1,214 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
Spitzer IRAC confirmation of z_850-dropout galaxies in the Hubble Ultra Deep Field: stellar masses and ages at z~7
Using Spitzer IRAC mid-infrared imaging from the Great Observatories Origins
Deep Survey, we study z_850-dropout sources in the Hubble Ultra Deep Field.
After carefully removing contaminating flux from foreground sources, we clearly
detect two z_850-dropouts at 3.6 micron and 4.5 micron, while two others are
marginally detected. The mid-infrared fluxes strongly support their
interpretation as galaxies at z~7, seen when the Universe was only 750 Myr old.
The IRAC observations allow us for the first time to constrain the rest-frame
optical colors, stellar masses, and ages of the highest redshift galaxies.
Fitting stellar population models to the spectral energy distributions, we find
photometric redshifts in the range 6.7-7.4, rest-frame colors U-V=0.2-0.4,
V-band luminosities L_V=0.6-3 x 10^10 L_sun, stellar masses 1-10 x 10^9 M_sun,
stellar ages 50-200 Myr, star formation rates up to ~25 M_sun/yr, and low
reddening A_V<0.4. Overall, the z=7 galaxies appear substantially less massive
and evolved than Lyman break galaxies or Distant Red Galaxies at z=2-3, but
fairly similar to recently identified systems at z=5-6. The stellar mass
density inferred from our z=7 sample is rho* = 1.6^{+1.6}_{-0.8} x 10^6 M_sun
Mpc^-3 (to 0.3 L*(z=3)), in apparent agreement with recent cosmological
hydrodynamic simulations, but we note that incompleteness and sample variance
may introduce larger uncertainties. The ages of the two most massive galaxies
suggest they formed at z>8, during the era of cosmic reionization, but the star
formation rate density derived from their stellar masses and ages is not nearly
sufficient to reionize the universe. The simplest explanation for this
deficiency is that lower-mass galaxies beyond our detection limit reionized the
universe.Comment: 4 pages, 3 figures, emulateapj, Accepted for publication in ApJ
Letter
A synoptic view of solar transient evolution in the inner heliosphere using the Heliospheric Imagers on STEREO
By exploiting data from the STEREO/heliospheric imagers (HI) we extend a well-established technique developed for coronal analysis by producing time-elongation plots that reveal the nature of solar transient activity over a far more extensive region of the heliosphere than previously possible from coronagraph images. Despite the simplicity of these plots, their power in demonstrating how the plethora of ascending coronal features observed near the Sun evolve as they move antisunward is obvious. The time-elongation profile of a transient tracked by HI can, moreover, be used to establish its angle out of the plane-of-the-sky; an illustration of such analysis reveals coronal mass ejection material that can be clearly observed propagating out to distances beyond 1AU. This work confirms the value of the time-elongation format in identifying/characterising transient activity in the inner heliosphere, whilst also validating the ability of HI to continuously monitor solar ejecta out to and beyond 1A
The Stellar Population of Lyman-alpha Emitting Galaxies at z ~ 5.7
We present a study of three Lyman-alpha emitting galaxies (LAEs), selected
via a narrow-band survey in the GOODS northern field, and spectroscopically
confirmed to have redshifts of z ~ 5.65. Using HST ACS and Spitzer IRAC data,
we constrain the rest-frame UV-to-optical spectral energy distributions (SEDs)
of the galaxies. Fitting stellar population synthesis models to the observed
SEDs, we find best-fit stellar populations with masses between ~ 10^9 - 10^10
M_sun and ages between ~ 5 - 100 Myr, assuming a simple starburst star
formation history. However, stellar populations as old as 700 Myr are
admissible if a constant star formation rate model is considered. Very deep
near-IR observations may help to narrow the range of allowed models by
providing extra constraints on the rest-frame UV spectral slope. Our
narrow-band selected objects and other IRAC-detected z ~ 6 i'-dropout galaxies
have similar 3.6 um magnitudes and z' - [3.6] colors, suggesting that they
posses stellar populations of similar masses and ages. This similarity may be
the result of a selection bias, since the IRAC-detected LAEs and i'-dropouts
probably only sample the bright end of the luminosity function. On the other
hand, our LAEs have blue i' - z' colors compared to the i'-dropouts, and would
have been missed by the i'-dropout selection criterion. A better understanding
of the overlap between the LAE and the i'-dropout populations is necessary in
order to constrain the properties of the overall high-redshift galaxy
population, such as the total stellar mass density at z ~ 6.Comment: 10 pages, 8 figures. Accepted for publication in Ap
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
Predictors of locating children participants in epidemiological studies 20 years after last contact: Internet resources and longitudinal research
