Abstract
Context. Star formation activity is an important driver of galaxy evolution and is influenced by the physical properties of the interstellar medium. Dwarf galaxies allow us to understand how the propagation of radiation and the physical conditions of the different ISM phases are affected by the low-metallicity environment. Aims. Our objective is to investigate the physical properties of the ionized gas of the low-metallicity dwarf galaxy, IC 10, at various spatial scales: from individual H II regions to the entire galaxy scale and examine whether diagnostics for integrated measurements introduce bias in the results. Methods. We modeled the ionized gas combining the mid- and far-infrared fine-structure cooling lines observed with Spitzer/IRS and Herschel/PACS, with the photoionization code CLOUDY. The free parameters of the models are the age of the stellar cluster, the density, and the ionization parameter of the ionized gas as well as the depth of the cloud. The latter is used to investigate the leakage of the ionizing photons from the analyzed regions of IC 10. We investigated H II regions in the main star-forming body, on scales of ~25 pc, three in the main star-forming region in the center of the galaxy and two on the first arc. We then considered larger sizes on the scale of ~200 pc. Results. Most clumps have almost-identical properties, density ~102–102.6 cm−3, ionization parameter between 10−2.2 and 10−1.6, and age of the stellar cluster ~5.5 Myr. All of them are matter-bounded regions, allowing ionizing photons to leak. The relatively uniform physical properties of the clumps suggest a common origin for their star formation activity, which could be related to the feedback from stellar winds or supernovae of a previous generation of stars. The properties derived for ~200 pc size “zones” have similar properties as the H II regions they encompass, but with the larger regions tending to be more radiation bounded. Finally, we investigated the fraction of [CII] 157.7 μm, [SiII] 34.8 μm and [FeII] 25.9 μm emission arising from the ionized gas phase and we find that most of the emission originates from the neutral gas, not from the ionized gas.
Highlights
The interstellar medium (ISM) plays a key role in understanding star formation (SF) process, as it is at the same time the reservoir of gas and dust and the repository of stellar ejecta, enriched by the elements produced by nucleosynthesis in massive stars
Since we do not have enough information on the age of the stellar population, we explored the effect of restricting the starburst age to either one of the two peaks, in order to witness the effects on the probability density function (PDF) of the other parameters
We have presented the Spitzer/IRS and Herschel/PACS spectroscopic observations of the infrared cooling lines tracing the ionized gas in the nearby irregular dwarf galaxy IC 10
Summary
The interstellar medium (ISM) plays a key role in understanding star formation (SF) process, as it is at the same time the reservoir of gas and dust and the repository of stellar ejecta, enriched by the elements produced by nucleosynthesis in massive stars. Metal-poor dwarf galaxies are not genuinely young (i.e., they are at least older than ∼1 Gyr) and they cannot be directly compared to highredshift galaxies. Some irregular dwarf galaxies harbour super star clusters (e.g., 30 Doradus in the Large Magellanic Cloud, Hunter 1999) and some of them host Wolf-Rayet (WR) stars (e.g., IC 10, Massey & Holmes 2002), hinting at intense SF activity within the last 10 Myr. The combination of a young massive stellar population which produces hard ultraviolet (UV) photons and a dust-poor ISM (more transparent) can result in a hard radiation field extending over galaxy-wide scales. Observations are consistent with the above picture, with ionized gas tracers detected throughout these galaxies (e.g., Kawada et al 2011; Lebouteiller et al 2012a; Cormier et al 2015)
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