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Garching bei München, Germany

Pirzkal N.,US Space Telescope Science Institute | Rothberg B.,US Space Telescope Science Institute | Rothberg B.,George Mason University | Nilsson K.K.,ST ECF | And 4 more authors.
Astrophysical Journal | Year: 2012

We have developed a new method for fitting spectral energy distributions (SEDs) to identify and constrain the physical properties of high-redshift (4 < z < 8) galaxies. Our approach uses an implementation of Bayesian based Markov Chain Monte Carlo that we have dubbed "πMC2." It allows us to compare observations to arbitrarily complex models and to compute 95% credible intervals that provide robust constraints for the model parameters. The work is presented in two sections. In the first, we test πMC2 using simulated SEDs to not only confirm the recovery of the known inputs but to assess the limitations of the method and identify potential hazards of SED fitting when applied specifically to high-redshift (z> 4) galaxies. In the second part of the paper we apply πMC2 to thirty-three 4 < z < 8 objects, including the spectroscopically confirmed Grism ACS Program for Extragalactic Science Lyα sample (4 < z < 6), supplemented by newly obtained Hubble Space Telescope/WFC3 near-IR observations, and several recently reported broadband selected z > 6 galaxies. Using πMC2, we are able to constrain the stellar mass of these objects and in some cases their stellar age and find no evidence that any of these sources formed at a redshift larger than z = 8, a time when the universe was 0.6 Gyr old. © 2012. The American Astronomical Society. All rights reserved. Source

Nilsson K.K.,ST ECF | Nilsson K.K.,Max Planck Institute for Astronomy | Nilsson K.K.,European Southern Observatory | Ostlin G.,The Oskar Klein Center | And 5 more authors.
Astronomy and Astrophysics | Year: 2011

We study the multi-wavelength properties of a set of 171 Lyα emitting candidates at redshift z = 2.25 found in the COSMOS field, with the aim of understanding the underlying stellar populations in the galaxies. We especially seek to understand what the dust contents, ages and stellar masses of the galaxies are, and how they relate to similar properties of Lyα emitters at other redshifts. The candidates here are shown to have different properties from those of Lyα emitters found at higher redshift, by fitting the spectral energy distributions (SEDs) using a Monte-Carlo Markov-Chain technique and including nebular emission in the spectra. The stellar masses, and possibly the dust contents, are higher, with stellar masses in the range logM * = 8.5-11.0 M⊙and AV = 0.0-2.5 mag. Young population ages are well constrained, but the ages of older populations are typically unconstrained. In 15% of the galaxies only a single, young population of stars is observed. We show that the Lyα fluxes of the best fit galaxies are correlated with their dust properties, with higher dust extinction in Lyα faint galaxies. Testing for whether results derived from a light-weighted stack of objects correlate to those found when fitting individual objects we see that stellar masses are robust to stacking, but ages and especially dust extinctions are derived incorrectly from stacks. We conclude that the stellar properties of Lyα emitters at z = 2.25 are different from those at higher redshift and that they are diverse. Lyα selection appears to be tracing systematically different galaxies at different redshifts. © 2011 ESO. Source

Nilsson K.K.,ST ECF | Nilsson K.K.,European Southern Observatory | Moller P.,European Southern Observatory
Astronomy and Astrophysics | Year: 2011

Of all the many ways of detecting high-redshift galaxies, the selection of objects by their redshifted Lyα emission has become one of the most successful. But what types of galaxies are selected in this way? Until recently, Lyα emitters were understood to be small star-forming galaxies, the possible building-blocks of larger galaxies. But with increased number of observations of Lyα emitters at lower redshifts, a new picture emerges. Lyα emitters display strong evolution in their properties from higher to lower redshift. It has previously been shown that the fraction of ultra-luminous infrared galaxies (ULIRGs) among the Lyα emitters increases dramatically between redshifts three and two. Here, the fraction of AGN among the LAEs is shown to follow a similar evolutionary path. We argue that Lyα emitters are not a homogeneous class of objects and that the objects selected with this method reflect the general star forming and active galaxy populations at that redshift. Lyα emitters should thus be excellent tracers of galaxy evolution in future simulations and modelling. © 2011 ESO. Source

Zafar T.,Copenhagen University | Moller P.,European Southern Observatory | Ledoux C.,European Southern Observatory | Fynbo J.P.U.,Copenhagen University | And 8 more authors.
Astronomy and Astrophysics | Year: 2011

