Astronomical Science Weighing Ultracompact Dwarf Galaxies in the Fornax Cluster Michael Hilker1 ade, thanks to several large spectro- ties. Unlike globular clusters which are Holger Baumgardt 2 scopic surveys in nearby galaxy clusters. characterised by a more or less constant Leopoldo Infante3 size (~ 3 pc half-light radius), the sizes Michael Drinkwater 4 Compact objects with masses greater of UCDs increase with luminosity rea- Ekaterina Evstigneeva 4 than those of normal globular clusters ching half-light radii of ~ 100 pc. Some of Michael Gregg 5 were only discovered about 10 years ago. the brighter UCDs exhibit a small low- In 1999, two bright compact objects surface-brightness envelope with exten- were confirmed as members of the For- sions up to several hundred parsecs. 1 ESO nax cluster in a spectroscopic survey Most of the brightest UCDs have slightly 2 Argelander-Institut für Astronomie, that was designed as a follow-up of a subsolar metallicities ([Fe/H] ~ –0.5 dex), Universität Bonn, Germany photometric investigation of dwarf ellipti- similar to the ‘red’, metal-rich bulge GCs 3 Departamento de Astronomía y Astrofí- cals in the Fornax cluster (Hilker et al. of giant galaxies. sica, Pontificia Universidad Católica de 1999). One year later, in 2000, a system- Chile, Santiago de Chile, Chile atic spectroscopic survey within a two- In summary, one might say that the name 4 Department of Physics, University of degree field centred on Fornax revealed ‘ultracompact dwarf galaxies’ (UCDs) ap- Queensland, Brisbane, Australia five compact members in the magnitude plies to old stellar systems in the transi- 5 Department of Physics, University of range –13.5 < MV < –12.0 (Drinkwater tion region between globular clusters and California, Davis, California, USA et al. 2000) which were one year later compact dwarf galaxies. dubbed ‘Ultracompact Dwarf Galaxies’ (UCDs) by Phillipps et al. (2001). In Fig- High-resolution spectra from the Ultra- ure 1 we show the location of the seven Formation scenarios for UCDs violet and Visual Echelle Spectrograph brightest UCDs in the Fornax cluster. (UVES) were used to derive internal An important question that is keeping velocity dispersions of Ultracompact We now know several physical properties UCD researchers busy is whether UCDs Dwarf galaxies (UCDs) in the Fornax of UCDs thanks to significant growth in should be regarded as galaxies or cluster of galaxies. The velocity disper- this research field. After the first discovery whether they are more closely linked to sions, together with highly spatially re- of UCDs in the Fornax cluster (summa- globular clusters. Various formation solved luminosity profiles from Hubble rised in a Nature article by Drinkwater et scenarios have been suggested that re- Space Telescope imaging (ACS cam- al. in 2003), many surveys were devel- flect the different viewpoints. The four era), allowed us to derive the dynamical oped to search for UCDs in different en- most promising are: masses of the UCDs. We show that the vironments and at fainter magnitudes. mass-to-light ratios of UCDs in Fornax Bright UCDs were also found in the Virgo 1. UCDs are the remnant nuclei of galax- are consistent with those expected for cluster, and fainter ones in both clusters. ies that have been significantly stripped pure stellar populations. No dark matter With absolute magnitudes in the range in the cluster environment (‘threshing’ contribution is needed. Thus, these MV = –13.5 to –11.0 they are up to 3 mag scenario, e.g. Bekki et al. 2003). Good UCDs seem to be the result of star-clus- brighter than ω Centauri, the most mas- candidates for isolated nuclei in the ter formation processes within galaxies, sive globular cluster (GC) of the Milky local environment are the Galactic rather than being compact dwarf galax- Way, but about 3 mag fainter than M32. globular clusters ω Centauri and the ies formed in dark-matter halos. Their sizes are related to their luminosi- giant star cluster G1 in Andromeda. Dwarf galaxies have only been exten- sively studied in the last three decades. Dwarf spheroidals are considered to be the faintest galaxies, having baryonic masses comparable to those of bright 6 globular clusters (~ 10 MA), but are 50– 200 times more extended. It has been believed that dwarf galaxies are diffuse Figure 1: Panoramic structures, with the exception of the com- view of the Fornax clus- pact elliptical M32, a companion of the ter with its population of UCDs. The back- Andromeda galaxy, which is ~ 8–10 times ground image was taken smaller than dwarf ellipticals of compara- with the Michigan Curtis ble luminosities, but about 150 times Schmidt Telescope more luminous than the brightest globular at the Cerro Tololo