MULTIOBJECT SPECTROSCOPY WITH EFOSC:

Observation of the Cluster A 370J. P. DUP/N, B. FORT, Y. MELLlER, J. P. P/CA T, G. SOUCA/L, Observatoire de Tou/ouse, FranceH. DEKKER, S. o'ODOR/CO, ESO

1. Introduction

Since the first experiment with Multi­ple Object Spectroscopy (MOS - seeThe Messenger 41, September 1985)made on EFOSC, the announced newfacility called PUMA 2 has been im­plemented. This is a smalI, computer­controlled punching machine with wh ichthe observer (on the site and during hisown run) can make the aperture maskshe or she needs.

The PUMA 2 system was developedby the Toulouse Observatory from aprototype PUMA first used at CFHT inHawaii (Fort et al. 1986). PUMA 2 wasimplemented for the first time on the3.6-m ESO telescope in February 1986during a technical run in order to checkthe machine operation on-site and todevelop and test the software facilities.Since then several astronomers haveused the system. Two of us (B. F. andG. S.) took part in a run in November1986 to measure velocities in clusters ofgalaxies. We believe it would be inter­esting for future users to comment onthe way MOS has been used, and topresent the performance which can beachieved with EFOSC. This papershould be considered as a run reportand will give first results from the datareduction made in Toulouse on the ob­servations of galaxies in the clusterAbell 370, using partially automatedsoftware. Results of MOS observationsare also presented by O'Odorico andOekker (1986).

2. Equipment and Procedures

2.1 EFOSC

For a detailed description of EFOSCwe refer to the ESO Operating Manual.The detector in use during theNovember run was the ESO CCO #8,which is a high resolution (15 ~lm pixel)RCA CCO that we used in 2 x 2 binnedmode. The quantum efficiency is about80 % and the read-out noise 35 elec­trons rms.

sponding to 2.1 and 3.6 arcseconds)allow the choice between two holesizes. It is also possible to punch slitswith these widths by punching aseriesof adjacent, partly overlapping holes.The relative positioning accuracy of theholes is 10 ~lm, given by stepper motorsof the Microcontrole XY tables. ThePUMA 2 is linked to the HP 1000 instru­ment control computer and the file withpositions to be punched may be sentdirectly to the PUMA 2 microprocessoror temporarily stored on disk orcassette.

2.3 Preparing the masks

To prepare a mask one needs anEFOSC image of the field of interest. Aninexperienced user should obtain it on aprevious night, so he/she can go at easethrough the interactive object selectionprocess and the preparation of the maskthe day before the observing night. Thisimplies the use of MOS on the secondnight of an observing run, except whenthe preceding observer agrees to takethe short exposures which are required.In our case Or. A. Pickles kindly agreedto take an image of A 370 during his run,thus allowing us to make the spectro­graphic observations during our firstnight. It was a real chance as it turnedout to be the best night of the whole run.

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2.3.1 Taking direct pictures of the field

Images of the field are usually taken inwhite light. 1-minute exposures are suf­ficient to detect all objects suitable forspectroscopy.

It is important to note that, dependingon the grism used and on the desiredspectral range, the field image may haveto be decentred in order to put thespectra of interesting objects in a suit­able place of the CCO. As an example,with the B 300 grism, and if the spectralrange 4500-6500 Ä is desired, the ob­jects must be chosen between the lines100 and 300 on the CCO frame (decen­tring of the field about 30" south).

2.3.2 Selecting the objects

The selection of the objects is doneinteractively on an image of the field atthe 2-0 display in the 3.6-m controlroom. The batch programme used forthis purpose runs within the IHAP datareduction system and requires pointingwith the cursors at the targets and at thepositions of free sky to be used forcomparison. Mistakes in the entering ofthe cursor positions can be correctedand the programme gives a warning ifthe spectra overlap. The proceduretakes less than half an hour if thenumber of objects is less than about 10.

2.2 PUMA 2

The PUMA 2 system is a micropro­cessor-controlled machine with whichholes and slits can be punched in thin(0.15 mm thick) copper sheets calledmasks or starplates. Two differentpunch heads (0.3 and 0.5 mm corre-

Figure 1: GGO frame obtained with MOS using the B 300 grism. The mask has 5 slits and 11holes and contains 12 object spectra. Note the number of radiation events in this 1h 3D-minuteexposure.

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Figure 2: Image of A 370 cluster of galaxies taken with a B filter on EFOSC (20-minuteexposure). The elongation of the images is due to the reproduction process from the TV screen.

