1,721,253 research outputs found
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Sizing Up Ly Alpha and Lyman Break Galaxies
No full textWe measure the sizes for a sample of 174 Ly alpha-selected galaxies with broadband imaging with the Hubble Space Telescope. Over the redshift range 2.25 < z < 6, Ly alpha-selected galaxies have a characteristic, constant, small size in rest-frame ultraviolet (UV) light. Coupled with a characteristic star Formation intensity (i.e., UV luminosity per unit area), this can explain their non-evolving ultraviolet continuum luminosity function. This is in contrast to Lyman break galaxies (LBGs) over the same redshift range, which have been previously shown to increase in linear size as H(z) (1). The compact physical size seems to be a critical determining factor in whether a galaxy will show Ly alpha emission or not. The L-* of LBGs and its evolution with redshift can be derived from a simple model where the star Formation intensity has an upper limit set by feedback processes, independent of redshift. The increase in L-* of LBGs is mainly driven by the increase in linear size over redshifts for z = 2-7. Since Ly alpha galaxies do not grow in linear size, they do not show an increase in L-*.US National Science Foundation AST-0808165Astronom
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Rapid Decline Of Ly Alpha Emission Toward The Reionization Era
No full textThe observed deficit of strongly Ly alpha emitting galaxies at z > 6.5 is attributed to increasing neutral hydrogen in the intergalactic medium (IGM) and/or to the evolving galaxy properties. To investigate this we have performed very deep near-IR spectroscopy of z greater than or similar to 7 galaxies using MOSFIRE on the Keck-I Telescope. We measure the Lya fraction at z similar to 8 using two methods. First, we derived N-Ly alpha/N-tot directly, using extensive simulations to correct for incompleteness. Second, we used a Bayesian formalism (introduced by Treu et al.) that compares the z > 7 galaxy spectra to models of the Ly alpha equivalent width (WLy alpha) distribution at z similar to 6. We explored two simple evolutionary scenarios: pure number evolution where Lya is blocked in some fraction of galaxies (perhaps due to the IGM being opaque along only some fraction of sightlines) and uniform dimming evolution where Ly alpha is attenuated in all galaxies by a constant factor (perhaps owing to processes from galaxy evolution or a slowly increasing IGM opacity). The Bayesian formalism places stronger constraints compared with the direct method. Combining our data with that in the literature, we find that at z similar to 8 the Lya fraction has dropped by a factor of > 3 (84% confidence interval) using both the dimming and number evolution scenarios, compared to the z similar to 6 values. Furthermore, we find a tentative positive Bayesian evidence favoring the number evolution scenario over dimming evolution, extending trends observed at z less than or similar to 7 to higher redshift. A comparison of our results with theoretical models implies the IGM volume averaged neutral hydrogen fraction greater than or similar to 0.3, suggesting that we are likely witnessing reionization in progress at z similar to 8.HST program by NASA through STScI GO-12060Astronom
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The Role Of Bulge Formation In The Homogenization Of Stellar Populations At Z Similar To 2 As Revealed By Internal Color Dispersion In CANDELS
No full textWe use data from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey to study how the spatial variation in the stellar populations of galaxies relates to the formation of galaxies at 1.5 < z < 3.5. We use the internal color dispersion (ICD), measured between the rest-frame UV and optical bands, which is sensitive to age (and dust attenuation) variations in stellar populations. The ICD shows a relation with the stellar masses and morphologies of the galaxies. Galaxies with the largest variation in their stellar populations as evidenced by high ICD have disk-dominated morphologies (with Sersic indexes < 2) and stellar masses between 10 < log (M/M-circle dot) < 11. There is a marked decrease in the ICD as the stellar mass and/or the Sersic index increases. By studying the relations between the ICD and other galaxy properties including size, total color, star formation rate, and dust attenuation, we conclude that the largest variations in stellar populations occur in galaxies where the light from newly, high star-forming clumps contrasts older stellar disk populations. This phase reaches a peak for galaxies only with a specific stellar mass range, 10 < log (M/M-circle dot) < 11, and prior to the formation of a substantial bulge/spheroid. In contrast, galaxies at higher or lower stellar masses and/or higher Sersic index (n > 2) show reduced ICD values, implying a greater homogeneity of their stellar populations. This indicates that if a galaxy is to have a quiescent bulge along with a star-forming disk, typical of Hubble sequence galaxies, this is most common for stellar masses 10 < log (M/M-circle dot) < 11 and when the bulge component remains relatively small (n < 2).NASA NAS5-26555NSF AST-0808133Astronom
