Astronomers suggest more galaxies were formed in the early universe than previously thought MizzouNews AAS_Office
1. IntroductionOver the past decade, the search for high-redshift galaxies using deep Hubble Space Telescope images in fields on both"blank sky" and lensing clusters seems to suggest that the number density of galaxies at z > 6 sharply decreases toward higher and higher redshifts and even a dearth of galaxies at z > 10 . This, if true, would create a severe problem for our understanding of the cosmic hydrogen reionization.
3. PhotometryThe final stacks are in the surface brightness units of MJy sr−1. At the scale of 006 pix−1, this translates to the magnitude zero-point of 26.581 for all. We derive that the 5σ depths in F090W, F150W, F200W, F277W, F356W, and F444W are 28.17, 28.54, 28.73, 29.67, 29.73, and 29.72 mag, respectively, for module A . The module B images are slightly less sensitive than the ones in module A, presumably due to the impact of the intracluster light .
Of course, the size of the decrement in a real galaxy depends on the actual SED, which can have a wide range of possibilities. This is especially true at such a high redshift, because the age of the universe is short enough that activities of short timescales are not averaged out. This is demonstrated in Figure 1 by the superposed model galaxy spectra that capture the extreme situations.
4.2. SED AnalysisThe six-band data afforded us the opportunity to perform SED fitting on our dropouts. The most important quantities derived from this analysis are the photometric redshifts , using which we can check whether our dropouts being at high-z can be supported by an independent method. We note that this is fitting the SEDs of dropouts that have already been selected, which is different from selecting high-z candidate galaxies using zph directly.
4.4. F200W DropoutsFor F200W dropouts, we consider the 2σ sensitivity limits in F200W, which are 29.73 and 29.69 mag in modules A and B, respectively. Therefore, the brightness threshold for a valid F200W dropout is m277 ≤ 28.93 and 28.89 mag in these two modules, respectively. There are 8 validated F200W dropouts in module A . An example object is shown in Figure 4, together with its SED fitting results . Two diagnostic color–color diagrams are also shown.
5. DiscussionFor the dropouts selected in module B, which is on the cluster field, it is necessary to consider the magnification by the cluster. Shortly before the ERO release, a lens model for this cluster was published . This is based on the Light-Traces-Mass approach and uses the RELICS images and redshifts measured by the Multi Unit Spectroscopic Explorer at the Very Large Telescope.
The details of the EAZY SED fitting for zph are presented in Appendix D.
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