This study examines predictors of locating participants that were last contacted 20 years ago using public web-search directories, in order to facilitate longitudinal environmental health research. Participants (n = 3,202) resided in four distinct geographical neighborhoods in Hamilton, Ontario during childhood; they were between 15 and 17 years old when they were last contacted in 1986. Data used for tracing included available addresses, telephone numbers, given names, and parental names. Reverse and forward search strategies were used to retrieve updated contact details. 43% of the sample was traced using online directories. Following ethical approval, participants were contacted using traced data and 29% of the original cohort was located. Predictors of locating participants were: availability of paternal names, being traced to original addresses or telephone numbers, gender (male), relatively higher socioeconomic status in childhood, and not being exposed to smoking in childhood. Where participants resided in childhood was not a significant predictor of locating participants. Although 13% of the sample was traced using forward search by name, only 4% were located. For participants traced to available addresses or telephone numbers, the difference between the proportions of traced and located participants was \u3c3%. Prospective studies on children may benefit from including the listed names that pertain to each child\u27s telephone number and full parental names at recruitment, thereby increasing the likelihood of locating participants using Internet resources. Integrating the use of Internet-based public directories for cohort reconstruction can reduce financial costs related to follow-up for longitudinal research. © 2009 Springer Science+Business Media B.V
The Role of the Dust in Primeval Galaxies: A Simple Physical Model for Lyman Break Galaxies and Lyman Alpha Emitters
We explore the onset of star formation in the early Universe, exploiting the
observations of high-redshift Lyman-break galaxies (LBGs) and Lyman alpha
emitters (LAEs), in the framework of the galaxy formation scenario elaborated
by Granato et al. (2004) already successfully tested against the wealth of data
on later evolutionary stages. Complementing the model with a simple, physically
plausible, recipe for the evolution of dust attenuation in metal poor galaxies
we reproduce the luminosity functions (LFs) of LBGs and of LAEs at different
redshifts. This recipe yields a much faster increase with galactic age of
attenuation in more massive galaxies, endowed with higher star formation rates.
These objects have therefore shorter lifetimes in the LAE and LBG phases, and
are more easily detected in the dusty submillimeter bright phase. The short UV
bright lifetimes of massive objects strongly mitigate the effect of the fast
increase of the massive halo density with decreasing redshift, thus accounting
for the weaker evolution of the LBG LF, compared to that of the halo mass
function, and the even weaker evolution between z~6 and z~3 of the LAE LF. LAEs
are on the average expected to be younger, with lower stellar masses, and
associated to less massive halos than LBGs. Finally, we show that the
intergalactic medium can be completely reionized at redshift z~6-7 by massive
stars shining in protogalactic spheroids with halo masses from a few 10^10 to a
few 10^11 M_sun, showing up as faint LBGs with magnitude in the range
-17<M_1350<-20, without resorting to any special stellar initial mass function.Comment: 13 pages, 8 figures, uses REVTeX 4 + emulateapj.cls and apjfonts.sty.
Title changed and text revised following referee's comments. Accepted by Ap
Optical to mid-IR observations of Lyman-alpha galaxies at redshift 5 in the HUDF: a young and low mass population
High redshift galaxies selected on the basis of their strong Lyman-alpha
emission tend to be young ages and small physical sizes. We show this by
analyzing the spectral energy distribution (SED) of 9 Lyman-alpha emitting
(LAE) galaxies at 4.0 < z < 5.7 n the Hubble Ultra Deep Field (HUDF).
Rest-frame UV to optical 700A < lambda < 7500A luminosities, or upper limits,
are used to constrain old stellar populations. We derive best fit, as well as
maximally massive and maximally old, properties of all 9 objects. We show that
these faint and distant objects are all very young, being most likely only a
few millions years old, and not massive, the mass in stars being ~10^6-10^8
M_sun. Deep Spitzer Infrared Array Camera (IRAC) observations of these objects,
even in cases where objects were not detected, were crucial in constraining the
masses of these objects. The space density of these objects, ~1.25x10^-4 Mpc^-3
is comparable to previously reported space density of LAEs at moderate to high
redshifts. These Lyman-alpha galaxies show modest star formation rates of ~8
M_sun yr^-1, which is nevertheless strong enough to have allowed these galaxies
to assemble their stellar mass in less than a few x10^6 years. These sources
appear to have small physical sizes, usually smaller than 1 Kpc, and are also
rather concentrated. They are likely to be some of the least massive and
youngest high redshift galaxies observed to date.Comment: 27 pages. Accepted. To appear in The Astrophysical Journa
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