A well-designed and properly-implemented Science Data Archive, as part of the capabilities of the Gemini observatories, would be a major contribution toward the full exploitation of the unique characteristics of the Gemini telescopes. An effective archive would boost scientific productivity and would ensure that maximum value was extracted from the expensive-to-obtain observational data. In the short term, an archive would contribute to the efficiency and effectiveness of observatory operations, scientific planning, and preparation of observing proposals. Archiving requirements are consistent with, and would help to optimise, queue mode observing requirements. In the long term, a science archive would enable many of the high level science goals outlined in the Gemini Science Requirements document. Some specific goals-- for example, understanding the relationship between quasars and their host galaxies, probing variable phenomena in stars and galaxies, dynamics of galactic nuclei, would be unattainable without archival access to the complete set of Gemini observations. The collective impact of the full Gemini observation database would far exceed what could be produced by the programs of single observers or teams, spanning a wider range in physically important parameters such as redshift and luminosity of active galactic nuclei or galaxies. Furthermore, many high-priority Gemini projects would use the Gemini data in conjunction with Hubble Space Telescope, HIPPARCOS, ROSAT, IRAS, NGST and other archival data.
A science data archive, in partnership with a well-designed engineering archive, would play an important role in characterising and monitoring the behavior of the science instruments. Optimising their performance would be made much easier with the direct modes of data access provided by the archive.
Access to Gemini data by the wider community, after the proprietary period, would ensure that the data are fully exploited and that maximum scientific value is returned to the astronomical communities and, ultimately, the citizens that support Gemini. The true legacy of the Gemini observatories would be the collection of excellent observations produced by its innovative instruments and this legacy, in the form of the first scientifically effective archive of ground-based observations, would play an important role in astrophysical research long after Gemini itself ceases operations.
The ultimate legacy of Gemini is the scientific data that it produces
and the full scientific exploitation of those data is the most important
goal of Gemini. The science requirements of the Gemini Data Archive
are defined to further the pursuit of that goal.
1) The scientific communities of the partner countries
must have online access to a complete catalogue of the Gemini observations
and to the data in a recognised and readily usable astronomical format.
2) All descriptors necessary to qualify and to quantify
the data must exist, be accessible, and be accurate. These descriptors
include a record of the complete optical path, observatory systems
status, and environmental conditions (e.g, logs of weather and instrument
temperatures). This requirement implies strong linkage between the
engineering archive and the scientific archive.
3) A mechanism must be in place to control access to the
data in order to enforce proprietary rights.
4) Security of the data must be guaranteed against loss
(for example, data should exist in duplicate in different locations).
5) The necessary elements must be in place to enable
calibration by an automated processing pipeline. These elements include
the existence of reliable calibration material for ALL observations
(whether queue-mode or classical) and pipeline processing software.
Calibration of data may be performed at the time of observation but the capability
must exist to re-calibrate all data at any future time to take advantage of increased
knowledge of the instruments and improved calibration material.
6) The Gemini archive facility must be considered to
be an integral part of the Gemini operations environment. Archive
requirements need to be considered alongside other important requirements
by all members of the team: instrument scientists, engineers, observatory
staff scientists, observing assistants, visiting astronomers, and archive
staff.
7) The Gemini archive must maintain
compatibility with evolving requirements for effective inter-archive access
because many science projects will use a combination of
archives as data sources.
8) A science archive is an evolving facility, and at all stages of its growth, it will require resources for continued development as well as operations. For instance, Gemini data must be transferred to new media as storage systems evolve. These data must be legible a century from now.
The Gemini Science Archive should provide the scientific community of the partner countries online access to all Gemini science data and supporting information in order to allow full scientific exploitation of those data. The Gemini Science Archive should guarantee that the valuable datasets obtained with the Gemini Telescopes are saved and preserved for use by future generations for research and education.
