GSMT Science Use Case
New science enabled by photonic OH suppression
S.C. Ellis & J. Bland-Hawthorn (University of Sydney)
Abstract:
The University of Sydney and the Anglo-Australian Observatory are close to completing
a new technological approach to OH suppression at near infrared wavelengths. The goal
is to remove the brightest 300 night sky lines in the J and H bands (1.0-1.8 m) at high
efficiency. This will impact a wide range of science cases over a wide range of redshift
intervals (0z13). We pay particular attention to the cosmological applications,
including “Epoch of Reionisation” science (7z13) and stellar populations (1z4). The
OH suppression is performed within multimode infrared fibres so the technology is well
adapted to multi-object and integral field spectroscopy.
Scientific Motivation:
Near-infrared astronomy is severely hampered by the extreme brightness of the night sky.
Photonic suppression (Bland-Hawthorn et al. 2004) will cleanly remove the atmospheric
emission lines, effectively making the J and H band background 4 mags fainter (Ellis &
Bland-Hawthorn 2008). This will have fundamental implications for all areas of nearinfrared astronomy, since it will allow unprecedentedly deep observations, giving access
to continuum and faint absorption and emission spectroscopic lines hitherto undetectable.
Cosmology in particular will benefit significantly since observations of the very early
Universe require observations of highly redshifted features that demand deep nearinfrared spectroscopy.
We identify eight science areas to exemplify the benefits of OH suppression on an ELT.
We have categorised these areas according to redshift/ rest-frame wavelength.
A: z=7-13: Ly in the early Universe. We will measure the evolution of the IGM from
7z13, i.e. the epoch of reionisation (Fan et al. 2006). We will achieve this via ultradeep near-infrared spectroscopy of high redshift Lyman- emitting sources and aim to
measure: (i) the redshifts of the sources, unambiguously, from a complete Gunn-Peterson
trough and asymmetric line profiles, (ii) the nature of the ionising sources via line widths,
(iii) the neutral fraction of the IGM via Ly damping wings and metal absorption lines,
(iv) the flux of the ionising sources via cosmic Strömgren spheres. This case is examined
in more detail below.
B: z=4-12: Wind diagnostics in high redshift galaxies. We will track the enrichment of
the IGM via wind diagnostics during the early Universe. It is possible that the IGM was
enriched via SNe ejecta even prior to reionisation (Madau, Ferrara & Ress 2001). We
will measure wind strengths from spectral features at restframe 1300-1800Å, e.g. SiIV,
CIV, etc, see figure 1.
C: z=3-6: Infrared redshift desert. This
epoch is a well known redshift desert in the
near-infrared due to a dearth of spectral
features. However, limited information is
available on late-type galaxies due to features
at 2400-2800Å such as FeII and MgII, see
fig 2.
Figure 1. A plot from Leitherer et al. (1995)
showing the characteristic P Cygni signature
of a strong starburst driven wind.
Figure 2. A plot from Abraham et al. (2004),
showing some UV spectral features which can
be used for fitting redshifts to 3<z<6 galaxies
in the NIR redshift desert.
D: z=1-4: The star-formation history of
galaxies I. This is a crucial epoch in the
evolution of galaxies; star-formation,
merging activity and AGN activity are all
thought to have peaked at 1<z<4. We will
measure the redshifts and star-formation rates
of galaxies during this period from nebular
line emission and continuum fitting of
features with 3200-5000Å (esp. the 4000Å
break). This case is described in more detail
below.
E: z=0.9-2.5: The star-formation history of
galaxies II. At this epoch H is redshifted
into the J and H bands allowing accurate starformation rates to be measured.
By
measuring the ‘twin-horn’ profiles of the
emission lines the Tully-Fisher relation can
be computed during this important period.
IFU spectroscopy will allow detailed studies
of stellar dynamics similar to the results of
Förster-Schreiber et al. see figure 3.
F: z=0.2-1.5: Stellar dynamics of galaxies.
Using features in the range 7000-9000Å,
esp. the Ca triplet, we will measure stellar
Figure 3. A plot from Förster-Schreiber et al. dynamics via IFU spectroscopy.
