150 research outputs found
A Glimpse of the First Galaxies
The recently refurbished Hubble Space Telescope reveals a galaxy from a time
when the Universe was just 500 million years old, providing insights into the
first throes of galaxy formation and the reionization of the Universe.Comment: Invited Nature "News and Views" Commentary on Bouwens et al. 2011,
Nature, 469, 504-507; 5 pages, 1 figur
A candidate redshift z ~ 10 galaxy and rapid changes in that population at an age of 500 Myr
Searches for very-high-redshift galaxies over the past decade have yielded a
large sample of more than 6,000 galaxies existing just 900-2,000 million years
(Myr) after the Big Bang (redshifts 6 > z > 3; ref. 1). The Hubble Ultra Deep
Field (HUDF09) data have yielded the first reliable detections of z ~ 8
galaxies that, together with reports of a gamma-ray burst at z ~ 8.2 (refs 10,
11), constitute the earliest objects reliably reported to date. Observations of
z ~ 7-8 galaxies suggest substantial star formation at z > 9-10. Here we use
the full two-year HUDF09 data to conduct an ultra-deep search for z ~ 10
galaxies in the heart of the reionization epoch, only 500 Myr after the Big
Bang. Not only do we find one possible z ~ 10 galaxy candidate, but we show
that, regardless of source detections, the star formation rate density is much
smaller (~10%) at this time than it is just ~200 Myr later at z ~ 8. This
demonstrates how rapid galaxy build-up was at z ~ 10, as galaxies increased in
both luminosity density and volume density from z ~ 8 to z ~ 10. The 100-200
Myr before z ~ 10 is clearly a crucial phase in the assembly of the earliest
galaxies.Comment: 41 pages, 14 figures, 2 tables, Nature, in pres
From z>6 to z~2: Unearthing Galaxies at the Edge of the Dark Ages
Galaxies undergoing formation and evolution can now be observed over a time
baseline of some 12 Gyr. An inherent difficulty with high-redshift observations
is that the objects are very faint and the best resolution (HST) is only ~0.5
kpc. Such studies thereby combine in a highly synergistic way with the great
detail that can be obtained for nearby galaxies. 3 new developments are
highlighted. First is the derivation of stellar masses for galaxies from SEDs
using HST and now Spitzer data, and dynamical masses from both sub-mm
observations of CO lines and near-IR observations of optical lines like Halpha.
A major step has been taken with evidence that points to the z~2-3 LBGs having
masses that are a few x 10^10 Msolar. Second is the discovery of a population
of evolved red galaxies at z~2-3 which appear to be the progenitors of the more
massive early-type galaxies of today, with dynamical masses around a few x
10^11 Msolar. Third are the remarkable advances that have occurred in
characterizing dropout galaxies to z~6 and beyond, < 1 Gyr from recombination.
The HST ACS has played a key role here, with the dropout technique being
applied to i & z images in several deep ACS fields, yielding large samples of
these objects. This has allowed a detailed determination of their properties
and meaningful comparisons against lower-z samples. The use of cloning
techniques has overcome many of the strong selection biases affecting the study
of these objects. A clear trend of size with redshift has been identified, and
its impact on the luminosity density and SFR estimated. There is a significant
though modest decrease in the SFR from z~2.5 to z~6. The latest data also allow
for the first robust determination of the LF at z~6. Finally, the latest UDF
ACS and NICMOS data has resulted in the detection of some galaxies at z~7-8.Comment: 18 pages, 8 figures. To appear in Penetrating Bars through Masks of
Cosmic Dust: The Hubble Tuning Fork Strikes a New Note, eds. D. Block, K.
