The Fornax Cluster Spectroscopic Survey: a sample of confirmed cluster dwarfs

Abstract

The Fornax Cluster Spectroscopic Survey (FCSS) project utilizes the Two‐degree Field (2dF) multi‐object spectrograph on the Anglo‐Australian Telescope (AAT). Its aim is to obtain spectra for a complete sample of all 14 000 objects with 16.5 ≤bj≤ 19.7 irrespective of their morphology in a 12 deg2 area centred on the Fornax cluster. A sample of 24 Fornax cluster members has been identified from the first 2dF field (3.1 deg2 in area) to be completed. This is the first complete sample of cluster objects of known distance with well‐defined selection limits. Nineteen of the galaxies (with −15.8 < MB < −12.7) appear to be conventional dwarf elliptical (dE) or dwarf S0 (dS0) galaxies. The other five objects (with −13.6 < MB < −11.3) are those galaxies which were described recently by Drinkwater et al. and labelled ‘ultracompact dwarfs’ (UCDs). A major result is that the conventional dwarfs all have scale sizes α≳ 3 arcsec (≃300 pc). This apparent minimum scale size implies an equivalent minimum luminosity for a dwarf of a given surface brightness. This produces a limit on their distribution in the magnitude–surface brightness plane, such that we do not observe dEs with high surface brightnesses but faint absolute magnitudes. Above this observed minimum scale size of 3 arcsec, the dEs and dS0s fill the whole area of the magnitude–surface brightness plane sampled by our selection limits. The observed correlation between magnitude and surface brightness noted by several recent studies of brighter galaxies is not seen with our fainter cluster sample. A comparison of our results with the Fornax Cluster Catalog (FCC) of Ferguson illustrates that attempts to determine cluster membership solely on the basis of observed morphology can produce significant errors. The FCC identified 17 of the 24 FCSS sample (i.e. 71 per cent) as being ‘cluster’ members, in particular missing all five of the UCDs. The FCC also suffers from significant contamination: within the FCSS's field and selection limits, 23 per cent of those objects described as cluster members by the FCC are shown by the FCSS to be background objects.

Ultracompact Dwarf Candidates in the Hydra Cluster

“…tribution, they are clearly within the Hydra Cluster (and possibly specifically associated with its cD galaxy, NGC 3311). Structurally, they are very compact: a normal dwarf-galaxy scale length of 300 pc (Deady et al 2002) would give FWHM ∼ 2Љ on our GMOS images, whereas our "superluminous GCs" are completely unresolved at the resolution of our GMOS 0.5 images, implying that their scale radii are pc. A better r Շ 50 eff limit on their scale sizes would, however, come from HST imaging with its 0.1Љ resolution.…”

Discovery of Ultracompact Dwarf Galaxies in the Virgo Cluster

“…Deady et al (2002) have already described the results for dwarf galaxies in the cluster core from the first of the 2dF fields (for B-band absolute magnitudes mostly in the range À11.6 to À14.8). The 2dF fields used in the survey are shown in Figure 10.…”

“…The template giving the highest R-coefficient ( Tonry & Davis 1979) provides the adopted radial velocity for an observed spectrum, as well as a simple indication of the type of spectrum. Further details of the observing and reduction were given in Drinkwater et al (2000b) and Deady et al (2002).…”

The Properties and Luminosity Function of Extremely Low Luminosity Galaxies

“…We find that in better seeing conditions, this corner of the plot is nearly complete. Neither of these features are of particular interest here (although it is worth noting that there is a class of compact galaxies that this incompleteness affects, among them M32 and the UCDs of Deady et al 2002). The gray scale shows that very few objects are observed in these regimes; in addition, since low-luminosity objects tend to have lower surface brightness than average, these incompletenesses at high surface brightness are unimportant for the low-luminosity galaxies.…”