Context. Q 0151+048 is a physical quasar (QSO) pair at z ∼ 1.929 with a separation of 3.3 arcsec on the sky. In the spectrum of the brighter member of this pair, Q 0151+048A, a damped Lyα absorber (DLA) is observed at a higher redshift. We have previously detected the host galaxies of both QSOs, as well as a Lyα blob whose emission surrounding Q 0151+048A extends over 5 × 3.3 arcsec. Aims. We seek to constrain the geometry of the system and understand the possible relations between the DLA, the Lyα blob, and the two QSOs. We also aim at characterizing the former two objects in more detail. Methods. To study the nature of the Lyα blob, we performed low-resolution, long-slit spectroscopy with the slit aligned with the extended emission. We also observed the whole system using the medium-resolution VLT/X-shooter spectrograph and the slit aligned with the two QSOs. The systemic redshift of both QSOs was determined from rest-frame optical emission lines redshifted into the NIR. We employed line-profile fitting technique, to measure metallicities and the velocity width of low-ionization metal absorption lines associated to the DLA and photo-ionization modeling to characterize the DLA further. Results. We measure systemic redshifts of zem(A) = 1.92924 ± 0.00036 and zem(B) = 1.92863 ± 0.00042 from the H β and H α emission lines, respectively. In other words, the two QSOs have identical redshifts within 2σ. From the width of Balmer emission lines and the strength of the rest-frame optical continuum, we estimate the masses of the black holes of the two QSOs to be 109.33 M⊙ and 108.38 M⊙ for Q 0151+048A and Q 0151+048B, respectively. We then use the correlation between black hole mass and dark matter halo mass to infer the mass of the dark matter halos hosting the two QSOs: 1013.74 M⊙ and 1013.13 M ⊙ for Q 0151+048A and Q 0151+048B, respectively. We observe a velocity gradient along the major axis of the Lyα blob consistent with the rotation curve of a large disk galaxy, but it may also be caused by gas inflow or outflow. We detect residual continuum in the DLA trough, which we interpret as emission from the host galaxy of Q 0151+048A. The derived H0 column density of the DLA is log NH0 = 20.34 ± 0.02 cm-2. Metal column densities are also determined for a number of low-ionization species resulting in an overall metallicity of 0.01 Z ⊙. We detect C ii, which allows us to make a physical model of the DLA cloud. Conclusions. From the systemic redshifts of the QSOs, we conclude that the Lyα blob is associated with Q 0151+048A rather than with the DLA. The DLA must be located in front of both the Lyα blob and Q 0151+048A at a distance greater than 30 kpc and has a velocity relative to the blob of 640 ± 70 km s-1. The two quasars accrete at normal Eddington ratios. The DM halo of this double quasar will grow to the mass of our local supercluster at z = 0. We point out that those objects therefore form an ideal laboratory to study the physical interactions in a z = 2 precursor of our local supercluster. © 2011 ESO. Source

Raiter A.,European Southern Observatory | Fosbury R.A.E.,ST ECF | Teimoorinia H.,European Southern Observatory | Teimoorinia H.,Institute for Advanced Studies in Basic Sciences
Astronomy and Astrophysics | Year: 2010

Context: The Great Observatories Origins Deep Survey (GOODS) has provided us with one of the deepest multi-wavelength views of the distant universe. The combination of multi-band photometry and optical spectroscopy has resulted in the identification of sources whose redshifts extend to values in excess of six. Amongst these distant sources are Lyα emitters whose nature must be deduced by clearly identifying the different components that contribute to the measured SED. Aims: From a sample of Lyα emitters in the GOODS-S field with uncontaminated photometry and optical (red) spectroscopy, we select a spatially compact object at a redshift of 5.563 (Lyα) that shows a second emission line, identified as Niv] 1486 Å. The SED is modelled in a way that accounts for both the Niv] line emission and the photometry in a self-consistent way. Methods: The photoionization code CLOUDY is used to calculate a range of nebular models as a function of stellar ionizing source temperature, ionization parameter, density and nebular metallicity. We compare the theoretical and observed magnitudes and search for the model parameters that also reproduce the observed Niv] luminosity and equivalent width. Results: A nebular model with a hot blackbody ionizing source of around 100 kK and a nebular metallicity of ∼5% of solar is able to fit the observed SED and, in particular, explain the large apparent Balmer break which is inferred from the pure stellar population model fitting conventionally applied to multi-band photometric observations. In our model, an apparent spectral break is produced by strong [Oiii] 4959, 5007 Å emission falling in one of the IR bands (IRAC1 in this case). A lower limit on the total baryonic mass of a model of this type is 3.2 × 108 M⊙. Conclusions: It is argued that objects with Lyα emission at high redshift that show an apparent Balmer break may have their SED dominated by nebular emission and so could possibly be identified with very young starbursting galaxies rather than massive evolved stellar populations. Detailed studies of these emission nebulæ with large telescopes will provide a unique insight into very early chemical evolution. © 2010 ESO. Source

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