The most luminous young SSCs of the Galactic neighborhood are the testbeds for study of the star formation process in large clusters and in starburst systems. What are the Orions of the SSC world? In Table 2 we have compiled from the literature properties of some well-studied and spatially resolved SSCs in the local universe. Included are Galactic center clusters and large Galactic star-forming regions. While smaller than many extragalactic SSCs, these Galactic clusters are close and more easily studied and should share many of the star-forming properties. The super star clusters of Table 2 reflect a wide range of environments and evolutionary stages in the formation and evolution of SSCs.
|Cluster||D||Ra||log L* / L||log M* / M||log NLyc||NO||MV||Age|
|Galaxy||NGC 3603||0.0076||4.5||7.0||3.4||50.1||> 50||...||1-4|
|NGC 1705||NGC1705-1||5.3||1.6||...||5.68||< 51||...||-14.0||12|
|He 2-10||He 2-10-1||3.8||1.5||...||5.7||...||1300||-14.3||5.2|
|He 2-10||He 2-10-A-4||3.8||3.9||...||...||52.4||...||...||...|
|He 2-10||He 2-10-A-5||3.8||1.7||...||...||51.9||...||...||...|
|He 2-10||He 2-10-B-1||3.8||1.8||...||...||51.9||...||...||...|
|He 2-10||He 2-10-B-2||3.8||1.8||...||...||52.0||...||...||...|
|M82||M82-F||3.6||2.8||7.73||5.8||...||...||-14.5||50 - 60|
|NGC 3125||NGC3125-A1||11.5||...||...||...||52.4d||250 - 3000e||...||3 - 4|
|NGC 3125||NGC3125-A2||11.5||...||...||...||...||550 - 3000||...||3 - 4|
|NGC 3125||NGC3125-B1,2||11.5||...||...||...||52.2||450||...||3 - 4|
|NGC 4214||NGC4214-1||4.1||< 2.5||...||...||...||280||-13.1||...|
|NGC 5253||NGC5253-5||3.8||...||5.8||...||51.9||155||~ -14||2|
|NGC 5253||NGC5253-IR||3.8||0.7||9.0||...||52.5||1200-6000g||...||2 - 3|
|aCluster radii are half-light radii. bCluster A is two clusters, de Marchi et al. 1997. cBright core of a larger, complex cluster, NGC 2070. dA1 and A2. eRange in O stars is due to differences in reddening. f MK. gResolved source; lesser number for r < 1 pc. References. Arches, Quintuplet, Galactic nuclear center clusters: Figer et al. 1999, Lang et al. 2001, Figer et al. 2005, Stolte 2003, Stolte et al. 2002, 2005, 2007, Najarro et al. 2004, Figer 2003, 2004, 2008. Kim et al. 2000, 2004, 2007, Kim & Morris 2003. Sgr B2: Dowell 1997, Gaume et al. 1995. NGC 3603: de Pree, Nysewander, & Goss 1999, Eisenhauer et al. 1998, Pandey et al. 2000, Drissen et al. 2002, Nürnberger & Petr-Gotzens 2002, Stolte et al. 2006, Harayama et al. 2008, Melena et al. 2008. W49: Smith et al. 1978, Welch et al. 1987, Conti & Blum 2002, Homeier & Alves 2005. Westerlund 1: Clark et al. 1998, 2005, Nürnberger et al. 2002, Nürnberger 2004, Crowther et al. 2006, Mengel & Tacconi-Garman 2007, 2008, Brandner et al. 2008. R136: Mills et al. 1978, Meylan 1993, Hunter et al. 1995, Massey & Hunter 1998, Noyola & Gebhardt 2007. M82-A1: Smith et al. 2006. M82-F: Smith & Gallagher 2001, O'Connell et al. 1995, McCrady et al. 2005. M82-L: McCrady & Graham 2007. NGC 1569 A and B: O'Connell et al. 1994, Sternberg 1998, Hunter et al. 2000, Ho & Filippenko 1996a, Smith & Gallagher 2001, Origlia et al. 2001, Gilbert 2002, Larsen et al. 2008. NGC1705-1: Ho & Filippenko 1996b, Sternberg 1998, Smith & Gallagher 2001, Johnson et al. 2003, Vázquez et al. 2004. He 2-10-1: Chandar et al. 2000. One of five clusters within He 2-10A. He 2-10-A, B: Vacca & Conti 1992, Johnson & Kobulnicky 2003. NGC 3125: Vacca & Conti 1992, Schaerer et al. 1999a, b, Stevens et al. 2002, Chandar et al. 2004, Hadfield & Crowther 2006. Region A has logQ0 = 52.39, for 4000 O stars, region B logQ0 = 52.19, for 3200 O stars, Hadfield & Crowther. Antennae: Gilbert et al. 2000, for 13.3 Mpc. NGC5253-5: Gorjian 1996, Calzetti et al. 1997, Schaerer et al. 1997, Tremonti et al. 2001, Chandar et al. 2004, Vanzi & Sauvage 2004, Cresci et al. 2005. NGC5253-IR: Obscured IR/radio source offset by ~ 0.5" from NGC5253-5. Beck et al. 1996, Turner et al. 1998, 2000, 2003, Mohan et al. 2001, Alonso-Herrero et al. 2004, Turner & Beck 2004, Martín-Hernandez et al. 2005, Rodríguez-Rico et al. 2007.|
The Galactic massive young clusters are readily resolved into stars and contain a wealth of information on young massive cluster evolution. However, even for these nearby clusters, confusion, contamination, and rapid dynamical evolution introduce great complexity into observational interpretations. Westerlund 1 is the closest of these large clusters, located in the Carina arm. NGC 3603 is a large southern cluster somewhat more distant. The Arches, Quintuplet, and Galactic Center nuclear clusters have formed in the immediate vicinity of a supermassive black hole, and may differ in structure and evolution from large clusters in more benign environments. The study of the Galactic Center clusters has been made possible by high resolution infrared observations. Included in Table 2 are luminous embedded