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Register a new userMomentum feedback from marginally-resolved HII regions in isolated disc galaxies
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Sarah May Rose Jeffreson
Publication date
2021
fields
Physics
and research's language is
English
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We present a novel, physically-motivated sub-grid model for HII region feedback within the moving mesh code Arepo, accounting for both the radiation pressure-driven and thermal expansion of the ionised gas surrounding young stellar clusters. We apply this framework to isolated disc galaxy simulations with mass resolutions between $10^3~{rm M}_odot$ and $10^5~{rm M}_odot$ per gas cell. Each simulation accounts for the self-gravity of the gas, the momentum and thermal energy from supernovae, the injection of mass by stellar winds, and the non-equilibrium chemistry of hydrogen, carbon and oxygen. We reduce the resolution-dependence of our model by grouping those HII regions with overlapping ionisation front radii. The Str{o}mgren radii of the grouped HII regions are at best marginally-resolved, so that the injection of purely-thermal energy within these radii has no effect on the interstellar medium. By contrast, the injection of momentum increases the fraction of cold and molecular gas by more than 50 per cent at mass resolutions of $10^3~{rm M}_odot$, and decreases its turbulent velocity dispersion by $sim 10~{rm kms}^{-1}$. The mass-loading of galactic outflows is decreased by an order of magnitude. The characteristic lifetime of the least-massive molecular clouds ($M/{rm M}_odot < 5.6 times 10^4$) is reduced from $sim 18$ Myr to $<10$ Myr, indicating that HII region feedback is effective in destroying these clouds. Conversely, the lifetimes of intermediate-mass clouds ($5.6 times 10^4 < M/{rm M}_odot < 5 times 10^5$) are elongated by $sim 7$ Myr, likely due to a reduction in supernova clustering. The derived cloud lifetimes span the range from $10$-$40$ Myr, in agreement with observations. All results are independent of whether the momentum is injected from a spherical or a blister-type HII region.
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Stellar feedback is needed to produce realistic giant molecular clouds (GMCs) and galaxies in simulations, but due to limited numerical resolution, feedback must be implemented using subgrid models. Observational work is an important means to test and anchor these models, but limited studies have assessed the relative dynamical role of multiple feedback modes, particularly at the earliest stages of expansion when HII regions are still deeply embedded. In this paper, we use multiwavelength (radio, infrared, and X-ray) data to measure the pressures associated with direct radiation ($P_{rm dir}$), dust-processed radiation ($P_{rm IR}$), photoionization heating ($P_{rm HII}$), and shock-heating from stellar winds ($P_{rm X}$) in a sample of 106 young, resolved HII regions with radii $lesssim$0.5 pc to determine how stellar feedback drives their expansion. We find that the $P_{rm IR}$ dominates in 84% of the regions and that the median $P_{rm dir}$ and $P_{rm HII}$ are smaller than the median $P_{rm IR}$ by factors of $approx 6$ and $approx 9$, respectively. Based on the radial dependences of the pressure terms, we show that HII regions transition from $P_{rm IR}$-dominated to $P_{rm HII}$-dominated at radii of $sim$3 pc. We find a median trapping factor of $f_{rm trap} sim$ 8 without any radial dependence for the sample, suggesting this value can be adopted in sub-grid feedback models. Moreover, we show that the total pressure is greater than the gravitational pressure in the majority of our sample, indicating that the feedback is sufficient to expel gas from the regions.
