GEMINI OBSERVATORY observing time request (HTML summary)

Semester: 2005A		Observing Mode: queue
Instruments: GMOS North		Gemini Reference: Not Available
Time Awarded: Not Available

Partner Submission Details (multiple entries for joint proposals)

				NTAC
Partner	Partner Lead Scientist	Time Requested	Minimum Time Requested	Reference number	Recommended time	Minimum Recommended Time	Rank
United Kingdom	Hook	15.0 hours	15.0 hours	Not Available	0.0 hours	0.0 hours
United States	Perlmutter	7.5 hours	7.5 hours	Not Available	0.0 hours	0.0 hours
Canada	Pritchet	22.5 hours	22.5 hours	Not Available	0.0 hours	0.0 hours
	Total Time	45.0 hours

This continuing QR (quick response) proposal is for GMOS-N time; a similar request for GMOS-S time has been submitted.

Science Justification

Observations of SNe Ia, the power spectrum of the cosmic microwave background (CMB; e.g., Spergel et al. 2003), the properties of massive clusters (e.g., Allen, Schmidt, & Fabian 2002), and dynamical redshift-space distortions (Hawkins et al. 2003) yield a complementary, yet consistent, picture of a flat universe with Omega_Matter=0.3 and Omega_Lambda=0.7. In the redshift range around z=0.3-0.9, the SN results are most sensitive to a linear combination of Omega_Matter and Omega_Lambda, close to (Omega_Matter-Omega_Lambda). By contrast, galaxy clustering and dynamics are sensitive primarily to Omega_Matter alone, while the CMB is most sensitive to (Omega_Matter+Omega_Lambda). Although combinations of other measurements can lead to a separate confirmation of the universe's acceleration (e.g., Efstathiou et al. 2002), taken alone it is the SNe that provide the best direct evidence for dark energy, with a confidence greater than 99%.

One of the most pressing questions in cosmology now is: "What drives this acceleration?". There is a fundamental difference between a Cosmological Constant and other proposed forms of dark energy -- the former being equivalent to the vacuum energy (constant in time and space) as opposed to a slowly-varying scalar field (e.g., "dynamical Lambda" models such as quintessence). The distinction can be addressed by measuring the dark energy's average equation-of-state, <w>= <p/rho>, where w=-1 corresponds to a Cosmological Constant -- most scalar-field models predict w>-0.8.

Current measurements of this parameter (e.g. Knop et al. 2003; Tonry et al. 2003; Riess et al. 2004) are consistent with a very wide range of dark energy theories (see Peebles and Ratra 2003 for a comprehensive review). The importance of improving measurements to the point where <w>=-1 could be excluded has led to a second-generation of supernova cosmology studies: large multi-year, multi-observatory programs benefiting from major commitments of dedicated time (i.e. the ESSENCE survey at CTIO and the more ambitious SNLS described here). These "rolling searches" find and follow SNe over many consecutive months of repeated wide-field imaging, with redshifts and SN type classification from coordinated spectroscopy. Our over-arching goal is to derive a constraint on <w> by building an order-of-magnitude larger statistical sample (i.e. ~700) of SNe in the redshift range z=0.3-0.9 where <w> is best measured. With this sample, we aim to answer the key question: Is the dark energy something other than Einstein's Lambda?

CFHT LEGACY SURVEY -- AN UNPRECEDENTED SN Ia DATASET TO MEASURE DARK ENERGY

The CFHT Legacy Survey (http://cfht.hawaii.edu/SNLS/ and http://legacy.astro.utoronto.ca/) is an ambitious repeat-imaging wide-field survey conducted in 4 filters (g'r'i'z'), utilising an imager field four times larger than used in the next largest survey (ESSENCE, at CTIO), with twice as much time devoted to the survey. The full five-year CFHT "Supernova Legacy Survey" (SNLS; which began in August 2003 with pre-survey from March 2003) will provide the biggest improvement in the determination of the dark energy parameters achievable over the next decade. The 700 well-measured SNe Ia, together with an Omega_Matter prior known to +/-0.03 (i.e. 10%), will allow us to determine w to a statistical precision of +/-0.07, distinguishing between w>-0.8 and w=-1 at 3-sigma.

