We present JHK s observations of the metal-poor ([Fe/H] < -1.40) Dwarf-irregular galaxies, Leo A and Sextans A obtained with the WIYN High-Resolution Infrared Camera at Kitt Peak. Their near-IR stellar populations are characterized by using a combination of colour-magnitude diagrams and by identifying long-period variable stars. We detected red giant and asymptotic giant branch stars, consistent with membership of the galaxy's intermediate-age populations (2-8 Gyr old). Matching our data to broadband optical and mid-IR photometry we determine luminosities, temperatures and dust-production rates (DPR) for each star. We identify 32 stars in Leo A and 101 stars in Sextans A with a DPR > 10 −11 M ⊙ yr −1 , confirming that metal-poor stars can form substantial amounts of dust. We also find tentative evidence for oxygen-rich dust formation at low metallicity, contradicting previous models that suggest oxygen-rich dust production is inhibited in metal-poor environments. The total rates of dust injection into the interstellar medium of Leo A and Sextans A are (8.2 ± 1.8) ×10 −9 M ⊙ yr −1 and (6.2 ± 0.2) ×10 −7 M ⊙ yr −1 , respectively. The majority of this dust is produced by a few very dusty evolved stars, and does not vary strongly with metallicity.

High ionization star forming (SF) galaxies are easily identified with strong emission line techniques such as the BPT diagram, and form an obvious ionization sequence on such diagrams. We use a locally optimally emitting cloud model to fit emission line ratios that constrain the excitation mechanism, spectral energy distribution, abundances and physical conditions along the star-formation ionization sequence. Our analysis takes advantage of the identification of a sample of pure star-forming galaxies, to define the ionization sequence, via mean field independent component analysis. Previous work has suggested that the major parameter controlling the ionization level in SF galaxies is the metallicity. Here we show that the observed SFsequence could alternatively be interpreted primarily as a sequence in the distribution of the ionizing flux incident on gas spread throughout a galaxy. Metallicity variations remain necessary to model the SF-sequence, however, our best models indicate that galaxies with the highest and lowest observed ionization levels (outside the range -0.37 < log [O III]/Hβ < -0.09) require the variation of an additional physical parameter other than metallicity, which we determine to be the distribution of ionizing flux in the galaxy.

We present an atlas of starburst galaxy emission lines spanning 10 orders of magnitude in ionizing flux and 7 orders of magnitude in hydrogen number density. Coupling SEDs from Starburst99 with photoionization calculations from Cloudy, we track 96 emission lines from 977 Å to 205 µm which are common to nebular regions, have been observed in H II regions, and serve as useful diagnostic lines. Each simulation grid displays equivalent widths and contains ~1.5x10 4 photoionization models calculated by supplying a spectral energy distribution, chemical abundances, dust content, and gas metallicity (ranging from 0.2 Z to 5.0 Z ). Our simulations will prove useful in starburst emission line data analysis, especially regarding local starburst galaxies that show high ionization emission lines. One sample application of our atlas predicts that C IV λ1549 will serve as a useful diagnostic emission line of vigorous star formation for coming James Webb Space Telescope observations predicting a peak equivalent width of approximately 316 Å. IntroductionStarburst galaxies, also known as H II galaxies or star-forming galaxies, feature strong emission lines primarily due to newly formed massive stars. As mentioned above, along the extreme "wings" of the BPT diagram, AGN and starforming galaxies are easily distinguished from one another. However, while starlight is often the dominant source of excitation in star-forming galaxies, several other excitation mechanisms can contribute to the production of emission lines. The most intensely star forming galaxies along the far left wing of the BPT diagram are often interacting or merging (Robaina et al. 2009). Strong shocks further contribute to the excitation of the gas in galaxies along the left wing.Similarly, at lower ionization, classifying galaxies as star-forming or AGN becomes difficult. For these galaxies, excitation and ionization of gaseous clouds could likely be the result of starlight, non-thermal sources, or a combination of the two. Historically, the presence of a radiation field hard enough to generate photons higher than 50 eV signified excitation from an AGN. However, modern models of stellar radiation fields that incorporate Wolf-Rayet (WR) stars produce a significant number of EUV photons capable of ionizing heavy elements through many excitation states.