P.Astier LSST@Belgrade (06/16)1
Cosmology with supernovae in LSST
Pierre Astier (LPNHE-Paris)
P.Astier LSST@Belgrade (06/16)2
Foreword
● These slides were not blessed by the Supernova working group....
● … but they are not orthogonal to the concerns being followed there.
● … and are somehow twisted towards possible European contributions in the SN distance arena.
P.Astier LSST@Belgrade (06/16)3
The role of supernovae in constraining cosmology
● Distances are a “must do”– Constrain the average background/expansion
history and in particular Dark Energy.
– A successful approach so far.
JLA sample(Betoule et al2014)
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No more spectroscopic confirmation
● The SuperNova Legacy Survey (2003-08) was “able” to spectroscopically identify all events in the Hubble-diagram
– As much 8-m time (spectroscopy) than CFHT time (imaging).
– Spectroscopy time was the bottleneck: some candidates could just not be observed.
– SDSS was in the same situation.
● DES-SN lives in this new paradigm:– Hosts redshifts on the AAT.
– “Live” SNe (~10%) on VLT and others .
P.Astier LSST@Belgrade (06/16)5
No more spectroscopic confirmation
● We need good (i.e low shot-noise) light curves:– For identification: we do not want the high-z end to
be more contaminated than the low-z end.
– At least 3 bands, for both light curve shapes and 2 colors or more.
– 3 bands improve the lever arm for color measurement.
● Good light curves come with other benefits:– Can effectively assemble a redshift-limited Hubble
diagram (no more Malmquist bias !)
– Efficient light curve model training.
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How well should light curves be measured (for distances)?
● Quality commensurate with natural spread...
– Brighter-bluer correction : c , with ~3
– color to 0.04 → distance modulus to ~ 0.12
Not very challenging:SNLS did it.(Guy et al 2010)
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How well should light curves be measured ?
● Precision of LC amplitudes
From Guy et al (2010)
Bottom-line:SNLS derived colorsessentially from gri
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Improving SN distances :light curve shot noise
Simulation for LSSTWith a 4-day cadence and ~10 mn per visit.
→ Significant improvement over SNLS and DES→ We might consider trading i and z depth in favor of y.
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Improving SN distances :rest-frame spectral coverage
● Rest frame UV of SNe is useless not very useful:– Only handful of nearby counterparts
– Large event to event scatter
– Suspicions of evolution
– Steep extinction curve
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Suspicions of UV evolution of SNeMaguire et al (2012) : - comparison of distant (z~0.5) and nearby (HST) spectra - empirical measurement of scatter
Marginal effectBut anyhow worrying
(Very) large scatter
P.Astier LSST@Belgrade (06/16)11
Suspicions of UV evolution of SNe
Marginal effectBut anyhow worrying
(Very) large scatter
i-band at z=0.95Maguire et al (2012)
P.Astier LSST@Belgrade (06/16)12
Redshift coverage for SNe with LSST
3 bands at >380nm→ z <= ~0.95
P.Astier LSST@Belgrade (06/16)13
The weight of high-z JLA events
JLA sample(Betoule et al2014)
(w) = 0.0575 (740 events)(w) = 0.0605 (z<0.8, 674 events)
Megacam has a low-sensitivity z band, and no y band.→ There is not much to gain at diving into deep UV
P.Astier LSST@Belgrade (06/16)14
What we would like for the LSST SN cosmology program:
● A deep enough “deep program” :– Means O(3) mn/day per band and per pointing.
– ~ Deep Drilling Fields observing scheme.
– We could think of exposing more in y and less in i and z.
● And a low-z SN program !– The average repetition in the wide is far too low to
measure distances.
– The often quoted number of SNe (O(105)) refers to detections, not distances.
P.Astier LSST@Belgrade (06/16)15
The LSST observing strategies
● Area● Time sampling
per band● Depth per visit
Wide Deep-Drilling Fields
~20 days ~5 days
23.5 → 24.5 ~20 to 50 timesmore
20,000 deg2 O(100) deg2
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Do we really care about low-z SNe ?
