CFHT Information Bulletin, number 38, First Semester 1998



Extragalactic Observations with Adaptive Optics

David Crampton, Dominion Astrophysical Observatory, HIA/NRC, Victoria, Canada (David.Crampton@hia.nrc.ca)

Introduction

Since the Adaptive Optics Bonnette (AOB, aka PUEO) provides excellent correction using much fainter guide stars than most previous systems, observations of many extragalactic targets are now possible, using either a nearby reference star or the object itself. Observations of the nuclei of nearby galaxies, high redshift galaxies, AGN, radiogalaxies, the host galaxies of quasars and gravitational lenses were observed during the first year of operation of AOB. Examples of these are used to highlight some of the unique aspects of observing with adaptive optics, and to demonstrate the potential of adaptive optics for studies of the high redshift universe.

For those unfamilar with PUEO, it is based on a 19 subaperture curvature wavefront system (Roddier et al. 1991). A complete description of the system is given by Arsenault et al. (1994), and a comprehensive discussion of its performance by Rigaut et al. (1997). If the target is to be observed in the infrared, a dichroic is used to reflect the visible light to the wavefront sensor (WFS) while transmitting the IR light to the science detector, but if the target is to be observed in the visible, a beamsplitter must be used to send a percentage of the light to the WFS. The delivered image quality depends on the brightness of the reference star, its distance from the target, the seeing at the time of observation and the wavelength of observation. To gain an appreciation of how these variables affect the images and what can be expected, the reader is encouraged to visit the "performance meter" at http://www.cfht.hawaii.edu/Instruments/Imaging/AOB/psf.html@. To achieve FWHM ~ 0.12" imaging in the near-IR (diffraction-limited) and at J and I under median seeing conditions, the reference star must be brighter than R~14 and within ~30" of the target. However, the system will provide good correction on stars as faint as R = 17 under good seeing conditions, and the reference star can be located anywhere within the 90" diameter field. AOB also offers considerable advantages in terms of operational efficiency, since it is literally a push-button operation that eliminates all the overhead of focussing and guiding associated with conventional observing.

Since WFPC-II on HST delivers excellent images in the visible spectral region, most of the observations with AOB to date have been carried out in the near-IR where there is less competition and also where it produces the best images. Hence, the observations reported here were mostly made with the University of Montreal infrared camera MONICA (Nadeau et al. 1994) which was modified to give a pixel scale of 0.034" and hence a field of 8.8"x 8.8". This very small field is a significant handicap for many observations, as is the fact that the detector suffers from persistence problems, i.e., a bright source leaves a residual image that slowly decays with time. Since the site seeing is variable, observations of relatively bright stars usually have to be carried out in order to monitor the PSF (point-spread function). The sequence of observations of very faint sources thus has to be carefully planned to minimize such problems while frequently observing the PSF star to attain the scientific goals. Veran et al. (1997) have recently demonstrated that statistics of the wavefront sensor signals can be used to derive an excellent model of the average PSF (as long as the guide star is m = 13 or brighter). This is obviously ideal in that the PSF estimate is simultaneous with the target observation, and it obviates the requirement of directly observing the bright star (saving time and avoiding persistence problems). A comparison demonstrating how well the model reproduces the observed PSF is reproduced in Figure 18. For each observation, statistical data on conditions at the time of observation are recorded in "psf" files and the atmospheric-corrected PSF is stored in "dph" files. The PSF corresponding to each exposure can then be calculated with the CFHT "DPH2PSF" package (see http://www.cfht.hawaii.edu/~veran/cfht_dph2psf.html@)

Examples of Self-referencing Targets

Objects such as the nuclei of nearby galaxies, Seyferts and AGN can be usually guided on directly, i.e., the nucleus is often sufficiently point-like to enable the WFS to function properly as long as there is sufficient flux. The cores of NGC 6946 and M31 are examples of bright spirals which were successfully observed. It is well-known that the nucleus of M31 is double, with the brightest component, P1, being offset from the fainter component (and dynamical centre) by ~0.5". In spite of the duplicity, guiding on the nucleus of M31 with AOB was straightforward, resulting in diffraction-limited images (FWHM = 0.12") of the stars in the nuclear bulge region (Figure 19) The near-IR image quality is comparable to those of WFPC, enabling accurate magnitudes and colors to be determined for a significant number of stars in the bulge (Davidge et al. 1997). Spectroscopy of the double nucleus of M31 with AOB + OASIS in the visible, or in the J and H bands with OSIS linked to AOB with an integral field unit (Felenbok & Crampton, in progress) should finally unravel the mystery of this double nucleus (see e.g., Bacon et al. 1994; Kormendy & Richstone 1995; Tremaine 1995).

