Supernovae and Cosmology with Future European Facilities
aa r X i v : . [ a s t r o - ph . C O ] N ov Supernovae and Cosmology with Future EuropeanFacilities
I. M. Hook
University of Oxford Astrophysics, Denys Wilkinson Building, Keble Road,Oxford OX1 3RH, UK andINAF Osservatorio Astronomica di Roma, via di Frascati 33, 00040 MontePorzio Catone, Italy
Key words: supernovae, future facilities.
Prospects for future supernova surveys are discussed, focusing on the ESA Euclidmission and the European Extremely Large Telescope (E-ELT), both expectedto be in operation around the turn of the decade. Euclid is a 1.2m spacesurvey telescope that will operate at visible and near-infrared wavelengths, andhas the potential to find and obtain multi-band lightcurves for thousands ofdistant supernovae. The E-ELT is a planned general-purpose ground-based 40m-class optical-IR telescope with adaptive optics built in, which will be capableof obtaining spectra of Type Ia supernovae to redshifts of at least four. Thecontribution to supernova cosmology with these facilities will be discussed in thecontext of other future supernova programs such as those proposed for DES,JWST, LSST and WFIRST.
1. Introduction
The discovery of the accelerating expansion of the Universe [Riess et al (1998),Perlmutter et al (1999)] was one of the biggest breakthroughs of the last twentyyears and was awarded the 2011 Nobel Prize for Physics. However the fundamentalquestion remains: what drives this acceleration? One possibility is that it is drivenby a mysterious component of the Universe, termed “Dark Energy” that exertsnegative pressure and that constitutes about of the energy density of theuniverse. Focus has now turned to measuring the equation-of-state parameter, w (= pressure / density) of this Dark Energy. Current measurements of w areconsistent with − , the value expected if the behaviour of Dark Energy can bedescribed by the cosmological constant ( Λ ) in Einstein’s field equations. Althoughthis would perhaps be the simplest explanation, large problems remain, forexample there is no natural explanation for such a small and yet non-zero valuefor the vacuum energy density. Other explanations such as “quintessence” andmodified gravity have been proposed, and distinguishing between these modelsis one of the major challenges of modern physics. A measurement of w notequal to − (at any redshift) would would rule out the cosmological constantexplanation and would have profound consequences for physics. Therefore there Phil. Trans. R. Soc. A (cid:13)
SN cosmology with future facilities
Figure 1. SNLS 3-year results [Conley et al. (2011), Sullivan et al (2011), Guy et al (2011)] Left:(reproduced from [Conley et al. (2011)]) Hubble diagram for 472 SNe Ia (123 low-z, 93 SDSS,242 SNLS, 14 HST). Right: (reproduced from [Sullivan et al (2011)]) cosmological constraintsfrom SNe Ia (blue), including systematics and assuming a flat universe. The grey contours showcombined constraints with WMAP7 and SDSS LRG power spectra constraints (green) and aprior on the Hubble constant H from SHOES. (Online version in colour) is considerable effort being directed towards improving measurements of w usingseveral techniques including weak lensing, Baryon Acoustic Oscillations (BAO),and Type Ia Supernovae (SNe Ia).The best constraints on Dark Energy to date measure w consistent with − at the ∼ level (including statistical and systematic uncertainties),assuming a flat universe and a constant equation of state [Sullivan et al (2011),Suzuki et al (2012)], see Fig 1. Presently systematic errors are estimated to becomparable to the statistical uncertainties in SN cosmology. The major sourcesof systematic error are related to photomeric calibration, particularly whencomparing distant SNe to nearby SN samples that have been compiled in adifferent way and with different rest-frame wavelength coverage. New nearbysearches are addressing this problem (for examples, see papers by B. Schmidtand J. Tonry at this meeting).A second source of systematic error arises from our lack of understanding ofthe colours of SNe Ia. SNe Ia show a range of colours, and the colour correlatesinversely with brightness. Such a relation could be caused by dust extinction inthe SN host galaxies or could be an intrinsic property of SNe Ia themselves, or acombination of the two. To make progress, any future SN survey should measureSNe Ia in multiple bands, particularly towards redder wavelengths where theeffects of dust are reduced and there is evidence for reduced intrinsic dispersion[Freedman et al (2009)].Finally, another potential systematic effect is evolution of the SN Iapopulation. We know that SN Ia spectra are similar at low and high redshift(up to z ∼ . ) [Bronder et al. (2008), Walker et al (2011)] although differencesin the in the UV have recently been seen [Maguire et al (2012)]. We also knowthat SN Ia rates and broadband SNe Ia lightcurve properties are dependenton the host galaxy type. The luminosity of SNe Ia depends on host galaxytype even after correction using the usual stretch and colour correction method[Sullivan et al (2010)]. Extending the redshift range over which SNe Ia aremeasured will allow better control of these effects. To date only a handful of N cosmology with future facilities z > at any wavelength. Such observations areextremely difficult from the ground because (a) the SNe are faint, and (b) thepeak of the SNe Ia spectral energy distribution moves into the NIR where the skybackground is higher. HST has therefore been the primary route for finding suchSNe [Riess et al (2007), Amanullah et al. (2010), Suzuki et al (2012)].
