First results from the Stockholm VIMOS Supernova Survey

Transcription

1 First results from the Stockholm VIMOS Supernova Survey - Detection efficiencies and photometric accuracy in supernova surveys

2 Outline The Stockholm VIMOS Supernova Survey, SVISS. First results from the SVISS. Detecting SNe in large surveys, testing the method

3 Supernovae as probes of the high redshift universe Core collapse SNe can be used as probes of ongoing star formation in distant galaxies. Star formation history of the universe. Thermonuclear SNe can be used as tools for obtaining cosmological parameters since they are standard candles. Requires that spectra have been obtained of the supernova. Requires assumptions on the initial mass function and dust content of galaxies at high redshift. Many SNe needed to beat down statistical errors.

4 Supernova rates, previous results CC Dahlen et al, 2004

5 Supernova rates, previous results Dahlén et al, 2008 Ia Dahlen et al, 2004

6 The Stockholm VIMOS Supernova Survey, SVISS

7 People Claes Fransson, Stockholm (PI) Göran Östlin, Stockholm, Stockholm, PhD student Seppo Mattila, Laia Mencia Trinchant, Stockholm, PhD student Tomas Dahlen, STScI Jesper Sollerman, Stockholm/Copenhagen Soroush Nasoudi-Shoar, previous MSc student + external collaborators: Bruno Leibundgut & Roberto Gilmozzi, ESO Tommy Wiklind & Duilia de Mello, Baltimore

8 Project Strategy ESO large programme, 114 hours Deep monthly imaging with VLT/VIMOS in R- (2 h) and I-bands (4h) of 2 fields Seeing < 0.8 Fields: parts of CDF-S and ELAIS-S1 7 epochs per field, + 2 reference epochs 5-sigma limit I=26.2, R=27.2 per epoch (Vega) Additional UBV images, not coincident with RI, to facilitate photo-z s (seeing < 0.8 ) Initial problems, but the survey is complete now.

9 Scientific objectives Detect more than 50 SNe at z > 0.5, roughly 2/3 CC, 1/3 Ia SFR through CC rate at z > 1 - Less sensitive to dust than restframe UV luminosity, no lum bias Ia progenitors and timescales Metal production history Comparison with host galaxy properties: SFR SFH Luminosity, stellar mass Morphology (HST), IR, etc - Deep field studies: LF, correlation, galaxy populations

10 SN Survey epochs 7 search epochs + 1 reference epoch in R and I Epoch Reference I (5σ)* FWHM (arcsec) *limiting magnitude calculated within an aperture of diameter 4*FWHM using the noise in the empty field in the unsubtracted epoch images

11 Colour composite of 2x2.5 arcmin part of ELAIS-S1, BVR filters Deep Field Science with coadded images 5-sigma limit mag: U* B* V* R I 10h 9h 6h 14h 30h * Elais only, VEGAMAG

12 SN classification Light curve with > 3 points Colour and colour evolution Photometric redshift of host galaxy 2 reference epochs facilitate AGN removal

13 SN light curve templates We have built a library of light curve templates for the different supernova types and redshifts, k-correcting and correcting to the VIMOS R and I filters. Ia IIP Rest-frame spectra and light curves are from P. Nugent. Peak magnitude scaling from Dahlén et al. 2004

14 SN classification II The observed light curves of our SN candidates will be matched to the light curve templates and the bestfitting template gives the type. This is done with straight forward chi square fitting. The free parameters of the fitting (for each type) is: - Peak magnitude (within a certain spread) - Redshift (uncertainty from photometric z estimate) - Dust extinction - Stretch parameter for Ia SNe.

15 Detecting supernovae Accurate registering Image subtraction using modified version of ISIS (Alard & Lupton 1998) Detect SNe in subtracted image using SExtractor (Bertin & Arnouts 1996) PSF (DAOPHOT) photometry in subtracted image Tests of detection efficiency and photometry using simulated sources (DAOPHOT/ADDSTAR)

16 Reference I-band Example SN candidate 1 Epoch 1 Difference

17 Reference I-band Example SN candidate 1 Epoch 1 Difference

18 Reference I-band Example SN candidate 2 Epoch 1 Difference

19 Reference I-band Example SN candidate 2 Epoch 1 Difference

20 I ref epoch I epoch 1 I subtracted V band, deep Example SN candidate 3

21 I ref epoch I epoch 1 I subtracted V band, deep Example SN candidate 3

22 Preliminary results 35 candidates detected in I, with 3+ points on the light curve. - Estimated redshift distribution (based on the depths), a mean z of ~ (depending on type). A few of these will most probably turn out to be false detections, and a few will be variable sources of non-sn type.

23 Simulations of SNe The method has been tested by simulating a large number of supernovae in the I band search epoch images (the real data images) both in- and outside galaxies. A total of 2660 galaxies were detected in the reference image. We simulated supernovae in 10 % of the galaxies (i.e. 266 supernovae) per magnitude bin (we used 17 magnitude bins, 22.0 < I < 27.0, with smaller stepsize in the fainter end). The total number of SNe simulated in each test run was approximately Additional test runs were made using only host galaxies of a certain brightness.

24 Detection and photometry tests All tests have been run with the same reference epoch and in the I band. SN position within host Compare results from different epochs Host galaxy brightness Stamp selection (more ISIS parameters)

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26 Comparing epochs, data from four epochs

27 Comparing epochs, data from four epochs

28 Host brightness

29 Pixel histograms, data from epoch 2 (upper) and epoch 5 (lower)

30 Results from the detection tests The two main conclusions we draw from the detection tests: The SN survey pipeline works fine, we are able to find and do photometry of SNe down to the limits given by the data quality. The subtraction method does a good job, in general, but has some problems for sources placed inside galaxies. There is a small flux offset for sources inside galaxies which affects both the photometry and the detection efficiency. This affects the faint end of the found SN distribution (i.e. the high-z end), and should be treated as a systematic error. It can be corrected for by using simulations.

31 Survey status and outlook All data in hand + system calibration Detection procedure tested out (Melinder et al submitted) SN catalogue and typing, work in progress Followup: NIR, host galaxy spectra SN detection tests has implications for planned SN surveys (e.g. PANSTARRs, IR SNe in LIRGs) Very deep galaxy catalogues for ELAIS-S1, useful for multiwavelength galaxy science.