Though uncertainties due to K-corrections, gravitational lensing, and Malmquist bias are small compared to the statistical error of current SN samples, our large SNLS sample will reduce statistical errors to the point where some systematics may again become important. The SNLS dataset itself will allow powerful tests and place constraints on several of these key systematics, as the following examples indicate.

Multi-colour lightcurves: The rolling search with multiple filters (g'r'i'z') will generate the first large high-redshift SN Ia dataset with complete colour coverage throughout the lightcurves (see Fig. 2 for examples of typical SNLS light-curves). This enables comprehensive extinction studies since all the SNe are sampled over a wide, rest-wavelength baseline. We will measure the stretch-corrected peak magnitudes across a range of rest filters, thus breaking the degeneracy between dust and SN intrinsic colour.

High-statistics subsamples: Our recent study (Sullivan et al. 2003) divided high-redshift SNe into subsamples based on host galaxy morphology. This is an important first test of evolutionary and dust effects that will differ in different host galaxy environments. The large SNLS sample will allow us to perform such tests with much better statistics and in much more detail. As in Sullivan et al., the narrow galaxy emission and absorption lines detectable with Gemini spectroscopy of SN+host provide valuable constraints on host galaxy stellar populations.

Tests for spectral evolution: Folatelli et al. (2004) have made quantitative measurements of spectral features in a sample of low-redshift Type Ia supernovae, and have found correlations between certain features and SN luminosity. Our Gemini spectra have proved to be of high enough quality to extend this work over a wide range of redshifts and test for evolution in the properties of SNe Ia. Final reductions of Gemini spectroscopy and typing are complete through August 2004 and will be presented in Howell et al. (2004; in prep.). Measurements of spectral features and tests for evolution using the Gemini dataset are also well advanced (Bronder et al. in prep).

Conclusion: This continuing proposal focuses on the extraordinary science opportunities presented by the CFHT Legacy Survey. With a large increase in statistics for SNe at redshifts 0.3-0.9, the range most sensitive to w, we have made major strides in our ongoing multi-semester campaign to build a well-measured SN Ia Hubble diagram. These data are crucial for studying the cosmological parameters and the nature of dark energy. They also serve to refine our evolution/dust checks on systematics. By supporting this program Gemini will continue to play a leading role in this fundamental science.

Attachments:

Technical Justification

Now at the end of the first full year of the survey, SNLS has matured into a highly focused survey whose technical characteristics are well understood. We now obtain 90% of our monthly CFHT allocation, demonstrating the robustness of the "rolling search" technique to weather and instrument-related problems. The survey is on track to locate and identify over 2500 SN candidates (~40 per month) during the survey lifetime (Fig. 1), of which ~1000 will be SNe Ia at z<0.9. Thus, the size of our confirmed SN Ia sample will be limited only by the amount of spectroscopic follow-up time available. Over 100 spectroscopically confirmed SNe Ia have been discovered to date (Fig. 1), comprising the largest-ever sample of high-z SNe discovered by one telescope. At the current rate of 11 confirmed SNe Ia per month, we will have approximately 700 confirmed SNe Ia by the end of the survey (Fig. 1); this will form the largest and most homogeneous high-z SN sample available over at least the next decade. With Legacy Survey given the single top priority out of all projects observed at CFHT when observations are due (every 4th night during dark and grey time), our unprecedented discovery rate and time-sampling will remain unparalleled.

We have developed a sophisticated and reliable technique to optimise our spectroscopic follow-ups. Using the real-time (in contrast to final) g'r'i'z' photometry (Fig. 2), we are able to predict candidate redshift and phase -- as well as a probability that the candidate is a SN Ia -- after only two or three epochs of CFHT data. These predictions allow us to schedule follow-up time when a SN is at maximum light, efficiently rejecting AGN, variable stars and SNe II from our follow-up program. The results are an overwhelming success: since implementation of this selection technique (essentially 2004A), the 22 candidates for which we have completed reductions have produced 18 SNe Ia (an 80% success rate, compared with 50% in classical searches e.g. Lidman et al. 2004) with a mean redshift of 0.72. Example spectra are shown in Fig. 3 -- Gemini is playing the defining role in the high-redshift science (see also Fig. 1).