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To Stack Or Not To Stack: Spectral Energy Distribution Properties Of Ly Alpha-Emitting Galaxies At Z=2.1
Full text linkWe use the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS) GOODS-S multi-wavelength catalog to identify counterparts for 20 Ly alpha emitting (LAE) galaxies at z = 2.1. We build several types of stacked spectral energy distributions (SEDs) of these objects. We combine photometry to form average and median flux-stacked SEDs, and postage-stamp images to form average and median image-stacked SEDs. We also introduce scaled flux stacks that eliminate the influence of variation in overall brightness. We use the SED fitting code SpeedyMC to constrain the physical properties of individual objects and stacks. Our LAEs at z = 2.1 have stellar masses ranging from 2 x 10(7) M-circle dot to 8 x 10(9) M-circle dot (median = 3 x 10(8) M-circle dot), ages ranging from 4 Myr to 500 Myr (median = 100 Myr), and E(B - V) between 0.02 and 0.24 (median = 0.12). Although still low, this represents significantly more dust reddening than has been reported for LAEs at higher redshifts. We do not observe strong correlations between Ly alpha equivalent width (EW) and age or E(B - V). The Ly alpha radiative transfer (q) factors of our sample are predominantly close to one and do not correlate strongly with EW or E(B - V). The absence of strong correlations with EW or q implies that Ly alpha radiative transfer is highly anisotropic and/or prevents Ly alpha photons from scattering in dusty regions. The SED parameters of the flux stacks match the average and median values of the individual objects, with the flux-scaled median SED performing best with uncertainties reduced by a factor of two. Median image-stacked SEDs provide a poor representation of the median individual object, and none of the stacking methods capture the large dispersion of LAE properties.NASA NAS5-26555NASA through a grant from the Space Telescope Science Institute 12060.57, 12445.56Eberly College of ScienceOffice of the Senior Vice President for Research at the Pennsylvania State UniversityNational Science Foundation AST-1055919Astronom
the UV Continuum of Z > 1 Star-forming Galaxies in the Hubble Ultraviolet Ultradeep Field
Full text linkWe estimate the UV continuum slope, beta, for 923 galaxies in the range 1 < z < 8 in the Hubble Ultradeep Field (HUDF). These data include 460 galaxies at 1 < z < 2 down to an absolute magnitude M-UV = -14(similar to 0.006L(z=1)(*); 0.02L(z=0)(*)), comparable to dwarf galaxies in the local universe. We combine deep HST/UVIS photometry in F225W, F275W, F336W wavebands (UVUDF) with recent data from HST/WFC3/IR (HUDF12). Galaxies in the range 1 < z < 2 are significantly bluer than local dwarf galaxies. We find theirmean (median) values (beta)= -1.382(-1.830)+/- 0.002 (random)+/- 0.1 (systematic). We find comparable scatter in beta (standard deviation = 0.43) to local dwarf galaxies and 30% larger scatter than z > 2 galaxies. We study the trends of beta with redshift and absolute magnitude for binned sub-samples and find a modest color-magnitude relation, d beta/dM= -0.11 +/- 0.01, and no evolution in d beta/dM with redshift. A modest increase in dust reddening with redshift and luminosity, Delta E(B -V) similar to 0.1, and a comparable increase in the dispersion of dust reddening atz < 2, appears likely to explain the observed trends. At z > 2, we find trends that are consistent with previous works; combining our data with the literature in the range 1 < z < 8, we find a color evolution with redshift, d beta/dz=-0.09 +/- 0.01 for low luminosity (0.05 L*z=3), and d beta/dz = -0.06 +/- 0.01 for medium luminosity (0.25 L-z=3(*)) galaxies.HST Program GO-12534NASA through grants from the Space Telescope Science InstituteNational Science Foundation 1055919Astronom
The Dynamical Masses, Densities, And Star Formation Scaling Relations Of Ly Alpha Galaxies
