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Searching for the first galaxies through gravitational lenses

Observing the first galaxies formed during the reionisation epoch, i.e. approximately within the first billion years after the Big Bang, remains one of the challenges of contemporary astrophysics. Several efforts are being undertaken to search for such remote objects. Combining the near-IR imaging power of the VLT and the natural effect of strong gravitational lensing our pilot program has allowed us to identify several galaxy candidates at redshift 6<~ z<~ 10. The properties of these objects and the resulting constraints on the star formation rate density at high redshift are discussed. Finally we present the status of follow-up observations (ISAAC spectroscopy, HST and Spitzer imaging) and discuss future developments.

had established itself as a simple and successful method to identify distant galaxies through the use of broad band photometry. Furthermore sensitive infrared instruments were now available on the large ground-based telescopes, able in principle to detect proto-galaxies out to even higher redshifts. Given these advances, we started few years ago a pilot-project with the aim of finding starforming galaxies at redshifts beyond 6-6.5 using lensing clusters as gravitational telescopes, a well-established technique nowadays. This project is mainly based on ISAAC and FORS2 data plus additional observations obtained at CFHT and HST.

Observations
Focussing on two well known gravitational lensing clusters, Abell 1835 and AC114, we obtained deep near-IR images in the SZ, J, H, and Ks bands with ISAAC and an additional z-band image with FORS2. These deep images, reaching e.g. a 1σ depth of 26.1 in H AB , were then used to search for objects which are detected at least in two near-IR bands, which show a blue near-IR colour, and which are undetected (i.e. "dropped out") in all optical bands. These criteria are optimised to select high redshift (z > 6) objects with intrinsically blue UV-restframe spectra, i.e. very distant starburst galaxies, and to avoid contamination from intrinsically faint and red cool stars. Different combinations of colour-colour plots allow a crude classification into several redshift bins. An example of such a diagram, showing the expected location of z ∼ 8-11 galaxies and of candidates found behind Abell 1835 is shown in Fig. 1. A complete report is given in Richard et al. (2006).

High-z galaxy candidates and the cosmic star formation density during reionisation
In spring 2003 the analysis of the candidates behind Abell 1835 yielded an intriguing, strongly lensed object whose spectral energy distribution was compatible with that of a galaxy at z ∼ 9-11. During our first spectroscopic followup run with ISAAC in summer 2003 we were able to secure J-band long-slit spectroscopy centered at this position under excellent seeing conditions. Interestingly, careful data reduction revealed at this location the presence of a single faint emission line detected at ∼ 4 − 8σ (depending on the integration aperture and stacking procedure), which if interpreted as Lyα would indicate z = 10.0! The report of these findings has been published in Pelló et al. (2004a).
What is the status of this fairly unique candidate? Weatherley et al. (2004) have questioned the reality of the emission line. However, their negative result could be due to the combination of two factors: an error in our absolute wavelength calibration discovered later, and their spectroscopic data reduction technique, where the information is only preserved at the original (wrong) wavelength position and smeared elsewhere (see Pelló et al. 2004b). On the imaging side, deep V -band observations with FORS2 (Lehnert et al. 2005) have confirmed its optical non-detection. Our ISAAC H-band images have been reanal-ysed by several groups using different methods (Bremer et al. 2005, Smith et al. 2006) yielding measurements compatible with ours typically within 1 σ. Surprisingly however, this object remained undetected in a deeper NIRI/GEMINI H-band image taken approximately 15 months after our ISAAC image (Bremer et al. 2005). In spring 2004 we obtained two SZ (∼ 1.06 µm) images with ISAAC, where this object is again detected (see Pelló et al. 2005). Taken together these spectroscopic and photometric detections, albeit individually of relatively low significance, indicate that this source is most likely not a spurious, but an intrinsically variable object, as discussed in Richard et al. (2006). Therefore, its nature and precise redshift remain puzzling, and we exclude this object from our list of high-z candidates, which we will discuss now.
Applying the above selection criteria to the observations of the two lensing clusters has yielded 13 candidates whose spectral energy distribution (SED) is compatible with that of star forming galaxies at z > ∼ 6 (see Richard et al. 2006). Images and the SEDs of some of them are shown in Fig. 2 for illustration. The typical lensing magnification reaches from 1.5 to 10, with an average of ∼ 6, i.e. nearly 2 magnitudes. Their star formation rate, as estimated from the UV restframe luminosity, is typically between ∼ 4 and 20 M ⊙ yr −1 after correcting for lensing.
We have used this data to attempt to constrain for the first time the density of star-forming galaxies present at 6 < ∼ z < ∼ 10 using lensing clusters. After taking into account the detailed lensing geometry, sample incompleteness, and correcting for false-positive detections we have constructed a luminosity function (LF) of these candidates assuming a fixed slope taken from observations at z ∼ 3. Within the errors the resulting LF is compatible with that of z ∼ 3 Lyman break galaxies. At low luminosities it is also compatible with the LF derived by Bouwens and collaborators for their sample of z ∼ 6 candidates in the Hubble Ultra Deep Field (UDF) and related fields. However, the turnover observed by these authors towards the bright end relative to the z ∼ 3 LF is not observed in our sample. Finally, from the LF we determine the UV star formation rate (SFR) density at z ∼ 6-10, shown in Fig. 3. Our values indicate a similar SFR density as between z ∼ 3 to 6, in contrast to the drop found from the deep NICMOS fields. Further observations are required to fully understand these differences. Taken at face value, this relatively high SFR density is in good agreement e.g. with the recent hydrodynamical models of Nagamine et al. (2005), with the reionisation models of Choudhury & Ferrara (2005), and also with the SFR density inferred from the past star formation history of observed z ∼ 6 galaxies (e.g. Eyles et al. 2006).