For survey-type observing pro­grammes which require an optimizationof the distribution of objects and skyapertures to optimally fill the CCO, asemi-automatic programme like the oneused by the Toulouse group at the CFHT(Fort et al. 1986) is more effective. Theobjects are automatically detected inthe field image, using for examplecriteria like magnitude, colour or size.The batch procedure then optimizes theselection, given boundary conditionslike length of the spectrum, minimumseparation between the objects, need ofsky reference. ESO is investigating thepossibility to implement such a proce­dure as weil but intertacing an existingFortran programme into IHAP is notstraightforward.

2.3.3 Punching the masks

This is a very efficient pracedure as 6masks with a given hole size can beprepared at a time. The size of the holesor slits can be chosen at this step. Holesand slits can be easily mixed on thesame mask by punching the same masktwice before dismounting it fram thePUMA 2 table. The hole shapes andsizes are very accurate, better than10 Ilm. Repeating the punching on thesame starplate may remove the burrsthat sometimes remain and then giveholes a slightly irregular shape.

The machine usually runs quitesmoothly except for an occasionalpunch break. Replacing the punch takesa few minutes and is taken care of bythe ESO maintenance staff.

2.3.4 Mounting and aligning the masks

The masks are put in place on EFOSCby the night assistant or by the as­tronomer after a short instruction. Thisoperation is easy and the masks aremaintained in a very accurate and re­petitive position.

The observer is assisted in the align­ment of the mask on the field by usingan IHAP batch pracedure that comparesan image of the mounted mask (illumi­nated with a calibration lamp) with animage of the field. Provided the guideprobe and telescope coordinates havebeen carefully noted when taking thedirect picture, it takes at most two itera­tions and about 10 minutes to align themask on the field with an accuracy asgood as 0.25 arcsec rms.

The operation of the instrumentrotator has been improved a few monthsago. It now sets to an accuracy of O'? 1.This is sufficient for accurate alignmentand makes the rotation of the EFOSCwheel (which is more complex since italso involves a translation of the mask)unnecessary.

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2.3.5 Taking the spectra

The procedure is described in theEFOSC Operating Manual and fram theexperience we obtained during our runwe would like only to mention someprecautions to be taken to secure accu­rate results after reduction.

The so-called blue halogen lamp cov­ers very weil the range 3500-7500 Aand is normally used for flat-fielding.

It is necessary to take calibration lampspectra thraugh each mask as the dis­persion is not exactly linear andchanges with the position in the field,along the columns.

It is useful to keep a short direct im­age of the mask (with the halogen lampor the dome lighting) for future auto­mated reduction. It gives also informa­tion about the transmission of the diffe­rent apertures.

All of these calibration exposures canbe taken during daytime. However, wepreferred to take them directly before orafter the science exposure since almostno telescope time is lost and one makessure that no important calibrations are

forgotten. Also, when using this method,data and related calibrations are storedconsecutively on the same tape, reduc­ing the chance of errors during datareduction.

As the exposure times are long, wealways chose to work at hour anglessmaller than 1.5 hour and zenith dis­tance less than 30° in order to minimizethe effects of atmospheric refraction.

Figure 1 is an example of a CCOframe obtained in MOS with a maskpunched with both slits and holes.

3. An Example of MOS Observa­tions: Galaxies in the ClusterA370

3.1 The astrophysical programme

Very little is known about the dynamicevolution of clusters of galaxies and theevolution of the galaxies in clusters.Multi-object spectroscopy is an idealtool to investigate these problems. Forexample, the normal evolution modelsfor galaxies (Bruzual 1981, Guiderdoni1986) do not explain the excess of blue

Table 1: Spectra obtained during the observing run at the 3.6-m with EFOSC in MOS mode(1-4 November 1986)

Cluster Number of apertures Number of objects exposure time

A 2444 28 holes 140bjects 1h

A 551 26 holes 130bjects 40 mn+ 45 mn

A 370 25 holes 130bjects 1h 30 mn+ 1h 30 mn

11 holes + 5 slits 120bjects 1h 30 mn

16 holes + 4 slits 130bjects 1h 30 mn

21 holes + 3 slits 14 objects 1h 30 mn

objects found in clusters with 2 ~ 0.2(Butcher-Oemler effect, hereafter re­ferred to as B. 0.). This effect raises a lotof questions on the physical processesinvolved, the time scales, the initial con­ditions and their role in evolution. It isclear that it is necessary to accumulatespectrographic observations on clustersof varying richness, shape and redshift.