AGN are obviously also easy to guide on if they are sufficiently bright. Several AGN and Seyfert 1 type galaxies with point-like nuclei and magnitudes in the range m = 11-14 (e.g., bright ones like NGC 4151 to fainter ones like NGC 6814) were observed during commissioning. However, guiding was not successful on more diffuse objects such as the V = 13 nucleus of the Seyfert 2 galaxy Mrk 266, or the large bright elliptical galaxy which hosts 3C296. Numerous "AGN" projects are currently underway and a few results have already been published or are in press, e.g., NGC 7469 (Lai, 1997), Mrk 273 (Knapen et al. 1997) and NGC 1068 (Rouan et al. 1997)

In general, observations of these targets was hampered by the small MONICA field since neither the sky nor PSF stars are included in the frames. An accurate estimate of the PSF is essential for these sources in order to adequately subtract the effects of the bright point source from the nuclear region, so both the sky and a nearby reference star of similar brightness must be monitored throughout the sequence of observations. PSFs derived from the WFS data using the Veran et al. method are obviously valuable in this context.

Targets with guide stars

Nearby guide stars must be used for targets which are not sufficiently concentrated or bright enough for the WFS to perform adequately. In this case, not only are observations of the guide star itself required to monitor the PSF variations, but measurements of starfields (e.g., globular cluster fields) are also required to estimate the degradation of the PSF due to isoplanatic effects. Steinbring (1997) has developed semi-empirical software that models this degradation over the field as a function of the seeing and produces an "off-axis PSF" that can be used to help analyse the target data.

The investigation of quasar host galaxies provides an example of a project where knowledge of the off-axis PSF is vital. Studies of QSO host galaxies are notoriously difficult due to the faintness of the galaxy itself plus the superposition of the bright nuclear source. The current quasar catalogues have been cross-correlated with the HST Guide Star Catalogue, yielding a sample of a few hundred quasars with suitable bright guide stars. A number of these have now been observed with AOB, both in the visible region (at I) with a CCD detector and in the near infrared (mostly at H). Unfortunately, our project has so far been plagued by poor weather and the data for only one, 1055.3+019, has been completely analysed and published (Hutchings et al. 1997). This z = 1.1 quasar is well-resolved in both I and H. The inner region (r < 0.65") of the galaxy exhibits a r1/4 profile, while the outer region has an exponential scale length of 0.35" (1.8 kpc) in the H band. The data suggest that the host galaxy has recently been subjected to a merger event.

The components of multiply-imaged gravitational lenses formed by galaxies in the line of sight to distant quasars typically have subarcsecond separations and consequently their study and monitoring could significantly benefit from the improved resolution offered by adaptive optics. Unfortunately, only a few of the ~35 such lenses currently known (according to the list maintained by Chris Kochanek http://cfa-www.harvard.edu/glensdata ) have nearby guide stars that are sufficiently bright to give good correction. One exception is SBS 1520+530 (Chavushyan et al. 1997), a doubly-imaged BAL quasar with a separation of 1.6 which happens to be only 13" from a m~12 star (see Figure 20). H band images with FWHM = 0.15" taken with AOB reveal the lensing galaxy 0.40" from the fainter component, offset 0.12" from the line joining the components (Crampton, Schecter and Beuzit 1997).

Registering invisible targets

Targets which are very faint, particularly those that are diffuse, often present additional problems since the MONICA field is so small that there aren't any objects or features that can be used to register the images. Near-IR images always have to be dithered to remove the effects of bad pixels and to improve the flat-field, offsets are often required to enlarge the area surveyed, and differential flexure between the WFS and the detector may produce additional shifts if the total exposure is long. Consequently, registering and superimposing the images is not trivial. Fortunately, this was forseen in the initial design of AOB and every effort was made to minimize the flexure between the WFS and the detector so that the WFS coordinates (recorded in the FITS header) can be used to determine the offsets between exposures. Observations of sequences of overlapping fields in globular clusters can be used to calibrate WFS offsets in terms of pixel location on the detector and/or equatorial coordinates. Since MONICA and the "AOB optics" have to be mounted/dismounted before/after each run these calibrations should be checked, but so far they appear to have been very repeatable. Through comparison of AOB + MONICA coordinates with positions of stars derived from HST images, the pixel scale is ~0.0342" per pixel at H and the image is rotated such that N is at ~ -10.6º from the top of the image and E is at -100.6º (this rotation varies slightly from run to run). Comparison of the WFS X and Y offsets with offsets of the frames derived from coordinates of stars in globular cluster fields gives the following calibration:

Shift_x = -0.3588 × (WFSX - 1900) - 0.0714 × (WFSY - 2150)

