612 research outputs found
Stripes in Quantum Hall Double Layer Systems
We present results of a study of double layer quantum Hall systems in which
each layer has a high-index Landau level that is half-filled. Hartree-Fock
calculations indicate that, above a critical layer separation, the system
becomes unstable to the formation of a unidirectional coherent charge density
wave (UCCDW), which is related to stripe states in single layer systems. The
UCCDW state supports a quantized Hall effect when there is tunneling between
layers, and is {\it always} stable against formation of an isotropic Wigner
crystal for Landau indices . The state does become unstable to the
formation of modulations within the stripes at large enough layer separation.
The UCCDW state supports low-energy modes associated with interlayer coherence.
The coherence allows the formation of charged soliton excitations, which become
gapless in the limit of vanishing tunneling. We argue that this may result in a
novel {\it ``critical Hall state''}, characterized by a power law in
tunneling experiments.Comment: 10 pages, 8 figures include
On the Clustering of Sub-millimeter Galaxies
We measure the angular two-point correlation function of sub-millimeter
galaxies (SMGs) from 1.1-millimeter imaging of the COSMOS field with the AzTEC
camera and ASTE 10-meter telescope. These data yields one of the largest
contiguous samples of SMGs to date, covering an area of 0.72 degrees^2 down to
a 1.26 mJy/beam (1-sigma) limit, including 189 (328) sources with S/N greater
than 3.5 (3). We can only set upper limits to the correlation length r_0,
modeling the correlation function as a power-law with pre-assigned slope.
Assuming existing redshift distributions, we derive 68.3% confidence level
upper limits of r_0 < 6-8 h^-1 Mpc at 3.7 mJy, and r_0 < 11-12 h^-1 Mpc at 4.2
mJy. Although consistent with most previous estimates, these upper limits imply
that the real r_0 is likely smaller. This casts doubts on the robustness of
claims that SMGs are characterized by significantly stronger spatial
clustering, (and thus larger mass), than differently selected galaxies at
high-redshift. Using Monte Carlo simulations we show that even strongly
clustered distributions of galaxies can appear unclustered when sampled with
limited sensitivity and coarse angular resolution common to current
sub-millimeter surveys. The simulations, however, also show that unclustered
distributions can appear strongly clustered under these circumstances. From the
simulations, we predict that at our survey depth, a mapped area of two
degrees^2 is needed to reconstruct the correlation function, assuming smaller
beam sizes of future surveys (e.g. the Large Millimeter Telescope's 6" beam
size). At present, robust measures of the clustering strength of bright SMGs
appear to be below the reach of most observations.Comment: 23 pages, 8 figures, accepted for publication in The Astrophysical
Journa
The SINS Survey: Broad Emission Lines in High-Redshift Star-Forming Galaxies
High signal-to-noise, representative spectra of star-forming galaxies at z~2,
obtained via stacking, reveal a high-velocity component underneath the narrow
H-alpha and [NII] emission lines. When modeled as a single Gaussian, this broad
component has FWHM > 1500 km/s; when modeled as broad wings on the H-alpha and
[NII] features, it has FWHM > 500 km/s. This feature is preferentially found in
the more massive and more rapidly star-forming systems, which also tend to be
older and larger galaxies. We interpret this emission as evidence of either
powerful starburst-driven galactic winds or active supermassive black holes. If
galactic winds are responsible for the broad emission, the observed luminosity
and velocity of this gas imply mass outflow rates comparable to the star
formation rate. On the other hand, if the broad line regions of active black
holes account for the broad feature, the corresponding black holes masses are
estimated to be an order of magnitude lower than those predicted by local
scaling relations, suggesting a delayed assembly of supermassive black holes
with respect to their host bulges.Comment: 11 pages, 5 figures. Accepted version, incorporating referee
comments, including changes to title, abstract, figures, and discussion
sectio
First Spectroscopic Evidence for High Ionization State and Low Oxygen Abundance in Lya Emitters
We present results from Keck/NIRSPEC and Magellan/MMIRS follow-up
spectroscopy of Lya emitters (LAEs) at z=2.2 identified in our Subaru
narrowband survey. We successfully detect Ha emission from seven LAEs, and
perform a detailed analysis of six LAEs free from AGN activity, two out of
which, CDFS-3865 and COSMOS-30679, have [OII] and [OIII] line detections. They
are the first [OII]-detected LAEs at high-z, and their [OIII]/[OII] ratios and