No full textWe present the first dynamical mass measurements for Ly alpha galaxies at high redshift, based on velocity dispersion measurements from rest-frame optical emission lines and size measurements from Hubble Space Telescope imaging, for nine galaxies drawn from four surveys. We use these measurements to study Ly alpha galaxies in the context of galaxy scaling relations. The resulting dynamical masses range from 109 to 10(10) M-circle dot. We also fit stellar population models to our sample and use them to place the Ly alpha sample on a stellar mass versus line width relation. The Ly alpha galaxies generally follow the same scaling relation as star-forming galaxies at lower redshift, although, lower stellar mass fits are also acceptable in similar to 1/3 of the Ly alpha galaxies. Using the dynamical masses as an upper limit on gas mass, we show that Ly alpha galaxies have unusually active star formation for their gas mass surface density. This behavior is consistent with what is observed in starburst galaxies, despite the typically smaller masses and sizes of the Ly alpha galaxy population. Finally, we examine the mass densities of these galaxies and show that their future evolution likely requires dissipational ("wet") merging. In short, we find that Ly alpha galaxies are low-mass cousins of larger starbursts.US National Science Foundation through NSF grant AST-0808165NASA Keck PI Data AwardW. M. Keck FoundationAstronom
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Constraining The Ly Alpha Escape Fraction With Far- Infrared Observations Of Ly Alpha Emitters
No full textWe study the far-infrared properties of 498 Ly alpha emitters (LAEs) at z = 2.8, 3.1, and 4.5 in the Extended Chandra Deep Field-South, using 250, 350, and 500 mu m data from the Herschel Multi-tiered Extragalactic Survey and 870 mu m data from the LABOCA ECDFS Submillimeter Survey. None of the 126, 280, or 92 LAEs at z = 2.8, 3.1, and 4.5, respectively, are individually detected in the far-infrared data. We use stacking to probe the average emission to deeper flux limits, reaching 1 sigma depths of similar to 0.1 to 0.4 mJy. The LAEs are also undetected at >= 3 sigma in the stacks, although a 2.5 sigma signal is observed at 870 mu m for the z = 2.8 sources. We consider a wide range of far-infrared spectral energy distributions (SEDs), including an M82 and an Sd galaxy template, to determine upper limits on the far-infrared luminosities and far-infrared-derived star formation rates of the LAEs. These star formation rates are then combined with those inferred from the Ly alpha and UV emission to determine lower limits on the LAEs' Lya escape fraction (f(esc)(Ly alpha)). For the Sd SED template, the inferred LAEs fesc(Ly alpha) are greater than or similar to 30% (1 sigma) at z = 2.8, 3.1, and 4.5, which are all significantly higher than the global fesc(Ly alpha) at these redshifts. Thus, if the LAEs fesc(Ly alpha) follows the global evolution, then they have warmer far-infrared SEDs than the Sd galaxy template. The average and M82 SEDs produce lower limits on the LAE fesc(Ly alpha) of similar to 10%-20% (1 sigma), all of which are slightly higher than the global evolution of fesc(Ly alpha), but consistent with it at the 2 sigma-3 sigma level.European Organisation for Astronomical Research in the Southern Hemisphere, Chile 078.F-9028(A), 079.F-9500(A), 080.A-3023(A), 081.F-9500(A)CSA (Canada)NAOC (China)CEA (France)CNES (France)CNRS (France)ASI (Italy)MCINN (Spain)SNSB (Sweden)STFC (UK)UKSA (UK)NASA (USA)Astronom
The Relation Between Star Formation Rate And Stellar Mass For Galaxies At 3.5 <= Z <= 6.5 In CANDELS
Full text linkDistant star-forming galaxies show a correlation between their star formation rates (SFRs) and stellar masses, and this has deep implications for galaxy formation. Here, we present a study on the evolution of the slope and scatter of the SFR-stellar mass relation for galaxies at 3.5 <= z <= 6.5 using multi-wavelength photometry in GOODS-S from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) and Spitzer Extended Deep Survey. We describe an updated, Bayesian spectral-energy distribution fitting method that incorporates effects of nebular line emission, star formation histories that are constant or rising with time, and different dust-attenuation prescriptions (starburst and Small Magellanic Cloud). From z = 6.5 to z = 3.5 star-forming galaxies in CANDELS follow a nearly unevolving correlation between stellar mass and SFR that follows SFR similar to M-a* with a = 0.54 +/- 0.16 at z similar to 6 and 0.70 +/- 0.21 at z similar to 4. This evolution requires a star formation history that increases with decreasing redshift (on average, the SFRs of individual galaxies rise with time). The observed scatter in the SFR-stellar mass relation is tight, sigma(log SFR/M-circle dot yr(-1)) < 0.3-0.4 dex, for galaxies with log M*/M-circle dot > 9 dex. Assuming that the SFR is tied to the net gas inflow rate (SFR similar to M-circle dot gas), then the scatter in the gas inflow rate is also smaller than 0.3-0.4 dex for star-forming galaxies in these stellar mass and redshift ranges, at least when averaged over the timescale of star formation. We further show that the implied star formation history of objects selected on the basis of their co-moving number densities is consistent with the evolution in the SFR-stellar mass relation.HST Program GO-12060Astronom