Freeman, R. Groess, I. Puerari, & E.K. Block (Dordrecht: Kluwer), in pres
z ⌠2â9 Galaxies Magnified by the Hubble Frontier Field Clusters. II. Luminosity Functions and Constraints on a Faint-end Turnover
We present new determinations of the rest-UV luminosity functions (LFs) at z = 2â9 to extremely low luminosities (>â14 mag) from a sample of >2500 lensed galaxies found behind the Hubble Frontier Fields (HFF) clusters. For the first time, we present faint-end slope results from lensed samples that are fully consistent with blank-field results over the redshift range z = 2â9, while reaching to much lower luminosities than possible from the blank-field studies. Combining the deep lensed sample with the large blank-field samples allows us to set tight constraints on the faint-end slope α of the z = 2â9 UV LFs and its evolution. We find a smooth flattening in α from â2.28 ± 0.10 (z = 9) to â1.53 ± 0.03 (z = 2) with cosmic time (dα/dz = â0.11 ± 0.01), fully consistent with dark matter halo buildup. We utilize these new results to present new measurements of the evolution in the UV luminosity density Ï
UV
brighter than â13 mag from z ⌠9 to z ⌠2. Accounting for the star formation rate (SFR) densities to faint luminosities implied by our LF results, we find that unobscured star formation dominates the SFR density at z âł 4, with obscured star formation dominant thereafter. Having shown we can quantify the faint-end slope α of the LF accurately with our lensed HFF samples, we also quantify the apparent curvature in the shape of the UV LF through a curvature parameter ÎŽ. The constraints on the curvature ÎŽ strongly rule out the presence of a turn-over brighter than â13.1 mag at z ⌠3, â14.3 mag at z ⌠6, and â15.5 mag at all other redshifts between z ⌠9 and z ⌠2
Mean Hα+[N ii]+[S ii] EW inferred for star-forming galaxies at z  âŒÂ 5.1â5.4 using high-quality Spitzer /IRAC photometry
Recent Spitzer/InfraRed Array Camera (IRAC) photometric observations have revealed that rest-frame optical emission lines contribute significantly to the broad-band fluxes of high-redshift galaxies. Specifically, in the narrow redshift range z ⌠5.1â5.4 the [3.6]â[4.5] colour is expected to be very red, due to contamination of the 4.5 ÎŒm band by the dominant Hα line, while the 3.6 ÎŒm filter is free of nebular emission lines. We take advantage of new reductions of deep Spitzer/IRAC imaging over the Great Observatories Origins Deep Survey-North+South fields (LabbĂ© et al. 2015) to obtain a clean measurement of the mean Hα equivalent width (EW) from the [3.6]â[4.5] colour in the redshift range z = 5.1â5.4. The selected sources either have measured spectroscopic redshifts (13 sources) or lie very confidently in the redshift range z = 5.1â5.4 based on the photometric redshift likelihood intervals (11 sources). Our zphot = 5.1â5.4 sample and zspec = 5.10â5.40 spectroscopic sample have a mean [3.6]â[4.5] colour of 0.31 ± 0.05 and 0.35 ± 0.07 mag, implying a rest-frame EW (Hα+[NâII]+[SâII]) of 665 ± 53 and 707 ± 74 Ă
, respectively, for sources in these samples. These values are consistent albeit slightly higher than derived by Stark et al. at z ⌠4, suggesting an evolution to higher values of the Hα+[NâII]+[SâII] EW at z > 2. Using the 3.6 ÎŒm band, which is free of emission line contamination, we perform robust spectral energy distribution fitting and find a median specific star formation rate of sSFR = 17+2â517â5+2 Gyrâ1, 7+1â2Ă7â2+1Ă higher than at z ⌠2. We find no strong correlation (<2Ï) between the Hα+[NâII]+[SâII] EW and the stellar mass of sources. Before the advent of JWST, improvements in these results will come through an expansion of current spectroscopic samples and deeper Spitzer/IRAC measurements
Rapid Evolution in the Most Luminous Galaxies During the First 900 Million Years
The first 900 million years (Myr) to redshift z~6 (the first seven per cent
of the age of the Universe) remains largely unexplored for the formation of
galaxies. Large samples of galaxies have been found at z~6, but detections at
earlier times are uncertain and unreliable. It is not at all clear how galaxies
built up from the first stars when the Universe was ~300 Myr old (z~12-15) to
z~6, just 600 Myr later. Here we report the results of a search for galaxies at
z~7-8, about 700 Myr after the Big Bang, using the deepest near-infrared and
optical images ever taken. Under conservative selection criteria we find only
one candidate galaxy at z~7-8, where ten would be expected if there were no
evolution in the galaxy population between z~7-8 and z~6. Using less
conservative criteria, there are four candidates, where 17 would be expected
with no evolution. This demonstrates that very luminous galaxies are quite rare
700 Myr after the Big Bang. The simplest explanation is that the Universe is
just too young to have built up many luminous galaxies at z~7-8 by the
hierarchical merging of small galaxies.Comment: Accepted for publication in Nature, 20 pages, 5 figures, 2 tables
(includes Supplementary Information), replaced to match version in pres
Dust penetrated morphology in the high redshift Universe
Images from the Hubble Deep Field (HDF) North and South show a large
percentage of dusty, high redshift galaxies whose appearance falls outside
traditional classification systems. The nature of these objects is not yet
fully understood. Since the HDF preferentially samples restframe UV light, HDF
morphologies are not dust or `mask' penetrated. The appearance of high redshift
galaxies at near-infrared restframes remains a challenge for the New
Millennium. The Next Generation Space Telescope (NGST) could routinely provide
us with such images. In this contribution, we quantitatively determine the
dust-penetrated structures of high redshift galaxies such as NGC 922 in their
near-infrared restframes. We show that such optically peculiar objects may
readily be classified using the dust penetrated z ~ 0 templates of Block and
Puerari (1999) and Buta and Block (2001).Comment: 4 pages, 2 figures. Presented at the conference "The Link between
Stars and Cosmology", 26-30 March, 2001, Puerto Vallarta, Mexico. To be
published by Kluwer, eds. M. Chavez, A. Bressan, A. Buzzoni, and D. Mayya.
