HII Regions In Galaxies Research Articles

Spectral Study of Some SBS Galaxies. Physical Conditions. Abundance of Oxygen and Nitrogen

Physical conditions and the oxygen and nitrogen abundances in 36 SBS galaxies with UV excesses and/or without UV excesses, but with emission lines, are determined using spectra from the SDSS DR7. It is found that SBS 0808+578 is an AGN, while the other objects are either HII galaxies or HII regions in galaxies. For all these objects the oxygen abundance 12+log(O/H) lies within a range of 7.85–8.61, and the ratio log(N/O) lies within a range of -1.45 to -0.4. On an N/O-O/H diagram they occupy the same region as high-excitation HII regions. No galaxies with a high metal deficit were found. The star formation rates (SFR) of these galaxies were determined from their Ha fluxes and found to lie within a range of 0.001÷6 M⨀ year-1, close to the values for typical star formation regions in spiral and irregular galaxies.

The physical conditions and oxygen and nitrogen abundance of 36 SBS galaxies from the SDSS DR7

AbstractPhysical conditions and oxygen and nitrogen abundances in 36 SBS UV-excess and/or emission-line galaxies from the SDSS DR7 were determined. We have found that SBS 0808+578 is AGN. The others are HII galaxies or HII regions in galaxies. For all objects the oxygen abundance 12+log(O/H) lies in the range of 7.85 ÷ 8.61 and log(N/O) ratio in the range of −1.45 ÷ −0.4. They occupy the same area in the diagram N/O O/H as the high-excitation HII regions. We found no extremely metal-deficient galaxy. Using H-alpha fluxes star formation rates (SFR) for our samples galaxies were determined. Determined SFRs, being in the range of 0.001 ÷ 6 o year−1, are similar of that observed in typical star forming regions in spiral and irregular galaxies.

Effects of shocks in stellar atmosphere models on the emission line spectrum of surrounding H ii regions

Emission line studies from Hii regions in galaxies require tools for the inversion of line ratios into desired physical properties. These tools generally come in form of diagnostic ratios/diagrams that are based on grids of photoionisation models. An important input to the photoionisation models is the stellar atmosphere spectrum of the ionising sources. The current omission of shocks in the calculation of the former set of models could threaten the accuracy of the physical interpretation of emission line ratios from Hii regions. Current stellar atmosphere models that are crucial inputs to the grid of photoionisation models used to generate nebular emission line diagnostic diagrams might produce significant biases due to the omission of shocks. We investigate whether a new generation of photoionisation model grids, taking shocks into account, is required to compensate for the biases. We make use of the WM-Basic stellar atmosphere code, which can account for the extra energetic emission in the stellar spectral energy distribution produced by shocks, in conjunction with the photoionisation code MOCASSIN to determine whether shocks produce significant biases in the determination of the physical parameters of the interstellar medium and/or ionising stellar parameters. We conclude that these effects are only important for stellar sources with effective temperatures lower than 30kK and in this case they yield artificially high stellar temperatures, electron temperatures and nebular ionisation parameters. The magnitude of the effect is also obviously dependent on the strength of the shock and is likely to be unimportant for the majority of stellar sources. Nevertheless, we find our 20kK and 30kK shock models to strongly enhance the He ii 4686 nebular emission line. This result is however not strong enough to explain previously observed He ii 4686 line emission in the spectra of Hii galaxies.

The electron temperatures of SDSS high-metallicity giant extragalactic H ii regions

Spectra of high-metallicity (12+log(O/H) > 8.2) HII regions where oxygen auroral lines are measurable in both the O+ and O++ zones, have been extracted from the Data Release 6 of the Sloan Digital Sky Survey (SDSS). Our final sample consists of 181 SDSS spectra of HII regions in galaxies in the redshift range from ~0.025 to ~0.17. The t_2,O-t_3,O diagram is examined. In the SDSS HII regions, the electron temperature t_2,O is found to have a large scatter at a given value of the electron temperature t_3,O. The majority of the SDSS HII regions lie below the t_2,O-t_3,O relation derived for HII regions in nearby galaxies, i.e. the positions of the SDSS HII regions show a systematic shift towards lower t_2,O temperatures or/and towards higher t_3,O temperatures. The scatter and shift of the SDSS HII regions in the t_2,O-t_3,O diagram can be understood if they are composite nebulae excited by two or more ionizing sources of different temperatures.

On the HII regions in galaxies

In the present paper we shall consider that the HII complexes observed in galaxies may be interpreted as the resulting Strömgren volume produced by a cluster or association of exciting stars. If ρH is the density of the hydrogen surrounding the cluster or association and I the number of ionizing quanta (λ < 912 å) emitted by the stars, the ionized volume becomes