Galaxy Luminosity Functions at z~7-10 from Four Years of JWST Observations

Abstract

We present galaxy luminosity functions at $z\approx 7.4$ and 10 based on a comprehensive search for candidate galaxies at $6.4\lesssim z\lesssim11.3$ in JWST's public NIRCam data four years after its operation. Our work utilises nine fields covering $\sim$1,045~arcmin$^2$ in total, including seven blank fields and two galaxy cluster fields. Using the dropout technique, we selected 3,733 dropouts from the F090W passband and 2,396 dropouts from the F115W passband, which constitute the largest samples of candidate galaxies at $z\approx 7.4$ and $z\approx 10$ to date. Among these objects, 338 F090W-dropouts and 99 F115W-dropouts already have secure redshifts from JWST NIRSpec spectroscopy over four years, which we use to form two large spectroscopic subsamples to assess the magnitude-dependent contamination rates of their parent dropout samples. Our luminosity functions are based on the results after statistically correcting for both incompleteness and contamination. We find that the luminosity function at $z\approx 7.4$ can be well described by a conventional Schechter function but with a faint-end slope of $α=-1.70\pm0.16$ that is significantly shallower than most of the previous studies at $z\approx 7$--8. To verify this result, more imaging and spectroscopy at $\sim$30~mag will be needed. The luminosity function at $z\approx 10$, on the other hand, can hardly be fitted by a Schechter function but is more consistent with a power law in the form of $Φ(M)\propto 10^{-0.4(β+1)M}$, with the best-fit slope of $β=-2.36\pm0.16$. While such a behavior is largely driven by the high number densities at $M_{\rm UV}\lesssim -21.8$~mag, it is supported by the currently existing spectroscopic confirmations. Further spectroscopy at the bright end($\lesssim 26$~mag) will be needed to strengthen this result.