Mid-Infrared Characteristics of Star-Forming Complexes in Irregular Galaxies*

ABSTRACTWe present Spitzer Space Telescope Infrared Spectrograph 10Y37 m high-resolution spectral maps of 11 star-formingregionsinﬁveirregulargalaxies.Wemeasureorplaceupperlimitsonnebularﬁne-structurelines,molecularlines from warm H 2 , and various polycyclic aromatic hydrocarbon (PAH) features in order to probe the hardness ofthestellar radiation ﬁeld,the electrondensity ofthe nebulae, andthephysicalenvironment in whichPAHsare beingheated.Thenebulaehavesimilarexcitations,correspondingtoamidtypeOstar,andelectrondensitiesarelowinmostregions. PAHs were detected in all but two nebulae, and PAH emission relative to stars or H emission tends to behigheringalaxieswithhigheroxygenabundances.Thepopulationof PAHsappearstobedifferentinImgalaxiesthanit is in spirals. We model emission from warm H 2 in one nebula to obtain a temperature of 800 þ440 340 K, a neutral gasdensity 310 5 cm ,andafar-UVradiationﬁeldof G 0 10 3 .Variationsinexcitationacrossthenebulaearesmall,butthePAH11.2 memissionvariesbyfactorsof 3Y4acrossthemaps.DifferencesinthelocalHiorHiienvironmentofstar-forming regions result in only mild differences in mid-IR ratios of nebular ﬁne-structure line emission amongIm galaxies. Variations in PAH emission, however, are more evident.Key words: galaxies: individual (NGC 1156, Haro 14, Holmberg II, NGC 1705, NGC 6822) —galaxies: irregular — galaxies: ISM — galaxies: kinematics and dynamics — stars: formation1. INTRODUCTIONNearbydwarf irregular(dIm)galaxiesprovideauniqueoppor-tunitytostudytheinterstellarmedium(ISM)of low-metallicitysystemsforuseininterpretingobservationsof high-redshiftgal-axies. This is especially urgent now, as we are beginning todirectly probe star formation in young galaxies (see review bySolomonVLisenfeldFBolattoetal.2000;Madden2000; Hunter et al. 2001; Lisenfeld et al. 2002; Hoopes et al.2004;Kiuchietal.2004).Inaddition,dImmolecularcloudsare,to a greater extent than in spirals, the dense cores of large Hiclouds. This can be understood as a result of the low metallic-ity of these galaxies and the higher density necessary to shieldthemoleculesfromtheambientstellarradiationﬁeld(Federmanet al. 1979; Franco & Cox 1986; Maloney & Black 1988;Elmegreen1989;Rubioetal.1991;Bolattoetal.1999;BlitzR Leroy et al. 2005, 2006; Blitz et al. 2007).The IR spectral region provides detailed diagnostics of theISMofgalaxies,andtheincreasedsensitivityof theSpitzerSpaceTelescopeenablesusfortheﬁrsttimetoextendIRstudiestotyp-ical dIm galaxies (rather than solely starbursts). For this reason,we have analyzed Spitzer Infrared Spectrograph (IRS; Houcket al. 2004a) high-resolution spectroscopy at 10Y37 mof11 star-forming regions in three Im galaxies (Holmberg II,NGC1156,andNGC6822)andtwobluecompactdwarf (BCD)galaxies(Haro14andNGC1705).Thespectraldataof NGC1156and Haro 14 were obtained as part of our own GO program, andthe other data are from theSpitzer Infrared Nearby GalaxiesSurvey (SINGS; Kennicutt et al. 2003) Legacy database.NGC 1156, NGC 6822, and Holmberg II have star-formingregionsscatteredovertheirdisks.Sevenstar-formingregionswereobservedinNGC6822andseveralinHolmbergII.InNGC1156the highest column density gas complex was targeted, and itcontainsonlymodestH emission(Hunter&Elmegreen2004).BycontrasttotheImgalaxies,theHiinHaro14isconcentratedat the center of the galaxy, forming an enormous gas and star-formingcomplex.AndNGC1705isfamousforhavingrecentlyformed a superstar cluster.Thus, with thesegalaxieswe can ex-aminethepropertiesof gasandstar-formingcomplexesindiffer-ent local environments.We use these data to examine the interaction of massive starswith the ISM in star-forming regions. We measure or place up-per limits on nebular ﬁne-structure lines, molecular lines fromwarm H