Context. The derived physical parameters for young HII regions are normally determined assuming the emission region to be optically thin. However, this assumption is unlikely to hold for young HII regions such as hyper-compact HII(HCHII) and ultra-compact HII(UCHII) regions and leads to the underestimation of their properties. This can be overcome by fitting the SEDs over a wide range of radio frequencies. Aims. The two primary goals of this study are (1) to determine the physical properties of young HII regions from radio SEDs in the search for potential HCHII regions, and (2) to use these physical properties to investigate their evolution. Method. We used the Karl G. Jansky Very Large Array (VLA) to observe the X-band and K-band with angular resolutions of ~1.7 and ~0.7, respectively, toward 114 HII regions with rising-spectra between 1-5 GHz. We complement our observations with VLA archival data and construct SEDs in the range of 1-26 GHz and model them assuming an ionization-bounded HII region with uniform density. Results. Our sample has a mean electron density of ne=1.6E4cm^{-3}, diameter diam=0.14pc, and emission measure EM = 1.9E7pc*cm^{-6}. We identify 16 HCHII region candidates and 8 intermediate objects between the classes of HCHII and UCHII regions. The ne, diam, and EM change as expected, but the Lyman continuum flux is relatively constant over time. We find that about 67% of Lyman-continuum photons are absorbed by dust within these HII regions and the dust absorption fraction tends to be more significant for more compact and younger HII regions. Conclusion. Young HII regions are commonly located in dusty clumps; HCHII regions and intermediate objects are often associated with various masers, outflows, broad radio recombination lines, and extended green objects, and the accretion at the two stages tends to be quickly reduced or halted.
We have observed 10 interacting galaxy pairs using the Fabry-Perot interferometer GH$alpha$FaS (Galaxy H$alpha$ Fabry-Perot system) on the $4.2rm{m}$ William Herschel Telescope (WHT) at the Observatorio del Roque de los Muchachos, La Palma. We present here the H$alpha$ surface brightness, velocity and velocity dispersion maps for the 10 systems we have not previously observed using this technique, as well as the physical properties (sizes, H$alpha$ luminosities and velocity dispersion) of 1259 HII regions from the full sample. We also derive the physical properties of 1054 HII regions in a sample of 28 isolated galaxies observed with the same instrument in order to compare the two populations of HII regions. We find a population of the brightest HII regions for which the scaling relations, for example the relation between the H$alpha$ luminosity and the radius, are clearly distinct from the relations for the regions of lower luminosity. The regions in this bright population are more frequent in the interacting galaxies. We find that the turbulence, and also the star formation rate, are enhanced in the HII regions in the interacting galaxies. We have also extracted the H$alpha$ equivalent widths for the HII regions of both samples, and we have found that the distribution of HII region ages coincides for the two samples of galaxies. We suggest that the SFR enhancement is brought about by gas flows induced by the interactions, which give rise to gravitationally bound gas clouds which grow further by accretion from the flowing gas, producing conditions favourable to star formation.
Throughout the Hubble time, gas makes its way from the intergalactic medium into galaxies fuelling their star formation and promoting their growth. One of the key properties of the accreting gas is its angular momentum, which has profound implications for the evolution of, in particular, disc galaxies. Here, we discuss how to infer the angular momentum of the accreting gas using observations of present-day galaxy discs. We first summarize evidence for ongoing inside-out growth of star forming discs. We then focus on the chemistry of the discs and show how the observed metallicity gradients can be explained if gas accretes onto a disc rotating with a velocity 20-30% lower than the local circular speed. We also show that these gradients are incompatible with accretion occurring at the edge of the discs and flowing radially inward. Finally, we investigate gas accretion from a hot corona with a cosmological angular momentum distribution and describe how simple models of rotating coronae guarantee the inside-out growth of disc galaxies.
Many more supernova remnants (SNRs) are now known in external galaxies than in the Milky Way. Most of these SNRs have been identified using narrow-band imaging, separating SNRs from HII regions on the basis of [SII]:H-alpha ratios that are elevated compared to HII regions. However, the boundary between SNRs and HII regions is not always distinct, especially at low surface brightness. Here we explore velocity structure as a possible criterion for separating SNRs from HII regions, using a sample of well-studied SNRs in the Large Magellanic Cloud (LMC) as well as a small number of SNRs in the galaxy M83. We find, perhaps not surprisingly, that even at large diameters, SNRs exhibit velocity broadening sufficient to readily distinguish them from HII regions. We thus suggest that the purity of most extragalactic samples would be greatly improved through spectroscopic observations with a velocity resolution of order 50~km/s$.
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