Method on Using Shadow Altitude to Remove Geocoronal H$α$

TL;DR

This work introduces a shadow-altitude–based method to subtract geocoronal H$\alpha$ contamination from intermediate-resolution sky spectra, enabling reliable Galactic H$\alpha$ studies in extended targets. By leveraging MaStar blank-sky fibres, the authors establish a broken-linear relation between geocoronal H$\alpha$ flux and shadow altitude in log space, and they compare it to a solar-altitude–based approach. The shadow-altitude method achieves a substantially lower fractional residual ($\delta=23.52\%$) than the solar-altitude method, highlighting improved predictive power, though intrinsic scatter persists due to geometric, solar-cycle, and DIG-related effects. The technique offers a practical path for accurate Galactic H$\alpha$ measurements in upcoming intermediate-resolution IFU surveys and can inform sky-subtraction strategies for related hydrogen recombination lines.

Abstract

Spectroscopic surveys allow spatially resolved spectroscopy of galaxies to study their interstellar medium (ISM). However, observations of Galactic H$α$ emission are contaminated by geocoronal H$α$ emission. The latter is known to depend on the shadow altitude, a geometric parameter relating the line of sight to Earth's shadow cone. Using fibres on blank skys from the SDSS-IV/MaStar survey, we established an empirical relation between the geocoronal H$α$ emission and the shadow altitude, with a root mean square fractional scatter of 23.52$\%$. This relation can be used to predict geocoronal H$α$ emission so that it can be removed from observed spectra. This removal method is advantageous when the observed targets are extensive in the sky, and it does not require a large velocity separation between the observed target and the local standard of rest. This will enable reliable studies of Galactic H$α$ in intermediate spectral resolution integral field spectroscopic surveys. We also find tentative evidences for the dependences of geocoronal emission on solar activity and the distance between the Earth and the Sun.

Figures (17)

• Figure 1: Full sky Galactic $\rm H\alpha$ map from finkbeiner2003full, with the MaStar visits filtered for quality flags in Table \ref{['table:visit_bitmask']} overplotted (red crosses).
• Figure 2: Histogram of Galactic $\rm H\alpha$ SB retrieved from the full sky $\rm H\alpha$ map using the unique pointing of each IFU, with the criterion used overplotted as the red vertical line.
• Figure 3: Hexagonal fibre arrangement within an example 127 fibres IFU (plate 8051, MJD 57057, mangaid 27-1858, exposure id 193985, IFU-design 12705). The excluded large-flux fibres are marked with black dots, their neighbouring fibres are marked with red dots, and the remaining fibres are marked with blue dots. The numbers above the dots are fibre ids. The excluded large-flux fibres are located at the centre as expected for the star location.
• Figure 4: The high spectral resolution solar template spectrum provided by BASS 2000 (blue curve) and the processed solar template spectrum in 1 Å resolution (orange curve) are shown in vacuum wavelength and normalised flux, with no solar template shift ($k_f = 0$).
• Figure 5: Solar amplitude, $a$, at different angular separation between the moon and the IFU (y-axis) and linear illumination fraction (x-axis). Colour bar represents the median $a$ in each pixel. A clear trend of increasing $a$ towards the lower right corner can be seen.