5 research outputs found
Lyman-break galaxies at z~5 -I. First significant stellar mass assembly in galaxies that are not simply z~3 LBGs at higher redshift
We determine the ensemble properties of z~5 Lyman break galaxies (LBGs)
selected as V-band dropouts to i(AB)<26.3 in the Chandra Deep Field South using
their rest-frame UV-to-visible SEDs. By matching the selection and performing
the same analysis that has been used for z~3 samples, we show clear differences
in the properties of two samples of LBGs which are separated by ~1Gyr in
lookback time. We find that z~5 LBGs are typically much younger (<100Myr) and
have lower stellar masses (10^9Msol) than their z~3 counterparts. The
difference in mass is significant even when considering the presence of an
older, underlying population in both samples. Such young and moderately massive
systems dominate the luminous z~5 LBG population (>70%), whereas they comprise
<30% of LBG samples at z~3. This result is robust under all reasonable
modelling assumptions. These intense starbursts appear to be experiencing their
first (few) generations of large-scale star formation and are accumulating
their first significant stellar mass. Their dominance in luminous LBG samples
suggests that z~5 witnesses a period of wide-spread, recent galaxy formation.
As such, z~5 LBGs are the likely progenitors of the spheroidal components of
present-day massive galaxies. This is supported by their high stellar mass
surface densities, their core phase-space densities, as well as the ages of
stars in the bulge of our Galaxy and other massive systems. Their high star
formation rates per unit area suggest that these systems host outflows or winds
that enrich the intra- and inter-galactic media with metals. Their estimated
young ages are consistent with inefficient metal-mixing on galaxy-wide scales.
Therefore these galaxies may contain a significant fraction of metal-free stars
as has been proposed for z~3 LBGs (Jimenez & Haiman 2006). [Abridged]Comment: Accepted for publication in MNRAS. 21 pages, 9 postscript figures.
For a PDF file with high resolution figures, see
http://www-astro.physics.ox.ac.uk/~averma
Direct Measurement of Dust Attenuation in z approx. 1.5 Star-Forming Galaxies from 3D-HST: Implications for Dust Geometry and Star Formation Rates
The nature of dust in distant galaxies is not well understood, and until recently few direct dust measurements have been possible. We investigate dust in distant star-forming galaxies using near-infrared grism spectra of the 3D-HST survey combined with archival multi-wavelength photometry. These data allow us to make a direct comparison between dust towards star-forming regions (measured using Balmer decrements) and the integrated dust properties (derived by comparing spectral energy distributions [SEDs] with stellar population and dust models) for a statistically significant sample of distant galaxies. We select a sample of 163 galaxies between 1.36 or = 5 and measure Balmer decrements from stacked spectra. First, we stack spectra in bins of integrated stellar dust attenuation, and find that there is extra dust extinction towards star-forming regions (AV,HII is 1.81 times the integrated AV, star), though slightly lower than found for low-redshift starburst galaxies. Next, we stack spectra in bins of specific star formation rate (log sSFR), star formation rate (log SFR), and stellar mass (logM). We find that on average AV,HII increases with SFR and mass, but decreases with increasing sSFR. The amount of extra extinction also decreases with increasing sSFR and decreasing stellar mass. Our results are consistent with the two-phase dust model - in which galaxies contain both a diffuse and a stellar birth cloud dust component - as the extra extinction will increase once older stars outside the star-forming regions become more dominant. Finally, using our Balmer decrements we derive dust-corrected H(alpha) SFRs, and find evidence that SED fitting produces incorrect SFRs if very rapidly declining SFHs are included in the explored parameter space. Subject headings: dust, extinction- galaxies: evolution- galaxies: high-redshif
A CANDELS-3d-HST Synergy: Resolved Star Formation Patterns at 0.7 less than z less than 1.5
We analyze the resolved stellar populations of 473 massive star-forming galaxies at 0.7 < z < 1.5, with multiwavelength broadband imaging from CANDELS andHalpha surface brightness profiles at the same kiloparsec resolution from 3D-HST. Together, this unique data set sheds light on how the assembled stellar mass is distributed within galaxies, and where new stars are being formed. We find the Halpha morphologies to resemble more closely those observed in the ACS I band than in the WFC3 H band, especially for the larger systems. We next derive a novel prescription for Halpha dust corrections, which accounts for extra extinction toward H II regions. The prescription leads to consistent star formation rate (SFR) estimates and reproduces the observed relation between the Halpha/UV luminosity ratio and visual extinction, on both a pixel-by-pixel and a galaxy-integrated level. We find the surface density of star formation to correlate with the surface density of assembled stellar mass for spatially resolved regions within galaxies, akin to the so-called "main sequence of star formation" established on a galaxy-integrated level. Deviations from this relation toward lower equivalent widths are found in the inner regions of galaxies. Clumps and spiral features, on the other hand, are associated with enhanced H alpha equivalent widths, bluer colors, and higher specific SFRs compared to the underlying disk. Their Halpha/UV luminosity ratio is lower than that of the underlying disk, suggesting that the ACS clump selection preferentially picks up those regions of elevated star formation activity that are the least obscured by dust. Our analysis emphasizes that monochromatic studies of galaxy structure can be severely limited by mass-to-light ratio variations due to dust and spatially inhomogeneous star formation histories
Ultradeep near-infrared imaging of the HDF-south: rest-frame optical properties of high redshift galaxies
We have obtained ultradeep J_s, H and K_s near-infrared imaging of the Hubble Deep Field South WFPC2 field with the ISAAC camera on the VLT. The total integration time of 100 hours resulted in the deepest ground-based infrared observations to date and the deepest K_s-band data ever taken. This depth allows us to determine the spectral energy distributions of the high-redshift galaxies with unprecendented accuracy. Together with existing optical observations, we use the multicolor data to select high-redshift galaxies by their rest-frame optical light, and study their statistical properties and morphologies. We find a wide variety of morphologies: some are large in the rest-frame optical and resemble normal spiral galaxies, others are barely detected in the observers optical and have red NIR colors. The latter belong to a new population of galaxies at redshifts z>2, that is notably absent in the HDF-North. The spectral energy distributions of many of such red galaxies show distinct breaks, which we identify as the balmer break/4000 Angstrom break, and their contribution to the stellar mass density is estimated to be substantial. At redshift z~3, we find a clear excess of superluminious galaxies (> 5 L*_B(z = 0)), which is consistent with 1 magnitude of luminosity evolution. Overall, the results show the necessity of deep near-infrared imaging to obtain a full census of the high redshift universe