The Tianlai-WIYN North Celestial Cap Redshift Survey

TL;DR

This work delivers a low-redshift optical redshift catalog in the North Celestial Cap to support 21 cm HI intensity mapping with the Tianlai array. It details target selection from the NCCS photometry, WIYN Hydra spectroscopy, and two redshift-determination approaches, resulting in 898 galaxies with redshifts and 11 cluster candidates over a $\sim$30 deg$^2$ area. The authors construct a formal selection function, compare clustering to SDSS-based mocks via the 2-point correlation function and 3D distributions, and identify HI mass fractions to template the HI content for cross-correlation with radio observations. Cross-matches with Gaia and 2MASS validate star contamination levels and catalog completeness, reinforcing the reliability of the tNCCSz sample for HI mapping applications. Overall, the study demonstrates the viability of optical redshift templates to calibrate and interpret low-$z$ HI intensity mapping from the Tianlai pathfinder and informs the distribution of HI within the surveyed volume.

Abstract

We present the results of a small, low redshift spectroscopic survey of galaxies within 3 degrees of the North Celestial Pole (NCP) selected using V-band photometry obtained from the North Celestial Cap Survey (NCCS) (Gorbikov & Brosch 2014). The purpose of the current survey is to create a redshift space template for 21 cm emission from neutral hydrogen with which to correlate radio line intensity observations by the Tianlai dish and cylinder interferometers. A total of 898 redshifts were obtained from the 2102 extended objects in the NCCS with m_V < 19 in the survey area. After accounting for extinction, the survey geometry and selection effects, the number density and clustering pattern of galaxies in the redshift catalog are consistent with other low redshift surveys. We were also able to identify 11 galaxy cluster candidates from this redshift catalog.

Figures (16)

• Figure 1: Left panel: Planck V-band extinction map showing $A_v$ (in magnitudes) in a 6$^\circ$ radius region around the NCP. We have used the Planck dust extinction map 2016AA...586A.132P to model the Galactic extinction. Note that the tNCCSz survey covers the central region to a radius of 3$^\circ$. One can notice that this area is subject to significant absorption by dust, reaching 1.7 magnitude in some directions covered by the survey. Right panel: Distribution of targets and associated tiling map for the tNCCSz survey. Color-coded by magnitude, the map highlights galaxies in a specific magnitude range for redshift analysis. The circular red outlines represent the 56 observational tiles, each corresponding to a unique pointing of the WIYN telescope to cover the declination range above $87^\circ$ and parts of the 86.6$^\circ$ to 87.0$^\circ$ range. The blued dashed circle represents the full width at half maximum of the primary beams of the TDA antennas at 1380 MHz ($z \sim 0.035$).
• Figure 2: Three spectra of $\delta$ Ursae Minoris obtained on March 7, 2022 are plotted in raw ADC counts per wavelength bin (scale on the left axis). For our configuration the wavelength bins are about 2.1 Å. The HST CALSPEC standard spectrum is plotted in physical units (scale on the right axis). The ratio of the average of the measured spectra to the standard establishes the normalization factor.
• Figure 3: Upper left: The spectrum of NCCS2343163 with a catalog magnitude of $m_V=14.56$ is the spectrum of a bright emission line galaxy. The clear $H_\alpha$ (6565 Å) emission line flanked by N[II] (6550, 6585 Å) dominates the spectrum. The measured redshift is $z=0.0158$. Upper right: The spectrum of NCCS3513807 with a catalog magnitude of $m_V=16.05$ is one of the brightest absorption line spectra that were obtained. The absorption lines are seen clearly at redshifted G-band (4307 Å), H$_\beta$ (4863 Å), Mg (5177 Å) and Na (5896 Å). The Ca-K (3935 Å) is present but difficult to discern from the noise until the wavelength scale is expanded while there is but a hint of the Ca-H (3970 Å) line. The measured redshift is $z=0.0543$. Lower left: The spectrum of NCCS3316833 with a catalog magnitude of $m_V=15.26$ is a typical spectrum of a bright star. There are clear hydrogen Balmer series lines as well as G-band (4307 Å) and Mg (5177 Å). Lower right: The spectrum of NCCS0424639 with a catalog magnitude of $m_V=17.91$ is a typical spectrum without any obvious features.
• Figure 4: Systematics template in the WIYN spectra. All spectra, after subtraction of a low-order polynomial, have been superimposed (blue dots). The red curve shows their median in wavelength bins of width 1 Å. The root-mean-square of the measurements distribution in each bin is shown in green. Areas where atmospheric or instrumental effects either cause large dispersion or prevent any measurements are clearly apparent. Note the "wiggles" at the long-wavelength end of the spectra.
• Figure 5: Results of the semi-automatic redshift determination procedures. The top panel shows the reduced spectrum of object NCCS426395, for which we measure $z=0.0956$ with template no. 24 from the SDSS collection. The bottom panel shows the reduced spectrum of object NCCS452010 for which we measure $z=0.1697$ using the template no. 41 from the AUTOZ collection, corresponding to an elliptical galaxy. See text for a detailed explanation.