For the ESO run, we chose A 370, avery rich cluster at z = 0.374 (Fig. 2), asthe first priority target. Some preliminarylow dispersion observations at CFHT(Mellier et al., 1987) have shown a veryunexpected high content of spiralswhich had to be confirmed at higherresolution.

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Figure 3: Spectrum of the brightest galaxy of the cluster A 370 (No. 20, cO type) with a redshiftof z = 0.379.

3.3 Data reduction

All the spectra were reduced in 6 fullweeks, using the software developed inToulouse for the PUMA 1 system on theCFHT (see Soucail et al., 1987 for a fulldescription). Changes needed for theESO images were minimal and no par­ticular problem arose in the reductionprocess. The only remark concerns arather large number of radiation eventson the CCO (ESO #8 has a frequency of5.2 events/minute/cm2). Twin exposureson the same field are useful to identifyand correct them. The reduction soft­ware has been developed on a VAXcomputer in Toulouse, and it could beintegrated in MIOAS, the ESO reductionpackage which runs on the same com­puter. It includes some interesting fea­tures related to the special format of thedata, Iike the different positions of theapertures on the CCO and the faintnessof the objects.

4. The Results

4. 1 Performance

Oespite the poor weather conditions,the limiting magnitudes are quite good:in one hour's exposure, with the B 300grism, we obtained spectra of B = 22,V = 21.2, R = 20.7 galaxies at a signal-

3.2 Observational results

Of the 3 nights given for our run only 2nights were useable for the MOS modebecause of bad weather conditions. 79spectra were obtained with a mean of13 per mask. Two other clusters werestudied besides A 370 and the observa­tions are summarized in Table 1.

No problems arose during the obser­vations but we feel that if it is to beefficiently exploited, this type of obser­vation demands two observers, espe­cially if most of the programme is de­voted to MOS spectroscopy and theyare not familiar with MOS or EFOSC.

NM

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2=0.379

600

600

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500

500

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Figure 4: Spectrum of a blue galaxy of the cluster identified as an irregular type (No. 41, Z =

0.379). Note the emission lines typical of H 11 regions and the strang Balmer absorption lines.

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Figure 6: Field of the A 370 cluster with the identification of all the objects from which aspectrum has been obtained at ESO or at CFHT with the PUMA system. The underlinednumbers correspond to objects of which the spectra were obtained at ESO in November 1986.

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5. Conclusion and Future De­velopments

As a conclusion we shall say that nowthe MOS mode on EFOSG in its presentstatus is certainly one of the most effi­cient systems used on 4-metre classtelescopes. It is clear from our experi­ence that the performance in terms oflimiting magnitude is quite good.

Further improvements could be madesoon in the object selection procedureby allowing a mixed (manual and auto­matic) procedure for selecting thetargets. With a more flexible and user­friendly procedure, the possibility ofmask preparation and observation in thesame night could be implemented.

An important question is the choicebetween holes and slits. The trade-off isthe number of objects per mask com­pared to the sky subtraction accuracywhich depends on the crowdedness of

galaxies (50 % as compared with 5 % inGoma) at a time 2/3 Ho-I.

Both more observations and theoreti­cal investigations have to be made toexplain why at about the same redshift,clusters showing about the same overallproperties seem to have so differentgalaxy contents. Gould the pressureeffect of the dense intergalactic gas inthe centre of rich clusters be a possibleanswer?

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active nuclei are present. By comparingthese observational results with starpopulation synthesis models with thesame resolution (Guiderdoni and Roc­ca-Volmerange, 1987) one obtains agood galaxy type distribution. The resultis an unexpectedly high rate of spiral

to-noise ratio on the continuum of 10.These performances are about 0.7 mag­nitude fainter than the ones obtained atGFHT with a focal reducer that has notbeen specially designed for this kind ofexperiment. It is possible to go evenfainter by co-adding several exposureswhich has the additional advantage ofbetter removal of radiation events. Ex­ampies of spectra of galaxies in the fieldof A 370 are given in Figures 3 to 5.

Figure 5: Spectrum of a background galaxy found in the field of the cluster (No. 14, Z = 0.548).

4.2 Astrophysical results

From a run at GFHT in 1985 and thelast run at ESO, we have 90 spectra inthe field of A 370, which represents anunusually large number for such a red­shift (z = 0.374, see Fig.6). About 55spectra are from cluster members andthey give a velocity dispersion of 1300(+ 230, - 150) km/sec and a M/L ratio of130 (with the virial approximation), verysimilar to those measured on closerclusters. A complete study of A 370 willbe given in a forthcoming paper (Mellieret al. , 1987) but we summarize here themost significant results.