Shift_y = -0.0825 × (WFSX - 1900) + 0.3715 × (WFSY - 2150)

where the WFS coordinates 1900, 2150 correspond to the approximate centre of the detector. The above equations can be used to determine the relative shifts required to stack a series of images. Unfortunately, the WFSY stage has developed more backlash than when tested in the lab, so that application of the above calibration leads to a dispersion in y of 2.9 pixels compared to 1.7 pixels in x. In practice, since the same dither pattern is normally used for sequences of exposures, empirical corrections for the backlash can be determined and applied. If this is done, repeated observations of the same point source (e.g., a quasar) over several hours show that the WFS positions can be used to register frames with a dispersion of 1.4 pixels or 0.05". Since the delivered images in the near-IR are usually diffraction-limited with FWHM ~0.12, this is clearly inadequate, so future systems should be designed to minimize this. One solution (which we have adopted for Gemini instruments) is to incorporate a tip/tilt/focus sensor on each instrument to eliminate the effects of flexure, backlash, etc. Ideally, one would like diffraction-limited images to be registered to better than ~10% of their FWHM; for 8m telescopes this will be of the order of only a few mas.

The 3C 273 jet provides a good example of a target which was barely visible, even on background-subtracted 300s exposures, and certainly no features are present which can be used to register the images. The jet is located between 11 - 21" from 3C273 which, of magnitude R = 13, was used as the guide star. Given the small field of the detector, the jet was observed at two different locations along the jet centered at 15 and 20" from 3C273. Repeated observations of point sources in other fields and in M13 were used to calibrate the WFS positions for this run. The (preliminary) result of stacking the exposures is shown in Figure 21. During these observations, the site seeing was poor on average and very variable (from 0.5" to 1.2") and so the compensated image quality is also variable. Although the results of the AOB commissioning observations indicate that images do not degrade as quickly with distance off axis as initially expected, the image quality at the end of the jet (~20" from the guide star) is not as good as on axis, particularly when the seeing is poor. Including registration errors (0.05"), the spatial resolution of the jet in Figure 21 is probably not better than ~0.3". The appearance of the jet in the H band is very different from that in the visible (Bahcall et al. 1995, Roser et al. 1997), but is similar to that seen at K' by Neumann et al. 1997.

Further examples of "invisible targets" are observations of high redshift galaxies. A pair of CFRS galaxies near a R = 15.3 guide star were observed during commissioning, and one of the z = 3.0 Steidel et al. (1996) galaxies was also attempted. In both cases, the goal was to examine the morphology of high redshift galaxies at more normal restframe wavelengths than is possible in the visible (H band corresponds to restframe I and B at z = 1 and 3 respectively). Observations of these very faint targets were significantly hampered by the detector remanence effect discussed above and also by the reduced sensitivity (three times lower than anticipated) of MONICA during the latter half of 1996. For example, it was initially estimated that each z = 3 galaxy (R ~ 24) would require a total exposure of ~8h, but when it became obvious that the object was barely visible after a few hours (because of the reduced sensitivity), the project had to be abandoned.

Devising observing strategies to minimize the effects of detector persistence yet still be confident of the registration in these tiny blank fields is quite a challenge. However, since the field of the new KIR detector for AOB is four times larger (and its sensitivity is three times higher) future observations of this type of target will be much easier. Although initial reports (Doyon, private communication) indicate that the detector remanence effects are absent or much reduced for KIR, it will still probably be wise to centre the observations of bright stars (for PSF and WFS calibrations) at different locations on the detector than faint targets.

Summary

Many extragalactic projects will benefit enormously from the improved spatial resolution that is now possible from the ground, provided that the targets are bright and sufficiently concentrated or can be selected near suitable guide stars. Since a large number of extragalactic projects don't involve point sources that can benefit from the ultimate resolution that an instrument like AOB can deliver and/or probably don't have a sufficiently bright reference source to achieve that resolution, critically sampling the diffraction core is less important than a reasonable field size (so that sky and PSF stars are included in the field). Hence a pixel scale of the detector for AOB of ~0.07" rather than 0.034" per pixel would be advantageous for most such projects. In addition to imaging, the forthcoming combination of "adaptive optic image quality" and spectrographs designed to capitalize on it will be very exciting. I, for one, was continually frustrated during commissioning by not being able to get spectra of, e.g., the 0.1" images of the nucleus of M31 to find out what was really going on.

Given a suitable guide star, AOB delivered near-IR images worse than ~0.1" on only 1 of the ~30 AOB nights that I've been involved with so far. Figure 22 shows typical results, from ~350 observations taken on the first 10 commissioning nights (Rigaut, Lai & Veran, 1996). It feels like we're really entering a new era: from now on ground-based astronomers will complain when the images are significantly worse than 0.1" rather than 1"!

Acknowledgment: While the above discussion is the result of my personal experience with AOB, the fantastic performance of Pueo is due to the combined efforts of teams from CFHT, Meudon, Laserdot, UH and DAO. Francois Rigaut deserves much of the credit for this superb implementation of Francois Roddier's curvature concept. It is a very exciting instrument to use.

References

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Editor: Dr. T. M. C. Abbott, tmca@cfht.hawaii.edu
Copyright © 1998, CFHT Corporation. All rights reserved.