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Extragalactic Astrophysics and Cosmology

Characterizing faint galaxies at the epoch of reionization

The epoch of reionization marks a major phase transition of the Universe, during which the intergalactic space became transparent to UV photons. Determining when this occurred, the physical processes involved and the sources of ionizing radiation represents one of the major goals in observational cosmology. This topic inevitably requires the investigation of distant (z>6) and faint sources (L<L*, i.e., observed magnitude > 26).


An ultradeep 52 hours VLT/FORS2 spectrum

We have collected a 52 hours spectrum obtained with FORS2 at the VLT of a solid z~7 galaxy candidate in the Hubble Ultra Deep Field with J125 ~ 26.1. This is the deepest spectrum ever obtained for a galaxy at the epoch of reionization. Neither emission lines nor a continuum were detected up to 10100Å, to a limiting equivalent width of 9Å (rest-frame). The combination of the superb HST photometry and the deep FORS2 spectrum provides a “spectro-photometric” redshift of 6.82±0.1. This increased redshift accuracy makes ALMA an interesting option for the confirmation of the redshift and investigation of the physical properties of this galaxy. The non-detection of Ly-α in even the best z~7 candidate (shown in Fig. 1) demonstrates the limitations of the current generation of 8-10 m class telescopes for these spectroscopic confirmations. Future facilities such as JWST and the E-ELT will be necessary to make decisive progress, especially for the detection of the ultraviolet continuum and the wavelength coverage up to the optical rest-frame.
[For more details, see Vanzella et al. 2014, A&A, 569, A78, and the September 2014 ESO/Messenger ]


Figure 1. Montage of images of the Hubble Ultra Deep Field focussing on GDS_1408. The background image is from the HST HUDF12 release of Wide Field Camera 3 (WFC3) near-infrared imaging (from STScI Release 2012-48), with enlarged views of GDS_1408 at the bottom. The upper right array of multi-band images (3 by 3 arcsec each) shows the appearance of GDS_1408, marked with a green colour, from U-band to H-band: VIMOS images in U and B; ACS images in B, V, i, I and z; WFC3 images in near-infrared bands (filters F105W, F125W [J-band], F140W and F160W [H-band]); and the detection in the narrow-band filter 973nm. This galaxy is the brightest z~7 candidate in the HUDF and has been continuously selected as a high-redshift candidate in the past 10-yr observations, at variance with other targets.




The faintest z>6 galaxy spectroscopically confirmed with LBT

As mentioned above, the limitation of the 8-10 m class telescopes is evident. One way to overcome this limit and assess faint Ly-α fluxes and magnitudes with reasonable signal to noise ratios (S/N>10) is to exploit the strong lensing effect. In this regard, we spectroscopically confirmed with LBT/MODS the faintest z>6 source currently known (z=6.4) behind the Frontier Fields galaxy cluster MACSJ0717 (see Fig. 2), with a de-lensed H-band magnitude > 29.3 (L<0.08L*(z=6)) and Ly-α flux <2×10-18 erg/s/cm2. The extremely blue ultraviolet slope (β<-2.6) suggests that this is a dust-free and/or young (<100 Myr) and/or metal poor galaxy. Intriguingly, the low star formation rate of ≤1 Msol/yr, a stellar mass <108 Msolar and the small size (<1.0 kpc2) may favour a possible leakage of ionizing radiation, that is an important  ingredient in the context of cosmic reionization.

[For more details see Vanzella et al. 2014, ApJ, 783, L12, and MEDIA-INAF].


Figure 2. Sixteen-band CLASH RGB false-color image of MACSJ0717.5+3745, with the two z=6.4 spectroscopically confirmed images marked with red circles (the insets show the F125W zoom). The critical curves (μ>100 here) for a source at z=6.4 from the revised Zitrin et al. model are overlaid in white. The green circles mark the multiple images used as constraints for the lens model. The proximity to the critical curves results in very high magnification, of the order of a few to a few dozen.





As reported above, long integration time with 8-10 m class telescopes in the field are capturing only the peak of the iceberg of z>6-7 galaxy population and from the “gravitational telescopes” we have a first glance of the faint luminosity regimes, thanks to the recent galaxy clusters surveys like CLASH and the ongoing Frontier Fields (ultradeep) initiative. However, future facilities like JWST and ELT are necessary to definitely probe the very faint domain of the luminosity (MUV<-15) and stellar mass functions (M«108 Msol) at z>6, in a statistically significant way. These regimes are critical in the framework of cosmic reionization and initial galaxy evolution.