“Exploring the NRO Opportunity for a Hubble-sized wide-field near-IR space telescope”

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Alan Dressler

--- presentation at the September 2012 Princeton Workshop on the NEW telescopes

September 4, 2012

DRM2 DRM1 NRO-WFIRST

Aperture 1.1m 1.3m 2.4m (m2) 0.9 1.3 4.8

Wavelength 0.6-2.4μm 0.6-2.4μm 0.6-2.0μm

PSF (1.5μm) 0.30 arcsec 0.24 arcsec 0.15 arcsec

FOV (sq deg) 0.58 0.375 0.375

Lifetime (yrs) 3 (5) 5 (10) 5 (10)

Cost (?) 1B$ $1.6B < $2.0B

description: WFIRST - - WFIRST WFIRST + +

PROB

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NWNH

HST-

like

In my opinion, DRM2 is a step back to a dark energy probe. There is not sufficient time to do the full WFIRST program, and what will suffer first, and most, is the GO program. This is not responsive to the NWNH Decadal Survey.

One more time: what was the idea behind WFIRST?

Was it really just a dark energy probe with a few bones thrown to exoplanet research, and to the astronomers who were mucking about in their gardens --- oblivious to the search for truth?

No, it was not. The EOS, and the Decadal Survey committee, embraced the notion that GO science was the key feature of the WFIRST program, recognizing that a modest-aperture wide-field near-IR telescope opened new opportunities across diverse fields of astronomy and astrophysics.

It would have been much simpler for the EOS to choose a couple of probe missions (in addition to enhancing the Explorer program) and be done with it, and the obvious candidates were dark energy and exoplanets. Why didn’t this happen?

arXiv:2012.2434

“The accelerating expansion of the universe is the most surprising cosmological discovery In decades, implying that the universe is dominated by some form of “dark energy” with exotic physical properties, or that Einstein’s theory of gravity breaks down on cosmological scales. The profound implications of cosmic expansion have inspired ambitious efforts to understand its origins, with experiments that aim to measure the history of expansion and growth of structure with percent-level precision or higher.”

I recommend this excellent review article! This quote expresses well the rank of dark energy science in our priorities. The discovery of the (apparent) cosmic acceleration is arguably the most important scientific discovery of our time.

The EOS Panel of NWNH agreed with this assessment. Nevertheless, the Panel did not decide to dedicate a space mission --- like JDEM ---- to this endeavor. In some part, this was because of the limited benefit to the broad astronomical community if most (all) of the new-mission funding went to a “dark energy probe.”

However, this was not the only reason. The Panel agreed that two other factors prevented the proposal of a dedicated dark energy mission as the highest priority:

(1) BALANCE: Already many other facilities engaged in this program: DES, PanSTARRs, BOSS, CFHT, Boss, Big-Boss, HET-DEX, Euclid, LSST… to name some major ones. How much of the available research resource should be allotted to this one program?

Just Weak

len

sing!

2: Decisiveness – what if the answer is “the cosmological constant?” By the time WFIRST flies, there will be a factor of 3 or more improvement in the precision of the cosmological parameters? The EOS thought that If – as is likely – it still looks like vacuum energy, many in the astronomical community might sour on the idea that the next major mission is dedicated to a further factor of 3 improvement. Again, from Weinberg et al. (2012):

”The future of cosmic acceleration studies depends partly on the facilities built to enable them, partly on the ingenuity of experimenters and theorists in controlling systematic errors and fully exploiting their data sets, and partly on the kindness of nature. The next generation of experiments could merely tighten the noose around w = −1, ruling out many specific theories but leaving us no more enlightened than we are today about the origin of cosmic acceleration. However, barely a decade after the first supernova measurements of an accelerating universe, it seems unwise to bet that we have uncovered the last “surprise” in cosmology. Equally important, the powerful data sets required to study cosmic acceleration support a broad range of astronomical investigations. These observational efforts are natural next steps in a long-standing astronomical tradition: mapping the universe with increasing precision over ever larger scales, from the solar system to the Galaxy to large scale structure to the CMB.“

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DRM2 DRM1 NRO-WFIRST

Photom Survey depth, 400 sec 600 sec 600 secHST equivalent exposure (3400 deg2) (3400 deg2) (10,000 deg2) (or 1800 sec for 3400 deg2

Spectroscopic Survey covers the same sky area and millions of galaxies, but this too is generally quite shallow. Only emission lines, usually Hα emission, will be detected, and for relatively luminous galaxies (not much fainter than L*). This is nothing at all like the rich spectroscopic data set from the Sloan Digital Sky Survey, which supported an enormous range of science programs. Again, smaller deeper grism surveys will be required for important GO programs.

How broad a range of astronomical investigations can be supported with just the dark energy surveys? Per exposure in a one band, the direct images are all equivalent to less than 1 orbit with HST + WFC3, for DRM1 and DRM2, only a fraction.