Space-based observatories have produced scientifically effective archives for over two decades. Data from IUE, IRAS, Einstein and other missions have made clearly important contributions to progress in astronomy. Hubble Space Telescope (HST) has broken new ground in the development of archives of optical data. The observations are all saved, pipeline processed and calibrated, catalogued and distributed. The HST archive has only recently begun to be heavily exploited and will be a valuable resource for decades to come. Hanisch (1998, SPIE, vol. 3349) recently reported that the data retrieval rate from the HST archive is now higher than the rate at which new data is being ingested; he also pointed out that, up to present, ten times more International Ultraviolet Explorer (IUE) data have been extracted from the IUE archive than was originally put in it.
Some large astronomical projects like the Palomar Sky Surveys, the Sloan Digital Sky Survey, or the 2Mass Survey are themselves archive projects. For example, The Sloan Digital Sky Survey (SDSS) -- using a telescope and instruments dedicated to that project-- will contain photometric, spectroscopic, and morphological parameters for several hundred million objects. Archiving is taking its place as one the most important resources that serve the astronomical research community.
It is important to appreciate the difference between a safe store for observatory data and a useful science archive. The science archive requires careful cataloging and effective search and retrieval tools as well as the capability of reliable calibration; see for example the AstroBrowse Web site at sol.stsci.edu/~hanisch/astrobrowse_form.html. These are the features which allow an archive to produce science and they are absent from basic data storage systems.
There are at least 3 classes of archive research project. The first consists of cases where the data are used for an entirely different scientific project than they were obtained for. The second is the case where new, improved, or otherwise different and more effective methods of analysis are brought to bear on the data. The third, and perhaps most important class exploits the collective effect of the archive where a larger and more comprehensive dataset (consisting of all of the archive observations taken to date) spanning a wider range in some important parameter is available to the archive researcher than could ever be available to an individual proposer. The whole of the archive dataset is worth far more than the sum of the parts, and the linkages across archives and across wavelength regimes adds still more value to archive data.
An excellent illustration of the effective use of archive resources is "The Demography of Massive Dark Objects in Galaxy Centres" (Magorrian et. al 1997, astro-ph/9708072). Nearly all of the leading workers in this field have collaborated to produce a study which uses imaging data from at least 6 HST programs and incorporates kinematic information from more than 10 separate ground-based observational programs. Clearly, this approach of combining many years worth of observational effort into a large homogeneous dataset is extremely effective. The existence of good archive facilities makes this type of substantial scientific progress possible.
As a second illustration, the CFHT archive was searched for observations of NGC 1068, an AGN which displays spectroscopic and photometric variability. The search took only a few minutes of effort and returned 189 exposures from 8 separate programs spanning 7 years. Spectra and images in the optical were obtained in 6 programs and infrared observations were made in 2 additional programs. The long time baseline makes this a very valuable archival dataset. A search of the JCMT archive revealed 613 observations of this object in the sub-millimeter regime from numerous programs. These could be combined with the 283 HST observations taken over a period of 6 years with 6 different instruments in 22 separate programs. All of these data are available from a single archive site at CADC. Over 50 observations are available from 6 different X-ray and gamma ray missions through the HESARC archive. This is a very well-observed object but many sources have been observed at multiple wavelengths at multiple observatories over long time baselines and archiving preserves the value of these data for future research.
The range of published archival research is impressive. Koesterke et al. (1998 A&A 330,1041) combine HST and IUE archival spectra to study mass loss in four PG1159 stars, Cagnoni et al. (1998 ApJ 494,54) use archival ASCA observations to evaluate the contribution to the X-ray background of discrete sources in the 2-10 keV energy band. Sodemann & Thomsen (1998 A&A Suppl. 127,327) perform an extension and re-analysis of earlier crowded-field photometry in M32 from archival HST imaging. Archival data from the Burst and Transient Source Experiment (BATSE) is reprocessed by Kommers et al. (1997 ApJ 491,704 ) to achieve higher sensitivity and is searched for low-significance transient signals in the 50-300 keV range. Ciliegi & Maccacaro (1997 MNRAS 292,338) study time and spectral variability of Einstein Extended Medium Sensitivity Survey (EMSS) active galactic nuclei. Serendipitous asteroid trails are detected by Evans et al. (1997 AAS 29.0701) by examining 30,568 frames from the HST Wide-Field Camera and 96 moving objects are found. Rigopoulou, Lawrence, & Rowan-Robinson (1996 MNRAS 278,1049) combine archival and other data from the sub-millimetre (JMCT) to the X-Ray (ROSAT) and for a set of ultra-luminous IRAS galaxies. IRAS archival data is used by Noriega-Crespo et al. (1997 AJ 113,780) to produce a survey for bow shock structures around OB runaway stars, and follow-up work uses re-processed IRAS high-resolution maps.