A 370 is a very rich cluster (richnesssimilar to Goma), X-ray emitting, andshows a large population of blue B.O.objects. The proportion of 21 % givenby the photometry (Butcher and Oemler,1983) has been reduced to about 11 %from our spectrographic measurements,because of a better evaluation of thecontamination by foreground objectsbut the B.O. effect is now weil confirmedby the spectroscopic measurements.

The B.O. objects are more preciselyspirals or Magellanic galaxies but no

2

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As an example, slits do not seem veryweil fitted for programmes where a lot ofspectra in a rather crowded field areneeded. The advantage of slits is cer­tainly a better sky subtraction in thecase of very faint objects. In the case ofbad seeing, slits are also more efficienl.More accurate extraction algoriths suchas e.g. the one proposed by Hornes(1986) could also be used on slit spec­tra. These might yield a considerableimprovement in the S/N ratios and in thelimiting magnitudes. An efficient futureEFOSC/MOS facility should allow bothround holed and rectangular slits to beused in a flexible way, depending on theastrophysical projecl. It should be pos­sible to interactively adapt the aperturesizes to the prevailing seeing.

Work is now being carried out at ESOand Toulouse Observatory to investi­gate other mechanisms for the makingof the mask, such as laser cutting and

the punching of precise rectangularslits.

On the data reduction side, furtherwork is necessary to develop optimizedsoftware in order to cope with the largeamount of data generated by this pow­erful observing technique.

RefereneesBruzual, A. G., 1981, PhO Thesis.Butcher, H., Oemler, A. and Wells, O. C.,

1983, Ap. J. Suppl. 52, 183.O'Odorico, S., Oekker, H., 1986, Proceedings

of the ESO-OHP Workshop on the Optimi-

NOTE ADDED IN PROOF

In arecent note in NATURE (Vol. 325,572), B. Paczynski discusses tl1e "disco­very" of a giant luminous arc in the core ofthe cluster A370, as announced in January1987 at the 169111 Meeting of the AAS. Itshould be pointed out that this object was

zation of the Use of CCO Oetectors inAstronomy, J. P. Baluteau and S.O'Odorico eds., published by ESO, 315.

Fort, B., Mellier, Y., Picat, J. P., Rio, Y. andLelievre, G., 1986, Proc. SPIE Instrumenta­tion in Astronomy VI, Vol. 627, 321.

Guiderdoni, B., 1986, PhO Thesis.Guiderdoni, B., and Rocca-Volmerange, B.,

1987, in preparation.Hornes, K., 1986, Publ. Astr. Soc. Pac., 98,

609.Mellier, Y., Fort, B., Mathez, G., and Soucail,

G., 1987, in preparation.Soucail, G., Mellier, Y., Fort, B., Picat, J. P.

and Cailloux, M., 1987, submitted to As­tron. Astrophys.

al ready identified in a poster paper whichwas presented at lAU Symposium No. 124in Peking in August 1986. A further discus­sion may be found in arecent paper bySoucail et al. (Astronomy & Astrophysics,172, L14; January 1987). More observationsof this interesting structure were obtained inOctober 1986 with EFOSC.

Nuevos meteoritos encontrados en ImilacH. PEOERSEN, ESD, and F. GARe/A, elo ESD(Traducido dei ingles por C. EULER, ESO)

Oesde tiempos prehistoricos han sido co­leccionadas piedras que caen dei cielo. Has­ta hace poco eran la unica fuente para hacerestudios de laboratorio de la materia extraga­I,ktica, e incluso en nuestra era espacial,siguen siendo una valiosa fuente de investi­gacion de la temprana historia dei sistemasolar.

Se estima que como termine medio cadakilometro cuadrado de la superficie terrestrees golpeada cada millon de aiios por unmeteorito con un pese superior a 500 gra­mos. La mayoria se pierden en los oceanos 0caen en regiones con escasa poblacion. Co­mo resultado, los museos en el munda reci­ben anualmente tan solo alrededor de 6 me­t€oritos cuya caida fuera atestiguada. OtrosIlegan por hallazgos casuales que en lamayoria de los casos son meteoritos que hancardo en tiempos prehistoricos.