GEMINI DATA STATUS -- PATHWAY TO SCIENCE

We have obtained spectroscopy of 59 candidates in 120 hours of observations at Gemini N/S during 2003B, 2004A, and the first two months of 2004B. Final spectroscopic reductions are done immediately after receipt of the data from Gemini using our custom-written pipeline, and we make types and redshifts available on our website. Analysis of the first year of spectroscopic data from Gemini is complete and we will submit the first spectroscopic paper from the survey before the end of the year.

SNLS has recently entered the second year of observations, producing deep reference images which allow final subtractions and the first science to emerge. CADC releases final reductions of CFHT data around two weeks after each month's run is complete; this reduction has a superior fringe removal and photometric flat-fielding (better than 1% across the entire array) when compared to real-time data. Our final photometry pipeline then delivers science quality light-curves a month after the reduced data are available, and we are able to produce Hubble diagrams, including Gemini-observed SNe, within 5-6 months of the spectroscopic observations (the SNe must be followed for around two months after maximum light to fit their light-curves). The vital role of Gemini is highlighted in the preliminary SNLS Hubble diagram in Fig. 1. The completed analysis of the first year SNLS cosmological results will be ready for publication in Spring 2005.

GMOS OBSERVATIONS

The Gemini Observatory plays the leading role in the observations of the highest-redshift targets. Each supernova must be spectroscopically identified, classified, and a precise redshift determined for the heavy investment in multi-colour lightcurves to pay off in a Hubble diagram. In the range 0.6<z<0.9 (where Gemini observations are focused), the key features for the identification of a SNe Ia are redshifted into the region of the spectrum dominated by sky emission. Nod and shuffle (available only at Gemini) virtually eliminates the systematic errors associated with sky and fringe subtraction, allowing reliable automated error-weighted fits of the candidate's spectrum against a library of SN templates (combined with an expert's eye!) to determine the SN type (Fig. 3).

The overheads associated with Gemini observations make deep exposures an efficient use of telescope time (brighter candidates are observed at VLT). Gemini targets have i'=23-24.2, with GMOS exposures of between 1 and 2 hr. With an average exposure time of 1.5 hr, and 30 minute overhead per object, we again request 60hr (total from all partners, both telescopes) in order to observe approximately 30 SN candidates in 2005A. This is in line with our observations to date -- see above. All observations will use a 0.75" slit, the R400_G5305 grating, and OG515 order sorting filter, with one of two central wavelengths depending on the predicted redshift of the target. The slit PA is chosen to pass through both SN and host galaxy to obtain a SN type and a host redshift. Our experience shows that the dispersion of the R400 grating (~2A/pix) gives excellent nod and shuffle sky subtraction, and we rebin the data ~10x afterwards for SN typing (see Fig. 3).

SCHEDULING AND LOGISTICS

As SNe Ia will be discovered and reach maximum light throughout the semester, observing time should be spread throughout 2005A. We obtain SN detections in real time at CFHT (<24 hr turnaround), and the Phase 2 proposal is typically updated daily during each dark period. The coordinates quoted here are the coordinates for the search fields -- exact target and guide star coordinates will be entered into the Phase II file when known. This is a QR (quick response) proposal, for which the triggers are SN discoveries from SNLS.

INTERNATIONAL COLLABORATION

Spectroscopy on 8m class telescopes is essential for this project to succeed; the total amount of spectroscopic time needed is well beyond the reach of any one group or nation. We are applying for 60 hrs of Gemini time this semester (30 hrs Canada, 20hrs UK, 10 hrs US). The Gemini-S request is for 15 hr (7.5 hr Canada, 5 hr UK, 2.5 hr US) and the Gemini-N request is for 45 hr (22.5 hr Canada, 15 hr UK, 7.5 hr US). Since we are now required to have a single PI, who will be the principal point of contact, we have chosen Isobel Hook due to her familiarity with GMOS and the Gemini queue observing system. We are also applying for 60 hr of VLT time (PI: Pain) in 2005A, and for 4 nights of classically scheduled Keck-LRIS time every "A" semester (PI: Perlmutter) when the Groth Strip (D3) cannot be seen by VLT or Gemini-S.

The co-investigators on this proposal represent the core collaboration; see http://snls.in2p3.fr/people/snls-members.html for a complete list.

