J/AJ/169/84 Line doublets in near-infrared sky emission spectra (Dauphin+, 2025)
Hydroxyl lines and moonlight: A high spectral resolution investigation of
near-infrared skylines from Maunakea to guide near-infrared spectroscopic
surveys.
Dauphin F., Petric A., Artigau E., Stephens A.W., Cook N.J., Businger S.,
Flagey N., Marshall J., Ntampaka M., Ravindranath S., Rousseau-Nepton L.
<Astron. J., 169, 84 (2025)>
=2025AJ....169...84D 2025AJ....169...84D
ADC_Keywords: Spectra, infrared ; Spectroscopy;
Keywords: Night sky brightness ; Time series analysis ; Calibration ;
Infrared spectroscopy
Abstract:
Subtracting the changing sky contribution from the near-infrared (NIR)
spectra of faint astronomical objects is challenging and crucial to a
wide range of science cases such as estimating the velocity
dispersions of dwarf galaxies, studying the gas dynamics in faint
galaxies, measuring accurate redshifts, and any spectroscopic study of
faint targets. Since the sky background varies with time and location,
NIR spectral observations, especially those employing fiber
spectrometers and targeting extended sources, require frequent
sky-only observations for calibration. However, sky subtraction can be
optimized with sufficient a priori knowledge of the sky's variability.
In this work, we explore how to optimize sky subtraction by analyzing
1075 high-resolution NIR spectra from the Canada-France-Hawaii
Telescope's SPIRou on Maunakea, and we estimate the variability of
481 hydroxyl (OH) lines. These spectra were collected during two sets
of three nights dedicated to obtaining sky observations every 5.5min.
During the first set, we observed how the Moon affects the NIR, which
has not been accurately measured at these wavelengths. We suggest
accounting for the Moon contribution at separation distances less than
10° when (1) reconstructing the sky using principal component
analysis, (2) observing targets at YJHK magnitudes fainter than ∼15,
and (3) attempting a sky subtraction better than 1%. We also
identified 126 spectral doublets, or OH lines that split into at least
two components, at SPIRou's resolution. In addition, we used
Lomb-Scargle periodograms and Gaussian process regression to estimate
that most OH lines vary on similar timescales, which provides a
valuable input for IR spectroscopic survey strategies. The data
(https://zenodo.org/records/13363061) and code
(https://github.com/FDauphin/spirou-sky-subtraction) developed for
this study are publicly available.
Description:
The spectra used for our investigation were obtained with SPIRou
(SPectropolarimetre InfraROUge), a high spectral resolution
(R∼75,000) NIR spectrograph at the 3.6m Canada-France-Hawaii
Telescope (CFHT) on Maunakea.
We collected 1075 sky observations, which spanned from 2018 July 28 to
2022 January 10, or approximately 3.5yr. The observations were also
unevenly spaced, ranging from 1min to 12 weeks between observations.
In 2019 December and 2020 January, observations included two sets of
3 days dedicated to sky measurement where each day, a sky spectrum was
observed approximately every 5.5min for 12hr. Due to technical issue,
all sky observations during those nights where taken at zenith. These
days occurred on 2019 December 14, 15, and 16 and 2020 January 22, 23,
and 25. We refer to the first three days as Event 1 and the last three
days as Event 2.
Event 1 contained 348 spectra (32% of the 1075 observations) and
Event 2 contained 372 spectra (35% of the 1075 observations).
Event 1 was split between (89.925°, 170.269°) and
(89.925°, 179.973°), while Event 2 was only observed at
(89.925°, 170.286°).
The wavelength range of the spectra was 0.965-2.500um containing
285,377 wavelength bins resampled on a uniform wavelength grid with a
step of 1km/s/pixel. All observations were affected by a steep
blackbody curve starting at 2.1um, which was caused by thermal
emission. The exposure times were 300.884s for both Events 1 and 2.
Although the airglow intensity depends on the zenith angle, we ignored
correcting for the zenith since there was minimal angle variation for
Events 1 and 2.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 48 372 Doublet Candidates
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See also:
J/A+A/549/A8 : Atmospheric extinction properties above Mauna Kea (Buton+, 2013)
J/A+A/555/A78 : High-resolution IR airglow spectrum (Oliva+, 2013)
J/A+A/568/A9 : 300-2500nm flux calibration reference spectra (Moehler+, 2014)
J/A+A/581/A47 : Lines and continuum sky emission in the NIR (Oliva+, 2015)
J/A+A/648/A48 : SPIRou wavelength calibration (Hobson+, 2021)
Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 14 A14 --- Trans Transition (1)
16- 24 F9.3 0.1nm mu0 [9975.0/22516.8] Initial Guess at line center
26- 34 F9.3 0.1nm mu1 [9974.2/22513.8] Best Fit line center, closest
local maxima
36- 44 F9.3 0.1nm mu2 [9975.0/22516.8] Best Fit line center, next
highest local max
46- 48 A3 --- Doublet Yes/No flag noting if the line was identified
as a doublet (2)
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Note (1): The transition from Oliva+2015a (J/A+A/581/A47) and
Rousselot+ (2000A&A...354.1134R 2000A&A...354.1134R)
Note (2): Occurrences as follows:
Yes = 107 occurrences
No = 265 occurrences
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History:
From electronic version of the journal
(End) Prepared by [AAS], Robin Leichtnam [CDS] 21-Nov-2025