Oesde el punto de vista mineralogico pue­den ser divididos en tres clases: piedras,hierros y hierros petreos. Los meteoritos quecaen son en su gran parte petreos, mientrasque aquellos que se encuentran tienen unalto porcentaje de hierro. Esto se debe a quelos meteoritos petreos tienen una erosionmas rapida y son menos visibles. Geografica­mente las ca/das de meteoritos estan muyrelacionadas con la densidad de la pobla­cion, la mayor parte descubiertos en Europay Norteamerica.

La mayoria de los meteoritos se encuen­tran por casualidad. La busqueda activa engeneral requiere demasiado tiempo para serde interes. Sin embargo, los glaciar s de laAntartica han demostrado ser un "buen terre­no de caza".

Meteoritos de Imilac

Otras areas donde se han hecho muchoshallazgos son algunas de las regiones deser-

ticas dei mundo, como el lade occidental deAustralia, las estepas de Norteamerica, y elOesierto de Atacama en Chile. En este ultimolas precipitaciones anuales son menores queen cualquier otra parte dei mundo, menos de5 mm, 10 que obviamente ayuda a la preser­vacion de los meteoritos. Como resultado,uno de los meteoritos atacameiios, encon­trado en el Tamarugal, tiene una edad terres­tre de 2.700.000 aiios, conocida como lamas antigua.

Mucl10S meteoritos chilenos pertenecen altipo "Pallasito'" y provienen muy probable­mente de una sola ca/da. L1evan el nombrede las localidades esparcidas geogratica­mente en un area de 100 por 100 km. En muypocos casos, sin embargo, se pudo indicarcon precision el lugar dei hallazgo y hastamuy reciente se creyo que los meteoritoshab/an sido encontrados dentro de un areade 100 por 500 m cerca dei pequeiio Salar deImilac, que se encuentra aproximadamente a170 km de Antofagasta. En este lugar existeuna excavacion similar a un crater con undiametro de 8 metros. Este puede haber sidocavado por indios en busca de la imaginadaveta de hierro. Varias excavaciones en coli­nas adyacentes muestran lugares donde en

. Los meleoritos se pueden dividir en lres c1ases:piedras. hierros y hierros petreos. Un sub-grupo deesle ultimo es bastante espeeial: una mezcla dehierro y niquel forma una eslructura de tipo espon­joso. Crislales olivinos, con un diametro de 1 a10 mm rellenan los orilieios. 10 que da una relaei6nde volum n melal/olivina de aproximadamente1 : 1. EI primer meteorilo de esta Indole fue eneon­trado en 1771/72 por el explorador aleman PeterSimon Pallas en sus viajes a traves de Rusia orien­tal. M loritos dei tipo "Pallasito" son muy eseasos:tan solo menos que un poreiento de todas laseaidas y 3.5 poreienlos de todes los hallazgosperteneeen a esle grupo.

el pasado se han coleccionado meteoritos.Aun la parte superior dei suelo contiene mu­chos pequeiios fragmentos de hierro quepesan tipicamente I gramo.

Los meteoritos de Imilac han lIegado amuchos museos y colecciones particularesen tode el mundo. EI ejemplar mas grandeconocido, de 198 kg, se encuentra en el Mu­sec Britanico. Otro fragmento, originalmentede 95 kg, esta en Copiapo. EI monto total deimaterial encontrado, plausiblemente de ori­gen de Imilac, se calcula en 500 kg.

Los principales hallazgos

Oespues de varias expediciones se pensoque todos los grandes meteoritos hab/an si­do coleccionados. Sin embargo, podemosinformar sobre el reciente descubrimiento detres meteoritos mas, totalizando 59 kg. EIhallazgo fue hecho por uno de los autores(F.G.), geologo. (Nota dei editor: F.G. es elesposo de una de las secretarias de la ESOen Santiago, Mariam G., a traves de quien loscientificos de La Silla fueron informados deidescubrimiento). Mientras buscaba agua pa­ra una empresa minera supo de la ca/da enImilac. Un poblador de la zona le informo deque algunos meteoritos hab/an sido encon­trados algunos kilometros al sur-oeste dei"crater". Oedicandose a la busqueda pudoencontrar otros tres con un peso de 5, 19 Y35 kg, respectivamente.

La Universidad dei Norte en Antofagastaexamino los fragmentos de 5 y 35 kg Y losclasifico como "Pallasitos". Por razones depese especifico creemos que tambien el hie­rro de 19 kg pertenece a ese grupo. Ya queen tode el mundo se han descrito tan solo 33hallazgos "Pallasitos" (y dos caidos), es unfuerte indicio que los nuevos ejemplares sonparte de la conocida caida de Imilac.

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