4.3. Modes of Star Formation

The ISM in dIrrs is highly inhomogeneous and porous, full of small and large shells and holes. The global gas density tends to be significantly below the Toomre criterion for star formation (van Zee et al. 1997). Stochastic, star formation may be driven by homogeneous turbulence, which creates local densities above the star formation threshold (e.g., Stanimirovic et al. 1999). Self-propagating stochastic star formation (Gerola & Seiden 1978; Gerola, Seiden, & Schulman 1980; Feitzinger et al. 1981) can lead to structures of sizes of up to 1 kpc, in which star formation processes remain active for 30-50 Myr, or to the formation of long-lived spiral features if an off-centered bar is present (Gardiner, Turfus, & Putman 1998). In the absence of shear, star formation continues along regions of high HI column density, fueled by the winds of recently formed stars and supernovae explosions.

Dense gas concentrations may, however, also remain inactive for hundreds of Myr, and there are not usually obvious triggers for the onset of star formation (see Dohm-Palmer et al. 2002). This may be different in nonquiescently evolving, starbursting dIrrs like IC10: gas accretion or other interactions may be triggering the starburst (see Section 3.2). The existence of isolated dIrrs with continuous star formation outside of groups shows that external triggers are not needed. Quiescently evolving dIrrs exhibit widely distributed star formation and have very small color gradients, whereas starbursting dIrrs show much more concentrated star formation and strong color gradients (van Zee 2001). The analysis of 72 dIrr galaxies in nearby groups and in the field revealed that the radial distribution of star-forming regions follows on average an annulus-integrated exponential distribution, and that secondary star-forming peaks at larger distances are consistent with internal triggering via stochastic, self-propagating star formation (Parodi & Binggeli 2003).

Quiescent dIrrs tend to form OB associations, while massive starbursts can lead to the formation of more compact star clusters. The number of massive clusters tends to correlate with galaxy mass (i.e., roughly with luminosity; see, e.g., Parodi & Binggeli 2003). For instance, in the dIrr NGC6822 on average one cluster is formed per 6 × 10⁶ years (a much smaller number than in the more massive LMC), while an OB association forms every 7 × 10⁵ years, similar to the LMC (Hodge 1980). The distinctive, well-separated peaks in the formation rate of populous clusters in the LMC, however, seem to be caused by close encounters with the Milky Way and the SMC (Gardiner, Sawa, & Fujimoto 1994; Girardi et al. 1995; Lin, Jones, & Klemola 1995); it is surprising that no corresponding enhancement in the SMC's fairly continuous cluster formation rate is seen. Generally, old globular clusters are rare in dIrrs. For the cluster census in Local Group Irr and dIrr galaxies, see Table 1.

On a global, long-term scale, star formation in dIrrs has essentially occurred continuously at a constant rate with amplitude variations of 2-3 (Tosi et al. 1991; Greggio et al. 1993), is largely governed by internal, local processes, and will likely continue for another Hubble time (Hunter 1997; van Zee 2001).

Table 1. Star Clusters in the Local Group Irr and dIrr Galaxies

Galaxy	Type	DSp	MV	NGC	SN	[Fe/H]	NOC
		[kpc]	[mag]			[dex]
LMC	IrIII-IV	50	-18.5	~ 13	0.5	-2.3, -1.2	4000
SMC	IrIV/IV-V	63	-17.1	1	0.1	-1.4	2000
NGC6822	IrIV-V	500	-16.0	1	0.4	-2.0	~ 20
WLM	IrIV-V	840	-14.4	1	1.7	-1.5	1
IC10	IrIV:	250:	-16.3	0	0	--	13
IC1613	IrV	500	-15.3	0	0	--	5
Phe	dIrr/dSph	405	-12.3	[4:]	[48:]	--	?
PegDIG	IrV	410	-11.5	0	0	--	3
LGS3	dIrr/dSph	280	-10.5	0	0	--	13
Notes: Only galaxies known to contain star clusters are listed. DSp denotes the distance to the nearest spiral galaxy (M31 or Milky Way, Col. 3). NGC and NOC (Cols. 5 & 8) list the number of globular clusters and open clusters, respectively. Note that the globular cluster suspects in Phoenix are highly uncertain. SN (Col. 6) is the specific globular cluster frequency. When two values are listed in Col. 7 (metallicity), these indicate the most metal-rich and most metal-poor globular clusters. For more details, a full list of galaxies with star clusters in the Local Group, and references, see Grebel (2002b).