It is evident from these examples--all involving space-based instrumentation in a key role-- that extremely valuable science over a wide range of subject areas can be done with a properly-implemented Science Data Archive and that much of this science would not be possible in its absence.
There is no fundamental reason why a Science Data Archive from a ground-based facility should be more difficult to implement or less valuable than an archive of data from a space-based observatory. There have historically been differences in facility design motivated by the fact that a higher level of planning and automation is required in space where real-time human intervention in observing procedures is much more difficult than on ground-based telescope. The Gemini observatories are being designed to operate in a mode that very much parallels that of space-based observatories and these design requirements will allow an effective archiving to be created. The motivation behind Gemini's design decisions is not the impossibility of human control (although the effect of Mauna Kea's altitude argues for a minimum of real-time decision-making), but the desire for optimum observatory performance which requires detailed planning of observations and queue-mode observing.
What are the unique difficulties of ground-based observing and archiving? The salient feature is varying weather conditions. The solution is to monitor and log transparency and seeing and maintain links between this information and the data. Queue-mode observing allows the observatory to respond to changing conditions by executing programs that are best-suited to those conditions thus optimising the scientific productivity of the Gemini facilities.
A number of other challenges faced by existing archives of ground-based observations are related to instrument and facility design as well as deficiencies in observing and logging procedures. Extensive experience with the archive of the Canada-France-Hawaii Telescope (CFHT) has brought these problems into sharp focus and has showed how to solve them. There have been four basic deficiencies. First, at CFHT there is no guarantee that adequate calibration material is obtained. Second, there is no requirement for adequate or uniform logging of observations and weather conditions. Third, there is no guarantee that data headers include ALL of the instrument, telescope, and other system configuration information that is essential to understand and reliably calibrate the data; furthermore there is no guarantee that some key components (e.g. filters) are in the right place, because they are not all encoded and monitored. These are the reasons that the CFHT archive--currently the best archive of ground-based data in existence--has not realized its full potential scientific productivity. Archive users, in general, simply lack sufficient information and confidence about what occurred during the execution of the observations to produce reliable science-quality data from the archive.
Gemini has been designed along the lines of a space-based observatory and this guarantees that most of the problems cited in the case of the CFHT archive are automatically resolved. An archive has been envisioned as an integral part of the Gemini facility. The success of the archive requires, above all, this element of integration with day-to-day operations of the observatory scientists and engineers, effective interactions with the instrument teams, and the contributions of the user community. In the ways it has designed its telescopes and instruments, the manner it has constructed its engineering archive and the plans it is setting for its operational modes, Gemini has already laid the foundations that are needed for the effective operation of a Science Data Archive.
The main argument in favor of allocating resources to a Science Data Archive is that it would increase the quality and the quantity of the science that is produced by the Gemini observatories. It would also preserve the scientific value of the Gemini data far into the future, not as part of an historical record but, rather, as data that would continue to contribute actively to scientific progress for decades to come.
An archive of science data would also play an important role in characterising, monitoring, and optimising the performance of the instruments. Comparison of newly-obtained data with those in the archive is the most reliable way to monitor performance.
A successful Gemini Science Data Archive would ensure that all scientists in all countries of the partnership would have access (following an appropriate proprietary period) to all of the data produced by the observatory. This would represent important added value to the Gemini partnership. The archive would ensure that the Gemini observations are fully exploited and that opportunities for doing Gemini research would be as widely distributed as possible. An archive would also be extremely valuable both for educational purposes and for public outreach activities (where HST has excelled). Furthermore, there is an issue of public accountability. The allocation of resources to an archive would demonstrate to the taxpaying public that all efforts were being taken to ensure that maximum value was being extracted from the Gemini facility and that these astronomical data were highly-valued and needed to be protected to preserve future research opportunities.
