Download poster

The VLT Spectroscopic Program
of the SuperNova Legacy Survey
BASA Stéphane, on behalf of the SNLS collaboration*
[email protected], Laboratoire d’Astrophysique de Marseille
*Complete member list on http://cfht.hawaii.edu/SNLS
The SuperNova Legacy Survey
Since August 2003, the SuperNova Legacy Survey
(SNLS) collaboration is engaged in a new high
redshift supernova survey using the newly
commissioned 1 square degree wide field imager
MegaPrime, a joint project of CFHT and
CEA/DAPNIA.
The imaging program of SNLS is part of the CFHT
Legacy Survey (CFHTLS) program, a large multi
purpose imaging survey. It will use 202 dark/grey
nights of observations over 5 years.
As a fundamental complement to these
observations, a spectroscopic program is carried out
on VLT, Gemini North and South, and Keck to
identify supernova type and measure the redshift,
but also to identify host galaxy type, study
supernovae evolution, observe UV emission, …
“Real time” analysis
Observations
D2
D3
D4
D1
Spectra are reduced with a Midas pipeline and
analyzed with home made programs:
• Production of 2D calibrated frames
(wavelength and flux calibrated).
• Extraction of the candidate and of the host
galaxy spectra if possible, with the associated
statistical errors.
Delay between the observation submission and
the spectrum analysis is reduced to less than 24 hr.
Sharing between the different spectroscopic
telescopes follow very simple rules:
Identification method
 Bright candidates sent to VLT: smaller
overheads.
 Faint candidates equitably sent to VLT and
Gemini.
 VLT replaced by KECK for D3 field which is
unobservable from Paranal.
It is based on a c2 fitting of the observed spectrum
with a model which is a linear combination of
supernova and galaxy templates.
2
.
c2=S
Sobs(lobs) – (aSsn(lrest [1+z])+bSgal(lrest [1+z]))
s(lobs)
l
The VLT spectroscopic program
ESO has allocated a Large Programme on FORS1
to the SNLS collaboration:
Candidates are observed at their luminosity maximum
to increase the Signal to Noise ratio.
All the VLT and Gemini observations are conducted in
Service Mode
Model composed of supernova
and galaxy templates
Each combination is then weighted by the c2
probability to measure the supernova properties and
the associated errors.
240 hours spread over 4 semesters.
Analysis
Example of observed supernovae
Ia at z=0.93
Peculiar Ia at z=0.28
Type and redshift distributions
Host type identification
65 candidates have been observed between June
2003 and May 2004.
Identification is mainly based on two methods:
Type
SNIa
/ SNIa?
SN
Ic
SN II /
SNII?
SN?
AGN/QSO
and high-z
galaxy
Number
37
1
3
12
12
Redshift is measured by using emission and/or
absorption lines of the host galaxy when it can be
extracted separately from the candidate (about 67%
of the observations). Otherwise, it is estimated by
using the supernovae features.
• A c2 fitting of the observed galaxy spectrum with
the templates defined by Kinney and al.
• A Principal Component Analysis (PCA). This method
decreases the number of variables and makes
possible a spectral classification of the galaxies.
Host Galaxy
Early E0,S0
Type
Fraction
Early Spiral
Sa, Sb
Late Spiral
& starburst
30.3%
42.4 %
27.3 %
UV emission
For high redshift supernovae, the blue part of the
optical spectrum correspond to the UV part of the
rest-frame spectrum.
SNIa /
SNIA?
Ic at z=0.17
Observations at an intermediate redshift, z=0.5-0.6,
offer a unique opportunity to observe UV emission.
SNII /
SNII ?
The redshift distribution of type Ia supernovae is
comprised between 0.2 and 0.95 (mean value = 0.55).
Ia at z=0.63
Texpo = 3600 sec.
Typical exposure time: 2160 to 3600 sec.
Conclusion
Between June 2003 and May 2004, 65 candidates have been observed by VLT
and 37 type Ia supernovae identified with a redshift comprised between 0.2 and
0.95.
Primary program of the VLT Large Programme (type identification and redshift
measurement) is fulfilled.
Thanks to the homogenous and large sample, many other studies are under way:
host galaxy identification, UV emission, …
The program is ramping up: at the end of the SNLS program, more than 250 type
Ia supernovae will be identified by VLT and 500 by VLT, Gemini and Keck.