star-forming regions SgrB2 and W49A; their relation to the massive, unembedded star clusters is unclear, although they have similar total luminosities. The other SSCs listed are in galaxies within ~ 20 Mpc, in which clusters can be spatially resolved. Many of these SSCs have been identified by their location within "Wolf-Rayet" galaxies, those galaxies with a strong He II 4686 line indicating the presence of significant numbers of Wolf-Rayet stars of age ~ 3 - 4 Myr (Conti 1991, Schaerer et al. 1999). The Wolf-Rayet feature is a relatively easy way to identify in systems with large clusters of young stars, and these are often found in SSCs. Many of the clusters in Table 2 have dwarf galaxy hosts. This may be a selection effect due to the difficulty of isolating clusters amid the higher confusion and extinctions in large spirals, since SSCs are definitely present in many local spirals, such as NGC 253, NGC 6946, and Maffei 2 (Condon 1992, Turner and Ho 1994, Watson et al. 1996, Maoz et al. 1996, Maoz et al. 2001, Rodríguez-Rico et al. 2006, Tsai et al. 2006, Roy et al. 2008). The dominance of dwarf galaxy hosts may also be due to "downsizing," the tendency for star formation to occur in smaller systems at later times.
It is evident from Table 2 that it can be difficult to compare these clusters because embedded and visible clusters are characterized in different ways. Embedded clusters are often characterized by photons that have been absorbed by gas or dust, with well-defined Lyman continuum fluxes and infrared luminosities. Visible clusters have star counts, cluster magnitudes, colors, and stellar velocity dispersions; these clusters can have good masses and ages. Putting together an evolutionary sequence of objects thus requires multiwavelength observations at high spectral resolution. Extinction is observed to decrease with increasing cluster age (Mengel et al. 2005), as one might expect from Galactic star-forming regions, so the embedded clusters are likely to also be the youngest clusters.
High angular resolution is key to the study of even the closest super star clusters, which are often found forming in large numbers. One of the first known super star clusters, NGC 1569-A, consists of two superimposed clusters (de Marchi et al. 1997), which is not immediately obvious even in the HST image (Figure 1). The embedded IR/radio SSC in the center of NGC 5253 was found to be offset by a fraction of an arcsecond from the brightest optical cluster, NGC 5253-5, (Calzetti et al. 1997) only 5 - 10 pc away (Turner et al. 2003, Alonso-Herrero et al. 2004).
Extinction can also be extreme in bright IR-identified starburst regions, and can obscure even the brightest clusters through the near-IR. Observed differential extinctions between the infrared Brackett lines at 2 and 4 µm indicate AV > 1 and even AK > 1 in many starbursts (Kawara et al. 1989, Ho et al. 1990). IR-derived extinctions are often higher than those derived from Balmer recombination lines toward the same regions (Simon et al. 1979) because of extinctions internal to the H II regions themselves. In M82, near-IR and mid-infrared spectroscopy indicates extinctions of AV ~ 25 (Willner et al. 1977, Simon et al. 1979) to Av ~ 50 (Förster Schreiber et al. 2001), similar to values observed in Galactic compact H II regions, but over much larger areas. The clusters in Arp 220 are heavily obscured, with estimated AV ~ 10 - 45 mag (Shioya et al. 2001, Genzel et al. 1998); regions behind the molecular clouds can reach AV ~ 1000 (Downes and Solomon 1998).
What does the next decade hold for the clusters of Table 2 and other nearby clusters like them? First, there are more SSCs to discover in the local universe, particularly embedded ones. IRAS is still a valuable tool for discovering young SSCs, but with arcminute resolution, it is not sensitive to bright, subarcsecond sources. There undoubtedly remain many compact, young ESF events to be found in the local universe. The WISE mission, an all sky mid-IR survey, will provide an extremely valuable dataset for discovering young and embedded SSCs. The enhanced sensitivities and high spatial resolution of the next generation of telescopes (JWST, EVLA, ALMA, SOFIA), will redefine our concept of "local" SSC formation, extending this list to more distant systems and to young SSCs within large, gas-rich spirals. The near-infrared, in particular, is an valuable link between visible and embedded SSCs. Subarcsecond resolutions are necessary to resolve individual clusters in regions of high and patchy extinction, and the closest galaxies are even now being pursued with adaptive optics. JWST and future extremely large ground-based telescopes will play an important role in connecting embedded clusters to their older, visible siblings to enable a longitudinal study of SSC evolution.