Gemini's first and subsequent generations of instrumentation will provide unprecedented observational capabilities and will open up new opportunities for study in fields as diverse as planetary searches and high-redshift clusters of galaxies. The uniqueness of Gemini science makes the provision of an archive of these data a compelling priority.
In the following sections, we discuss how a Science Archive would help to realize several specific scientific goals of the Gemini observatories. In some cases the main benefit is derived from the larger and more comprehensive database represented by the Gemini observations accumulated over a period of time. Sometimes the increased time resolution is important, for example, for the study of variable phenomenon and proper motions. In some cases, Gemini observations will be combined with those from other facilities to provide a wider baseline in time or in wavelength. In some examples observations will be used for a completely different scientific purpose from that for which they were obtained and sometimes they will be used for the same purpose, but the new results will be due to fresh viewpoints and more effective methods of analysis used by archival researchers. Gemini archival observations will be used as the basis for new proposals to Gemini and other facilities, and also will be employed in conjunction with data from sister archives. In all of these cases, the Gemini archive would help ensure that the best is made of the available information content.
Star formation occurs in molecular clouds in regions with large amounts
of extinction and the spatial scales involved are small (1-104 AU
which translates to subarcsecond even for nearby regions). High spatial
resolution and infrared capabilities are most important. Imaging and
moderate to high spectral resolution (requiring good light gathering power)
are needed to answer questions
about environmental effects on the initial mass function, the physical
state of molecular clouds and grain chemistry. Accretion and outflow
processes around Young Stellar Objects can be investigated, and the basic
parameters of circumstellar disks in nearby regions will be measurable
using a combination of adaptive optics, coronagraphy, and polarimetry.
The Gemini archive could be useful in several aspects of the study of star formation. For example:
The authors believe they had witnessed the first light emerging from newly born star. The superb IR capabilities of Gemini will often lead astronomers to point at star forming regions, and any phenomena like the one described above should be readily detected. The use of archival data of the same regions will be crucial in the interpretation of the observations leading to a better understanding of the stellar formation process.
Using VLA archive data, Rodríguez et al (1989; ApJ, 346, L85) found that the outer components of the triple source in Serpens FIRS1 are moving away from the central object. The Gemini archive could be helpful in identifying and determining the kinematics of outflow phenomena at even earliest stages after its onset and certainly of the kinematics of Herbig-Haro objects at later stages of evolution.
The archive data could allow investigation of the wiggling motions (changes in flow direction) of the highly collimated jets associated with recently formed stars.
Pre-main sequence stars are known to be highly variable objects, particularly at X-ray wavelengths. Of course the Gemini archive could be useful in assessing their variability at IR wavelengths, particularly in the H2 luminosity from the surroundings thought to be associated with the X-ray luminosity.
Massive stars evolve through the Wolf-Rayet phase to explode as supernovae. It is believed that the most massive (M greater than or equal to 40 Solar Masses) go through a relatively short (less than or equal to 105 yr) but active phase of instability after leaving the main sequence. Stars in this stage--known as luminous blue variables (LBVs)--are very rare (Fig. 1); the most luminous infrared source in the Galaxy, eta Carina, for example erupted between 1837 and 1860. Such events are probably more frequent in starbursting galaxies, where many very massive stars are formed in super stellar clusters. Because of insufficient image resolution and because these events are most conspicuous in the infrared, very few LBV events have been studied. With Gemini's excellent imaging capability in the infrared, an archive would easily allow the search for, and subsequent study of, erupting LBV stars in massive star forming regions within D < 10 Mpc.