2. Future facilities
In the following sections, several upcoming projects are decribed that will makedramatic advances in SN Ia cosmology. These approach the problem in two mainways. The first is improving statistics (which can also lead to improved controlof systematics through construction of sub-samples). The second is extending thewavelength coverage of high-redshift SN samples into the near-infrared (near-IR).( a ) DES and VISTA
The Dark Energy Survey (DES) is an international private-public partnershipinvolving institutions from USA, Spain, UK, Brazil and Germany with the goalof mounting and operating a 3 sq deg CCD camera on the CTIO Blanco 4mtelescope. DES will carry out a 5000 sq deg survey over 5 years starting in late2012. It will survey the sky in g,r,i,z,Y bands.The SN component of DES has been described in detail by[Bernstein et al. (2012)]. 30 square degress will be imaged repeatedly inthe g,r,i,z bands and is expected to result in 4000 well measured SNe Ia in theredshift range . < z < . . An external spectroscopy program is being plannedthat will provide robust redshifts of the host galaxies (which aid SN photometricclassification and are necessary for accurate fitting of cosmological parameters).In addition, spectra will be obtained for a subset of the SNe themselves ( < ),some of which will have detailed multi-epoch spectroscopy.The 4m VISTA telescope, equipped with a 67 Mpix NIR camera and operatedby ESO, has been carrying out public NIR surveys in Z,Y,J,H,Ks bands since 2009.The SN component within the VIDEO survey [Jarvis et al (2012)] is expected toproduce about 100 SNe Ia with z < . observed in Y and J bands. These SNe willoverlap with the DES sample and will therefore have both optical and near-IRlightcurves. ( b ) LSST
The Large Synoptic Survey Telescope (LSST) is a U.S.-based public-privatepartnership that aims to construct and operate a 8.4m survey telescope (6.7meffective diameter) equipped with a very wide field camera (9.6 sq deg) withu,g,r,i,z,y filters. The main survey will cover ∼ every 3-4 days, andby the end of the survey each field will have been imaged over 1000 times. The“Deep Drilling Fields” will cover a smaller area but reaching a deeper limitingmagnitude per visit and with faster cadence (to be determined), thereby producingSN lightcurves of higher quality and reaching higher redshift than the main survey. SN cosmology with future facilities ∼ year survey lifetime.The SNe Ia case has been described in detail in the LSST Science book(Chapter 11, Wood-Vasey et al). For SN Ia cosmology the key gain is massivestatistics: 50000 SNe Ia per year are expected, with redshift up to ∼ . (mainsurvey) and up to ∼ (Deep Drilling Fields). Such statistics also improvesystematics by allowing the sample to be split into subsets, for example dependingon host galaxy type. However because of the very large sample size, only a smallfraction will have spectroscopic redshifts and classifications. The LSST SN surveywill also allow tests on isotropy and homogeneity, and tests of SN Ia evolution.LSST will also find core-collapse SNe and measure SN rates (for all Types).( c ) JWST
The James Webb Space Telescope (JWST) is a joint project between NASA,the European Space Agency and the Canadian Space Agency. It consists of a 6.5mspace observatory with four instruments optimised to the infrared wavelengthrange. Launch is currently expected in 2018. Being an observatory as opposedto a survey instrument, JWST’s science use will be determined mainly fromPI programs. JWST’s key advantage for SN cosmology is sensitivity at infraredwavelengths, allowing it to reach well above z = 1 . Several possible ways thatJWST could advance SN cosmology have been suggested. z > : In the white paper “James Webb Space Telescope Studies of DarkEnergy” [Gardner et al. (2010)] it is demonstrated that in 1 year using 1080 hours,JWST could find and follow 60 z > SNe, including obtaining spectra with JWSTitself. z > : Riess & Livio (2006) make the case in that observing at z > , wherethe effects of Dark Energy are expected to be very modest, will provide a key testfor evolution in the SN Ia population. They estimate that ∼ . SNe Ia could befound within each NIRCAM field reaching 10nJy in K. They suggest monitoringa few NIRCAM fields with cadence ∼ days. z ∼ : The JWST exposure time calculator predicts that JWST can reach 2magnitudes below peak brightness of a typical SN Ia lightcurve at z = 4 in 10 000s.By monitoring 10 NIRCAM fields for 5 years, a sample of ∼ objects could befound. Spectroscopic confirmation could be obtained using future ground-basedextremely large telescopes (see next section).( d ) The European ELT
