Triangulum Extended (TREX) Survey: I. An Automated Method for Precise Airglow Subtraction for Keck DEIMOS Multislit Spectra

The Deep Imaging Multi-Object Spectrograph (DEIMOS) on the Keck II 10-meter telescope on Manua Kea in Hawaii is one of the most powerful instruments in the world for obtaining spectra of faint objects at visible wavelengths. As such, DEIMOS has been a workhorse instrument for a large community of Keck observers since it was commissioned on the telescope nearly two decades ago. Since Keck and DEIMOS are ground based, the spectra are affected by the Earth’s atmospheric emission lines, or airglow. In this the first of a series of three AAS posters, we present an automated method for optimizing atmospheric airglow subtraction which is useful for spectroscopic studies of faint astrophysical targets. Using DEIMOS multi slit spectroscopic data from the Triangulum Extended (TREX) survey that contains emission lines associated with ionized gas in the disk of M33, we have developed mask design procedures, observing strategies, and empirical data reduction techniques that account for temporal variations in the atmospheric airglow (which are especially large for certain emission lines), residuals in the vignetting correction (flat fielding), residual wavelength calibration errors, and variations in the spectrograph focus across its field of view in order to optimize the quality of the atmospheric airglow subtraction. In particular, this optimization results in a significant reduction of the systematic errors associated with the subtraction of the strongest atmospheric airglow emission lines. This improvement in the airglow subtraction in turn enables the detection of very faint and rare emission lines associated with M33’s ionized gas disk and improves the measurement precision for emission line fluxes and centroid wavelengths (and therefore Doppler shifts and line-of-sight velocities). We plan to make our automated airglow subtraction software available to the Keck community.

This research was supported in part by the National Science Foundation and NASA/STScI. This research was conducted under the auspices of the Science Internship Program (SIP) at the University of California Santa Cruz. We wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to work with observations from this mountain.