If we use HST itself as a guide, it is evident that most of the GO science – the very thing that has come to define the extraordinary success of HST – comes from deeper exposures. At first even these short exposures will support a wealth of science, but I predict the demand would shift over the lifetime of the mission to a range of deeper, targeted individual or small-survey programs. The dark energy surveys are not enough!

What about the other “core program,” a demographic survey for exoplanets through microlensing? The EOS thought that this was a very important step in the exoplanet program, because it would complete the Kepler survey “beyond the snow line” – key information that is necessary to inform a theory of planet formation.

In general, the exoplanet community did not agree. Take, for example, the “Planetary Systems and Star Formation” Panel of NWNH) decision to give low priority to the microlensing program, preferring instead: (1) astronometry (SIM-lite) to search for planets, particularly Earth-like planets, around nearby with, (2) planet searches through transits -- again focusing on Earth-like planets nearby K&M stars (and subsequent spectroscopy), and (3) attempts to directly image planets, for example, giant planets around FGK stars in the local neighborhood. A full-on TPF, also supported by many in the exoplanet community, was apparently thought too much a stretch by this Panel (and the EOS).

However, the EOS saw that neither SIM-lite, transit searches, or “modest” planet imagers used hardware that was also compatible or adaptable to deep and broad range of astrophysics programs, so like a “dark energy probe,” the EOS thought that one of these choices -- though compelling -- would have been a relatively narrow ones for the (likely) ‘only new mission of the decade.’

Dan Stern’s NIRSS telescope, which rated highly because of its great improvement on basic astronomical data with a modest-sized telescope --- the last of the ‘low hanging fruit.’

But the EOS was less persuaded by the importance of covering the entire sky, compared to deeper, surveys and pointed observations the NIRSS could make, examples of which are described in the “NRO-as-WFIRST” paper.

Combining these elements became the key of the WFIRST mission as conceived by the EOS: send up a JDEM-Omega like space telescope that could perform a dark energy survey and a microlensing search (a la Dave Bennett’s MPF), but --- after initial guaranteed allocations --- let these complete with GO proposals from the whole community. The GO science would come partly from the dark energy surveys, but we expected that, more and more as the mission continued, from individual small surveys and pointed observations.

This is what DRM1 tries to preserve, what DRM2 will lose, and what NRO-WFIRST will make extraordinary – like Hubble.

Dennis Crabtree’s 2012 update on ground-based optical-IR facility productivity compared to the Hubble Space Telescope

The Hubble is arguably the most productive science tool of all time. An NRO-WFIRST telescope can aspire to this kind of productivity for a wealth of science programs, as Hubble has done.

100x the Area as HST’s WFC3/IR cam

0.25 sq deg FOV

HST WFC3/IR FOV

Existing telescope has a 2.4-m f/1.2 primary with a 9% obscuration secondary that produces a 1/20 wave near-IR optical system at about f/8 assembled and tested.

A preliminary 3-mirror design could use 16 Hawaii 4RG HgCdTe IR- detectors (10μm pixels) to cover 0.375 deg2 at 0.11”/pixel, compared to this telescope’s 0.15” diffraction limit at 1.5μm

This is slightly better sampling than the SDT 1.5-m WFIRST with 0.18” pixels for a 0.24” diffraction limit.

A “proof of concept” optical design study by Erin Elliott (STScI) that uses an NRO telescope “as-is” to produce a FOV=0.375 deg2.

This is the same field as DRM1 with the same relative sampling: 0.11 arcsec pixels sampling a diffraction-limied 0.15 arcsec at 1.5μm. With suitable dithering, diffraction limited observations can be made at 1.2μm up to 2.0μm.

In the “NRO as WFIRST” paper, telescope is assumed to be at at room temperature, like the Hubble.

I suggest that the criteria for preferring an NRO-WFIRST to DRM1 should be: (1) equal or better performance in the dark energy program --- `overall,’ in the figure-of- merit sense; (2) equal or better performance in the microlensing planet search program -- in the number of planets and determinations of their masses.

Assuming a comparable field-of-view for NRO-WFIRST, a long wavelength cutoff of around 2.0um, and a suitable orbit to carry out the above survey programs, I take as a given the ability of NRO-WFIRST to perform a far superior “other deeper surveys” and GO program compared to DRM1.

So, as a prelude to what we will hear at this workshop about what an NRO telescope can do for the WFIRST program, let me briefly sum up the results given in the paper for the two core programs, dark energy and microlensing.

Highpoints of Weinberg, Hirata, et al’s writeup in the “NRO as WFIRST” paper

Dark Energy programs: DRM1 to NRO-WFIRST comparison

Supernova: NRO-WFIRST can find 3x more SN in same DRM1 redshift range. (1) if systematic errors dominate (mismatch of SN types,host galaxy type, dust, uncertain k-corrections) then more SN won’t improve SN contribution to DE parameter determination compared to DRM1, however, the NRO advantage could be used to extend the redshift range to better overlap with BAO (which measures real Mpc compared to scaled Mpc of SN). (2) If systematics don’t dominate (suggest using SN spectrophotometry (IFU) instead of simple photometry to match samples, then gain is substantial: 2-3X DRM1.

