correspond to a cooling time of about 1.6
yr. In addition, an emission line from the hotspot, possibly Fe xxv, was
detected at rest wavelength 10.04\AA.
The thermal X-ray emission from the first hotspot is offset from the radio
emission but is coincident with optical filaments detected with broadband
filters of HST/WFPC2. The best explanation for the X-ray, radio, and optical
emission is that of a `jet-cloud' interaction.
The diffuse emission around the nucleus of PKS 2152-699 can be modeled as a
thermal plasma with a temperature of 1.2 K and a luminosity of
1.8 erg s. This emission appears to be asymmetric with a
small extension toward Hotspot A, similar to a jet. An optical hotspot (EELR)
is seen less than an arcsecond away from this extension in the direction of the
core. This indicates that the extension may be caused by the jet interacting
with an inner ISM cloud, but entrainment of hot gas is unavoidable. Future
observations are discussed.Comment: To appear in the Astrophysical Journal 21 pages, 5 Postscript
figures, 1 table, AASTeX v. 5.
Observations of the Structure and Dynamics of the Inner M87 Jet
M87 is the best source in which to study a jet at high resolution in
gravitational units because it has a very high mass black hole and is nearby.
The angular size of the black hole is second only to Sgr A*, which does not
have a strong jet. The jet structure is edge brightened with a wide opening
angle base and a weak counterjet. We have roughly annual observations for 17
years plus intensive monitoring at three week intervals for a year and five day
intervals for 2.5 months made with the Very Long Baseline Array (VLBA) at 43
GHz. The inner jet shows very complex dynamics, with apparent motions both
along and across the jet. Speeds from zero to over 2c are seen, with
acceleration observed over the first 3 milli-arcseconds. The counterjet
decreases in brightness much more rapidly than the main jet, as is expected
from relativistic beaming in an accelerating jet oriented near the
line-of-sight. Details of the structure and dynamics are discussed. The roughly
annual observations show side-to-side motion of the whole jet with a
characteristic time scale of about 9 years.Comment: 11 pages, 7 figures. Published in a special issue of Galaxies which
is the proceedings of "Blazars through Sharp Multi-Wavelength Eyes" edited by
J. L. Gomez, A. P. Marscher, and S. G. Jorsta
The H-alpha Luminosity Function and Star Formation Rate Volume Density at z=0.8 from the NEWFIRM H-alpha Survey
[Abridged] We present new measurements of the H-alpha luminosity function
(LF) and SFR volume density for galaxies at z~0.8. Our analysis is based on
1.18m narrowband data from the NEWFIRM H-alpha Survey, a comprehensive
program designed to capture deep samples of intermediate redshift emission-line
galaxies using narrowband imaging in the near-infrared. The combination of
depth ( erg s cm in H-alpha at
3) and areal coverage (0.82 deg) complements other recent H-alpha
studies at similar redshifts, and enables us to minimize the impact of cosmic
variance and place robust constraints on the shape of the LF. The present
sample contains 818 NB118 excess objects, 394 of which are selected as H-alpha
emitters. Optical spectroscopy has been obtained for 62% of the NB118 excess
objects. Empirical optical broadband color classification is used to sort the
remainder of the sample. A comparison of the LFs constructed for the four
individual fields reveals significant cosmic variance, emphasizing that
multiple, widely separated observations are required. The dust-corrected LF is
well-described by a Schechter function with L*=10^{43.00\pm0.52} ergs s^{-1},
\phi*=10^{-3.20\pm0.54} Mpc^{-3}, and \alpha=-1.6\pm0.19. We compare our
H-alpha LF and SFR density to those at z<1, and find a rise in the SFR density
\propto(1+z)^{3.4}, which we attribute to significant L* evolution. Our H-alpha
SFR density of 10^{-1.00\pm0.18} M_sun yr^{-1} Mpc^{-3} is consistent with UV
and [O II] measurements at z~1. We discuss how these results compare to other
H-alpha surveys at z~0.8, and find that the different methods used to determine
survey completeness can lead to inconsistent results. This suggests that future
surveys probing fainter luminosities are needed, and more rigorous methods of
estimating the completeness should be adopted as standard procedure.Comment: 19 pages (emulate-ApJ format), 16 figures, 5 tables, published in
ApJ. Modified to match ApJ versio
Physical conditions of the interstellar medium in star-forming galaxies at z~1.5
We present results from Subaru/FMOS near-infrared (NIR) spectroscopy of 118
