248 research outputs found
Clustering of i-dropout galaxies at z=6 in GOODS and the UDF
We measured the angular clustering at z~6 from a large sample of i-dropout
galaxies (293 with z<27.5 from GOODS and 95 with z<29.0 from the UDF). Our
largest and most complete subsample (having L>0.5L*) shows the presence of
clustering at 94% significance. For this sample we derive a (co-moving)
correlation length of r_0=4.5^{+2.1}_{-3.2} h_{72}^{-1} Mpc and bias
b=4.1^{+1.5}_{-2.6}, using an accurate model for the redshift distribution. No
clustering could be detected in the much deeper but significantly smaller UDF,
yielding b<4.4 (1 sigma). We compare our findings to Lyman break galaxies at
z=3-5 at a fixed luminosity. Our best estimate of the bias parameter implies
that i-dropouts are hosted by dark matter halos having masses of ~10^11 M_sun,
similar to that of V-dropouts at z~5. We evaluate a recent claim that at z>5
star formation might have occurred more efficiently compared to that at z=3-4.
This may provide an explanation for the very mild evolution observed in the UV
luminosity density between z=6 and z=3. Although our results are consistent
with such a scenario, the errors are too large to find conclusive evidence for
this.Comment: minor changes to match published versio
Keck Deep Fields. III. Luminosity-dependent Evolution of the Ultraviolet Luminosity and Star Formation Rate Densities at z~4, 3, and 2
We use the Keck Deep Fields UGRI catalog of z~4, 3, and 2 UV-selected
galaxies to study the evolution of the rest-frame 1700A luminosity density at
high redshift. The ability to reliably constrain the contribution of faint
galaxies is critical and our data do so as they reach to M*+2 even at z~4 and
deeper still at lower redshifts. We find that the luminosity density at high
redshift is dominated by the hitherto poorly studied galaxies fainter than L*,
and, indeed, the the bulk of the UV light in the high-z Universe comes from
galaxies in the luminosity range L=0.1-1L*. It is these faint galaxies that
govern the behavior of the total UV luminosity density. Overall, there is a
gradual rise in luminosity density starting at z~4 or earlier, followed by a
shallow peak or a plateau within z~3--1, and then followed by the well-know
plunge at lower redshifts. Within this total picture, luminosity density in
sub-L* galaxies evolves more rapidly at high redshift, z>~2, than that in more
luminous objects. However, this is reversed at lower redshifts, z<~1, a
reversal that is reminiscent of galaxy downsizing. Within the context of the
models commonly used in the observational literature, there seemingly aren't
enough faint or bright LBGs to maintain ionization of intergalactic gas even as
late as z~4. This is particularly true at earlier epochs and even more so if
the faint-end evolutionary trends we observe at z~3 and 4 continue to higher
redshifts. Apparently the Universe must be easier to reionize than some recent
studies have assumed. Nevertheless, sub-L* galaxies do dominate the total UV
luminosity density at z>~2 and this dominance further highlights the need for
follow-up studies that will teach us more about these very numerous but thus
far largely unexplored systems.Comment: Accepted for publication in the Astrophysical Journal. Abstract
abridge
Number Density of Bright Lyman-Break Galaxies at z~6 in the Subaru Deep Field
We report on the bright Lyman-break galaxies (LBGs) selected in a 767
arcmin^2 area of the Subaru Deep Field. The selection is made in the i-zR vs
zB-zR plane, where zB and zR are new bandpasses with a central wavelength of
8842A and 9841A, respectively. This set of bandpasses enables us to separate
well z~6 LBGs from foreground galaxies and Galactic cool stars. We detect 12
LBG candidates down to zR=25.4, and calculate the normalization of the
rest-frame far-ultraviolet (FUV: 1400A) luminosity function at MFUV = -21.6 to
be \phi(-21.6) = (2.6+/-0.7) x 10^{-5} mag^{-1} Mpc^{-3}. This must be the most
reliable measurement ever obtained of the number density of bright z~6 LBGs,
because it is more robust against both contamination and cosmic variance than
previous values. The FUV luminosity density contributed from LBGs brighter than
MFUV = -21.3 is (2.8+/-0.8) x 10^{24} ergs/s/Hz/Mpc^3, which is equivalent to a
star formation rate density of (3.5+/-1.0) x 10^{-4} Msun/yr/Mpc^3. Combining