Weak lensing: NRO-WFIRST could cover 3X the area (104 deg minus the overhead for more exposures.) Alternatively, a deeper 3400 deg2. PSF is better by ratio of 2.4/1.5 (obstructed/off-axis-unobstructed). Systematic errors scale as the ratio of the psf (squared?). Retain 3 bands – K (Kshort?) to reduce systematics by cross comparison. More complex PSF, but active control of wavefront errors through secondary mirror.

BAO: NRO-WFIRST would have a lower redshift cutoff, z=2 compared to the z=2.7 of of a cold DRM1 at L2. But, with a factor of 3 gain in speed, over 104 deg can be sampled with the figure of merit nP > 1 over the entire volume 1.3<z<2.0. This is far superior to Euclid’s BAO survey, which has nP>1 over 0.7<z<1.0 (much smaller volume) and has 0.3>nP>0.15 over the NRO-WFIRST redshift range. Both BAO and RSD should gain substantially compared to DRM1, the only loss being the 2.0<z<2.7 volume due to the warmer operating temperature.

Summary: For dark energy research, NRO-WFIRST is equivalent to, or several times better, than DRM1.

NRO-WFIRST is not as complementary to Euclid as DRM1 or DRM2. (It’s just better.)

Supernova: DRM1 and NRO-WFIRST have it, Euclid doesn’t. Complementary?

Weak lensing & BAO: DRM1’s higher redshift volume is complementary to Euclid. NRO-WFIRST covers more of the same redshift volume as Euclid, but with (1) infrared sampling of galaxy shapes (at same resolution as Euclid’s visible imaging), with the advantage of more galaxian light and smoother shapes; (2) BAO sampling nP > 1 over full NRO-WFIRST volume, compared to 0.15< nP<0.5 over same volume.

Highpoints of Gaudi, Penny, and Bennett’s writeup in the “NRO as WFIRST” paper

Microlensing: to yield 200 Earth-like planets requires 200 million star years of monitoring. Photometric precision of a few percent is required for the faintest stars in the program. Stellar density in Galactic bulge is 100M stars per sq deg.

Given the same observing time as DRM1, and the same detector area, the number of planetary detections increases 2-3 times simply by increasing the survey area. In addition, the data will be superior because of the better PSF in such crowded fields --- adding usable stars to the sample.

In addition, the smaller PSF of NRO-WFIRST yields additional gains in the determination of masses of host stars.

Because of the ~0.15 arcsec resolution of NRO-WFIRST, nearly all background stars are resolved from the lens+source signal, and the proper motion between host star and the lensing star can be determined for most bright lensing events (over a 5-year mission lifetime)

So, my test to preferring NRO-WFIRST to DRM1 is passed.

To repeat, I take as QED that NRO-WFIRST will excel at GO science programs, in the tradition on Hubble, compared to DRM1

Some examples of this kind of deeper, large-field near-IR science will follow. Most of the material at this meeting shows how even the dark energy surveys are deep enough to accomplish major astronomical programs in a variety of subjects. It will be important to study the record with Hubble to see how much of its scientific accomplishments come from science that required several orbits or more, because I suspect that this kind of program will come to dominated the GO program as the mission continues. Design reference missions of NRO-WFIRST should include longer pointed observations (and small surveys) as an important part of the plan!

Jason Kalirai and Julianne Dalcanton will discuss the power of NRO-WFIRST in studies of nearby galaxies and the Milky Way

Other examples of GO science to be discussed at the workshop:

Marc Postman will talk on cosmology with rich clusters, and strong and weak lensing by rich clusters that allows cluster mass determinations and as a probe of lensed galaxies that will reveal the properties of z = 8-10 galaxies!

Dan Stern will talk on some of the things he included in the NIRSS, including QSOs at z>7, pop III SNe, mapping large scale structure at z=7, the diffuse background due to the first generation of stars, and more!

James Rhoads will talk about astronomical probes to trace the process of reionization 6<z<12 with NRO-WFIRST surveys.

GO science of another flavor!Please check out Jeremy Kasdin’s presentation on the possibility of adding a coronagraphic imager to NRO-WFIRST, to accomplish high priority exoplanet science as described in NWNH.

…a closing thought: in the 1980s I served on a committee that considered possible key projects for the “space telescope,” chaired by Vera Rubin. We went through many papers and exercises to imagine what the important areas of research would be. The things we identified did of course turn out to be key programs, but they represented a tiny fraction of the (apparently) unimaginable diversity of science that would be dreamed-up by the astronomical community. I am still amazed by it.

JWST is such an opportunity, and the NRO telescopes are another. I believe that, if we fail to use one to them build a wide-field near-IR imaging telescope, and opt instead for a probe class DRM2-like mission, this will be one of the greatest missed opportunities of the history of the storied NASA astrophysics program.