The NTT Provides the Deepest Look Into Space 6. A. PETERSON, Mount Stromlo Observatory,Australian National University, Canberra S. D'ODORICO, M. TARENGHI and E. J. WAMPLER, ESO The ESO New Technology Telescope r on La Silla has again proven its extraor- - dinary abilities. It has now produced the "deepest" view into the distant regions of the Universe ever obtained with ground- or space-based telescopes. Figure 1 : This picture is a reproduction of a I.1 x 1.1 arcmin portion of a composite im- age of forty-one 10-minute exposures in the V band of a field at high galactic latitude in the constellation of Sextans (R.A. loh 45'7 Decl. -0' 143. The individual images were obtained with the EMMI imager/spectrograph at the Nas- myth focus of the ESO 3.5-m New Technolo- gy Telescope using a 1000 x 1000 pixel Thomson CCD. This combination gave a full field of 7.6 x 7.6 arcmin and a pixel size of 0.44 arcsec. The average seeing during these exposures was 1.0 arcsec. The telescope was offset between the indi- vidual exposures so that the sky background could be used to flat-field the frame. This procedure also removed the effects of cos- mic rays and blemishes in the CCD. More than 97% of the objects seen in this sub- field are galaxies. For the brighter galax- ies, there is good agreement between the galaxy counts of Tyson (1988, Astron. J., 96, 1) and the NTT counts for the brighter galax- ies. However, the limiting magnitude for this image is - I mag fainter than for previous galaxies is shown on the accompanying map This V-image represents the deepest im- work. A magnitude sequence of a few of the on page 2. age that has ever been published, The new picture reaches magnitude 29 A Magnitude Sequence in the NTT Picture and shows enormous numbers of ex- tremely faint and remote galaxies whose images almost completely fill the field of view. The Observations Beginning in March 1991, and to- gether with Yuzuru Yoshii of the Nation- al Astronomical Observatory in Tokyo and Joseph Silk of University of Califor- nia, Berkeley, we have embarked upon an observing programme with the NTT aimed at detecting and measuring ex- tremely faint galaxies. To avoid problems with bright objects in the field, we pointed the NTT towards an "empty field" in Sextans. Previous observations had shown that no objects brighter than about magnitude 20 were visible in this direction. As will be seen, this first attempt has been highly suc- cessful. Using a Thomson high-quality CCD detector in the ESO Multi-Mode Instru- ment (EMMI) at one of the NlT Nasmyth foci, Bruce Peterson took forty-one ex- posures of this field, totalling 6 hours 50 O loblo' '05 llo 0 109 minutes. The individual pictures were OO 108 0 0 ll~dl~l~114 116 registered and co-added to produce a 0 115 0 117 119 combined image of which a small part 123 1'927 (about 2% of the total area) is repro- - - ,129 1 0121126;5 lglZ2 c-h 126 , 01 5 duced on the photo on the front page of F~gure2: This map identifies some of the galaxies which are seen in the deep NTT frame on the this issue of The Messenger, front page The computer-drawn ellipses reflect their sizes and shapes Note that the identifying numbers are located to the right of the objects they refer to. The following sequence of measured V-magnitudes illustrates the extraordinary depth of th~s Characteristics of the Picture picture: V = 20.3 (galaxy no 72), 21.1 (73), 21 5 (64), 23.0 (62), 23.4 (80), 24 9 (1 14), 26.3 (58), It has been known for some time that 26.8 (92), 27.5 (1 05), 27 7 (1 1O), 28.0 (1 3), 28.4 (1 0), 29.0 (70), 29 1 (45). on very deep sky exposures, most re- corded objects fainter than about mag- nitude 24 are galaxies, rather than indi- vidual stars. In simple terms, this is be- extent overlap each other. As a matter than the Milky Way Galaxy, then their cause, as we look further and further out of fact, it is not even certain that there is distances would be even larger. in space, we see more and more galax- any place where we are able to see the ies, while there is only a limited number sky background. Already this simple ob- Follow-up Observations of foreground stars in the Milky Way. servation is of great cosmological sig- More than 97% of the objects in the nificance: the number of galaxies still A number of follow-up observations frame are galaxies. The brightest ones, appears to be increasing at these very are now being undertaken. of about magnitude 21 -25, can clearly faint magnitudes. It seems that we have First of all, reasonably accurate col- be seen to have different shapes and not yet reached a point where we begin ours of most of the observed galaxies can be accordingly classified. Thanks