Ob- servatory. Insets are clusters of the Local Group. The gap in HST/STIS images of five luminosity between globular clusters and UCDs in Fornax and a compact dwarf galaxies has started to be nucleated dwarf elliptical filled in observationally over the last dec- UCD1 UCD2 UCD3 UCD4 UCD5dEN galaxy in Fornax (far right). The Messenger 129 – September 2007 49 Astronomical Science Hilker M. et al., Weighing Ultracompact Dwarf Galaxies If this formation channel is viable one Figure 2: A composite image of the would not expect a dark matter com- central region of the Fornax cluster based on data acquired at the 2.5-m ponent in UCDs since ‘threshing’ sim- DuPont telescope at the Las Cam- ulations show that the dark matter panas Observatory. Images in the fil- halo of dwarf galaxies is completely ters B, V, I and were combined to stripped within a few Gyrs. In Figure 2 make the colour composite. The insets show HST/STIS images of a UCD we show a UCD and a nucleated dwarf (upper) and a nucleated dwarf elliptical elliptical in comparison. Both are lo- galaxy (lower). cated in the very heart of the Fornax cluster. 2. UCDs were formed from the agglom- eration of many young, massive star clusters that were created during merg er events (e.g. Fellhauer and Kroupa 2002). Such super-star cluster complexes are observed in interacting galaxies like the Antennae. In this for- mation process no dark matter would be involved. Visual Echelle Spectrograph (UVES) at was used with some slight modifications. 3. UCDs are the brightest globular clus- the VLT is an ideal instrument to solve the The spectral resolution of the final, re- ters and were formed in the same GC UCD riddle. duced spectra was ~ 8 km s–1. The sig- formation event as their less massive nal-to-noise ranged between 10 and counterparts (e.g. Mieske et al. 2004). 20 at 5 900 Å. In Figure 3 we show spec- The most massive GCs then suppos- UVES spectroscopy tra of the four UCDs and four reference edly formed from the most massive stars in the wavelength region of the So- molecular clouds (MCs) of their host High-resolution spectra were obtained dium (Na) doublet absorption lines. Such galaxy. The luminosity-size relation of for four Fornax UCDs, one nucleus of a absorption lines are important features the most massive clusters suggests dwarf elliptical and several reference to measure the internal velocity disper- that there is a break of the formation/ stars (mostly red giant stars) in service sion of the UCDs. One can clearly see collapse physics at a critical MC mass. mode at the VLT in 2000/2001. The in- that UCD absorption lines are broadened Also in this case no dark matter in tegration times were between one and six with respect to the ones of the reference UCDs is expected. hours, depending on the brightness of stars. This broadening is caused by the the UCD. For the reduction of the spec- internal velocity dispersion of stars popu- 4. UCDs are genuine compact dwarf tra, the UVES pipeline, provided by ESO, lating the UCDs. galaxies, formed in small dark-matter halos at the low mass end of cosmo- logical substructure. This scenario has 1 the advantage that no external proc- esses, like mergers or tidal disruption, 0.5 are needed. A considerable dark- [Fe/H] = –1.18 HD41667 UCD2 0 matter component is expected if this formation channel applies. 1 How can a massive star cluster be dis- 0.5 cerned from a low-mass compact gal- [Fe/H] = –0.87 HD20038 UCD3 0 axy? The answer may be hidden in the seen and unseen mass of UCDs. Dwarf 1 galaxies are expected to be dark matter Normalised Flux dominated, globular clusters are not. 0.5 Thus, the best way to distinguish be- [Fe/H] = –0.60 HD17233 UCD4 0 tween these two possibilities is to meas- ure the masses of UCDs. In order to 1 do so we need to measure the motion of Figure 3: UVES spectra 0.5 of four standard stars stars within UCDs with high-resolution (left) and four UCDs [Fe/H] = –0.14 HR296 UCD5 spectroscopy. The faster their motion, the 0 (right) around the wave- larger the UCD mass. The Ultraviolet and 5860 5880 5900 5920 5940 5880 5900 5920 5940 5960 length region of the Na λ(Å) λ(Å) doublet. 50 The Messenger 129 – September 2007 Internal velocity dispersions Figure 4: Illustration of velocity dis persion measurement for UCD3. 1 The kinematic analysis of the spectra was performed using a direct-fitting method 0.5 (van der Marel and Franx 1993). First, the Normalised Flux spectra were placed on a logarithmic UCD3 0 wavelength scale and normalised. Then, the reference star spectra were con- 1 volved with Gaussian velocity dispersion profiles in the range 2 to 60 km s–1. All 0.5 UCD spectra were fitted with all sets of Normalised Flux NaI (D2) NaI (D1) smoothed ‘template’ spectra.