(2006) showing H- emission, velocity and
G: z=0-0.2: Nearby galaxies. We will measure the infrared properties of galaxies in the
SDSS/2dFGRS realm. NIR spectroscopy allows the measurement of AGN fine structure
lines (e.g. OI, FeII) as well as broad lines such as Pa allowing unambiguous
classification of AGN (Ramos Almeida et al 2008), see figure 4. In galaxies
measurement of the CO bandhead allows an accurate determination of galaxy mass. We
note that restricting NIR spectroscopy to OH
suppressed J and H bands removes the need
for cryogenic spectrographs and allows very
wide-field MOS and ADC.
H: z=0: Galactic astronomy. Low mass
stars emit most of their light at NIR
wavelengths. Understanding the low mass end
of the IMF and the formation of L, T and Y
F
Figure 4. A plot from Ramos
W Almeida et al. dwarfs as well as unbound planets therefore
(2008) showing the narrow
H lines in the NIR requires NIR spectroscopy. OH suppression
which allow accurate AGNMclassification.
will allow observations to probe much further
within our own Galaxy.
m
a
p
Approach:
s is achieved with fibre Bragg gratings and photonic lanterns
Photonic OH suppression
(Bland-Hawthorn et al. 2004; Leon-Saval, Birks & Bland-Hawthorn 2005), thus the most
o
natural application of OH
f suppression is fibre-fed spectroscopy. This can be applied as
either wide-field MOS, a monolithic IFU or even deployable IFUs.
z
Photonic OH suppression can be incorporated as an upgrade to planned IR spectrographs
2
by designing an appropriate
fibre positioner or IFU which feeds an OH suppression unit
consisting of FBG photonic
lanterns,
which in turn feeds an existing instrument via relay
g
optics (e.g. FLEX on an 8m;
see Horton et al. 2008 and Ellis et al. 2008).
a
l
a
Figure 5 shows the measured
transmission of a FBG which suppresses 63 doublets over
x
200nm in the H band. The interline losses are <5% everywhere. The maximum
i
suppression achieved is 55dB,
i.e. a factor of >300,000. This is the most complex optical
e
filter ever produced in any
s field of science (Bland-Hawthorn et al. 2008a,b).
.
Figure 5. The measured transmission of a FBG suppressing 63 doublets over 200nm in the H band. The
top panel shows a model of the NIR sky-brightness which is dominated by atmospheric OH emission lines.
The J and H photometric bands are marked. The middle panel shows the sky-brightness for the region
covered by the FBG. The bottom panel shows the transmission of the FBG. The blue and red lines show
the response of two separate printings. The green dots show the target wavelengths and depths. The dotted
lines show printed notches that were inaccessible to measurement with the current apparatus. The notches
early exceed requirements whilst the interline losses are still less than 5% across the entire spectrum. On
close examination, there are faint blue “aliases” mixed in with the red notches, and vice versa; these will
not appear in the next generation of filters.
Limiting Factors and the Current State of the Art:
The limiting factors to photonic OH suppression are limited A, the number and depth of
notches and the true brightness of the interline continuum.
The brightness of the interline continuum restricts OH suppression to wavelengths at
which the OH lines are the dominant source of background. For most sites at temperate
latitudes this means <1.8m, but at Antarctic sites such as Dome C OH suppression
could be effective out to <2.5m.
Current developments are pushing FBGs to 150 notches over each band. In the J and H
bands the faintest lines suppressed are 0.2 and 1 ph s-1 m-2 arcsec-2 respectively. Over
the width of a notch (1Å) the zodiacal scattered light is  0.01 ph s-1 m-2 arcsec-2, so even
with this level of suppression the OH lines are brighter than the interline continuum. The
number and depth of notches in a FBG, combined with the spectral resolution, therefore
place some limit on the performance of OH suppression. We note however that unlike
high–dispersion masking OH suppression, instrumental scattering of OH light is not a
problem in FBGs, and therefore the benefits in reduced background are available across
the entire spectrum and not just at the line cores.
Multi-notch aperiodic FBGs can currently only be printed into single-mode fibres. In
order to make the fibres useful for astronomy the photonic lantern was developed which
converts a multi-mode fibre into an array of single-mode fibres, and back again (LeonSaval, Birks, Bland-Hawthorn et al. 2005; Horton & Bland-Hawthorn 2006). The FBGs
are inserted into the SMF array, providing MMF performance from SMFs. Currently
photonic lanterns are being developed with up to 61 modes. This means that for a fibre
with NA of 0.1 we require fibre core diameter of no more than 60m to avoid modal
dispersion.
