James Webb Space Telescope Feed Post

Unveiling the distant Universe: Characterizing z>=9 Galaxies in the first epoch of COSMOS-Web

For each galaxy, we show a 2” stamp image centered on our detections on the upper line and the residuals on the lower line after subtraction of the galaxy models found by SE++ for the four NIRCam filters, the MIRI filter and the HST/F814W filter. Black squares for MIRI indicate that the galaxy is out of the MIRI coverage. Abstract: We report the identification of 15 galaxy candidates at z>=9 using the initial COSMOS-Web JWST observations over 77 arcmin2 through four NIRCam filters (F115W, F150W, F277W, F444W) with an overlap with MIRI (F770W) of 8.7 arcmin2. We fit the sample using several publicly-available SED fitting and photometric redshift codes and determine their redshifts between z=9.3 and z=10.9 (?z?=10.0), UV-magnitudes between MUV = -21.2 and -19.5 (with ?MUV?=-20.2) and rest-frame UV slopes (?ß?=-2.4). These galaxies are, on average, more luminous than most z=9 candidates discovered by JWST so far in the literature, while exhibiting similar blue colors in their rest-frame UV. The rest-frame UV slopes derived from SED-fitting are blue (ß~[-2.0, -2.7]) without reaching extremely blue values as reported in other recent studies at these redshifts. The blue color is consistent with models that suggest the underlying stellar population is not yet fully enriched in metals like similarly luminous galaxies in the lower redshift Universe. The derived stellar masses with ?log10(M?/M?)?˜8-9 are not in tension with the standard ?CDM model and our measurement of the volume density of such UV luminous galaxies aligns well with previously measured values presented in the literature at z~9-10. Our sample of galaxies, although compact, are significantly resolved.