A simple computer program for designing a NIR space telescope for a cosmological survey

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A simple computer program for designing a NIR
space telescope for a cosmological survey

• Are high z objects at z gt10 detectable by a
  small NIR space imager?

Tadashi Nakajima (NAOJ)
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Is there a niche for a small wide field NIR
surveyor?

• Small budget
• Short term
• Wide field of view
• Characteristics listed above are of merit, if a
  wide field imager has enough sensitivity to be
  complementary to great observatories like JWST or
  TMT.

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Choice of near infrared

• Zodiacal background is minimum in NIR.
• Dust scattering in the visible.
• Dust thermal in the mid infrared.
• Extragalactic scientific objectives
• To go beyond z gt 7, NIR is inevitable due to
  foreground absorption.
• Redshift limits of Balmer break seen in post
  starburst are z5 at K and z11 at M.
• If high z objects are rare and spatially
  localized, a wide-field survey may be necessary.
• Galactic scientific objectives
• Probably many (e.g. Cold brown dwarfs), but I
  ignore them here.

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Candidates of UV luminous objects in their rest
frames

• Early forming galaxies in their bright phases.
• AGNs, if black hole formation and growth are
  early enough.
• Afterglow of gamma-ray bursters, if first
  generation metal-poor stars are massive.

How bright were they and how many were there?
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Sensitivity calculation

• Noise sources
• Zodiacal background photons (median value from
  Allen)
• Instrumental thermal background photons (3
  warm surfaces, emissivity 5 each,
  T50K(L2),150K(satellite orbit of the Earth)
• Detector HgCdTe(1-2.5?m) or InSb(2.5-5)
• Dark current (0.05 e / sec / pix)
• Read noise (6 e / pix after multiple sampling)
• This level of noise performance is realized on
  the ground.
• Source photons

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Sensitivity depends on pixel-wise sampling

• Suppose the rest-frame diameter of a circular
  emitting region is fixed to, say 1 kpc, SNR after
  aperture photometry depends on how many pixels
  (Npix) are included in this emitting region.
• Npix depends on
• Pixel-wise sampling (arcsec/pix)
• Angular diameter distance, once the source is
  resolved.

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Other assumptions

• Standard constant Lambda cosmology
• This fixes DA(z) and DL(z) at a given z.
• Fixed fractional bandwidth of 30.
• Overall throughput of 30.
• Size of a detector array is fixed to 2k x 2k and
  FOV per array is calculated as an output
  parameter.
• Wavelength bands are similar to
  J(1.2),H(1.6),K(2.2),L(3.5),M(4.6) except for
  their wider fractional bandwidths.
• 5 sigma detection after aperture photometry.

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Input parameters

• Diameter of the telescope (central
  obscuration neglected)
• Band (J,H,K,L,M)
• Integration time
• Pixel-wise sampling
• Source diameter
• Temperature of warm optics

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Sample 1-1 (D0.7m,K,Earth)
Age Age of the Universe, APsizeApparent
source diameter, DL Diffraction limit
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Sample 1 2 output instrumental parameters
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Sample 2-1 (0.7m,L,Earth)
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Sample 2-2 Instrumental output
Instrumental Thermal Background Limited,
if T(mirror)150K.
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Gap between K and L

• To go beyond 2.5 microns, the choice of detector
  is InSb whose operating temperature is low.
• To be sensitive at L and especially at M, the
  telescope optics must be cooled to, say, 50K. As
  long as radiation cooling is assumed, this
  implies that the satellite orbit of the Earth is
  not favorable.
• JWST and SPICA will go to L2.
• Larger communication antenna is necessary for L2.
  .. Bigger spacecraft expensive.

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Speculation on potential UV luminous targets

• Integrated Star light
• Active Galactic Nuclei
• Afterglow of gamma-ray bursts

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SED of a forming galaxy?
Bruzual Chalot 93
Numbers are ages in Gyr Note that Lyman break is
eminent after 0.1 Gyr.
But high z galaxies have not been detected down
to K24 within a small field of less than 100
arcmin2 (Subaru SDF with AO)
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Life time and Lbol of a star
Data for solar metallicity.
Log L/Lsun
Mass
Life (Gyr)
50 (O5) 0.006 5.0
20 (O8) 0.010 4.5
10 (O9) 0.030 3.5
5 (B3) 0.100 --- LB ---- 2.5
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Lyman break galaxies at zlt7 --- useful reference?

• UV luminosities of LBGs lt 109 Lsun.
• Lyman break means the presence of neutral H in
  photosphere (predominance of B stars).
• They may not be in the earliest phase more of a
  post-starburst phase.
• At zgt10 (tUNIVlt0.4Gyr), what is interesting is
  the integrated UV luminosity within a few pixels
  (kpc scale), or the number of very young (tlt0.03
  Gyr) and massive stars (Mgt10Msun) in a forming
  bulge or pre-galactic cluster, if such thing
  existed.
• Very young objects must have been missed by the
  LB technique at lower z, but the drop-out
  technique based on foreground Lyman ? absorption
  should still be effective.

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Possibilities other than galaxies

• AGNs
• Presence of QSOs(1012 Lsun) is known up to z6.
• Smaller less luminous AGNs(1011 Lsun) may be
  present at higher z.
• Time scale for a SMBH growth 0.1Gyr or less?
• May depend on accreted material (gas or stars).
• BH formation after SNe/GRBs or directly from
  ISM?
• Gamma-ray bursters
• Bright ones at lower z have UV luminosities in
  excess of 1011 Lsun for more than 3 days.
• Observed bright phase should be extended for a
  higher z object by the factor of (1z).
• They should be detectable, if there are many of
  them.

