Cosmology with supernovae in LSST · P.Astier LSST@Belgrade (06/16) 4 No more spectroscopic...

P.Astier LSST@Belgrade (06/16)1

Cosmology with supernovae in LSST

Pierre Astier (LPNHE-Paris)

mailto:LSST@Belgrade


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.



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)



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 .



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.



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)



How well should light curves be measured ?

● Precision of LC amplitudes

From Guy et al (2010)

Bottom-line:SNLS derived colorsessentially from gri



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.



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



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



Suspicions of UV evolution of SNe

Marginal effectBut anyhow worrying

(Very) large scatter

i-band at z=0.95Maguire et al (2012)



Redshift coverage for SNe with LSST

3 bands at >380nm→ z <= ~0.95



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



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.



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



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.



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.



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)



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



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 ?



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)



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



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 !



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 ?



The redshift lever arm : z>1

Proposal fo

r Eucli

d

left-o

ver tim

e

(if any)

LSST+ EuclidJoint SN survey



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)



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 (!)



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.



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.



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



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


Cosmology with supernovae in LSST · P.Astier LSST@Belgrade (06/16) 4 No more spectroscopic...

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