Complexes of triggered star formation in supergiant shell of Holmberg II

Abstract

We report a detailed analysis of all regions of current star formation in the walls of the supergiant H i shell (SGS) in the galaxy Holmberg II based on observations with a scanning Fabry-Perot interferometer at the 6-m SAO RAS telescope. We compare the structure and kinematics of ionized gas with that of atomic hydrogen and with the stellar population of the SGS. Our deep Hα images and archival images taken by the HST demonstrate that current star formation episodes are larger and more complicated than previously thought: they represent unified star-forming complexes with sizes of several hundred pc rather than 'chains' of separate bright nebulae in the walls of the SGS. The fact that we are dealing with unified complexes is evidenced by identified faint shell-like structures of ionized and neutral gas which connect several distinct bright H ii regions. Formation of such complexes is due to the feedback of stars with very inhomogeneous ambient gas in the walls of the SGS. The arguments supporting an idea about the triggering of star formation in SGS by the H i supershells collision are presented. We also found a faint ionized supershell inside the H i SGS expanding with a velocity of no greater than 10 − 15 km s −1 . Five OB stars located inside the inner supershell are sufficient to account for its radiation, although a possibility of leakage of ionizing photons from bright H ii regions is not ruled out as well.

Probing star formation and feedback in dwarf galaxies

“…These ages, together with the distance between the different SF episodes, imply velocities of 20 to 100 km s −1 for the blast wave caused by the SN explosions from the central post-starburst, to be able to ignite the current SF, in good agreement with the expansion velocities found for shells in dwarf galaxies (Walter 1999;Egorov et al 2014Egorov et al , 2017. Hence, the morphology and age distribution of the SF episode both indicate triggered SF in Tololo 1937-423, with the most recent SF episode triggered by the collective effects of stellar winds and SN explosions from the central post-starburst.…”

“…Holes in H i are thought to be created by feedback from massive stars: the supersonic flows generated by distant SN blast waves and/or stellar winds sweep up a shell of shocked neutral gas (Weaver et al 1977;McCray & Kafatos 1987;Tenorio-Tagle & Bodenheimer 1988;Weisz et al 2009b,a;Warren et al 2011;Bagetakos et al 2011). Evidence is accumulating that in the rims of these bubbles, new (secondary) star formation can occur (Lozinskaya 2002;Cannon et al 2005;Egorov et al 2014Egorov et al , 2017Cairós & González-Pérez 2017). In line with these findings, the ionized gas emission in Tololo 1937-423 could be tracing a secondary SF episode.…”

Evolution of clustered supernovae

“…A useful diagnostic of the superbubble evolution is commonly thought to come from line emission of different elements that probe the physical conditions in gas. Giant holes in the gas distribution in galactic disks of several nearby dwarf galaxies have been studied with the help of Hα emission from the gas ionized by Ly-continuum photons produced by the underlying stellar population (e.g., Martinez-Delgado et al 2007;Moiseev & Lozinskaya 2012;Egorov et al 2014Egorov et al , 2017. In this environment the spatial Hα distribution traces the regions with a higher emission measure EM = n 2 e dl, and thus reflects variations of gas density and radiation field under an implicit assumption that photoionized gas is kept at T ≃ 10 4 K. Under such conditions the ratio of Hα to Hβ intensities is practically fixed at 2.86 (e.g., Draine 2011) provided the dust extinction is weak.…”

“…It became clear from the seminal paper by McKee & Ostriker (1977) that SN remnants may stay isolated during only a limited period in the beginning, and afterwards they merge with neighbouring remnants to form a percolating network. Depending on the environment, such merged SNe remnants can build giant supershells (as seen, e.g., in Holmberg galaxies Walter & Brinks 1999;Stewart & Walter 2000;Egorov et al 2014Egorov et al , 2017, and large scale galactic outflows, called galactic winds. Galactic winds in turn act to enrich the Universe with heavy elements.…”

Star formation in shells of colliding multi-SNe bubbles

“…Such shells can fragment into molecular clouds, which in turn may become gravitationally unstable and give rise to the next episode of star formation (McCray & Kafatos 1987). Such structures are widely observed both in dwarf and spiral galaxies (e.g., Egorov et al 2014Egorov et al , 2017.…”