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Usage of GRBs

• For the study of GRBs themselves, the surveyor
  must be coeval with a high energy observatory.
• If the objective is to study IGM (re-ionization),
  spectroscopic follow-up by giant telescopes is
  necessary (may be observable with 8-10m LGAO
  for brightest ones and certainly observable with
  TMT or JWST).
• For the purpose of pre-location of over-density
  regions in the high z universe, GRB surveys
  without follow up will have significance.

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Simplest surveyor?(D0.7m,HgCdTe,Earth orbit)

• J/H/K simultaneous imager (Crude z estimation)
• HgCdTe 2k x 2k array each
• Diffraction limited at K.
• FOV 0.23 x 0.23 deg2
• 5 sigma limit K 24(Vega) in 1 hour
• 18 hrs to cover 1 deg2 for 100 duty cycle.
• Data rate 6.7 Kbytes / sec.
• 460 deg2 / yr for K24, 46 deg2 / yr for K25.
• Life time --- probably many years.

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Simplest surveyor 2

• Scientific uncertainty
• Luminosity functions of high z objects are
  unknown.
• Trade-off between depth and area of the survey is
  unknown.
• However, this uncertainty is the very reason
  that the observational astronomy is important.
• When we find something Qualitatively New, at
  that point, we will be slightly ahead of the rest
  of the world.

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Some strategic speculations(Personal view)

• Is there competition in the world?
• Coherence among domestic projects.
• Technological hurdle and quick solution.

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World wide situations?

• NASA
• HST service mission in 2008.
• Replacement of NICMOS with a 1k x 1k HgCdTe
  imager sensitive to 1.7 microns. (HST mirror is
  heated for temperature control.)..till 2013?
• Currently no plan other than JWST (t 2014).
• ESA
• I do not know of any plan for a wide-field NIR
  surveyor.
• Even when JWST flies or TMT is built, nobody
  knows where to point the telescope to observe
  extreme high z objects, since high density
  regions may be localized in the sky.

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Creating a domestic road map

• Road map is a term used in US to show logical
  coherence among different projects primarily to
  persuade authorities of funding agencies.
• Here I show a logic to unify Subaru, ELT, and a
  small NIR surveyor.

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Roll of Subaru

• Wide field survey in the visible up to z7.
• Morphology of galaxies by LGAO imaging up to 2.3
  microns.
• Picking up z-drop objects by combing the Subaru
  wide field survey in the visible and NIR survey
  at JHK.

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Usage of ELT

• If the goal of Japanese astronomers is using
  ELT instead of participating in ELT project,
  there may be some strategic consideration.
• ELT will be a powerful follow up instrument up to
  2.3 microns (or up to z17 in Lyman ?).
• Strategy barter time between ELT and the NIR
  surveyor.

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Summary of coherence

• ELT
• Spectroscopy up to z17.
• Subaru
• Wide field survey at zlt7
• Morphology (and possibly spectroscopy) at zlt17 by
  LGAO.
• NIR surveyor
• Wide filed survey between z 7 and 17.
• One way to barter time with ELT or even with
  JWST?

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Consideration about university based data center

• Creative usage and software development are often
  made by university based data centers in US (eg.
  HST/JHU, Chandra/CfA, IPAC-SSC/Caltech.)
• Presence of a data center located in a city with
  many universities may help expand the research
  opportunities for astronomers not associated with
  astronomy departments.

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Technological hurdles?

• Not a small-sized mission in JAXA standard
• Approximate weight 700 kg
• Medium-sized mission in JAXA standard.
• Cannot close domestically for quick launch.
• Pointing stability ( 0.1 arcsec)
• 0.2 arcsec accuracy is achieved by HINODE using
  tip-tilt correction.

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Cheap and quick solution?

• Rocket
• Russian, Indian, Chinese, American?
• E.g. Delta II (Launch vehicle for Spitzer)
• Payload to LEO 30006000 kg, GTO900-2000kg.
• Price 37M
• Spacecraft
• American? Russian? Chinese?
• Science instrument
• Japanese
• Technology proven (AKARIHINODE)
• Smaller than AKARI (Payload950kg, Total120M)
• Data center --- I think this is critical.
• Japanese
• Usage of AKARI data New data Previous ones
• University based? (Importing IPAC expertise?)

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Time scale consideration

• ALMAs first fringe (2010) . Not in time.
• JWST TMT( 2014) 2014 may be the target launch
  year.
• A potential future project for young astronomers.
• Forming a well defined plan by 2008 or 9.
• Long life is a key.
• Cryogenics No cryogen (LHe/LN2) is the best.
• Redundancy of moving parts such as actuators.

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Apart from my imaginations

• Since my simple program appears to give
  reasonable numbers (e.g. Reproduces Spitzers
  SWIRE survey sensitivity),
• It may be of use for a conceptual design of a
  NIR surveyor for theorists and young
  astronomers.
• It is found at
• http//optik2.mtk.nao.ac.jp/tnakajima

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Acknowledgments

• My thanks to
• H. Matsuhara
• N. Gouda
• T. Nakagawa
• H. Shibai
• for introduction to space astronomy and
• comments on the manuscript.