206 research outputs found
AAT Imaging and Microslit Spectroscopy in the Southern Hubble Deep Field
We present a deep photometric (B- and R-band) catalog and an associated
spectroscopic redshift survey conducted in the vicinity of the Hubble Deep
Field South. The spectroscopy yields 53 extragalactic redshifts in the range
0<z<1.4 substantially increasing the body of spectroscopic work in this field
to over 200 objects. The targets are selected from deep AAT prime focus images
complete to R<24 and spectroscopy is 50% complete at R=23. There is now strong
evidence for a rich cluster at z\simeq 0.58 flanking the WFPC2 field which is
consistent with a known absorber of the bright QSO in this field. We find that
photometric redshifts of z<1 galaxies in this field based on HST data are
accurate to \sigma_z/(1+z)=0.03 (albeit with small number statistics). The
observations were carried out as a community service for Hubble Deep Field
science, to demonstrate the first use of the `nod & shuffle' technique with a
classical multi-object spectrograph and to test the use of `microslits' for
ultra-high multiplex observations along with a new VPH grism and deep-depletion
CCD. The reduction of this new type of data is also described.Comment: From the better late than never department: AJ in press (2006). 16
pages, 2 tables, 6 figures, final data release + Appendix at
http://www.aao.gov.au/hdfs/Redshifts
Size-scaling of clump instabilities in turbulent, feedback regulated disks
We explore the scaling between the size of star-forming clumps and rotational
support in massively star-forming galactic disks. The analysis relies on
simulations of a clumpy galaxy at and the observed DYNAMO sample of rare
clumpy analogs at to test a predictive clump size scaling
proposed by \citet{Fisher2017ApJ...839L...5F} in the context of the Violent
Disk Instability (VDI) theory. We here determine the clump sizes using a
recently presented 2-point estimator, which is robust against resolution/noise
effects, hierarchical clump substructure, clump-clump overlap and other
galactic substructure. After verifying Fisher's clump scaling relation for the
DYNAMO observations, we explore whether this relation remains characteristic of
the VDI theory, even if realistic physical processes, such as local asymetries
and stellar feedback, are included in the model. To this end, we rely on
hydrodynamic zoom-simulations of a Milky Way-mass galaxy with four different
feedback prescriptions. We find that, during its marginally stable epoch at
, this mock galaxy falls on the clump scaling relation, although its
position on this relation depends on the feedback model. This finding implies
that Toomre-like stability considerations approximately apply to large
() instabilities in marginally stable turbulent disks,
irrespective of the feedback model, but also emphasizes that the global clump
distribution of a turbulent disk depends strongly on feedback.Comment: Accepted by ApJ, no changes made. 11 pages, 4 figure
The Redshift One LDSS-3 Emission line Survey (ROLES) II: Survey method and z~1 mass-dependent star-formation rate density
Motivated by suggestions of 'cosmic downsizing', in which the dominant
contribution to the cosmic star formation rate density (SFRD) proceeds from
higher to lower mass galaxies with increasing cosmic time, we describe the
design and implementation of the Redshift One LDSS3 Emission line Survey
(ROLES). ROLES is a K-selected (22.5 < K_AB < 24.0) survey for dwarf galaxies
[8.5<log(M*/Msun)< 9.5] at 0.89 < z < 1.15 drawn from two extremely deep fields
(GOODS-S and MS1054-FIRES). Using the [OII]3727 emission line, we obtain
redshifts and star-formation rates (SFRs) for star-forming galaxies down to a
limit of ~0.3 Msun/yr. We present the [OII] luminosity function measured in
ROLES and find a faint end slope of alpha_faint ~ -1.5, similar to that
measured at z~0.1 in the SDSS. By combining ROLES with higher mass surveys, we
measure the SFRD as a function of stellar mass using [OII] (with and without
various empirical corrections), and using SED-fitting to obtain the SFR from
the rest-frame UV luminosity for galaxies with spectroscopic redshifts. Our
best estimate of the corrected [OII]-SFRD and UV SFRD both independently show
that the SFRD evolves equally for galaxies of all masses between z~1 and z~0.1.