Follow-up observations
Compared to the dataset just described several additional observations could be secured on these clusters in the meantime. For example, deep z-band images of both clusters were obtained with the ACS camera onboard HST. These observations confirm that the vast majority (all except one of the above 13) of our high-z candidates are optical dropouts as expected, remaining undetected down to a 1σ limiting magnitude of 28.-28.3 mag AB (Hempel et al. 2006). In collaboration with Eiichi Egami we have also access to IRAC/Spitzer GTO images at 3.6, 4.5, 5.8 and 8.0 µm of large sample of lensing clusters including Abell 1835 and AC114. Again, none of our high redshift galaxy candidates are detected. This is easily understood, since extrapolation of their intrinsically blue SED to IRAC wavelengths shows that their expected fluxes fall below the Spitzer sensitivity. It implies that these objects do not host "old" stellar populations with strong Balmer breaks, and that they are not affected by significant extinction. A more detailed account of these observations will be presented elsewhere. For comparison a brighter lensed galaxy at z ∼ 7 has been detected earlier by Spitzer (Egami et al. 2005).
In parallel several attempts were made to detect emission lines from selected candidates using near-IR long-slit spectroscopy with ISAAC on the VLT. See Pelló et al. (2005) for a preliminary report. Presently such observations are tedious, fairly time-consuming, and require excellent seeing conditions. Indeed, given the faintness of the expected lines, the strong IR background, and the need for the highest spectral resolution possible to minimise the impact of the numerous sky lines, a "scan" of the entire J-band at R ∼ 3000 for example requires 5 settings and a total of ∼ 10 ksec to detect an unresolved line of (6 − 8) × 10 −18 erg s −1 cm −2 flux at 5 σ with ISAAC.
One or more emission lines could be detected in few objects, as shown in Fig. 4. For example, one of our secondary targets turned out to be a very faint z = 1.68 emission line galaxy. Other lines are clearly identified as the [O ii] λ3727,3729 doublet from an intermediate redshift galaxy. Finally several objects show a single emission line, which -if identified as Lyα -yields a redshift compatible with the (high) estimated photometric redshift. However, in none of these cases a clear asymmetry of the line, as typical for Lyα from lower z starbursts, was found. Searches for additional lines in these objects (e.g. C iv λ1550 or He ii λ1640 if at high z, or for [O iii] λ5007 or Hα if at low z) have so far been negative. Therefore the redshift of these objects is currently difficult to establish, but high-z cannot be excluded on the present grounds. Forthcoming, more efficient near-IR spectrographs should allow a significant breakthrough in this field.

Spin-off projects on EROs and dusty intermediate-z galaxies
Our search for optical dropout galaxies behind lensing clusters yields also other interesting objects, such as extremely red objects (EROs; see Richard et al. 2006). In contrast to the high-z candidates most of them are detected by IRAC/Spitzer. These objects, most likely all at intermediate redshift (z ∼ 1-3), turn out to have similar properties as e.g. faint IRAC selected EROs in the Hubble UDF, other related objects such as the putative post-starburst z ∼ 6.5 galaxy of Mobasher et al. (2005), and some sub-mm galaxies (see Hempel et al. 2006) Spectroscopic observations with FORS2 and X-ray Chandra observations are also being secured to clarify the redshift and nature of these interesting optical dropout objects.

Future
With our pilot program it has been possible to find several very high redshift candidate galaxies by combining the power of strong gravitational lensing with the large collecting area of the VLT. However, differences with other studies based on deep blank fields are found, and already differences between our two clusters indicate that these could at least partly be due to field-to-field variance. Given the relatively low S/N ratio of the high-z candidates and the large correction factors applied to this sample, it is of great interest to increase the number of lensing clusters observed with this technique.
Furthermore, rapidly upcoming new spectrographs such as the second generation near-IR VLT instruments XShooter and KMOS, the EMIR spectrograph on the Spanish GRANTECAN telescope and others will provide a huge efficiency gain for spectroscopic follow-up of faint candidate sources, thanks to their increased spectral coverage and multi-object capabilities. Observations at longer wavelengths, e.g. with HERSCHEL, APEX and later ALMA, are also planned to search for possible dust emission in such high-z galaxies and to characterise more completely other populations of faint optical dropout galaxies. Finally the JWST and ELTs will obviously be powerful machines to study the first galaxies. Large territories remain unexplored in the early universe!     Richard et al. (2003, A&A 412, L57). The black vertical lines correspond to sky lines.