Lines and continuum sky emission in the near infrared: observational constraints from deep high spectral resolution spectra with GIANO-TNG

Abstract

Aims. Determining the intensity of lines and continuum airglow emission in the H-band is important for the design of faint-object infrared spectrographs. Existing spectra at low/medium resolution cannot disentangle the true sky-continuum from instrumental effects (e.g. diffuse light in the wings of strong lines). We aim to obtain, for the first time, a high resolution infrared spectrum deep enough to set significant constraints on the continuum emission between the lines in the H-band. Methods. During the second commissioning run of the GIANO high-resolution infrared spectrograph at the La Palma Observatory, we pointed the instrument directly to the sky and obtained a deep spectrum that extends from 0.97 to 2.4 µm. Results. The spectrum shows about 1500 emission lines, a factor of two more than in previous works. Of these, 80% are identified as OH transitions; half of these are from highly excited molecules (hot-OH component) that are not included in the OH airglow emission models normally used for astronomical applications. The other lines are attributable to O 2 or unidentified. Several of the faint lines are in spectral regions that were previously believed to be free of line emission. The continuum in the H-band is marginally detected at a level of about 300 photons/m 2 /s/arcsec 2 /µm, equivalent to 20.1 AB-mag/arcsec 2 . The observed spectrum and the list of observed sky-lines are published in electronic format. Conclusions. Our measurements indicate that the sky continuum in the H-band could be even darker than previously believed. However, the myriad of airglow emission lines severely limits the spectral ranges where very low background can be effectively achieved with low/medium resolution spectrographs. We identify a few spectral bands that could still remain quite dark at the resolving power foreseen for VLT-MOONS (R≃6,600).

First demonstration of OH suppression in a high-efficiency near-infrared spectrograph

“…In Bland-Hawthorn et al (2004) and Ellis & Bland-Hawthorn (2008) we argued that OH suppression should significantly lower the interline continuum, since some large fraction of typical measured values will be due to scattered OH light, some support for which was found in a detailed study of the NIR night sky spectrum by Sullivan & Simcoe (2012). Oliva et al (2015) stated that following the GNOSIS results we retracted this claim in Ellis et al (2012); in fact the thermal emission in both GNOSIS and PRAXIS have prevented an accurate characterisation of the true interline continuum, and this issue is not yet settled.…”

“…The lack of reduction in the interline continuum could be due to the presence of real emission of unknown origin (see e.g. Content 1996;Sullivan & Simcoe 2012;Oliva et al 2015;Nguyen et al 2016), which could be dependent on the observing site, but interpretation is confounded by the low signal-to-noise of the measurements.…”

The CARMENES search for exoplanets around M dwarfs

“…In the J band, no prominent features seem to be missing in the models, and none of the features predicted by the models are missing in the spectra. The long exposure of the M7 star shows a number of sky emission lines from OH airglow (Oliva et al 2015). …”

Technical note: Bimodality in mesospheric OH rotational population distributions and implications for temperature measurements

“…Both groups also determined that the rotational temperature of OH(v = 8) was significantly higher than that for OH(v = 9). Figure 1 summarizes the results on OH rotational temperatures reported by Cosby and Slanger (2007), Oliva et al (2015), and Noll et al (2016). Despite some variation, possibly due to the different location, time, and instrument for these measurements, these data sets show a similar trend for the vibrational level dependence of the OH rotational temperatures.…”

“…Despite some variation, possibly due to the different location, time, and instrument for these measurements, these data sets show a similar trend for the vibrational level dependence of the OH rotational temperatures. Additional details and the Oliva et al (2015) data set can be found in the original publications as well as in Kalogerakis (2017).…”