R23-indices provide the first simultaneous determinations of ionization
parameter and oxygen abundance for LAEs. CDFS-3865 has a very high ionization
parameter (q_{ion}=2.5^{+1.7}_{-0.8}x10^8 cm s^{-1}) and a low oxygen abundance
(12+log(O/H)=7.84^{+0.24}_{-0.25}) in contrast with moderate values of other
high-z galaxies such as Lyman-break galaxies (LBGs). COSMOS-30679 also
possesses a relatively high ionization parameter (q_{ion}=8^{+10}_{-4}x10^7 cm
s^{-1}) and a low oxygen abundance (12+log(O/H)=8.18^{+0.28}_{-0.28}). Both
LAEs appear to fall below the mass-metallicity relation of z~2 LBGs. Similarly,
a low metallicity of 12+log(O/H)<8.4 is independently indicated for typical
LAEs from a composite spectrum and the [NII]/Ha index. Such high ionization
parameters and low oxygen abundances can be found in local star-forming
galaxies, but this extreme local population occupies only ~0.06% of the SDSS
spectroscopic galaxy sample with a number density ~100 times smaller than that
of LAEs. With their high ionization parameters and low oxygen abundances, LAEs
would represent an early stage of galaxy formation dominated by massive stars
in compact star-forming regions. High-q_{ion} galaxies like LAEs would produce
ionizing photons efficiently with a high escape fraction achieved by
density-bounded HII regions, which would significantly contribute to cosmic
reionization at z>6.Comment: 19 pages, 11 figures. Accepted for publication in Ap
GMASS Ultradeep Spectroscopy of Galaxies at redshift z~2. I. The stellar metallicity
Context: Galaxy metallicities have been measured to redshift z~2 by gas-phase
oxygen abundances of the interstellar medium using the R23 and N2 methods.
Galaxy stellar metallicities provide crucial data for chemical evolution models
but have not been assessed reliably much outside the local Universe. Aims: We
determine the iron-abundance, stellar metallicity of star-forming galaxies
(SFGs) at redshift z~2, observed as part of the Galaxy Mass Assembly ultra-deep
Spectroscopic Survey (GMASS). Methods: We compute the equivalent width of a
rest-frame mid-ultraviolet, photospheric absorption-line index, the 1978 index
found to vary monotonically with stellar metallicity by Rix and collaborators.
We normalise and combine 75 SFG spectra from the GMASS survey to produce a
spectrum corresponding to a total integration time 1652.5 hours (and a
signal-to-noise ratio ~100 for our 1.5 angstrom binning) of FORS2 spectroscopic
observations at the Very Large Telescope. Results: We measure an
iron-abundance, stellar metallicity of log (Z/Zsolar) = -0.574+/-0.159 for our
spectrum representative of a galaxy of stellar mass 9.4 x 10^9 Msolar assuming
a Chabrier IMF. We find that the R04 model SFG spectrum for log (Z/Zsolar) =
-0.699 solar metallicity provides the best description of our GMASS coadded
spectrum. For similar galaxy stellar mass, our stellar metallicity is ~0.25 dex
lower than the oxygen-abundance, gas-phase metallicity quantified by Erb and
collaborators for UV-selected star-forming galaxies at z=2. Conclusions: We
conclude that we are witnessing the establishment of a light-element
overabundance in galaxies as they are being formed at redshift z~2. Our
measurements are reminiscent of the alpha-element enhancement seen in
low-redshift, galactic bulges and early-type galaxies. (Abridged)Comment: Accepted for publication in Astronomy and Astrophysics on 18 December
2007, 9 pages, 8 figures, aa.bst and aa.cls A&A style file
A Multiwavelength Study of a Sample of 70 micron Selected Galaxies in the COSMOS Field I: Spectral Energy Distributions and Luminosities
We present a large robust sample of 1503 reliable and unconfused 70microm
selected sources from the multiwavelength data set of the Cosmic Evolution
Survey (COSMOS). Using the Spitzer IRAC and MIPS photometry, we estimate the
total infrared luminosity, L_IR (8--1000 microns), by finding the best fit
template from several different template libraries. The long wavelength 70 and
160 micron data allow us to obtain a reliable estimate of L_IR, accurate to
within 0.2 and 0.05 dex, respectively. The 70 micron data point enables a
significant improvement over the luminosity estimates possible with only a 24
micron detection. The full sample spans a wide range in L_IR, L_IR ~ 10^8-10^14
L_sun, with a median luminosity of 10^11.4 L_sun. We identify a total of 687
luminous, 303 ultraluminous, and 31 hyperluminous infrared galaxies (LIRGs,
ULIRGs, and HyLIRGs) over the redshift range 0.01<z<3.5 with a median redshift
of 0.5. Presented here are the full spectral energy distributions for each of
the sources compiled from the extensive multiwavelength data set from the
ultraviolet (UV) to the far-infrared (FIR). Using SED fits we find possible
evidence for a subset of cooler ultraluminous objects than observed locally.