Thus even with photonic lanterns the A of an OH suppressing fibre is smaller than that
of a typical astronomical fibre. Therefore in order to increase the A of a fibre we
envisage using bundles of fibres, e.g. as a lenslet IFU.
Current state of the art
OH suppression with current technology (e.g. Iwamuro et al. 2001) is fundamentally
limited by instrumental scattering including diffraction by the aperture stops, bulk
scattering, dispersing elements, etc. This is manifested as broad Lorentzian wings in the
PSF of any instrument. To avoid this problem requires that the OH lines be suppressed
prior to dispersion and/ or in a manner that is solely dependent on wavelength. FBGs
fulfil both these criteria. See Ellis & Bland-Hawthorn (2008) for a detailed discussion on
the effects of scattering on OH suppression.
OH suppression on an ELT will be competitive with JWST; JWST will have lower
backgrounds but a much smaller collecting area. Furthermore JWST will be confined to
a single NIR spectrograph operating either in MOS mode with R=100 or R=1000, or
single object spectroscopy with R=3000 with a modest FoV 9 arcmin2. OH suppression
is a ground-based technology and therefore adaptable to the scientific needs (FoV,
resolution, etc.) as required in the era of ELTs.
Technical Details:
OH suppressed spectroscopy will be carried out in a similar manner to current NIR
observations today. No specialised mode of observation is envisaged.
Preparatory, Supporting, and Followup Observations:
The science cases are too diverse to discuss this in detail.
Anticipated Results:
We now discuss two of the science cases in detail, viz. studies of the reionisation era,
7<z<13, and studies of galaxy evolution at 1<z<4. We concentrate on the results of
simulations of OH suppression on a 30m telescope. The simulations follow those of Ellis
& Bland-Hawthorn (2008), which is a complete end-to end system, including
atmospheric, thermal and detector backgrounds, background variability and systematic
calibration errors.
A. Epoch of reionisation
The unambiguous identification of very high redshift galaxies is a necessary first step in
studying the era of reionisation. Candidate galaxies at z>7 are already being found by
means of narrow-band surveys, continuum drop-out surveys and spectroscopic surveys
(e.g. Bouwens et al. 2008ab; Henry et al. 2008; Stark et al. 2007). Future deep NIR
imaging surveys (e.g. VISTA, JWST) should uncover many more.
The unambiguous confirmation of the redshift of candidate galaxies requires either the
detection of at least two spectroscopic features, or, less stringently, identification of a
single idiosyncratic feature. At 7<z<13 the most promising primary feature to identify is
either a Lyman- emission line or a Lyman break due to the accessibility of these
features to NIR observations; there are no suitable features bluewards of the Lyman break
and suitable features to the red of Ly are redshifted to mid-IR wavelengths making
ground-based observations impracticable. For the same reason identification of a
secondary spectroscopic feature also requires mid-IR observations. This may be possible
with the MIRI instrument on JWST which will offer R100 spectroscopy at MIR
wavelengths.
Due to the lack of supporting spectroscopic features, we must instead identify high
redshift galaxies via the asymmetry of the Ly emission line or via the presence of a
Gunn-Peterson absorption profile. This requires deep, moderate resolution NIR
spectroscopy.
Figure 6 shows a simulated observation of the best fitting model of the candidate z=9
LBG found by Henry et al. (2008). The exposure time was 2hrs and the spectral
resolution was R=1000. 150 doublets were suppressed across the J band, with a
maximum suppression of 30dB. The complete lack of any emission bluewards of the
break provides possible evidence that the galaxy is at z=9.
Figure 6. A simulated observation (left) of the z=9 LBG candidate discovered by Henry et al. (2008). The
exposure time is 2hrs and the resolution is R=1000. The blue spectrum in the background shows the same
source observed via longslit spectroscopy (i.e. perfect background subtraction) with no OH suppression.
The right hand plot shows a comparision of the object (black) and sky (blue) spectra at the resolution of the
observations.
Gunn-Peterson absorption saturates at neutral fractions of only 10-4. Therefore in order to
measure the ionised fraction of the IGM beyond z=7 we must turn to other methods. One
method is to measure the damping wing in the Ly break (Miralda-Escude 1998).