to to look through the system of galaxies, will be measured. With the NTT, this will the good angular resolution, it is possi- as we can look through the stars in the be possible for those which are brighter ble to see that some of the fainter im- Milky Way system. than magnitude 28. An R-frame similar ages are more or less elongated. This A single frame, however, cannot with to the V-frame shown here has already may be due to the galaxy type or to the certainty discern between relatively been obtained; a comparison may en- inclination. nearby dwarf galaxies, very distant "nor- able us to cast some light on the ages of Calibration exposures were made on mal" galaxies, or extremely remote the galaxies observed. the same nights and we determined that superluminous galaxies. If some of the The NTT will also be able to obtain the limiting magnitude of the frame is faintest images here seen belong to spectra of the galaxies brighter than fainter than magnitude 29. This is more dwarf galaxies like the Magellanic about magnitude 24. This will make pos- than one magnitude, i.e. at least 2.5 Clouds, then their redshifts are likely to sible the measurement of their redshifts, times, fainter than any other image ob- be 0.5 to 0.7, corresponding to look- i.e, their velocities and cosmological tained so far by any optical telescope, back times of 38-48 % of the age of the distances. on the ground as well as in space. We Universe. Those which are normal The differentiation between relatively have indicated a magnitude sequence galaxies like the Milky Way will have nearby dwarf galaxies and much more on the drawing. redshifts of the order of 3-3.5 and the distant normal galaxies will also be pos- The frame shows enormous numbers look-back time would be 88-91 %. sible by means of continued observa- of faint galaxies whose images to a large However, if any of them are brighter tions of the same sky field. Dwarf galax- ies at redshift 0.5 would be close and therefore immediately give impor- into regions of the Universe, so remote enough for individual supernovae to be tant information about the nature of the in space and time that they have never observed in large numbers. Contrarily, objects seen. before been explored. supernovae in normal galaxies at red- This NTT picture has given us a tan- This is the type of work that will be at shift 3 or more would be too faint to be talizing, first glimpse of what can be the frontline of optical observational observed. A comparison of pictures ob- done with the new and improved obser- cosmology during the coming years. tained at different times will tell whether vational means which are now at our short-lived supernovae are seen or not, disposal. It has given us a unique look HST - the First Year R. FOSBURY, ST-ECF At the end of April, the Hubble Space restrictions on the observing pro- servations of the flare star AD Leo coor- Telescope completed its first year in or- grammes. Communications with the dinated with a variety of ground- and bit. With a projected lifetime, however, ground, both direct and via the TDRS space-based observatories. of some fifteen years, the observational system, are no problem with a bit-error- After an equivocal start, the pointing programme of the spacecraft is just be- rate some thousand times better than performance has been improved to a ginning. After the announcement in June the minimum specification. The data level where most target acquisitions are 1990 of the spherical aberration in the management is also operating well, al- successful. The spacecraft motions in- primary mirror, the first assessment by though a failed memory unit (one of six, duced by the solar arrays during ter- astronomers tended to relegate the ob- four of which are currently used) caused minator transitions mean that the "jitter" servatory to the role of a "large IUE with the HST to enter a deep sleep - "hard- does not meet specifications although some UV imaging". ware sunpoint safemode" state - re- large parts of the orbit are extremely Now that the science verification cently. A five-day recovery process was quiet (- 5 milliarcseconds rms). Noisy phase of the commissioning process is completed just in time to intercept the periods, which would degrade certain nearing completion and the Guaranteed observing schedule for some GHRS ob- observations, can be rendered benign Time and General Observer pro- grammes have started, it is a good time to assess what we have learned so far and try to paint a somewhat more realis- tic picture of the current capabilities of the instruments as they will be until the optical correctors are installed late in 1993 (current estimate).