● Consider 5 DDF over 4 seasons (conservative)
→ yields ~ O(8000) SNe to z~0.95, very well measured.
● Add low-z events
~SDSS : z<0.35
– natural z distribution
For forecast hypotheses, seeArXiv:1409.8562.
P.Astier LSST@Belgrade (06/16)17
So,
● The high-z part of the LSST Hubble diagram is reasonably secured in the current observing plan
● Some part of the Wide survey should be “unevenly” scheduled to get a few thousand SNe in the SDDS redshift range (standard 30s visit).
– O(3000) deg2 for 6 months at a 4 day cadence.
– The DESC SN WG is indeed working on the cadence of the Wide LSST Survey.
– Without it, SN constraints are going to be poor.
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What are the identified limitations of LSST SN cosmology?
● Photometric calibration – dominates systematic uncertainties now
● Getting (precise) redshifts● The “host correction” (a.k.a “mass step”)● Identification (Ia vs others)● The redshift lever arm (the longer, the better)
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Photometric calibration
● Today, the zero points uncertainties are ~5 mmag and translate to (w) = 0.03 (Betoule et al, 2014)
● DESC (PCWG) is committed to transferring the NIST photometric physical scale to stellar standards.
DE FoM for a SN Hubble diagram, as a function of calibrationaccuracy.
~ 8k DDF SNe~ 8k Nearby SNeDetails in arXiv:1409.8562
Now
Goal
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Redshifts for the Hubble diagram
● The emerging standard way: collecting host redshifts after the fact.
● Requires a large multiplex because needed exposure times are O(20-50) ks on a 4m (z~1).
● For the high-z part , it requires dedicated time on a wide-field MOS instrument : 4most for south.
● Low-z : rely on the 4most survey footprint ?● Other suggestions ?
P.Astier LSST@Belgrade (06/16)21
The host correction (1)
● Since ~2010, all SN papers apply a host-dependent distance correction.
Residuals to the Hubble diagramare related tosome host galaxyproperty: e.g. the stellar mass
“The mass step”(Betoule+ 2014)
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The host correction (2)
M. Rigault & the SNfactory (2013) : the brightness of SNe Ia is related tothe local H emission.
M. Roman & co (2016, in prep) : the brightness of SNe is relatedto the local restframe U-V color (SFR indicator):
● 7 significance.● Makes more sense than some host mass ● Relies on imaging only
SNLS5 sample
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SN photometric identification
● Do we really need a very pure HD ?– Redshift-independent contamination is OK.
● Quality of light curves helps:– Low shot-noise
– 3 bands (to rely on color relations)
● A lot of work currently underway in DESC SN WG.
● Requires “live SN” spectroscopy to assess contamination: VLT !
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The redshift lever arm : z<0.1
Adding events to the base sample:● z<0.1 is the best deal● it is also the most difficult in practice
A few hundred of SNe @ z~0.05 are very useful for HD cosmology.● Detection by LSST is guaranteed● Follow up by LSST ? How ?● Follow-up with small (<2m) telescopes ? ●VST will soon be de commissioned● Will ZTF have done that by 202X ?
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The redshift lever arm : z>1
Proposal fo
r Eucli
d
left-o
ver tim
e
(if any)
LSST+ EuclidJoint SN survey
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What else can we do with SNe ?LSS constraints !
● Lensing (DDF SN sample).– Skewness of the distribution of residuals (or
evolution of the skewness with redshift).
– Angular correlation of residuals.
● Velocity correlations (nearby SN sample z<~0.1)– Requires a “sizable” time*volume at low z
– Constraints on and
e.g. Castro et al (2016)
P.Astier LSST@Belgrade (06/16)27
Velocity correlations from nearby SNe
1511.08695
3 realizations of HubbleResiduals (400 deg2), ignoring “intrinsic scatter”
Constraints from 1000 nearby SNe Uniform over 0.01<z<0.1 and 400 deg2
→ all events over ~ 50 years (!)