Three extremely large telscope projects are currently underway: the GiantMagellan Telescope (GMT) , the Thirty Meter Telescope (TMT) and theEuropean Extremely Large Telescope (E-ELT) . These projects are at acomparable stage of development and all aim for first light around the turn of http://tmt.org/ N cosmology with future facilities Figure 2. Artist’s impression of the European Extremely Large Telescope (E-ELT). Image credit:ESO (Online version in colour) the decade. In this paper I focus on the European ELT, but the broad conclusionsare similar, or scalable, to all three projects.The E-ELT project aims to design and construct a 39m diameter optical-IR telescope, which will be the largest optical/IR telescope in the World. Theproject is run by ESO on behalf of its member states. The design is a novel 5mirror concept with adaptive optics built in, and the telescope will be situatedon Cerro Armazones in Chile. The first three instruments have been selected: adiffraction-limited camera and an integral-field spectrograph for first light, and amid-IR imager-spectrograph to arrive shortly afterwards. The full instrumentationsuite will be built up over first decade of operation. Since the time of this RoyalSociety Discussion meeting, the ESO Council has approved the E-ELT programmesubject to confirmation of funding from some member states.The E-ELT is a general-purpose observatory and the science case is verygeneral [Hook (ed) 2005]. In additon to its clear power for identification ofvarying sources from other facilities (e.g. GRBs), examples of time domain sciencewith E-ELT include Solar System observations (including study of weather andvolcanic activity), Exo-planets (incuding radial velocity, direct detection andtransit measurements) and study of the motions of stars and stellar flares in thevicinity of the Galactic centre. Furthermore, with specialised high-time resolutiondetectors, E-ELT could study extreme physics (pulsars, neutron stars, black holes)stellar phenomena, transits and occultations [Shearer et al (2010)].The E-ELT will be particularly powerful for spectroscopic observations ofSNe Ia at high redshift. Because they are point sources, SN observations benefitfrom the telescope’s adaptive optics capability. Such spectroscopic observations
SN cosmology with future facilities are needed in order to unambiguously classify the SNe and to determine theirredshifts. Simulations show that with the HARMONI integral-field spectrograph[Thatte et al (2010)] and using the telescope’s adaptive optics system, the SiIIfeature near a rest-frame wavelength of 4000Å is clearly detected even at z = 4 (at which redshift the feature is observed in the K-band; Hook, 2010). Since thepresence of SiII and absence of hydrogen is the defining signature of SNe Ia, thismeans that it will be possible to confirm SN Types even out to z = 4 . To obtain E-ELT spectroscopy of a sample of 50 objects < z < (which could be reasonablydiscovered by JWST, see section c) would require of order 400 hours spread overthe over 5 years of the survey. ( e ) Euclid
Euclid is a 1.2m optical-IR space telescope within ESA’s Cosmic Vision2015-2025 programme [Laureijs et al (2011)]. Its primary science goal is precisonmeasurement of cosmological parameters via weak lensing and galaxy clusteringtechniques. ESA’s downselection process in October 2011 resulted in selectionof Euclid for the second M-class launch slot. In June 2012 the mission passedthe important milestone of adoption of the mission by ESA. Launch is currentlyscheduled to take place in Q4 2019.The instrumentation consists of an optical imager and a near infrared imagerand spectrograph, both with field of view of 0.5 sq deg. The satellite will belaunched by a Soyuz rocket and will operate at the L2 Lagrange point for a 6 yearmission duration.Euclid’s Wide survey will cover ≥ sq deg, with imaging in a single broadoptical band (R+I+Z) to a depth of AB=24.5 (10 σ for a point source), imagingin three near infrared bands (Y, J, H) to a depth of AB=24 (5 σ , extended source)and NIR slitless spectroscopy to a depth of × − cm − s − (3.5 σ unresolvedline flux).Additional Deep fields will cover ≥ sq deg, reaching 2 magnitudes deeperthan the Wide survey in both optical and NIR imaging and NIR Spectroscopy.The Deep fields are primarily for calibration purposes but the ∼ repeat visitswill enable a vast range of additional science incuding detection and study ofvariable, moving and transient objects.In addition, ideas for specialised dedicated surveys for SNe and microlensing ofexoplanets are being considered, although these are not currently in the baseline.These will be further explored as the survey design is optimised.For SNe Ia cosmology, one example strategy has been developed that wouldallow exploration of a new redshift range, to z ∼ . . Simulations show that 6months of Euclid survey time could be used to carry out a survey of 20 sq degwith 4 day cadence. When combined with simiultaeous ground-based observationsin I and z bands, this results in 1700 well measured SNe Ia in the redshift range . < z < . with measurements covering a consistent rest-frame wavelengthrange. Such a survey would make a significant improvement in the measurementof cosmological parameters from existing surveys at the time, and would add anindependent method to Euclid’s primary cosmological probes, thereby enhancingthe mission’s overall impact. N cosmology with future facilities f ) WFIRST
