3,279 research outputs found
A New Measurement of the Stellar Mass Density at z~5: Implications for the Sources of Cosmic Reionization
We present a new measurement of the integrated stellar mass per comoving
volume at redshift 5 determined via spectral energy fitting drawn from a sample
of 214 photometrically-selected galaxies with z'<26.5 in the southern GOODS
field. Following procedures introduced by Eyles et al. (2005), we estimate
stellar masses for various sub-samples for which reliable and unconfused
Spitzer IRAC detections are available. A spectroscopic sample of 14 of the most
luminous sources with =4.92 provides a firm lower limit to the stellar mass
density of 1e6 Msun/Mpc^3. Several galaxies in this sub-sample have masses of
order 10^11 Msun implying significant earlier activity occurred in massive
systems. We then consider a larger sample whose photometric redshifts in the
publicly-available GOODS-MUSIC catalog lie in the range 4.4 <z 5.6. Before
adopting the GOODS-MUSIC photometric redshifts, we check the accuracy of their
photometry and explore the possibility of contamination by low-z galaxies and
low-mass stars. After excising probable stellar contaminants and using the z'-J
color to exclude any remaining foreground red galaxies, we conclude that 196
sources are likely to be at z~5. The implied mass density from the unconfused
IRAC fraction of this sample, scaled to the total available, is 6e6 Msun/Mpc^3.
We discuss the uncertainties as well as the likelihood that we have
underestimated the true mass density. Including fainter and quiescent sources
the total integrated density could be as high as 1e7 Msun/Mpc^3. Using the
currently available (but highly uncertain) rate of decline in the star
formationhistory over 5 <z< 10, a better fit is obtained for the assembled mass
at z~5 if we admit significant dust extinction at early times or extend the
luminosity function to very faint limits. [abridged]Comment: Accepted for Publication in ApJ, 39 page
Keck Spectroscopy of Faint 3<z<8 Lyman Break Galaxies:- Evidence for a Declining Fraction of Emission Line Sources In the Redshift Range 6<z<8
Using deep Keck spectroscopy of Lyman break galaxies selected from infrared
imaging data taken with WFC3/IR onboard the Hubble Space Telescope, we present
new evidence for a reversal in the redshift-dependent fraction of star forming
galaxies with detectable Lyman alpha emission in the redshift range 6.3 < z <
8.8. Our earlier surveys with the DEIMOS spectrograph demonstrated a
significant increase with redshift in the fraction of line emitting galaxies
over the interval 4 < z < 6, particularly for intrinsically faint systems which
dominate the luminosity density. Using the longer wavelength sensitivities of
LRIS and NIRSPEC, we have targeted 19 Lyman break galaxies selected using
recent WFC3/IR data whose photometric redshifts are in the range 6.3 < z < 8.8
and which span a wide range of intrinsic luminosities. Our spectroscopic
exposures typically reach a 5-sigma sensitivity of < 50 A for the rest-frame
equivalent width (EW) of Lyman alpha emission. Despite the high fraction of
emitters seen only a few hundred million years later, we find only 2 convincing
and 1 possible line emitter in our more distant sample. Combining with
published data on a further 7 sources obtained using FORS2 on the ESO VLT, and
assuming continuity in the trends found at lower redshift, we discuss the
significance of this apparent reversal in the redshift-dependent Lyman alpha
fraction in the context of our range in continuum luminosity. Assuming all the
targeted sources are at their photometric redshift and our assumptions about
the Lyman alpha EW distribution are correct, we would expect to find so few
emitters in less than 1% of the realizations drawn from our lower redshift
samples. Our new results provide further support for the suggestion that, at
the redshifts now being probed spectroscopically, we are entering the era where
the intergalactic medium is partially neutral.Comment: 8 pages, 5 figures, Accepted to ApJ 10/1/1
Spectroscopy of CASSOWARY gravitationally-lensed galaxies in SDSS: characterisation of an extremely bright reionization-era analog at
We present new observations of sixteen bright () gravitationally
lensed galaxies at selected from the CASSOWARY survey. Included
in our sample is the galaxy CSWA-141, one of the brightest known
reionization-era analogs at high redshift (g=20.5), with a large sSFR (31.2
Gyr) and an [OIII]+H equivalent width
(EW=730~\r{A}) that is nearly identical to the average
value expected at . In this paper, we investigate the rest-frame
UV nebular line emission in our sample with the goal of understanding the
factors that regulate strong CIII] emission. Whereas most of the sources in our
sample show weak UV line emission, we find elevated CIII] in the spectrum of
CSWA-141 (EW=4.6~\r{A}) together with detections of
other prominent emission lines (OIII], Si III], Fe II, Mg II). We
compare the rest-optical line properties of high redshift galaxies with strong
and weak CIII] emission, and find that systems with the strongest UV line
emission tend to have young stellar populations and nebular gas that is
moderately metal-poor and highly ionized, consistent with trends seen at low
and high redshift. The brightness of CSWA-141 enables detailed investigation of
the extreme emission line galaxies which become common at . We find that
gas traced by the CIII] doublet likely probes higher densities than that traced
by [OII] and [SII]. Characterisation of the spectrally resolved Mg II emission
line and several low ionization absorption lines suggests neutral gas around
the young stars is likely optically thin, potentially facilitating the escape
of ionizing radiation.Comment: 20 pages, 9 figures, Accepted for publication in MNRA
Spitzer and Hubble Constraints on the Physical Properties of the z~7 Galaxy Strongly Lensed by Abell 2218
We report the detection of a z~7 galaxy strongly lensed by the massive galaxy
cluster Abell 2218 (z=0.175) at 3.6 and 4.5 um using the Spitzer Observatory
and at 1.1 um using the Hubble Space Telescope. The new data indicate a refined
photometric redshift in the range of 6.6-6.8 depending on the presence of
Ly-alpha emission. The spectral energy distribution is consistent with having a
significant Balmer break, suggesting that the galaxy is in the poststarburst
stage with an age of at least ~50 Myr and quite possibly a few hundred Myr.
