Public Release of 2dF data from the Fornax Cluster Spectrosc

Thanks to the 2dF spectrograph on the Anglo-Australian Telescope, we have recently completed the first stage of a complete spectroscopic survey more than one order of magnitude larger than any previous study, measuring 7000 spectra in a 6 sq.deg. area as p

a r X i v :a s t r o -p h /0106374v 1 20 J u n 2001Public Release of 2dF data from the Fornax Cluster Spectroscopic

Survey

M.J.Drinkwater 1 C.Engel 1S.Phillipps 2J.B.Jones 3M.J.Meyer 1

To appear in the Anglo-Australian Observatory Newsletter

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School of Physics,University of Melbourne,Victoria 3010,Australia 2

Department of Physics,University of Bristol,Bristol BS81TL,UK 3

School of Physics &Astronomy,University of Nottingham,Nottingham NG72RD,UK 1Spectroscopy of Everything Almost 20years ago Morton,Krug &Tritton (1985)made the first complete spectroscopic survey of a given region of sky.They surveyed stellar objects in a 0.3deg 2region of sky to a limit of B =20,measuring 600objective prism spectra with the UKST and 100slit spectra with the AAT.All the objects measured were normal stars except for a small number of QSOs and white dwarfs,the latter being used to put new constraints on the white dwarf luminosity function.Thanks to the arrival of the 2dF spectrograph on the AAT,we have recently completed the first stage of a complete spectroscopic survey more than one order of magnitude larger than the Morton et al.study,measuring 7000spectra in a 2πdeg 2area as part of our study of the Fornax Cluster.In this article we describe the public release of 3600spectra from our first field.We hope that this public release will encourage colleagues making surveys for rare objects to choose these fields,as much of the follow-up spectroscopy that might be required is available from our data.Our 2dF Fornax Cluster Spectroscopic Survey (FCSS;see Drinkwater et al.2000a)was designed to make the most complete census possible of low-luminosity galaxies in the Fornax Cluster.As well as the conventional low-surface brightness dwarfs,we already had evidence (Drinkwater &Gregg 1998)that very compact,high surface brightness dwarf cluster galaxies had been missed in previous work,so we took the unusual step of observing all objects in each 2dF field,both resolved (“galaxies”)and unresolved

(“stars”).In this way we avoided any morphological bias as to what a cluster galaxy should look like.Observing all the “stars”in each 2dF field leads to a large increase in the number of targets observed:within our magnitude limits (16.5<b J <19.8)over 50%of the objects in each 2dF field are stars.Thanks to the flexibility of 2dF,however,this only leads to a small increase in total observing time as we schedule the stars at times when we could not usefully observe galaxies,such as twilight or at high airmass or through cloud.

2Status of the Survey

As of 2001January we have completed observations of two of our four planned 2dF fields centred on the Fornax Cluster.We have analysed the data from our first field and have released it for public access as described below.The properties of the sample are summarised in Fig.1,a plot of surface brightness against magnitude for all the objects we successfully observed.The main all-object sample was selected in the magnitude range 16.5<b J <19.8.We extended our observations of the unresolved objects (“stars”)to a slightly fainter magnitude limit of b J ≈20.2.There is some incompleteness for galaxies with surface

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Thanks to the 2dF spectrograph on the Anglo-Australian Telescope, we have recently completed the first stage of a complete spectroscopic survey more than one order of magnitude larger than any previous study, measuring 7000 spectra in a 6 sq.deg. area as p

Figure1:Surface brightness-magnitude diagram for the objects we have successfully observed in our first2dFfield.Resolved objects(“galaxies”)are plotted as(blue)filled triangles and unresolved objects (“stars”)as(red)open circles.The surface brightness measurements are based on exponentialfits to the photographic data and are only indicative for the stars(see Drinkwater et al.2000a).

brightness lower than about23.5b J mag arcsec−2as we were unable to measure their redshifts(see Drinkwater et al.2000a).

The diversity of objects in our sample is illustrated by Fig.2,a cone diagram of the sample in which the distance axis is scaled(as z1/4)so as to display both Galactic stars and the most distant QSOs. Note that although we avoid the use of conventional morphological classifications in our analysis(we use redshift,spectral signatures and luminosity instead),we do show the morphological classifications in both Fig.1and Fig.2.The presence of unresolved objects(“stars”)in both the Fornax Cluster and among the backgroundfield galaxies demonstrates the importance of our all-object strategy.These objects would have been missed by conventional galaxy surveys only looking at objects that are resolved on UKST sky survey plates.

The scientific highlight of our results to date has been the discovery of a new population of Fornax Cluster dwarf galaxies so compact they were previously mistaken for Galactic stars(Drinkwater et al. 2000b).They can be seen among the more normal,resolved Fornax Cluster dwarf galaxies in Fig.2. These”ultra-compact dwarf”(UCD)galaxies are unlike any known type of stellar system.They are smaller and more concentrated than any known dwarf galaxy,but are2-3magnitudes more luminous than the largest Galactic globular clusters.Numerical simulations have shown that the UCDs could have been formed by tidal stripping of nucleated dwarf galaxies in close orbits around the central galaxy of the cluster,NGC1399(Bekki,Couch&Drinkwater2001).We are making detailed follow-up observations of the UCDs with Hubble Space Telescope imaging to measure their sizes along with high re …… 此处隐藏：8691字，全部文档内容请下载后查看。喜欢就下载吧 ……