Figure 1: A new LBV in the giant HII region NGC 2363 located at the SW of the magellanic irregular NGC 2366 (D = 3.7 Mpc). | |
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a) HST WFPC2 B(F439W) (upper) and H alpha (F656N) (lower) images of NGC 2363 in 1996 January 8. The Luminous Blue Variable star is identified. The field of view is 13'' x 9''. |
b) Time sequence of `archival' R-Band ground-based images of NGC 2363. The LBV appeared in early 1994, and it is now the most luminous object in the galaxy NGC 2366. (Drissen, L., Roy, J.-R., & Robert, C. 1997, ApJ, 474, L35) |
One of the more thorny astrophysical problems is AGN unification. By this we mean deciding which classes of AGN objects are in fact identical, but simply viewed from different orientations, and also which classes differ in only minor respects and should be considered part of a spectrum with minor changes in some parameter. Opinions on this topic have swung wildly from the days when the (innumerable) classes of AGN were developed, to recent times when many people argue that all AGN are explicable by a single model with only minor changes.
In order to arrive at the true answer (most probably somewhere between
the above extremes), one needs data of comparable quality on a wide
range of AGN types, covering a wide range in redshift and luminosity.
Such a large dataset would never be produced by a single observer
because of time allocation limitations, but
would inevitably accumulate in a Gemini archive. Two examples of
pieces of the unification puzzle that illustrate the potential
of an archive are:
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Figure 2: An example of multi-wavelength archive research combining Adaptive Optics with other existing data. This is an example of an X-ray selected AGN at z=0.037 observed at 4 different wavelengths from near-infrared to B-band (Schade & Crampton 1998 in preparation). All of these data are or will be available through archives. At left is an Adaptive Optics image in the H-band (pixel size 0.0375 arcseconds) taken at CFHT and next to it is an HST PC I-band image of the same object (scale 0.0455 arcseconds/pixel). These are displayed so as to show the diffraction rings in both images. The next two images are R-band and B-band images taken in seeing of 1.5 arcseconds (with pixel size 0.31 arcseconds) at La Palma with the JKT 1 meter telescope. The 1-meter integrations show the low surface brightness outer regions of the galaxies while the high-resolution images probe the inner structures with a good wavelength baseline. The boxes are each 101 pixels on a side. These data were simultaneously analysed to obtain structural parameters of the inner region of the AGN and of the host galaxy. |
1. Radio loud QSOs and luminous radio galaxies:
Both AO and non-AO imaging with NIRI will allow comparison of the host
galaxies of the two classes. IFU spectroscopy with GMOS and NIRS will
show gas motions, both inflow and outflow over the galaxy nuclei,
while polarimetry with NIRI and GMOS will show locations where
scattered nuclear light may be found. To be successful, a unification
model would have to explain any observed difference in the properties
of the host galaxies or nuclear gas. It would also be supported by the
discovery of strong scattered nuclear light in objects where it is not
seen directly.
2. Seyfert 1 and 2 galaxies:
Several classic cases are known of Seyfert 2 galaxies which show broad lines when studied in polarised light. This has led some to claim that all Seyfert 2 galaxies are in fact Seyfert 1 galaxies, where the nucleus is obscured. There are however a number of problems with this simple hypothesis. Large well defined samples of Seyferts are needed, with high resolution imaging and IFU spectroscopy (NIRI, GMOS) to test this hypothesis.
There are many other promising unification candidates including: Broad absorption line and normal radio quiet QSOs, or BL Lac objects and low luminosity radio galaxies. Only by combining data from many different Gemini proposals will a large enough sample be obtainable. One can then start to assemble the entire picture, and finally, determine how many physical parameters are really needed to determine what an AGN will appear like to us. Being able to analyse uniformly NIRI imaging for all AGN, for example, would be a powerful use of a Gemini archive.
Quasars and AGN often attain new interest as result of discoveries in varied wavelength regimes, for example, the X-ray and radio. Figure 2 shows a set of data from various telescopes at various wavelengths that can be combined to derive information about the AGN and its host galaxy. Variability in some band, lensing, membership in large scale structure, or the presence of foreground absorbers all represent auxiliary information that might not be available when original Gemini observations are made, but which would renew interest in those observations. An archive thus plays a crucial role in maintaining the value of Gemini data for AGN science.
