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Register a new userLyman-alpha emitters as tracers of the transitioning Universe
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Kim K. Nilsson
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Of the many ways of detecting high redshift galaxies, the selection of objects due to their redshifted Ly-alpha emission has become one of the most successful. But what types of galaxies are selected in this way? Until recently, Ly-alpha emitters were understood to be small star-forming galaxies, possible building-blocks of larger galaxies. But with increased number of observations of Ly-alpha emitters at lower redshifts, a new picture emerges. Ly-alpha 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 Ly-alpha emitters increases dramatically between redshift three and two. Here, the fraction of AGN among the LAEs is shown to follow a similar evolutionary path. We argue that Ly-alpha 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. Ly-alpha emitters should hence be excellent tracers of galaxy evolution in future simulations and modeling.
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This publication contains the conference summary of the Understanding Lyman-alpha Emitters conference held at the Max Planck Institute for Astronomy in Heidelberg October 6 - 10, 2008. The scope of the conference was to bring together most of the scientists working in the field of Lyman-alpha emitters, whether at low or high redshift, or on observational or theoretical aspects, and to summarise how far the field of study of galaxies with Lyman-alpha emission has come. An outlook towards the future of the field was also desired. As part of the conference, two days were dedicated to in total six discussion sessions. The topics were i) new methods and selection methods, ii) morphology, iii) what can the local Universe observations tell us about the high redshift Universe?, iv) clustering, v) SED fitting and vi) Ly-alpha blobs. The chairs of those sessions were asked to summarise the discussions, as presented in these proceedings.
We study the multi-wavelength properties of a set of 171 Ly-alpha 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 Ly-alpha emitters at other redshifts. The candidates here are shown to have different properties from those of Ly-alpha 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 log M_* = 8.5 - 11.0 M_sun and A_V = 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 Ly-alpha fluxes of the best fit galaxies are correlated with their dust properties, with higher dust extinction in Ly-alpha 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 Ly-alpha emitters at z = 2.25 are different from those at higher redshift and that they are diverse. Ly-alpha selection appears to be tracing systematically different galaxies at different redshifts.
We study the far-infrared properties of 498 Lyman Alpha Emitters (LAEs) at z=2.8, 3.1 and 4.5 in the Extended Chandra Deep Field-South, using 250, 350 and 500 micron data from the Herschel Multi-tiered Extragalactic Survey (HerMES) and 870 micron data from the LABOCA ECDFS Submillimeter Survey (LESS). 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 $1sigma$ depths of ~0.1 to 0.4 mJy. The LAEs are also undetected at $ge3sigma$ in the stacks, although a $2.5sigma$ signal is observed at 870 micron for the z=2.8 sources. We consider a wide range of far-infrared spectral energy distributions (SEDs), including a 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 Ly$alpha$ escape fraction ($f_{rm esc}($Ly$alpha$)). For the Sd SED template, the inferred LAEs $f_{rm esc}($Ly$alpha$) are $gtrsim30%$ ($1sigma$) at z=2.8, 3.1 and 4.5, which are all significantly higher than the global $f_{rm esc}($Ly$alpha$) at these redshifts. Thus, if the LAEs $f_{rm esc}($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 $f_{rm esc}($Ly$alpha$) of ~10 to 20% ($1sigma$), all of which are slightly higher than the global evolution of $f_{rm esc}($Ly$alpha$) but consistent with it at the 2 to 3$sigma$ level.
We present a simple physical model for populating dark matter halos with Lyman Alpha Emiiters(LAEs) and predict the physical properties of LAEs at z~3-7. The central tenet of this model is that the Ly-alpha luminosity is proportional to the star formation rate (SFR) which is directly related to the halo mass accretion rate. The only free parameter in our model is then the star-formation efficiency (SFE). An efficiency of 2.5% provides the best-fit to the Ly-alpha luminosity function (LF) at redshift z=3.1, and we use this SFE to construct Ly-alpha LFs at other redshifts. Our model reproduce the Ly-alpha LFs, stellar ages, SFR ~1-10; Msun/yr, stellar masses ~ 10^7-10^8 Msun and the clustering properties of LAEs at z~3-7. We find the spatial correlation lengths ro ~ 3-6 Mpc/h, in agreement with the observations. Finally, we estimate the field-to-field variation ~ 30% for current volume and flux limited surveys, again consistent with observations. Our results suggest that the star formation, and hence Ly-alpha emission in LAEs is powered by the accretion of new material, and that the physical properties of LAEs do not evolve significantly over a wide range of redshifts. Relating the accreted mass, rather than the total mass of halos, to the Ly-alpha luminosity of LAEs naturally gives rise to the duty cycle of LAEs.
The Lyman-alpha (Lya) recombination line is a fundamental tool for galaxy evolution studies and modern observational cosmology. However, subsequent interpretations are still prone to a number of uncertainties. Besides numerical efforts, empirical data are urgently needed for a better understanding of Lya escape process. We empirically estimate the Lyman-alpha escape fraction fesc(Lya) in a statistically significant sample of z ~ 0 - 0.3 galaxies in order to calibrate high-redshift Lyman-alpha observations. An optical spectroscopic follow-up of a sub-sample of 24 Lyman-alpha emitters (LAEs) detected by GALEX at z ~ 0.2-0.3, combined with a UV-optical sample of local starbursts, both with matched apertures, allow us to quantify the dust extinction through Balmer lines, and to estimate the Lyman-alpha escape fraction from the Halpha flux corrected for extinction in the framework of the recombination theory. The global escape fraction of Lyman-alpha radiation spans nearly the entire range of values, from 0.5 to 100 %, and fesc(Lya) clearly decreases with increasing nebular dust extinction E(B-V). Several objects show fesc(Lya) greater than fesc(continuum) which may be an observational evidence for clumpy ISM geometry or for an aspherical ISM. Selection biases and aperture size effects may still prevail between z ~ 0.2-0.3 LAEs and local starbursts, which may explain the difference observed for fesc(Lya).
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