REFERENCES

Allen, Schmidt & Fabian, 2002, MNRAS 334, 11 Efstathiou et al., 2002, MNRAS 330, 29 Hawkins et al., 2003, MNRAS 346, 78 Knop et al., 2003, ApJ 598, 102 Lidman et al., 2004, A&A, in press Peebles & Ratra, 2003, Reviews of Modern Physics 75, 559 Perlmutter et al., 1999, ApJ 118, 1766 Riess et al., 1998, AJ 116, 1009 Riess et al., 2004, ApJ 607, 665 Spergel et al., 2003, ApJS 148, 175 Sullivan et al., 2003, MNRAS 340, 1057 Tonry et al., 2003, ApJ 594, 1

Observation Details

Resources

• Gemini North
  • GMOS North
    • Focal Plane Unit
      • Longslit 0.75 arcsec
      • Nod and Shuffle 0.75 arcsec
    • Filter
      • GG455_G0305
      • OG515_G0306
    • Disperser
      • R400_G5305

Observing Conditions

Name	Image Quality	Sky Background	Water Vapor	Cloud Cover
SN Spec	70%	50%	Any	50%

Scheduling Information:

Synchronous dates:

Optimal dates:

Impossible dates:

Allocation Committee Comments

Additional Information

Keywords: Cosmological distance scale, Survey

Publications:

• Hook, I. et al. (2004), "The Gemini North Multi-Object Spectrograph: Integration, Tests, and Commissioning", PASP, 116, 425
• R.Knop, et al, (2003), New Limits on Cosmological Parameters Omega_M and Omega_Lambda from HST Observations of Eleven Supernovae at Redshifts z=0.36-0.86, ApJ, 589, 102
• Sullivan, M. et al. (2003), "The Hubble Diagram of Type Ia Supernovae as a Function of Host Galaxy Morphology", MNRAS, 340, 1057
• Wang, L., Goldhaber, G., Aldering, G., and Perlmutter, S. (2003), "Multi-Color Light Curves of Type Ia Supernovae on the Color-Magnitude Diagram: a Novel Step Torward More Precise Distance and Extinction Estimates", ApJ, in press. astro-ph/0302341
• van den Bergh et al (2002), "Classifications of the Host Galaxies of Supernovae", PASP, 114, 820, 2002
• Howell, D.A. (2001), "Progenitors of Subluminous Type Ia SNe", ApJL, 554, 193
• Howell, D. A., Hoeflich, P. Wang, L., and Wheeler, J. C. (2001), "Evidence for Asphericity in a Subluminous Type Ia Supernova: Spectropolarimetry of SN 1999by", ApJ, 556, 302
• Pain R. et al (2001) : "The Distant Type Ia SN rate" 2002, ApJ, 577, 120
• Howell, D. A., Wang, L., Wheeler, J. C. (2000) "The Distribution of High- and Low-Redshift Type Ia Supernovae", ApJ, 530, 166
• Perlmutter S. et al (1999) "Measurements of Omega and Lambda from 42 High-z Supernovae", ApJ, 517, 565
• Perlmutter S. et al, (1998) "Discovery of a supernova explosion at half the age of the Universe and its cosmological implications", Nature 391, 51.
• Perlmutter S. et al (1997) "Measurement of the cosmological parameters Omega and Lambda from the first 7 supernovae at z>0.35", ApJ, 483, 565

Allocations:

Reference	Time	% Useful	Comment
GN-2004B-Q-16	45.0 hours		Followup spectroscopy for CFHTLS supernovae. 2/6 months completed, 22 hours used to date.
GS-2004B-Q-31	15.0 hours		Followup spectroscopy for CFHTLS supernovae. Band 2; no time used to date.
ESO VLT 20003A/3B/04A/04B	240.0 hours		French + other European collaborators time on FORS1 (VLT) for CFHTLS SN followup. 60 hr allocated in 2003A/03B/04A/04B. The PI is R. Pain.
GN-2004A-Q-19	43.0 hours	100	Followup spectroscopy for CFHTLS supernovae.
GS-2004A-Q-11	17.0 hours	7	Followup spectroscopy for CFHTLS supernovae. Weather and competition with other Band 1 programs led to a lower than expected completion rate.
GN-2003B-Q-9	45.0 hours	100	Followup spectroscopy for CFHTLS supernovae.
GS-2003B-Q-8	15.0 hours	63	Followup spectroscopy for CFHTLS supernovae. Weather and competition with other Band 1 programs led to a lower than expected completion rate.

Proposal Contents