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Spectral Templates

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Spectral templates (usually late type stars) are required for the analysis of kinematical data on external galaxies (or other stellar ensembles). In the near-IR, the most commonly used features are the CO overtone bands at λ > 2.3µm. Although observational and theoretical libraries exist at lower spectral resolutions (R<3000), no comprehensive set of stellar kinematic templates was available to be used with two of the configurations of Gemini NIR instruments used for stellar population kinematic studies in external galaxies - NIFS and GNIRS 111 l/mm grating  (both longslit and IFU) - and all programmes using those configurations would invariably spend some science time taking a small set of stellar spectra to use as templates. This led to a constant duplication of data taking, since those targets are programme calibrations and are not made available to other users until the end of the default 18 months proprietary period.

During semester 2006B at Gemini South, given the extraordinarily poor conditions over the whole semester, and the eventual end of true "poor weather" programmes in the queue, a Director's Discretionary "poor weather" GS-2006B-DD-3 programme was carried out to provide the NIR community with a larger set of late (F7 to M3 types I, II, III and V) stellar spectra, with intermediate S/N (30-50), including the four CO overtone bands (2.24-2.42µm) at R~6000 resolution. A subset of the targets was also observed at a slightly bluer spectral range to improve usefulness for NIFS users, overlapping with the red setting on the first two CO bands.

To the original sample of 29 stars observed with GNIRS, another 11 were added from NIFS observations obtained as part of programmes GN-2006A-SV-123, GN-2006B-Q-107, GN-2007A-Q-25, covering the full range 2.1 to 2.5µm at a similar resolution.

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Library Description

GNIRS sample
Instrument configuration
 GNIRS IFU+111 l/mm grating in the K band
"Red" setting
 λc=2.335µm (2.24-2.43µm), dλ=1.84Å/pixel
FWHM (arc lines) = 1.82 pixel
"Blue" setting
 λc= 2.245µm (2.15-2.33µm), dλ=1.85Å/pixel
FWHM (arc lines) = 1.88 pixel
Combined spectrum
 2.15-2.43µm, rebinned to dλ = 1Å/pixel
Observed sample
 29 objects in the "red" setting, from F7III to M3III
Of these, 23 objects were also observed in the "blue" setting
NIFS sample
Instrument configuration
 NIFS IFU+K grating+HK filter, 3.0arcsec or KG3+ND masks
Original sampling
 λc=2.20 and 2.25µm (2.07-2.47µm), dλ=2.13Å/pixel
FWHM (arc lines) ~ 3.2
Observed sample
 3 objects centred at 2.20µm, 8 centred at 2.25µm, spectral types from G8II to M5III


The GNIRS sample

The observed sample was selected from a list kindly provided by Greg Doppmann, compiled from the literature (mostly based in Cayrel de Strobel et al 1997), and the selection was based exclusively on observability: targets which were visible for as long as possible during the 06B semester, bright enough to provide the desired S/N on a reasonable on-source time under CC=90, IQ=ANY conditions, and having a hot (A0-A7) star close enough (and bright enough!) to be used for telluric correction. The fact that both target and telluric stars also had to have a bright (V<13mag) guide star available as well, restricted even more the choices.

The observing conditions also determined the instrument configuration: to achieve R=5900 with GNIRS in long slit mode, one would have to use the 0.30" slit - implying in very large slit losses under IQ=Any (FWHM>0.80" in K) seeing. Given the superior GNIRS IFU performance in the K band (over 90% of that of the equivalent long slit mode), there was only a small loss in sensitivity by using the IFU+111 l/mm grating configuration. On the other hand, the small field-of-view of the IFU required the observations to be carried out in a object-sky-sky-object pattern, but even with a 50% reduction in on-source time,  this setup still proved to be very efficient at the telescope.

The NIFS sample:

The data were obtained either as programme calibrations for GN-2006A-SV-123 and GN-2007A-Q-25, and therefore based solely in observability and brightness; or as part of another "poor weather" programme GN-2006B-Q-107, and in this case following the same rationale as the GNIRS sample (bright enough for poor conditions, with proper telluric and guide stars available). The stars observed as part of 06A-SV-123 and 07A-Q-25 used the AOWFS for guiding, while for 06B-Q-107, the AO fold was parked and guiding was done using the PWFS2.

All the GNIRS data collected under programme GS-2006B-DD-3 has been made public from the start in the Gemini Science Archive. The NIFS data are subject to the standard proprietary period. This page contains links to the table of final processed spectra, details of the data reduction, and some examples of the template use with actual GNIRS and NIFS science data. The community at large is welcome to download all or part of the library as needed, and users of GNIRS or NIFS are encouraged to explore its use as an alternative to requesting further observation of spectral standards with their science programmes. If the raw data are re-processed to be used in papers or publications, please use the standard Gemini acknowledgement text and the above programme IDs. For the processed data contained in this page, the authors would appreciate acknowledgement of the library use (Winge, Riffel and Storchi-Bergmann (in preparation), please point to this URL until the actual paper appears).

The data are presented in standard FITS format, and the user can select either the GNIRS red (2.24-2.42µm) or blue (2.15-2.32µm) spectral ranges at their native spectral binning; or the combined spectrum (when both ranges were observed), rebinned to 1 Å/pix. The NIFS spectra are also presented at native binning and rebinned to 1 Å/pix.

The authors would like to thank the Gemini Deputy Director and Head of Science, Jean-Rene Roy, and the former Gemini South Head of Science Operations, Michael West, for the support and time allocation for our GNIRS programme. Many thanks as well to all the Gemini South observers and SSAs that so positively believed that no conditions were ever too poor to give GS-2006B-DD-3 a chance!

Current release is V 1.5 - NIFS data included, full headers added to all spectra, data corrected to rest wavelength.

History and release notes:

2009 Jan 31 - Version 1.5 uploaded to the Gemini Web Site

  • Highlights
    • NIFS data added!
    • Complete headers from the last step of reduction before extracting the spectra to 1-D added back to all spectra.
    • Continuum shape removal improved prior to combining the GNIRS settings.
    • Final spectra are corrected to rest velocity. This was done by taking a strong, isolated line at 2.2814µm and using that as a reference point to all the remaining spectra. This was done to correct in just one step the intrinsic radial velocity and any zero-point offset that could exist from the wavelength calibration.
  • Still ToDo:
    • Flux calibration.
    • Telluric correction in the Brγ region.
    • Error vectors.

2007 Apr 10 - Version 1.0 uploaded to the Gemini Web Site.

  • Highlights and ToDo list
    • The "blue" setting spectral range currently starts at 2.18µm. After tinkering with the telluric correction for a while, I'm still not satisfied with the result in the Brγ region.
    • The image headers are NOT complete. The original GNIRS files are MEF, so most of the information is located in the primary header unit (extension [0]), and gets lost when the spectrum is extracted. This is also in the list for the next release.
    • Final spectra are:
      • NOT flux calibrated. The continuum shape has been removed by fitting a low order polynomial. I decided to go this way rather than tackle the issue of flux calibrating the spectra for this release.
      • NOT corrected for Galactic extinction.
      • NOT corrected for heliocentric radial velocity.
    • Error vectors (variance planes) NOT included. This is critical for a good evaluation of the errors in the cross-correlation results, but the variance planes are not being propagated correctly through the reduction process.
Last update 2009 January 31; Claudia Winge