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Velocity correlations from low-z SNe
● The forecast from 1511.08695 seems unrealistic:– But we can aim at a few thousand events on a few
thousand deg2.
– Detection with LSST + follow-up elsewhere ?
– Worth the effort after ZTF ?
● But SN cosmology benefits a lot from nearby events:
– Effectively increases the redshift lever arm
– Light-curve fitter training.
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The LSST SN Hubble diagram
Wide : negociate adequatecadencingof some patches/seasons
Nearby : Considerpointed follow-upwith small telescopes
z>1 : remind Euclidheads that Euclidis about cosmology
Redshifts:4most, DESI,….
Distances:bridge laboratoryand stellar standards
DDF: monitorthe observing strategy. y bandis vital.
P.Astier LSST@Belgrade (06/16)30
The LSST SN Hubble diagram
DDF: monitorthe observing strategy. y bandis vital. Wide :
negociate adequatecadencingof some patches/seasons
Nearby : Considerpointed follow-upwith small telescopes
z>1 : remind Euclidheads that Euclidis about cosmology
Redshifts:4most, DESI,….
Distances:bridge laboratoryand stellar standards
Where EuropeCouldcontribute
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Summary/conclusions
● LSST is well equipped to assemble the best supernova Hubble Diagram of the next decade:
– Wide: SN statistics
– Fast: allows to collect low-z events
– Deep : actually reach z~1 with good light-curves.
● Collecting low-z light curves (z<0.35) requires some alterations to the base observing plan (“Rolling cadence”, within a constant allocation !)
● European facilities could play a leading role in follow-up
P.Astier LSST@Belgrade (06/16)33
Simulated surveys
Typically 20,000 SN events in total,with ~2,000 measured by Euclid NISP.
The assumed exposure timesdeliver precise distancesat the highest redshifts.
Euclid+LSST
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Lightcurve examples
The contribution ofshot noise to distanceuncertainty is at most 0.09,when the “natural” scatteraround the Hubble diagramis ~0.12
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Forecast approach
● Consider major sources of uncertainty :– SN statistics and photometric shot noise (including
finite depth of SN-free data).
– “natural” Hubble diagram scatter
– Photometric calibration uncertainties
– SN empirical “modeling” uncertainties● Finite training statistics● Irreducible residual scatter
– Ground distance error floor
● Conservative uncertainties: current state of the art.
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Forecast method
● Consider all parameters simultaneously and marginalise.
● Parameter sets:
– SNe lightcurve parameters (brightness, color, decline rate, date of maximum)
– SN model parameters (average SN restframe flux and color variations)
– Calibration offsets
– Distance estimator coefficients
– Cosmological parameters
● Typically 50,000 parameters
● Evaluate a sub-block of the inverse of the Fisher matrix
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The importance of calibration uncertainties
No calibration uncertaintyNo model uncertainty
The drop happens with both calibration and modeltraining turned on
With all uncertaintysources
FoM obtained using SNe+Planck (geometry only).
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Sensitivity to systematics
Irreducible distance error
NIRZPaccuracy
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Model-free forecast (1)
● We propose a covariance matrix of distances in redshift bins.
● If the tested model is not widely different from the vanilla cosmology, the obtained precision is accurate.
● We tested that we recover the direct fit precision for CDM, at an accuracy better than 1%.
● Of course, no external prior added: just SNe.
P.Astier LSST@Belgrade (06/16)40
Model-free forecast (2)
● Paper : A&A 572 A80 (2014) http://arxiv.org/abs/1409.8562
● The appendix E describes the binned distance prior.
● We provide the covariance matrix, short note, and some code example at : http://supernovae.in2p3.fr/~astier/desire-paper/SN-prior/