However, until direct observations at longer wavelengths are obtained, the peak
of emission and the dust temperature cannot be well constrained. We use these
SEDs, along with the deep radio and X-ray coverage of the field, to identify a
large sample of candidate active galactic nuclei (AGN). We find that the
fraction of AGN increases strongly with L_IR, as it does in the local universe,
and that nearly 70% of ULIRGs and all HyLIRGs likely host a powerful AGN.Comment: 31 pages including 31 figures and 6 tables. Accepted for publication
in ApJ. The full resolution version is available here:
http://www.ifa.hawaii.edu/~jeyhan/paperI/Kartaltepe_70mic_PaperI.pd
The star formation history of mass-selected galaxies in the COSMOS field
We explore the evolution of the specific star formation rate (SSFR) for
3.6um-selected galaxies of different M_* in the COSMOS field. The average SFR
for sub-sets of these galaxies is estimated with stacked 1.4GHz radio continuum
emission. We separately consider the total sample and a subset of galaxies (SF)
that shows evidence for substantive recent star formation in the rest-frame
optical SED. At 0.2<z<3 both populations show a strong and M_*-independent
decrease in their SSFR towards z=0.2, best described by a power- law (1+z)^n,
where n~4.3 for all galaxies and n~3.5 for SF sources. The decrease appears to
have started at z>2, at least above 4x10^10M_Sun where our conclusions are most
robust. We find a tight correlation with power-law dependence, SSFR (M_*)^beta,
between SSFR and M_* at all z. It tends to flatten below ~10^10M_Sun if
quiescent galaxies are included; if they are excluded a shallow index beta_SFG
-0.4 fits the correlation. On average, higher M_* objects always have lower
SSFRs, also among SF galaxies. At z>1.5 there is tentative evidence for an
upper SSFR-limit that an average galaxy cannot exceed. It is suggested by a
flattening of the SSFR-M_* relation (also for SF sources), but affects massive
(>10^10M_Sun) galaxies only at the highest z. Below z=1.5 there thus is no
direct evidence that galaxies of higher M_* experience a more rapid waning of
their SSFR than lower M_* SF systems. In this sense, the data rule out any
strong 'downsizing'. We combine our results with recent measurements of the
galaxy (stellar) mass function in order to determine the characteristic mass of
a SF galaxy (M_*=10^(10.6\pm0.4)M_Sun). In this sense, too, there is no
'downsizing'. Our analysis constitutes the most extensive SFR density
determination with a single technique to z=3. Recent Herschel results are
consistent with our results, but rely on far smaller samples.Comment: 37 pages, 14 figures, 7 tables; accepted for publication in the
Astrophysical Journal; High resolution versions of all figures available at
www.mpia-hd.mpg.de/homes/karim/research.htm
The History of Galaxy Formation in Groups: An Observational Perspective
We present a pedagogical review on the formation and evolution of galaxies in
groups, utilizing observational information from the Local Group to galaxies at
z~6. The majority of galaxies in the nearby universe are found in groups, and
galaxies at all redshifts up to z~6 tend to cluster on the scale of nearby
groups (~1 Mpc). This suggests that the group environment may play a role in
the formation of most galaxies. The Local Group, and other nearby groups,
display a diversity in star formation and morphological properties that puts
limits on how, and when, galaxies in groups formed. Effects that depend on an
intragroup medium, such as ram-pressure and strangulation, are likely not major
mechanisms driving group galaxy evolution. Simple dynamical friction arguments
however show that galaxy mergers should be common, and a dominant process for
driving evolution. While mergers between L_* galaxies are observed to be rare
at z < 1, they are much more common at earlier times. This is due to the
increased density of the universe, and to the fact that high mass galaxies are
highly clustered on the scale of groups. We furthermore discus why the local
number density environment of galaxies strongly correlates with galaxy
properties, and why the group environment may be the preferred method for
establishing the relationship between properties of galaxies and their local
density.Comment: Invited review, 16 pages, to be published in ESO Astrophysics
Symposia: "Groups of Galaxies in the Nearby Universe", eds. I. Saviane, V.