The exact evolution in normalisation depends on the indicator used, with the
[OII]-based estimate showing a change of a factor of ~2.6 and the UV-based a
factor of ~6. We discuss possible reasons for the discrepancy in normalisation
between the indicators, but note that the magnitude of this uncertainty is
comparable to the discrepancy between indicators seen in other z~1 works. Our
result that the shape of the SFRD as a function of stellar mass (and hence the
mass range of galaxies dominating the SFRD) does not evolve between z~1 and
z~0.1 is robust to the choice of indicator. [abridged]Comment: Resubmitted to MNRAS following first referee report. 20 pages, 16
figures. High resolution version available at
http://astro.uwaterloo.ca/~dgilbank/papers/roles2.pd
Extreme gas fractions in clumpy, turbulent disk galaxies at z~0.1
In this letter we report the discovery of CO fluxes, suggesting very high gas
fractions in three disk galaxies seen in the nearby Universe (z ~ 0.1). These
galaxies were investigated as part of the DYnamics of Newly Assembled Massive
Objects (DYNAMO) survey. High-resolution Hubble Space Telescope imaging of
these objects reveals the presence of large star forming clumps in the bodies
of the galaxies, while spatially resolved spectroscopy of redshifted Halpha
reveals the presence of high dispersion rotating disks. The internal dynamical
state of these galaxies resembles that of disk systems seen at much higher
redshifts (1 < z < 3). Using CO(1-0) observations made with the Plateau de Bure
Interferometer, we find gas fractions of 20-30% and depletion times of tdep ~
0.5 Gyr (assuming a Milky Way-like CO conversion factor). These properties are
unlike those expected for low- redshift galaxies of comparable specific star
formation rate, but they are normal for their high-z counterparts. DYNAMO
galaxies break the degeneracy between gas fraction and redshift, and we show
that the depletion time per specific star formation rate for galaxies is
closely tied to gas fraction, independent of redshift. We also show that the
gas dynamics of two of our local targets corresponds to those expected from
unstable disks, again resembling the dynamics of high-z disks. These results
provide evidence that DYNAMO galaxies are local analogues to the clumpy,
turbulent disks, which are often found at high redshift.Comment: Accepted to ApJ Letter
Connecting Clump Sizes in Turbulent Disk Galaxies to Instability Theory
In this letter we study the mean sizes of Halpha clumps in turbulent disk
galaxies relative to kinematics, gas fractions, and Toomre Q. We use 100~pc
resolution HST images, IFU kinematics, and gas fractions of a sample of rare,
nearby turbulent disks with properties closely matched to z~1.5-2 main-sequence
galaxies (the DYNAMO sample). We find linear correlations of normalized mean
clump sizes with both the gas fraction and the velocity dispersion-to-rotation
velocity ratio of the host galaxy. We show that these correlations are
consistent with predictions derived from a model of instabilities in a
self-gravitating disk (the so-called "violent disk instability model"). We also
observe, using a two-fluid model for Q, a correlation between the size of
clumps and self-gravity driven unstable regions. These results are most
consistent with the hypothesis that massive star forming clumps in turbulent
disks are the result of instabilities in self-gravitating gas-rich disks, and
therefore provide a direct connection between resolved clump sizes and this in
situ mechanism.Comment: Accepted to Apj Letter
Z-FIRE: ISM properties of the z = 2.095 COSMOS Cluster
We investigate the ISM properties of 13 star-forming galaxies within the z~2
COSMOS cluster. We show that the cluster members have [NII]/Ha and [OIII]/Hb
emission-line ratios similar to z~2 field galaxies, yet systematically
different emission-line ratios (by ~0.17 dex) from the majority of local
star-forming galaxies. We find no statistically significant difference in the
[NII]/Ha and [OIII]/Hb line ratios or ISM pressures among the z~2 cluster
galaxies and field galaxies at the same redshift. We show that our cluster
galaxies have significantly larger ionization parameters (by up to an order of
magnitude) than local star-forming galaxies. We hypothesize that these high
ionization parameters may be associated with large specific star formation
rates (i.e. a large star formation rate per unit stellar mass). If this
hypothesis is correct, then this relationship would have important implications
for the geometry and/or the mass of stars contained within individual star
clusters as a function of redshift.Comment: 11 pages, 5 figures, accepted for publication in Ap
ZFIRE: The Evolution of the Stellar Mass Tully-Fisher Relation to Redshift 2.0 < Z < 2.5 with MOSFIRE
Using observations made with MOSFIRE on Keck I as part of the ZFIRE survey,
we present the stellar mass Tully-Fisher relation at 2.0 < z < 2.5. The sample
was drawn from a stellar mass limited, Ks-band selected catalog from ZFOURGE
over the CANDELS area in the COSMOS field. We model the shear of the Halpha
emission line to derive rotational velocities at 2.2X the scale radius of an
exponential disk (V2.2). We correct for the blurring effect of a
two-dimensional PSF and the fact that the MOSFIRE PSF is better approximated by
a Moffat than a Gaussian, which is more typically assumed for natural seeing.