High-resolution spectroscopy of nearby galaxy nuclei frequently provide evidence for the presence of massive black holes. Light gathering power and high spatial resolution give Gemini a unique capability to extend these studies in terms of data quality and in terms of target distance. Gemini will produce much larger samples of observations of galaxy nuclei, and permit the reliable assessment of the true frequency of the black hole phenomenon and its relation to galaxy properties. Multiple groups will be involved in galaxy nuclei investigations, and one can expect many more studies like that of Magorrian et. al (1997) mentioned is section 1.1 to use archive data in the future.
The Galactic Centre provides an unprecedented laboratory for investigating the central regions of a late-type spiral galaxy. Early studies with single-element infrared detectors revealed a number of bright point sources, hinting that the stellar content near the Galactic Centre may be different from the surrounding bulge. Using state-of-the-art instrumentation, such as the CFHT Adaptive Optics system, it has been possible to obtain near-diffraction limited images of the central parsec of the Galaxy, such as the image shown in Figure 3. It is now recognised that there is a population of young stars near the Galactic Centre that are centered around a supermassive object, corresponding to the radio source SgrA*. Efforts to study the region around SgrA* are confounded by (1) crowding, which limits efforts to resolve stars fainter than K ~14, even with angular resolutions corresponding to the diffraction limit of 4-metre class facilities (Davidge et al. 1997, AJ, 114, 2686), (2) the potentially complicated kinematics of stars in this region, which introduces uncertainties in dynamical studies using only radial velocities, and (3) contamination from foreground (and possibly even background) disk objects, which becomes progressively more significant towards fainter magnitudes.
The second and third problems can be overcome by measuring proper motions, and archival data from Gemini (as well as other 8 metre telescopes) will provide the means of obtaining homogeneous stellar positions over moderately long time scales. When combined with radial velocities, proper motions can be used to deduce true space velocities, so that the orbits of stars about SgrA* can be constructed, and an accurate mass determined. Data of this nature will permit an extension of the pioneering work of Eckart & Genzel (1996, Nature, 383, 415). Proper motion measurements can also be used to distinguish between bulge and disk stars, and thereby establish whether or not the faint blue objects detected by Davidge et al. 1997) are bona fide main sequence turn-off stars associated with SgrA, or foreground interlopers.
Figure 3: The central 12.4 x 11.9 arcsec field of the Galaxy, as observed in K with the CFHT AOB. The Airy pattern can be seen around the brightest sources. SgrA* is at the approximate center of the image. Various IRS sources are labelled.
Bars within bars are recognised to be a possible mechanism to fuel AGN. Stellar bars allow gas to crowd and collide, thus loosing angular momentum and sinking to the center. The search for secondary and tertiary bars in the centers of disk galaxies (Wozniak et al 1995, A&AS, 111, 115; Friedli et al. 1996, A&AS, 118, 461) is ideally served by archives. Shlosman et al. (1989, Nature, 338, 45) has predicted that a sequence of bars of different orders may cascade down in scale, in ratios such as 7:1. Large bars have typical size of 5-7 kpc, while secondary bars are less than or equal to 1 kpc. Bars are better seen in the near-infrared, probably because of reduced absorption at longer wavelengths. High-order (e.g., tertiary) bars would allow gas to get closer to the nucleus, e.g. r less than or equal to 20 pc for a tertiary bar. The large number of disk galaxies that will be observed with Gemini in the infrared will provide a priceless source of images for searching for tertiary bars at the scale of 100 pc and for establishing the statistics of such features with respect to AGN properties.
Gemini will be heavily involved in surveys for high-redshift clusters and field galaxies and in spectroscopic and imaging follow-up for these surveys. Many images of faint galaxies will be obtained. Adaptive Optics will provide superb spatial resolution for both imaging and spectroscopy.
In the infrared, Gemini will produce the best ground-based imaging ever done and wide field is not a requirement for discoveries of small scale gravitational lensing phenomena. An example of the type of serendipitous discoveries that should be expected comes from HST archive data. Figures 4a and 4b show two cases where archival data was used to discover new gravitational lenses producing Einstein crosses. In the first case (Fischer et al. 1998 in preparation) the lens is a luminous cluster elliptical galaxy at z=0.46. The required lensing mass is consistent with the velocity dispersion implied by fundamental plane considerations if the source (most likely a quasar) is at z ~ 3. In the second case (Ratnatunga et al. 1995 ApJ 453, L5) the lens is an isolated field galaxy at z=0.81, and the source shows an emission feature interpreted as Ly alpha at z=3.4 (Crampton et al. 1996 A&A 307, L53). These discoveries were made as a result of re-processing of archive data with more sophisticated techniques than were used by the original observers. The Einstein crosses were not apparent until good modelling and subtraction of the lensing galaxy image was performed rendering the multiple images obvious.
