Free Access
Volume 519, September 2010
Article Number L4
Number of page(s) 9
Section Letters
Published online 14 September 2010

Online Material

Appendix A: Photometric selection of z $\sim $ 2.2 LAE candidates from ALHAMBRA survey in GOODS-North

The method used for finding LAEs is based on a color-magnitude diagnostic diagram. The filter which samples the emission line is the ON filter and those used to define the continuum constitute the OFF filters set. The choice of the ON filter determines the range where the redshifts of the candidates will be. In this way, we seleted the LAE candidates at 2.10<z<2.37 by using the filters A394M as ON filter (owing to the official naming of ALHAMBRA filters, the number between letters gives the rounded central wavelength in nanometers; the last letter is the acronym of ``Medium'' bandpass), and a sum of A425M and A457M as OFF filters, in order to increase the S/N in the continuum. Figure A.1 (inset) shows the transmission curve of the chosen filters for LAE selection. Assuming an effective filter width equal to its FWHM, and taking into account the sky area surveyed, as well as the emission line of interest, we have explored in this way a comoving volume of about $6.63\times10^{4}~\textrm{Mpc}^3$.

\end{figure} Figure A.1:

ALHAMBRA expected OFF-ON color of the VIMOS LAEs spectra at ${z} \sim 3$, shifted to z=2, as a function of their equivalent widths. The relation between both variables allows us to have a color selection criterion. The dashed line represents the basic color threshold which corresponds to a ${EW}_{\rm{rest}}= 35 ~ \rm{\AA}$. Inset: Bandpasses of the filters used to find LAEs at 2.1<z<2.37. The curves represent the ON ( A394M) and OFF ( A425M+ A457M) filters transmissions, multiplied by the detector quantum efficiency and the atmospheric transmission at airmass = 1. A typical spectrum of a LAE at z=2.2 is overlaid to show the basis of the photometric selection criterium.

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Once the ON and OFF filters were chosen, and according with the ALHAMBRA survey sensitivity, we adopt a limiting magnitude of 25.0 in the ON-band. Then, we elaborated a color selection criterion to find LAEs as efficiently as possible. We used the data from the GOODS/VIMOS Spectroscopy DR 2.0.1 (Popesso et al. 2009) to simulate the behavior of the SED from a typical LAE against the selected ALHAMBRA filters. From the whole spectroscopic catalog we selected the objects that show a line which corresponds to a Ly$\alpha $ emission. Then the spectra were de-redshifted so that the central wavelength of the lines were within the transmission range of the ON filterin each case, as shown in the example of Fig. A.1, and their rest frame equivalent widths were calculated. After this we were able to compute the OFF-ON color by convolution of the filter profiles with each spectrum. In Fig. A.1 we represent this color against the rest frame equivalent width of the lines, measured with splot in IRAF. As can be seen, there exists a relation between both variables, which allows us to define a color selection criterion. Our LAE candidates were selected to have rest-frame equivalent widths ${EW}_{\rm {rest}}\ga 35~\rm {\AA}$. According to Fig. A.1, this value corresponds to a threshold color of approximately 0.3 for both filter pairs. But taking into account the photometric errors, our adopted color selection criterion is $m_{\rm OFF}-m_{\rm ON}-0.3\geq 3\sqrt{\sigma_{\rm OFF}^2+\sigma_{\rm ON}^2}$. This translates the minimum rest-frame equivalent width to ${EW}_{\rm {rest}}\ga 35~\rm {\AA}$ at the limiting magnitude in the ON-band, corresponding to Ly$\alpha $ luminosities of $\log L=42.29$ erg s-1 at z=2.2.

All photometric measures were performed with SExtractor (Bertin & Arnouts 1996) on the full processed and stacked images of the field ALHAMBRA-5, pointing 1. To calculate the color we used three arcsec aperture magnitudes and MAG_AUTO ones to plot the magnitudes of the objects. Sources with SExtractor nonzero flags were discarded.

In order to study the nature of the possible continuum contaminants we carried out simulations with galaxy templates of BC03 (Bruzual & Charlot 2003) and SWIRE (Polletta et al. 2007). We took all the templates and redshifted them from z=0 to 3 in bins of $\Delta z=0.05$. We calculated their expected color and checked whether they satisfy the color selection criterion. As a result, we found that the contaminants appear to be mainly starburst galaxies at $1.0 \la z \la 2.0$, whose UV rest-frame drop is sampled with the filters. As an example, Fig. A.2 shows a set of spectra of different starburst templates that satisfy the color criterium, although they are false positive candidates. More details of this study are included in a forthcoming paper (Oteo et al., in prep.).

\end{figure} Figure A.2:

Spectra of possible contaminants. The transmission curves of the filter set used to select LAEs and spectra of possible contaminants at different redshifts, built from BC03 and SWIRE templates, are shown. The main contaminants are those whose UV continuum slope is sampled by the filters, resulting in the appearance of false-positive candidates. For the sake of clarity, the spectra and the transmission of the filters have been scaled.

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Appendix B: Data of z $\sim $ 2.2 LAE candidates in GOODS-North

\end{figure} Figure B.1:

Optical pseudo-spectra of $z\sim 2.2$ LAE candidates from ALHAMBRA survey. ACS cutouts ($3\times 3$ sq-arcsec) of the objects, when available, are shown in the right side of each pseudo-spectrum. Vertical line inside each panel represent the position of Ly$\alpha $ emission line center at the corresponding photometric or spectroscopic redshift given in Table B.1. Fluxes are in 10-18 erg cm-2 s-1 Å-1 units. The horizontal bars represent the effective widths of ALHAMBRA filters. Assumming a concordant cosmology, with ${H}_0=70~{\rm km}~{\rm s}^{-1}~{\rm Mpc}^{-3}$, the mean scale of the images is $\sim $8.2 kpc arcsec-1.

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Table B.1:   Combined data list of $z\sim 2.2$ LAE candidates in the north-east half of the GOODS-North field.

Table B.2:   Number counts of LAEs redshifts 2.2 and 3.1, limited in Ly$\alpha $ luminosity and rest-frame EW.

\begin{figure}\par\resizebox{18cm}{!}{\includegraphics{14719fgC2.eps}}\end{figure} Figure B.1:

Rest-frame EW against Ly$\alpha $ luminosity for the LAE samples referred in Table B.2. Star-forming like LAEs are represented by stars and the AGNs (or AGN/Composite objects) by triangles. Circled triangles are LAEs with FIR counterparts from this work. An interpretation of the AGN distribution in this diagram is given in the main text.

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