our measurement with those at z<6 in the literature, we find that the FUV
luminosity density of bright galaxies increases by an order of magnitude from
z~6 to z~3 and then drops by 10^3 from z~3 to the present epoch, while the
evolution of the total luminosity density is much milder. The evolutionary
behavior of bright LBGs resembles that of luminous dusty star-forming galaxies
and bright QSOs. The redshift of z~3 appears to be a remarkable era in the
cosmic history when massive galaxies were being intensively formed.Comment: 12 pages, accepted for PASJ, a high resolution version is available
at http://hikari.astron.s.u-tokyo.ac.jp/~shima/z6LBGs
The Subaru/XMM-Newton Deep Survey (SXDS). IV. Evolution of Lya Emitters from z=3.1 to 5.7 in the 1 deg^2 Field: Luminosity Functions and AGN
We present luminosity functions (LFs) and various properties of Lya emitters
(LAEs) at z=3.1, 3.7, and 5.7, in a 1 deg^2 sky of the Subaru/XMM-Newton Deep
Survey (SXDS) Field. We obtain a photometric sample of 858 LAE candidates based
on deep Subaru/Suprime-Cam imaging data, and a spectroscopic sample of 84
confirmed LAEs from Subaru/FOCAS and VLT/VIMOS spectroscopy in a survey volume
of ~10^6 Mpc^3 with a limiting Lya luminosity of ~3x10^42 erg/s. We derive the
LFs of Lya and UV-continuum (~1500 \AA) for each redshift, taking into account
the statistical error and the field-to-field variation. We find that the
apparent Lya LF shows no significant evolution between z=3.1 and 5.7 within
factors of 1.8 and 2.7 in L* and phi*, respectively. On the other hand, the UV
LF of LAEs increases from z=3.1 to 5.7, indicating that galaxies with Lya
emission are more common at earlier epochs. We identify six LAEs with AGN
activities from our spectra combined with VLA, Spitzer, and XMM-Newton data.
Among the photometrically selected LAEs at z=3.1 and 3.7, only ~1 % show AGN
activities, while the brightest LAEs with logL(Lya) >~ 43.4-43.6 erg/s appear
to always host AGNs. Our LAEs are bluer in UV-continuum color than dropout
galaxies, suggesting lower extinction and/or younger stellar populations. Our
stacking analyses provide upper limits to the radio luminosity and the
f(HeII)/f(Lya) line fraction, and constrain the hidden star formation
(+low-luminosity AGN) and the primordial population in LAEs.Comment: 75 pages, 27 figures; ApJS in press. High resolution version at
http://www.ociw.edu/~ouchi/work/astroph/sxds_LAEs/ouchi_SXDSLAE_ApJS.pd
Large Cosmic Variance in the Clustering Properties of Lyman Alpha Emitters at z~5
We reported in a previous paper the discovery of large-scale structure of
Lyman Alpha emitters (LAEs) at z=4.86+-0.03 with a projected size of 20 Mpc x
50 Mpc in narrow-band data of a 25' x 45' area of the Subaru Deep Field
(Omega_0=0.3, lambda_0=0.7, H0=70 km/s/Mpc). However, the surveyed area, which
corresponds to 55 Mpc x 100 Mpc, was not large enough that we can conclude that
we are seeing a typical distribution of z~5 LAEs. In this Letter, we report the
results of follow-up imaging of the same sky area using a new narrow-band
filter (NB704, lambda_c=7046 A and FWHM=100 A) to detect LAEs at z=4.79, i.e.,
LAEs lying closer to us by 39 Mpc on average than the z=4.86 objects. We detect
51 LAEs at z=4.79+-0.04 down to NB704=25.7, and find that their sky
distribution is quite different from the z=4.86 LAEs'. The clustering of z=4.79
LAEs is very weak on any scales and there is no large-scale high- contrast
structure. The shape and the amplitude of the angular correlation function are
thus largely different between the two samples. These results demonstrate a
large cosmic variance in the clustering properties of LAEs on scales of ~ 50
Mpc.Comment: 4 pages (uses emulateapj5.sty), accepted for ApJ
Effects of Dust Geometry in Lyman Alpha Galaxies at z = 4.4
Equivalent widths (EWs) observed in high-redshift Lyman alpha galaxies could
be stronger than the EW intrinsic to the stellar population if dust is present
residing in clumps in the inter-stellar medium (ISM). In this scenario,
continuum photons could be extinguished while the Lyman alpha photons would be
resonantly scattered by the clumps, eventually escaping the galaxy. We
investigate this radiative transfer scenario with a new sample of six Lyman
alpha galaxy candidates in the GOODS CDF-S, selected at z = 4.4 with
