* Astronomy

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RE: Hubble Ultra Deep Field

Blobrana · L

Title: Spitzer IRAC confirmation of z_850-dropout galaxies in the Hubble Ultra Deep Field: stellar masses and ages at z~7
Authors: Ivo labbe (1), Rychard Bouwens (2), G.D. Illingworth (2), M. Franx (3) ((1) Carnegie Observatories, (2) UC Santa Cruz, (3) Leiden Observatory)

Using Spitzer IRAC mid-infrared imaging from the Great Observatories Origins Deep Survey, we study z850-dropout sources in the Hubble Ultra Deep Field. After carefully removing contaminating flux from foreground sources, we clearly detect two z850-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 million years old. The IRAC observations allow us for the first time to constrain the rest-frame optical colours, 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 colours U-V=0.2-0.4, V-band luminosities LV=0.6-3 x 10^10 solar luminosity, stellar masses 1-10 x 10^9 solar mass, stellar ages 50-200 million years, star formation rates up to ~25 solar masses per year, and low reddening AV<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 solar masses 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 reionisation, but the star formation rate density derived from their stellar masses and ages is not nearly sufficient to reionise the universe. The simplest explanation for this deficiency is that lower-mass galaxies beyond our detection limit reionised the universe.

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Title: The Hubble Ultra Deep Field
Authors: Steven V. W. Beckwith, Massimo Stiavelli, Anton M. Koekemoer, John A. R. Caldwell, Henry C. Ferguson, Richard Hook, Ray A. Lucas, Louis E. Bergeron, Michael Corbin, Shardha Jogee, Nino Panagia, Massimo Robberto, Patricia Royle, Rachel S. Somerville, Megan Sosey

This paper presents the Hubble Ultra Deep Field (HUDF), a one million second exposure of an 11 square minute-of-arc region in the southern sky with the Hubble Space Telescope. The exposure time was divided among four filters, F435W (B435), F606W (V606), F775W (i775), and F850LP (z850), to give approximately uniform limiting magnitudes mAB~29 for point sources. The image contains at least 10,000 objects presented here as a catalogue. Few if any galaxies at redshifts greater than ~4 resemble present day spiral or elliptical galaxies. Using the Lyman break dropout method, we find 504 B-dropouts, 204 V-dropouts, and 54 i-dropouts. Using these samples that are at different redshifts but derived from the same data, we find no evidence for a change in the characteristic luminosity of galaxies but some evidence for a decrease in their number densities between redshifts of 4 and 7. The ultraviolet luminosity density of these samples is dominated by galaxies fainter than the characteristic luminosity, and the HUDF reveals considerably more luminosity than shallower surveys. The apparent ultraviolet luminosity density of galaxies appears to decrease from redshifts of a few to redshifts greater than 6. The highest redshift samples show that star formation was already vigorous at the earliest epochs that galaxies have been observed, less than one billion years after the Big Bang.

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-- Edited by Blobrana at 09:06, 2006-07-31

Blobrana · L

Spoiler: Expand (1911 X 1912)

Part of middle of the Hubble Ultra Deep Field, 255 X 255 pixels in the original HUDF, (cropped from its original 823 X 823 pixels)