This suggests the possibility that a mature stellar population is already in
place at such a high redshift. Compared with typical Lyman break galaxies at
z~3-4, the stellar mass is an order of magnitude smaller (~10^{9} Msun), but
the specific star formation rate (star formation rate/M_{star}) is similarly
large (> 10^{-9} yr^{-1}), indicating equally vigorous star-forming activity.Comment: 11 pages, 2 figures, 2 tables; Accepted for publication in ApJ
Galaxies at z = 6 - 9 from the WFC3/IR imaging of the HUDF
We present the results of a systematic search for galaxies in the redshift
range z = 6 - 9, within the new, deep, near-infrared imaging of the Hubble
Ultra Deep Field provided by the Wide Field Camera 3 (WFC3) on HST. We have
performed full SED fitting to the optical+infrared photometry of all
high-redshift galaxy candidates detected at greater than 5-sigma in at least
one of the WFC3/IR broad-band filters. After rejection of contaminants, the
result is a sample of 49 galaxies with primary redshift solutions z > 5.9. Our
sample, selected without recourse to specific colour cuts, re-selects all but
the faintest one of the 16 z-drops selected by Oesch et al. (2009), recovers
all 5 of the Y-drops reported by Bouwens et al. (2009), and adds a further 29
galaxy candidates, of which 12 lie beyond z = 6.3, and 4 lie beyond z = 7. We
also present confidence intervals on our photometric redshift estimates, and
caution that acceptable low-redshift (z < 2) solutions exist for 28 out of the
37 galaxies at z > 6.3, and for all 8 galaxy candidates at z > 7.5.
Nevertheless, the very highest redshift candidates appear to be strongly
clustered in the field. We derive new estimates of the ultraviolet galaxy
luminosity function at z = 7 and z = 8. Where our results are most robust, at a
characteristic luminosity M(1500) ~ -19.5 (AB), we find that the comoving
number density of galaxies declines by a factor of ~ 2.5 between z = 6 and z =
7, and by a further factor of ~ 2 by z = 8. These results suggest that it is
difficult for the observed population of high-redshift star-forming galaxies to
achieve reionisation by z ~ 6 without a significant contribution from galaxies
well below the detection limits, plus alterations in the escape fraction of
ionising photons and/or continued vigorous star formation at z > 15.Comment: 25 Pages, Accepted for publication in MNRA
Lyα and CâIII] emission in z = 7â9 Galaxies: accelerated reionization around luminous star-forming systems?
We discuss new Keck/MOSFIRE spectroscopic observations of four luminous galaxies at z â 7â9 selected to have intense optical line emission by Roberts-Borsani et al. Previous follow-up has revealed Lyα in two of the four galaxies. Our new MOSFIRE observations confirm that Lyα is present in the entire sample. We detect Lyα emission in the galaxy COS-zs7-1, confirming its redshift as zLyα = 7.154, and we detect Lyα in EGS-zs8-2 at zLyα = 7.477, verifying an earlier tentative detection. The ubiquity of Lyα in this sample is puzzling given that the IGM is expected to be significantly neutral over 7 7 is expected to be strongly luminosity-dependent, with transmission accelerated in systems with intense star formation
High Scale Physics Connection to LHC Data
The existing data appears to provide hints of an underlying high scale
theory. These arise from the gauge coupling unification, from the smallness of
the neutrino masses, and via a non-vanishing muon anomaly. An overview of high
scale models is given with a view to possible tests at the Large Hadron
Collider. Specifically we discuss here some generic approaches to deciphering
their signatures. We also consider an out of the box possibility of a four
generation model where the fourth generation is a mirror generation rather than
a sequential generation. Such a scenario can lead to some remarkably distinct
signatures at the LHC.Comment: 23 pages, no figures. Based on invited lectures at the 46th Course at
the International School of Subnuclear Physics- Erice -Sicily: 29 August -7
September, 200
An Observational Test for the Anthropic Origin of the Cosmological Constant
The existence of multiple regions of space beyond the observable Universe
(within the so-called "multiverse") where the vacuum energy density takes
different values, has been postulated as an explanation for the low non-zero
value observed for it in our Universe. It is often argued that our existence
pre-selects regions where the cosmological constant is sufficiently small to
allow galaxies like the Milky Way to form and intelligent life to emerge. Here
we propose a simple empirical test for this anthropic argument within the
boundaries of the observable Universe. We make use of the fact that dwarf
galaxies formed in our Universe at redshifts as high as z~10 when the mean
matter density was larger by a factor of ~10^3 than today. Existing technology
enables to check whether planets form in nearby dwarf galaxies and globular
clusters by searching for microlensing or transit events of background stars.
The oldest of these nearby systems may have formed at z~10. If planets are as
common per stellar mass in these descendents as they are in the Milky Way
galaxy, then the anthropic argument would be weakened considerably since
planets could have formed in our Universe even if the cosmological constant was
three orders of magnitude larger than observed. For a flat probability
distribution, this would imply that the probability for us to reside in a
region where the cosmological constant obtains its observed value is lower than
\~10^{-3}. A precise version of the anthropic argument could then be ruled-out
at a confidence level of ~99.9%, which constitutes a satisfactory measure of a
good experimental test.Comment: JCAP, in pres
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