star-forming galaxies at in the Subaru Deep Field. These galaxies
are selected as [OII]3727 emitters at 1.47 and 1.62 from
narrow-band imaging. We detect H emission line in 115 galaxies,
[OIII]5007 emission line in 45 galaxies, and H,
[NII]6584, and [SII]6716,6731 in 13, 16, and 6
galaxies, respectively. Including the [OII] emission line, we use the six
strong nebular emission lines in the individual and composite rest-frame
optical spectra to investigate physical conditions of the interstellar medium
in star-forming galaxies at 1.5. We find a tight correlation between
H and [OII], which suggests that [OII] can be a good star formation
rate (SFR) indicator for galaxies at . The line ratios of
H/[OII] are consistent with those of local galaxies. We also find that
[OII] emitters have strong [OIII] emission lines. The [OIII]/[OII] ratios are
larger than normal star-forming galaxies in the local Universe, suggesting a
higher ionization parameter. Less massive galaxies have larger [OIII]/[OII]
ratios. With evidence that the electron density is consistent with local
galaxies, the high ionization of galaxies at high redshifts may be attributed
to a harder radiation field by a young stellar population and/or an increase in
the number of ionizing photons from each massive star.Comment: Fixed a minor issue with LaTeX table numberin
A Search for Molecular Gas toward a BzK-selected Star-forming Galaxy at z = 2.044
We present a search for CO(3-2) emission in SDF-26821, a BzK-selected
star-forming galaxy (sBzK) at z = 2.044, using the 45-m telescope of the
Nobeyama Radio Observatory and the Nobeyama Millimeter Array. We do not detect
significant emission and derive 2 \sigma limits: the CO luminosity of L'CO <
3.1 x 10^10 K km s^{-1} pc^{-2}, the ratio of far-infrared luminosity to CO
luminosity of L_FIR/L'CO > 57 Lsun (K km s^{-1} pc^{-2})^{-1}, and the
molecular gas mass of M_H2 < 2.5 x 10^10 Msun, assuming a velocity width of 200
km s^{-1} and a CO-to-H2 conversion factor of alpha_CO=0.8 Msun (K km s^{-1}
pc^{-2})^{-1}. The ratio of L_FIR/L'CO, a measure of star formation efficiency
(SFE), is comparable to or higher than the two z ~ 1.5 sBzKs detected in
CO(2-1) previously, suggesting that sBzKs can have a wide range of SFEs.
Comparisons of far-infrared luminosity, gas mass, and stellar mass among the
sBzKs suggest that SDF-26821 is at an earlier stage of forming stars with a
similar SFE and/or more efficiently forming stars than the two z ~ 1.5 sBzKs.
The higher SFEs and specific star formation rates of the sBzKs compared to
local spirals are indicative of the difference in star formation modes between
these systems, suggesting that sBzKs are not just scaled-up versions of local
spirals.Comment: 4 pages, 4 figures, Accepted for publication in PAS
Optical and near-IR spectroscopy of candidate red galaxies in two z~2.5 proto-clusters
We present a spectroscopic campaign to follow-up red colour-selected
candidate massive galaxies in two high redshift proto-clusters surrounding
radio galaxies. We observed a total of 57 galaxies in the field of MRC0943-242
(z=2.93) and 33 in the field of PKS1138-262 (z=2.16) with a mix of optical and
near-infrared multi-object spectroscopy.
We confirm two red galaxies in the field of PKS1138-262 at the redshift of
the radio galaxy. Based on an analysis of their spectral energy distributions,
and their derived star formation rates from the H-alpha and 24um flux, one
object belongs to the class of dust-obscured star-forming red galaxies, while
the other is evolved with little ongoing star formation. This result represents
the first red and mainly passively evolving galaxy to be confirmed as companion
galaxies in a z>2 proto-cluster. Both red galaxies in PKS1138-262 are massive,
of the order of 4-6x10^11 M_Sol. They lie along a Colour-Magnitude relation
which implies that they formed the bulk of their stellar population around z=4.
In the MRC0943-242 field we find no red galaxies at the redshift of the radio
galaxy but we do confirm the effectiveness of our JHK_s selection of galaxies
at 2.3<z<3.1, finding that 10 out of 18 (56%) of JHK_s-selected galaxies whose
redshifts could be measured fall within this redshift range. We also
serendipitously identify an interesting foreground structure of 6 galaxies at
z=2.6 in the field of MRC0943-242. This may be a proto-cluster itself, but
complicates any interpretation of the red sequence build-up in MRC0943-242
until more redshifts can be measured.Comment: 17 pages, 14 figures, accepted for publication in Astronomy and
Astrophysic
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