James Webb Space Telescope Feed Post

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

Paper 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 M_{UV}=-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.