The proposed NASA mission WFIRST (Wide-Field Infra-Red SurveyTelecope) emerged as the top priority large space mission in the U.S. “NewWorlds New Horizons” 2010 Decadal Survey. Its science drivers are measurementof the expansion history of the Universe and galaxy clustering (exploring DarkEnergy and modified gravity theories via the weak lensing, supernova and BAOtechniques); Exoplanets (via microlensing); Deep NIR surveys; a Galactic planesurvey; High-z QSOs and a guest observer program.Definition of the hardware is in progress. The interim report of the ScienceDefinition Team study [Green et al. (2011)] described a 1.3m off-axis telescopeoperating from . − . µ m , and a ∼ . sq deg FOV covered by imaging in 5filters and slitless spectroscopy with R ∼ from . − . µ m . An additional R ∼ prism would be available for SN spectroscopy. Launch is anticipated for ∼ , assuming phase A starts in 2013.The WFIRST SN Ia program goals are stated as > SNe per ∆ z = 0 . redshift bin in the range . < z < . per dedicated 6 months (spread out duringthe mission lifetime). The goal is to achieve an error on distance modulus of < . mag per ∆ z = 0 . redshift bin. They also consider an ‘optimistic’ case where theredshift range is extended to 1.5 and systematic effects are reduced.
3. Conclusions
There is a spectacular suite of new facilities on the horizon that will make dramaticadvances in the quantity and quality of SN samples for cosmology. Several of thesefacilities are already planning SN programs and have made detailed predictionsof the achievable results in terms of numbers of objects and in some casescosmological constraints (e.g. in terms of the Dark Energy Task Force Figureof Merit). Such figure of merit calculations depend critically on the assumptionsmade for systematic effects and priors, so comparison is not straightforward andis not attempted here. However by comparing the N(z) distributions of existingand future samples we can obtain a simple visual impression of the gains we canexpect (Figure 3).In summary, the gains likely to emerge are: • an enormous gain in statistics (at least 2 orders of magnitude) of opticalSN Ia measurements, from a combination of current nearby searches, DESand then LSST • an enormous gain (2 orders of magnitude) in numbers of observed-frameNIR observations of distant SNe, from VISTA and then space-based surveyswith Euclid (survey strategy permitting) and WFIRST, potentially allowingextension of the SN Ia Hubble diagram up to z ∼ . . • better control of systematic effects, arising from (a) the ability to create sub-samples from large parent samples, and (b) from the improved wavelengthcoverage resulting from combined optical-NIR observations of the same SNe • extension of the SN Ia Hubble diagram to currently unexplored redshiftsabove 2 and up to about 4, from JWST and ELTs SN cosmology with future facilities
Figure 3. (Online version in colour) Redshift distributions from existing and planned SN surveysin (left panel) linear and (right panel) log space. Optical surveys are shown in blue: the existingUnion 2.1 sample [Suzuki et al (2012)] (filled histogram); DES (solid line); LSST wide surveyand Deep Drilling fields (dot-dashed and dashed lines respectively). Note that the LSST valuesare for only one year of operation and may be a factor ∼ higher for the final sample. Near-IRsamples are shown in red: the existing CSP sample [Freedman et al (2009)] (filled histogram);VISTA sample (assumed to be 100 objects sampled from the DES dristribution up to z=0.5,dashed line); a possible Euclid survey if scheduling allows (hatched histogram); the WFIRSTSN goals (horizontal hatched histogram); an example very high-redshift sample that could becompiled by JWST and ELTs (diagonal hatched histogram at z > ). N cosmology with future facilities
Acknowledgment
IH is supported by grants from STFC, OPTICON (EC FP7 grant number 226604) and theUK Space Agency for work on the European ELT and Euclid projects. The E-ELT/HARMONIsimulations in Section d were carried out by Tim Goodsall and IH. Calculations for JWSTobservations of z > SNe (Section c) were carried out by W. Taylor and IH. Section e is basedon work undertaken within the Euclid SN & transients SWG (part of the Euclid Consortium),and makes use of calculations performed by P. Astier, S. Spiro, K. Maguire and J. Guy. Helpfulinput on the LSST SN search (section b) was provided by M. Wood-Vasey.
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