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Figure 4a: Einstein cross discovered in archival HST data. (HST PI was Dressler, analysis by Fischer, Schade, Barrientos 1998 in preparation.) This object is in a cluster at z=0.46 and the lens is a luminous elliptical galaxy. The lensing was not detected in the original investigation but was revealed by applying new processing techniques. |
Figure 4b: A second case of lensing discovered in archival data is this Einstein cross produced by a very luminous field elliptical galaxy at z=0.81. (HST PI Groth, analysis by Ratnatunga et al. 1995 ApJ 453, L5.) The source in this case is a quasar with redshift z=3.4. The right portion of each frame shows the image with the modelled galaxy profile removed. |
A second example of archive science illustrates the case where the aggregate effect of accumulated archive observations provided greater opportunities for discoveries than those available to the original proposers of the observations. Figure 5 shows the evolution in luminosity (Delta MB) of elliptical galaxies in clusters as a function of redshift (Schade et al. 1997 ApJ 477, L17). A collection of nine clusters (from seven distinct observational programs), spanning the redshift range 0.17 < z < 1.21, was extracted from the HST archive. A uniform analysis technique was applied to trace the shift in luminosity of the sequence of giant elliptical galaxies with increasing look-back time. Each point on this diagram represents not a single galaxy, but a sequence of galaxies in a single cluster (from 4 to 28 individual galaxies). The results indicate that elliptical galaxies undergo luminosity evolution consistent with models of the passive aging of a single-burst stellar population formed at z > 3.
Figure 5: Luminosity evolution of elliptical galaxies from archival HST data (Schade et al. 1997 ApJ 477, L17). Delta MB is the shift in luminosity at a given size as measured from the sequence of giant elliptical galaxies. Each point represents not a single galaxy but the shift of a sequence of galaxies at a given redshift or in a given cluster. Solid symbols are for cluster elliptical galaxies using HST imaging and open symbols are from ground-based CFHT fields. Open circles are cluster E's and open squares field E's.
A third example is the study of spectrophotometric evolution of distant galaxies with cluster gravitational optics: cluster lenses are ``high z filters'' which select background galaxies and distort them, making their shapes easy to identify while foreground galaxies are undistorted. Only objects with z greater than or equal to 2 zlens can be selected with gravitational ``telescopes''
For each lensing cluster, a set of arclets gives a subsample of a global population of faint field galaxies. Thus a large number of clusters, observed under fine imaging conditions, are needed to be able to follow the spectrophotometric evolution of all morphological types of galaxies. Imaging and spectroscopy of many high-redshift clusters will be conducted with the Gemini telescopes, and this will provide an opportunity to assemble a statistically reliable sample of high redshift lensed galaxies within a reasonable timeframe.
Figure 6a:
HST image (F702W) of galaxy cluster A2218.
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Figure 6b) Density of arclets in cluster A2218 (with B less than or equal to 24.5) for a given model of mass distribution. |
The efficient and effective operation of a modern observatory requires (for both technical and scientific reasons):
These considerations suggest that a facility with all of the
features of a Science Data Archive is required as an integral part of the
efficient functioning and optimal science output of the Gemini telescopes.
The Science Data Archive would allow proposers to evaluate quickly the feasibility of their program by study of actual data retrieved from the archive. There is no substitute for seeing results from the same instrument and configuration that an observer is considering using. Access to real data would allow proposers to estimate reliably the required integration times, and allow them to evaluate the effect of different atmospheric emissivity and image quality on the data needed for their science.
