Astronomical Spectroscopy with Skipper CCDs: First Results from a Skipper CCD Focal Plane Prototype at SIFS

We present the first on-sky results from an ultra-low-readout-noise Skipper CCD focal plane prototype for the SOAR Integral Field Spectrograph (SIFS). The Skipper CCD focal plane consists of four 6k 1k, 15 m pixel, fully-depleted, p-channel devices that have been thinned to 250 m, backside processed, and treated with an anti-reflective coating. These Skipper CCDs were configured for astronomical spectroscopy, i. e. , a single-sample readout noise < 4. 3\, e^-\, rms/pixel, the ability to achieve multi-sample readout noise 1\, e^-\, rms/pix, full-well capacities 40, 000--65, 000 e^-, low dark current and charge transfer inefficiency (2 10^-4 e^-/pixel/s and 3. 44 10^-7, respectively), and an absolute quantum efficiency of 80\% between 450\, nm and 980\, nm (90\% between 600\, nm and 900\, nm). We optimize the readout sequence timing to achieve sub-electron noise (0. 5\, e^-\, rms/pix) and photon-counting (0. 22\, e^-\, rms/pix) readout noise over an area of 2k 4k and 110 4k pixels, respectively in a readout time of 17min. We observed two Lyman- emitting quasars (HB89\, 1159+123 and QSO\, J1621–0042) at redshift z 3. 5, two moderate redshift galaxy clusters (CL\, J1001+0220 and SPT-CL\, J2040-4451), an emission line galaxy at z = 0. 3239, a candidate member star of the Bo\"otes~II ultra-faint dwarf galaxy, and five CALSPEC spectrophotometric standard stars (HD074000, HD60753, HD106252, HD101452, HD200654). We present charge-quantized, photon-counting observation of the quasar HB89\, 1159+123 and show the detector sensitivity increase for faint spectral features. We demonstrate signal-to-noise performance improvement for SIFS for observations in the low-background, readout-noise-dominated regime. We outline preliminary scientific studies that will leverage the SIFS-Skipper CCD data to derive new cosmological measurements in the context of dark matter and galaxy evolution.