However, if the ionising sources are luminous enough they may ionise a local HII region,
making the red damping wing difficult to observe (Madau & Rees 2000). Figure 7
shows the effect of the damping wing with variously sized cosmic Strömgren spheres.
Detecting the presence of a damping wing would allow the neutral fraction of the IGM to
be measured (Miralda-Escude 1998). The presence of a Strömgren sphere would hinder
the measurement of the neutral fraction but would allow important information to be
discovered on the total ionising flux of the sources within the HII region. If some
assumptions are made on the fluxes of individual sources, then the presence of a
Strömgren sphere should allow calculations on the clustering of primordial galaxies and
AGN to be made.
Figure 7. The Lyman- line and the effect of the IGM. In each sub-figure the top panel shows the
transmission through the IGM, and the main panel shows the Lyman- line. In black is shown the intrinsic
line profile taken to have FWHM=20Å. The red lines show the absorbed line due to a completely neutral
surrounding medium. Notice the effect of the red damping wing on the red side of the line and the
complete absorption due to resonant scattering on the blue side of the line. The green lines show the
modification of the absorbed profile if the source is located within a locally ionised HII region of either
1Mpc, 5Mpc or 50Mpc radius. The blue side is still strongly absorbed due to residual HI within the
Strömgren sphere, but the red side shows much less damping, since only the tail end of the damping wing
reaches sufficiently red wavelengths.
Figure 8 shows simulated observations of various z=9 Ly sources including the effects
of the damping wing and cosmic Strömgren spheres. If the redshift of the source is
known a priori (as indicated by the dashed red lines in the figure) then the effect of the
damping wing or Strömgren sphere is easily discerned. In the case of a completely
neutral medium the peak is shifted to the red of 1216Å (restframe); however for a large
Strömgren sphere the line is very asymmetrical but peaks at 1216Å.
Figure 8. Simulated observations of z=9 Ly emission line sources. In all cases the exposure time was
8hrs and the resolution was R=3000. The top row has f=10-18 erg s-1 cm-2 and FWHM=150 km s-1, the
middle row has f=10-18 erg s-1 cm-2 and FWHM=300 km s-1, the bottom row has f=10-17 erg s-1 cm-2 and
FWHM=1500 km s-1. The columns from left to right are for Strömgren spheres of 0, 1, 5, 10 and 50 Mpc.
The black lines show the OH suppressed spectrum and the blue lines show a long-slit spectrum with no OH
suppression, dashed red lines show the redshifted wavelength of the unabsorbed line centre.
Analysing the neutral fraction of the IGM in this way therefore requires an independent
measure of redshift. In the era of JWST this is best achieved using MIR spectroscopy of
nebular lines in the case of galaxies or narrow forbidden lines in the case of AGN.
Figure 8 also shows that it would be possible to distinguish between accretion and starformation as the ionisation mechanism, since the accretion sources will have much larger
velocity dispersion (FWHM > 1000 km s-1) compared to galaxies (FWHM < 500 km s-1).
Oh (2002) has proposed using metal absorption lines to measure the neutral fraction of
the IGM beyond z=7. Both SiII and OI have ionisation energies close to that of HI, but
both have much lower transition probabilites. Providing these metals are present in the
IGM at sufficiently high redshift, absorption from these lines can thus be used as a proxy
for measuring the neutral fraction of HI, and will not saturate until much higher redshifts.
Suitable lines are OI 1302.17Å and SiII 1260.42Å, both of which have been previously
observed in high-z QSO spectra (Pettini 2001).
Thus a single observation could
incorporate Ly, OI and SiII.
Figure 9 shows a simulated stacked spectrum of an H=26 AB mag, z=11 QSO, with a
total exposure time of 70hrs and R=1000 (model QSO spectrum provided by R.
McMahon private communication). Metal lines (unidentified) are clearly visible,
showing the viability of this method.
Figure 9. A simulated 70hr stacked spectrum of an H=26 AB mag, z=11 QSO at R=1000. Plots and lines
are arranged as for figure 5. The red dashed line shows the actual object spectrum. Metal absorption
features are clearly visible.