CANDELS Observations Of The Environmental Dependence Of The Color-Mass-Morphology Relation At Z=1.6
Full text linkWe study the environmental dependence of color, stellar mass, and morphology by comparing galaxies in a forming cluster to those in the field at z = 1.6 with Hubble Space Telescope near-infrared imaging in the CANDELS/UDS field. We quantify the morphology of the galaxies using the effective radius, r(eff), and Sersic index, n. In both the cluster and field, approximately half of the bulge-dominated galaxies (n > 2) reside on the red sequence of the color-magnitude diagram, and most disk-dominated galaxies (n < 2) have colors expected for star-forming galaxies. There is weak evidence that cluster galaxies have redder rest-frame U - B colors and higher stellar masses compared to the field. Star-forming galaxies in both the cluster and field show no significant differences in their morphologies. In contrast, there is evidence that quiescent galaxies in the cluster have larger median effective radii and smaller Sersic indices compared to the field with a significance of 2 sigma. These differences are most pronounced for galaxies at clustercentric distances 1 Mpc < R-proj < 1.5 Mpc, which have low Sersic indices and possibly larger effective radii, more consistent with star-forming galaxies at this epoch and in contrast to other quiescent galaxies. We argue that star-forming galaxies are processed under the influence of the cluster environment at distances greater than the cluster-halo virial radius. Our results are consistent with models where gas accretion onto these galaxies is suppressed from processes associated with the cluster environment.NASA NAS5-26555HST program GO-12060NASA through a grant from the Space Telescope Science Institute GO-12060NASA through Hubble Fellowship grant HF-51269.01-ASpace Telescope Science InstituteSouthern California Center for Galaxy EvolutionUniversity of California Office of ResearchQEII Fellowship from theAustralian GovernmentAstronom
CANDELS: The Evolution Of Galaxy Rest-Frame Ultraviolet Colors From Z=8 To 4
Full text linkWe study the evolution of galaxy rest-frame ultraviolet (UV) colors in the epoch 4 less than or similar to z less than or similar to 8. We use new wide-field near-infrared data in the Great Observatories Origins Deep Survey-South field from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, Hubble Ultra Deep Field (HUDF) 2009, and Early Release Science programs to select galaxies via photometric redshift measurements. Our sample consists of 2812 candidate galaxies at z greater than or similar to 3.5, including 113 at z similar or equal to 7-8. We fit the observed spectral energy distribution to a suite of synthetic stellar population models and measure the value of the UV spectral slope (beta) from the best-fit model spectrum. We run simulations to show that this measurement technique results in a smaller scatter on beta than other methods, as well as a reduced number of galaxies with catastrophically incorrect beta measurements (i.e.,. Delta beta > 1). We find that the median value of beta evolves significantly from -1.82(-0.04)(+0.00) at z = 4 to -2.37(-0.06)(+0.26) at z = 7. Additionally, we find that faint galaxies at z = 7 have beta = -2.68(-0.24)(+0.39) (similar to-2.4 after correcting for observational bias); this is redder than previous claims in the literature and does not require "exotic" stellar populations (e. g., very low metallicities or top-heavy initial mass functions) to explain their colors. This evolution can be explained by an increase in dust extinction, from low amounts at z = 7 to A(V) similar to 0.5 mag at z = 4. The timescale for this increase is consistent with low-mass asymptotic giant branch stars forming the bulk of the dust. We find no significant (<2 sigma) correlation between beta and M-UV when measuring M-UV at a consistent rest-frame wavelength of 1500 angstrom. This is particularly true at bright magnitudes, though our results do show evidence for a weak correlation at faint magnitudes when galaxies in the HUDF are considered separately, hinting that dynamic range in sample luminosities may play a role. We do find a strong correlation between beta and the stellar mass at all redshifts, in that more massive galaxies exhibit redder colors. The most massive galaxies in our sample have similarly red colors at each redshift, implying that dust can build up quickly in massive galaxies and that feedback is likely removing dust from low-mass galaxies at z >= 7. Thus, the stellar-mass-metallicity relation, previously observed up to z similar to 3, may extend out to z = 7-8.NASA through HST Cycle 18 HST-AR-12127NASA through Hubble Fellowship HST-HF-51288.01HST 12060, 12127Space Telescope Science InstituteNASA NAS 5-26555Astronom
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