James Webb Space Telescope Feed Post

A Steep Decline in the Galaxy Space Density Beyond Redshift 9 in the CANUCS UV Luminosity Function

Example SEDs for CANUCS galaxies in the range 7.5 < zphot_ML < 8.5. The galaxies are ordered by increasing absolute magnitude. Left panels show the observed photometry (0.3 arcsec apertures on images convolved to the F444W PSF; black circles with errorbars), best-fit EAZY-py template (blue line) and template photometry integrated over filter bandpasses (cyan squares). Middle panels show the photometric redshift probability density with a purple line at zphot_ML. Right panels show 1.6 arcsec cutouts in all the HST optical (where available) and NIRCam filters. The RGB cutouts use colors R: all filters above 2.9µm, G: all filters at 1.7 to 2.9µm, B: all NIRCam filters below 1.7µm. Abstract: We present a new sample of 158 galaxies at redshift z > 7.5 selected from deep JWST NIRCam imaging of five widely-separated sightlines in the CANUCS survey. Two-thirds of the pointings and 80% of the galaxies are covered by 12 to 14 NIRCam filters, including seven to nine medium bands, providing accurate photometric redshifts and robustness against low redshift interlopers. A sample of 28 galaxies at z > 7.5 with spectroscopic redshifts shows a low systematic offset and scatter in the difference between photometric and spectroscopic redshifts. We derive the galaxy UV luminosity function at redshifts 8 to 12, finding a slightly higher normalization than previously seen with HST at redshifts 8 to 10. We observe a steeper decline in the galaxy space density from z=8 to 12 than found by most JWST Cycle 1 studies. In particular, we find only eight galaxies at z>10 and none at z > 12.5, with no z>10 galaxies brighter than F277W AB=28 or MUV=-20 in our unmasked, delensed survey area of 53.4 square arcminutes. We attribute the lack of bright z > 10 galaxies in CANUCS compared to GLASS and CEERS to intrinsic variance in the galaxy density along different sightlines. The evolution in the CANUCS luminosity function between z=8 and 12 is comparable to that predicted by simulations that assume a standard star formation efficiency, without invoking any special adjustments.