Ivanov, J. Borissov
Normal-state conductivity in underdoped La_{2-x}Sr_xCuO_4 thin films: Search for nonlinear effects related to collective stripe motion
We report a detailed study of the electric-field dependence of the
normal-state conductivity in La_{2-x}Sr_xCuO_4 thin films for two
concentrations of doped holes, x=0.01 and 0.06, where formation of diagonal and
vertical charged stripes was recently suggested. In order to elucidate whether
high electric fields are capable of depinning the charged stripes and inducing
their collective motion, we have measured current-voltage characteristics for
various orientations of the electric field with respect to the crystallographic
axes. However, even for the highest possible fields (~1000 V/cm for x=0.01 and
\~300 V/cm for x=0.06) we observed no non-linear-conductivity features except
for those related to the conventional Joule heating of the films. Our analysis
indicates that Joule heating, rather than collective electron motion, may also
be responsible for the non-linear conductivity observed in some other 2D
transition-metal oxides as well. We discuss that a possible reason why moderate
electric fields fail to induce a collective stripe motion in layered oxides is
that fairly flexible and compressible charged stripes can adjust themselves to
the crystal lattice and individual impurities, which makes their pinning much
stronger than in the case of conventional rigid charge-density waves.Comment: 10 pages, 10 figures, accepted for publication in Phys. Rev.
On the Stellar Populations and Evolution of Star-Forming Galaxies at 6.3 < z < 8.6
We study the physical characteristics of galaxies at 6.3 < z < 8.6, selected
from deep near-infrared imaging with the Wide Field Camera 3 (WFC3) on board
the Hubble Space Telescope. Accounting for the photometric scatter using
simulations, galaxies at z ~ 7 have bluer UV colors compared to typical local
starburst galaxies at > 4 sigma confidence. Although these colors necessitate
young ages (<100 Myr), low or zero dust attenuation, and low metallicities,
these are explicable by normal (albeit unreddened) stellar populations, with no
evidence for near-zero metallicities and/or top-heavy initial mass functions.
The age of the Universe at these redshifts limits the amount of stellar mass in
late-type populations, and the WFC3 photometry implies galaxy stellar masses ~
10^8 - 10^9 Msol for Salpeter initial mass functions to a limiting magnitude of
M_1500 ~ -18. The masses of ``characteristic'' (L*) z > 7 galaxies are smaller
than those of L* Lyman break galaxies (LBGs) at lower redshifts, and are
comparable to less evolved galaxies selected on the basis of their Lyman alpha
emission at 3 < z < 6, implying that the 6.3 < z < 8.6 galaxies are the
progenitors of more evolved galaxies at lower redshifts. We estimate that Lyman
alpha emission is able to contribute to the observed WFC3 colors of galaxies at
these redshifts, with an estimated typical line flux of ~ 10^-18 erg s^-1
cm^-2, roughly a factor of four below currently planned surveys. The integrated
UV specific luminosity for the detected galaxies at z ~ 7 and z ~ 8 is within
factors of a few of that required to reionize the IGM assuming low clumping
factors, implying that in order to reionize the Universe galaxies at these
redshifts have a high ( ~ 50%) escape fraction of Lyman continuum photons,
possibly substantiated by the very blue colors of this population.Comment: Accepted to the Astrophysical Journal; replaced with accepted
version. Minor modifications to sample, conclusions are unchange
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