B. The star-formation history of galaxies
The period between 1<z<3 is hugely important for studies of galaxy evolution. During
this time it is thought that star-formation (Hopkins 2004; Doherty et al. 2006), merging
(Hopkins et al. 2006) and AGN activity (Croom et al. 2004) were all at their peak. This
is the epoch in which the galaxies of today were assembled and shaped, their evolution
progressing more quiescently at z<1. We will measure the redshifts, star-formation rates,
and metallicities of galaxies at 1<z<4 by means OH suppressed near-infrared
spectroscopy.
This period is a well known redshift desert at optical wavelengths due to the lack of
suitable spectroscopic features. The extension of accurate luminosity functions and
scaling relations (e.g. Faber et al. 2007) to higher redshifts clearly requires accurate
redshifts.
It is possible to measure the star-formation histories of both late and early type galaxies
throughout this period from emission lines for the former (e.g. H, H, OII, OIII) and
from the Balmer break for the latter. Using these features has the significant advantage
that they are the same features used to determine the star-formation rates and histories of
galaxies at lower redshifts, thus measurements from both epochs will be directly
comparable.
Figure 10 shows a simulated 0.5hr observation of an H=19 Vega mag, z=1.4 emission
line galaxy. It is clear that the task of fitting redshifts is straightforward with OH
suppression on a 30m telescope. Furthermore the H and NII emission lines are clearly
visible and distinguishable allowing accurate star-formation rates to be determined.
Figure 10. A simulated spectrum of an H=19 Vega mag, z=1.4 emission line galaxy, the exposure time
was 0.5hr and the resolution R=1000. The full spectrum (left) shows that identification of features is
undemanding. The middle plot shows a close-up of the region around H from the same spectrum. In the
left and middle plots the blue spectrum shows the equivalent long-slit spectrum with no OH suppression.
The right hand spectrum compares the object (black) and sky (blue) spectrum at the resolution of the
observations.
Figure 11 shows a simulated R=1000, 6hr observation of an H=21 Vega mag early type
galaxy at z=3. Spectral fitting of the Balmer break will allow age dating of the stellar
populations, while individual absorption lines will allow the metallicity history of
galaxies to be determined.
Figure 11. A simulated observation of an H=21 Vega mag, z=3 early-type galaxy with exposure time 6hrs
and R=1000. Plots and lines are arranged as for figure 5.
Requirements and Goals Beyond the GMT and TMT Baseline Instrument Designs:
This programme requires the completion of the development of FBG OH suppression and
photonic lantern MMF-SMF converters. Both these developments are being carried out
at the AAO and the University of Sydney. Both new technologies have been
demonstrated in principle and in practice in laboratory tests. Single fibre on-sky tests
will take place at the AAO within a year suppressing 150 lines across the H band, with
161 fibre converters. The first tests will employ the fibre pointing directly at the sky to
calibrate the performance of the system. This will be followed by single fibre pointed
observations using the auxiliary Cassegrain focus of the AAT with IRIS2.
The first fully optimised instrument employing FBG OH suppression will be FLEX, an
IFU feed for an existing NIR spectrograph on an 8m class telescope (Ellis, BlandHawthorn, Horton et al 2008; Horton, Ellis, Bland-Hawthorn et al. 2008). This
technology will have been fully tested and optimised by the era of ELTs.
No special observing conditions or operations will be required for OH suppression.
There will be huge potential for data-mining surveys from OH suppression observations.
OH suppression is naturally combined with MOS surveys since both require fibre fed
spectrographs.
Similarly we envisage multi-object IFU spectroscopy with OH
suppression, again with the potential to create large databases.
No other facilities will be able to carry out such deep NIR MOS surveys. JWST will
provide complementary data, especially deep NIR imaging and deep MIR spectroscopy
as mentioned above.
Summary:
The synthesis of the collecting power of ELTs with photonic OH suppression promises
unprecedented views of the Universe. We have presented two cases to exemplify the
power of OH suppression on an ELT. Observations of the very early Universe will allow
study of the epoch of reionisation – ‘a great last problem’ of cosmology. The epoch of
reionisation, the nature of the ionising sources and the evolution of the IGM will all be
made possible with photonic OH suppression. OH suppression will open up the redshift
desert to observations similar to those presently achieved in the SDSS and 2dFGRS
surveys. This epoch is immensely important for the understanding of galaxy evolution,
since this is the era at which star-formation, merging and AGN activity are all though to
have peaked. Although we have only simulated observations for these two science cases,
we note that OH suppression should benefit astronomical observations from z=0 to z=13.
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