ground-based narrow-band imaging obtained at CTIO. Grism spectra from the HST
PEARS survey confirm that three objects are at z = 4.4, and that another object
contains an active galactic nuclei (AGN). If we assume the other five (non-AGN)
objects are at z = 4.4, they have rest-frame EWs from 47 -- 190 A. We present
results of stellar population studies of these objects, constraining their
rest-frame UV with HST and their rest-frame optical with Spitzer. Out of the
four objects which we analyzed, three objects were best-fit to contain stellar
populations with ages on the order of 1 Myr and stellar masses from 3 - 10 x
10^8 solar masses, with dust in the amount of A_1200 = 0.9 - 1.8 residing in a
quasi-homogeneous distribution. However, one object (with a rest EW ~ 150 A)
was best fit by an 800 Myr, 6.6 x 10^9 solar mass stellar population with a
smaller amount of dust (A_1200 = 0.4) attenuating the continuum only. In this
object, the EW was enhanced ~ 50% due to this dust. This suggests that large EW
Lyman alpha galaxies are a diverse population. Preferential extinction of the
continuum in a clumpy ISM deserves further investigation as a possible cause of
the overabundance of large-EW objects that have been seen in narrow-band
surveys in recent years.Comment: Submitted to the Astrophysical Journal. 35 pages, 7 figures and 4
table
A contribution to the selection of emission-line galaxies using narrow-band filters in the optical airglow windows
Emission line galaxies are an invaluable tool for our understanding of the
evolution of galaxies in the Universe. Imaging of deep fields with narrow-band
filters allows not only the selection of these objects, but also to infer the
line flux and the equivalent width of the emission line with some assumptions.
The narrow-band filter technique provides homogeneous samples of galaxies in
small comoving volumes in the sky. We present an analysis of the selection of
emission-line galaxies using narrow-band filters. Different methods of
observation are considered: broad-band -- narrow-band filters and two
broad-band and one narrow-band filters.
We study also the effect of several lines entering simultaneously inside the
filters (this is the case of Halpha). In each case the equations to obtain the
equivalent width and line flux from the photometry are obtained. Candidates to
emission-line objects are selected by their color excess in a magnitude-color
diagram. For different narrow-band filters, we compute the mean colors of stars
and galaxies, showing that, apart from galaxies, some types of stars could be
selected with certain filter sets. We show how to compute the standard
deviation of the colors of the objects even in the usual case when there are
not enough objects to determine the standard deviation from the data. We
present also helpful equations to compute the narrow-band and the broad-band
exposure times in order to obtain minimum dispersion in the ratio of fluxes of
both bands with minimum total exposure time.Comment: Accepted for publication in PASP 48 pages, 10 figures Corrected
typos, fixed references. Updated reference to T
The DEEP2 Redshift Survey: Lyman Alpha Emitters in the Spectroscopic Database
We present the first results of a search for Lyman-alpha emitters (LAEs) in
the DEEP2 spectroscopic database that uses a search technique that is different
from but complementary to traditional narrowband imaging surveys. We have
visually inspected ~20% of the available DEEP2 spectroscopic data and have
found nine high-quality LAEs with clearly asymmetric line profiles and an
additional ten objects of lower quality, some of which may also be LAEs. Our
survey is most sensitive to LAEs at z=4.4-4.9 and that is indeed where all but
one of our high-quality objects are found. We find the number density of our
spectroscopically-discovered LAEs to be consistent with those found in
narrowband imaging searches. The combined, averaged spectrum of our nine
high-quality objects is well fit by a two-component model, with a second,
lower-amplitude component redshifted by ~420 km/s with respect to the primary
Lyman-alpha line, consistent with large-scale outflows from these objects. We
conclude by discussing the advantages and future prospects of blank-sky
spectroscopic surveys for high-z LAEs.Comment: Accepted for publication in Ap
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