a.6. OH Emission Line Suppression for DETF Stage IV Supernovae

6.a.6. OH Emission Line Suppression for DETF Stage IV Supernovae

The sky background in the NIR is an obstacle to a major leap forward in supernova cosmology. While new surveys such as DES and LSST, and precision low-z measurements such as Supernova Factory, will make substantial progress in the next decade, they will not alone achieve the “Stage IV” goal of the Dark Energy Task Force. The August 2012 report [2] of the DOE/HEP Dark Energy Science Committee outlined the needed steps for each dark energy technique. In terms of improving supernovae constraints significantly, the report says “… a space mission capability may not be likely to be achieved in the next decade; there is a need to explore ground-based alternatives, combining near-IR technology with atmospheric-sky-line suppression…”

Supernovae cosmology constraints benefit from precise measurements in the NIR in three ways:

1. 
   In order to reduce supernova brightness systematics, it is critical that observations cover the same portion of the rest-frame SN spectrum at high and low redshifts. Already at redshifts z>0.7 (DES and LSST range), this requires some observations in the NIR. Note that the farthest NIR filter in DES, Y-band, is not used in supernova measurements since the sky background from OH lines is too large and the signal-to-noise is too small to be useful.
   
2. 
   Supernovae observations with good statistics at redshifts z > 1 significantly improve the constraints on the time dependence of dark energy. Figure 6 (right) shows the rest-frame SN spectrum, as well as the same spectrum at z=2.0, each normalized to their peak flux. Note that the z=2.0 spectrum lies entirely in the NIR region.
   
3. 
   It is well documented that the effects of dust on brightness measurements are reduced in the NIR.
   

The background from OH emission is not a new problem, what is the current state-of-the-art in suppression? This issue has been the focus of Australian astronomy for many years, led by Joss Bland-Hawthorn at the University of Sydney, and Simon Ellis at the Australian Astronomical Observatory (AAO). They have built a prototype system using optical fiber devices called Fiber Bragg Gratings (FBG), but it only suppressed ~100 of the ~1000 OH lines. In addition, in a recent astronomy photonics review by Bland-Hawthorn [3] this system was described as “too elaborate for mass production.” The same review paper states “One promising technology is the micro-ring resonator used in telecommunication devices for adding or dropping discrete wavelengths.”

• 
  We are committed, long-term, to advance the supernova dark energy technique to its ultimate sensitivity.
  
• 
  We are also investigating, along with Saul Perlmutter at LBNL, a near-term goal of implementing a demonstration system, in the southern hemisphere, to follow-up detections of DES supernovae.
  
• 
  Kyle Barbary, current Argonne Director’s Fellow and Saul Perlmutter’s student, worked on OH suppression while at LBNL and brings a lot of expertise.
  
• 
  Kyler Kuehn, the ANL post-doc that led the PreCam project, took a staff position at AAO working with Simon Ellis part-time on OH suppression. We are currently working with Kuehn and Ellis on ring resonator designs.
  
• 
  The Argonne Center for Nanoscale Materials (CNM), has experience with ring resonators and is an excellent place to investigate custom designs. As a Basic Energy Sciences “user facility”, approved proposals are cost-free for users. We submitted, and have received approval, a user proposal to fabricate ring resonators for FY14.
  
• 
  Other facilities at Argonne, such as our Material Sciences Division, could be useful for the fabrication of custom designs of other devices such as Volume Bragg Gratings.
  

• 
  Modified our DECam CCD test-stand to test OH technologies.
  
• 
  Working with Kuehn and Ellis at AAO, developed an initial ring resonator design.
  
• 
  Submitted a successful user proposal, to the ANL Center for Nanoscale Materials, to fabricate ring resonators in FY14.
  
• 
  Working with SULI student Nick Miller, tested several off-the-shelf Fiber Bragg Gratings in this test-stand (details below).
  
• 
  Miller also wrote a Python-based simulation of Bragg Gratings, both fiber and non-fiber, that we are using to guide future designs.
  

[1] S.C. Ellis and J. Bland-Hawthorn, “The case for OH suppression at near-infrared wavelengths”, Monthly Notices of the Royal Astronomical Society, Volume 386, (2008).

[2] Albrecht et al., “The DOE/HEP Dark Energy Science Program: Status and Opportunities”, http://science.energy.gov/~/media/hep/hepap/pdf/20120827/DE_Science_Program_FINAL_Aug2012.pdf

[3] J. Bland-Hawthorn and Pierre Kern, “Molding the flow of light: Photonics in astronomy”, Physics Today Volume 65, (2012)