Nominal performance of the telescope and instruments could be verified by comparing newly acquired data with archival data; instrumental setup, verification, and performance monitoring would be facilitated. The observatory and its instruments could be monitored to study trends in behavior and fine-tuned to ensure optimal performance.
The provision of quick-look tools requires the existence of good calibration material and automated processing in real time. Thus a processing pipeline needs to be defined for each instrument where quick-look tools are implemented. Minimal calibration steps should include bias correction, flat-fielding and wavelength calibration. Additional effort would provide image distortion correction, atmospheric refraction correction and fluxing.
Simulations (Mountain, Simons, & Boroson, 1995 RPT-PS-G0053) provide evidence for substantial scientific gain from queue scheduling; MSB discuss models where between 70% and 100% of observing time is dedicated to this mode of operation. The most compelling science is often the most challenging technically, and the observations that fully exploit the best atmospheric conditions are likely to be obtained in queue mode.
Queue-mode observing automatically satisfies the requirements of a Science Data Archive. The data are obtained and accompanied by a) sufficient calibration material, b) electronic logs of the events that took place during data acquisition, and c) weather monitoring information. These products delivered to the proposers are sufficiently complete and reliable to allow them to carry out the desired analysis with confidence. There is no difference between delivering these products to the original proposer several hours after they were obtained and delivering them, following the proprietary period, to an archive user.
In order that data be ``archive-able'', observations must always be accompanied by sufficient calibration material and logs of weather and other events. The definition of specific calibration requirements, which largely determine the value of the Gemini science archive, requires ongoing consultation with the user community. The information should be complete enough to redo the observations in exactly the same way. This requirement applies equally to queue-mode and classical-mode observing. There are several benefits to the classical-mode observer of obtaining the minimum calibration data. First, instrument teams and observing staff who use the instrument frequently possess a great depth of expertise and would advise which calibration procedures are necessary and would implement those procedures correctly and efficiently so that the minimum necessary amount of time is expended on calibration. Secondly, calibration data should be treated as ``shared'' data. This will often result in further reductions in calibration effort, since each observer is not required to obtain a complete calibration dataset where redundant material already exists. An additional benefit is that the quality of the calibration provided to the classical (as well as the queue-mode) observer would be as good as or better than they would otherwise have obtained. Finally, the calibration material would be tuned for integration into both the quick-look evaluation tools and into the processing pipeline that would exist for each instrument.
The observer in either queue or classical mode would benefit from pipeline processing whether they use it to produce their final data products or whether they use it as a benchmark to evaluate the results of their own processing software. The production of pipeline software is an integral part of the archive process, but it is also an integral part of the implementation of quick-look tools for real-time evaluation of data quality. As is the case for calibration material, the instrument teams, and Gemini staff astronomers, will have considerable depth of expertise on processing of data from each instrument and would develop processing algorithms in consultation with users. The standard pipeline processing software will not satisfy all telescope users but, on balance, it would save most observers a considerable amount of time normally spent on routine data reduction and software development.
A Science Data Archive would represent a major contribution to the scientific productivity of the Gemini observatories in a number of ways. First, we have given a number of examples where it would enable first-rate scientific research that would never be done in the absence of an archive. Second, a data archive carries benefits for proposal preparation, instrument performance verification and optimisation, queue-mode observing, and Gemini operations in general. This is because it is more efficient to operate in an environment where information is managed and processed with the best existing technological tools. An archive is a major component of such an environment. Third, it would help to keep us competitive. The VLT and Subaru projects realize the power of effective archiving and are investing large resources in archive development. We know how to develop and run a highly productive science archive with a fraction of the resources these projects are expending. Finally, the Gemini Science Archive would distribute scientific opportunities among astronomers in the partner countries, would help to inform the public about the excitement and importance of astrophysical research, and would demonstrate to the taxpayers who support Gemini that we were acting responsibly in ensuring that these valuable data are being handled with the greatest care to ensure that they are fully exploited and that their value is being preserved for future generations of scientists.
We are grateful to Andy Woodsworth, Jim Hesser, Séverin Gaudet, David Bohlender, Tim Davidge, Laurent Drissen, Phil Puxley and Gordon Walker for their comments and help in preparing this paper.
30 March 1998