We find for the Tully-Fisher relation at 2.0 < z < 2.5 that logV2.2 =(2.18 +/-
0.051)+(0.193 +/- 0.108)(logM/Msun - 10) and infer an evolution of the
zeropoint of Delta M/Msun = -0.25 +/- 0.16 dex or Delta M/Msun = -0.39 +/- 0.21
dex compared to z = 0 when adopting a fixed slope of 0.29 or 1/4.5,
respectively. We also derive the alternative kinematic estimator S0.5, with a
best-fit relation logS0.5 =(2.06 +/- 0.032)+(0.211 +/- 0.086)(logM/Msun - 10),
and infer an evolution of Delta M/Msun= -0.45 +/- 0.13 dex compared to z < 1.2
if we adopt a fixed slope. We investigate and review various systematics,
ranging from PSF effects, projection effects, systematics related to stellar
mass derivation, selection biases and slope. We find that discrepancies between
the various literature values are reduced when taking these into account. Our
observations correspond well with the gradual evolution predicted by
semi-analytic models.Comment: 21 pages, 14 figures, 1 appendix. Accepted for publication by Apj,
February 28, 201
ZFIRE: Using H equivalent widths to investigate the in situ initial mass function at z~2
We use the ZFIRE survey (http://zfire.swinburne.edu.au) to investigate the
high mass slope of the initial mass function (IMF) for a mass-complete
(log10(M/M)~9.3) sample of 102 star-forming galaxies at z~2 using
their H equivalent widths (H-EW) and rest-frame optical
colours. We compare dust-corrected H-EW distributions with predictions
of star-formation histories (SFH) from PEGASE.2 and Starburst99 synthetic
stellar population models. We find an excess of high H-EW galaxies that
are up to 0.3--0.5 dex above the model-predicted Salpeter IMF locus and the
H-EW distribution is much broader (10--500 \AA) than can easily be
explained by a simple monotonic SFH with a standard Salpeter-slope IMF. Though
this discrepancy is somewhat alleviated when it is assumed that there is no
relative attenuation difference between stars and nebular lines, the result is
robust against observational biases, and no single IMF (i.e. non-Salpeter
slope) can reproduce the data. We show using both spectral stacking and Monte
Carlo simulations that starbursts cannot explain the EW distribution. We
investigate other physical mechanisms including models with variations in
stellar rotation, binary star evolution, metallicity, and the IMF upper-mass
cutoff. IMF variations and/or highly rotating extreme metal poor stars
(Z~0.1Z) with binary interactions are the most plausible explanations
for our data. If the IMF varies, then the highest H-EWs would require
very shallow slopes (>-1.0) with no one slope able to reproduce the
data. Thus, the IMF would have to vary stochastically. We conclude that the
stellar populations at z~2 show distinct differences from local populations and
there is no simple physical model to explain the large variation in
H-EWs at z~2.Comment: Accepted to MNRAS. 43 pages, 27 Figures. Survey website:
http://zfire.swinburne.edu.au
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