High-resolution version of Figure 2 can be found at
http://www.inaoep.mx/~puerari/conf_puertovallart
Early star-forming galaxies and the reionization of the Universe
Star forming galaxies represent a valuable tracer of cosmic history. Recent
observational progress with Hubble Space Telescope has led to the discovery and
study of the earliest-known galaxies corresponding to a period when the
Universe was only ~800 million years old. Intense ultraviolet radiation from
these early galaxies probably induced a major event in cosmic history: the
reionization of intergalactic hydrogen. New techniques are being developed to
understand the properties of these most distant galaxies and determine their
influence on the evolution of the universe.Comment: Review article appearing in Nature. This posting reflects a submitted
version of the review formatted by the authors, in accordance with Nature
publication policies. For the official, published version of the review,
please see http://www.nature.com/nature/archive/index.htm
A distortion of very--high--redshift galaxy number counts by gravitational lensing
The observed number counts of high-redshift galaxy candidates have been used
to build up a statistical description of star-forming activity at redshift z >~
7, when galaxies reionized the Universe. Standard models predict that a high
incidence of gravitational lensing will probably distort measurements of flux
and number of these earliest galaxies. The raw probability of this happening
has been estimated to be ~ 0.5 percent, but can be larger owing to
observational biases. Here we report that gravitational lensing is likely to
dominate the observed properties of galaxies with redshifts of z >~ 12, when
the instrumental limiting magnitude is expected to be brighter than the
characteristic magnitude of the galaxy sample. The number counts could be
modified by an order of magnitude, with most galaxies being part of multiply
imaged systems, located less than 1 arcsec from brighter foreground galaxies at
z ~ 2. This lens-induced association of high-redshift and foreground galaxies
has perhaps already been observed among a sample of galaxy candidates
identified at z ~ 10.6. Future surveys will need to be designed to account for
a significant gravitational lensing bias in high-redshift galaxy samples.Comment: Nature, Jan. 13, 2011 issue (in press
Spectroscopy of z ⌠6 i-dropout galaxies : frequency of Lyα emission and the sizes of Lyα-emitting galaxies
We report on deep spectroscopy, using LRIS on Keck I and FORS2 on the VLT, of a sample of 22 candidate z similar to 6 Lyman break galaxies (LBGs) selected by the (i)775 - z(850) > 1: 3 dropout criterion. Redshifts could be measured for eight objects. These redshifts are all in the range z = 5: 5-6.1, confirming the efficiency of the i775 - z850 color selection technique. Six of the confirmed galaxies show Ly alpha emission. Assuming that the 14 objects without redshifts are z similar to 6 LBGs that lack detectable Ly alpha emission lines, we infer that the fraction of Ly alpha-emitting LBGs with Ly alpha equivalent widths greater than 20 angstrom among z similar to 6 LBGs is approximate to 30%, similar to that found at z similar to 3. Every Ly alpha-emitting object in our sample is compact, with half-light radii r(hl) 97% confidence. We speculate that the small sizes of the Ly alpha-emitting LBGs are due to these objects being less massive than other LBGs at z similar to 6
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