Internal motions of HII regions and giant HII regions

We report new echelle observations of the kinematics of 30 HII regions in the LMC, including the 30 Doradus giant HII region. All of the HII regions possess supersonic velocity dispersions, which can be attributed to a combination of turbulent motions and discrete velocity splitting produced by stellar winds and/or embedded supernova remnants (SNRs). The core of 30 Dor is unique, with a complex velocity structure that parallels its chaotic optical morphology. We use our calibrated echelle data to measure the physical properties and energetic requirements of these velocity structures. The most spectacular structures in 30 Dor are several fast expanding shells, which appear to be produced at least partially by SNRs.     

Turbulent mixing in wind-blown HII regions

We describe the process of turbulent mixing, occuring at the interface between the stellar wind bubble and the overlying, compressed HII layer. It is shown that hot electrons from the bubble can penetrate into the mixing region, where they deposit some fraction of their thermal energy. This first leads to an expansion of the gas at constant pressure, which soon becomes too inefficient to carry away the additional energy. Therefore pressure can build up in the mixing layer, forcing the gas to expand into the bubble. We also discuss the effect of the over-pressure on the HII shell and find that it is possible to punch a hole at the polar caps, where mixing is most efficient. This may lead to the formation of bipolar outflows. We apply our model to the wind-blown HII region NGC 6334(A), and show that it can explain the observed distortion of the bubble and the bipolar ionization cones.     

Chemical abundances and ionizing clusters of H ii regions in the LINER galaxy NGC 4258

We present long-slit observations in the optical and near-infrared of eight H  ii regions in the spiral galaxy NGC 4258. Six of the observed regions are located in the south-east inner spiral arms, and the other two are isolated in the northern outer arms. A detailed analysis of the physical conditions of the gas has been performed. For two of the regions, an electron temperature has been derived from the [S  iii ] λ 6312 line. For the rest, an empirical calibration based on the red and near-infrared sulphur lines has been used. The oxygen abundances derived by both methods are found to be significantly lower (by a factor of 2) than previously derived by using empirical calibrations based on the optical oxygen lines.
In the brightest region, 74C, the observation of a prominent feature caused by Wolf–Rayet (WR) stars provides an excellent constraint over some properties of the ionizing clusters. In the light of the current evolutionary synthesis models, no consistent solution is found to explain at the same time both the WR feature characteristics and the emission-line spectrum of this region. In principle, the presence of WR stars could lead to large temperature fluctuations and also to a hardening of the ionizing radiation. None of these effects is found in region 74C, for which the electron temperatures found from the [S  iii ] λ 6312 line and the Paschen discontinuity at 8200 Å are equal within the errors, and the effective temperature of the ionizing radiation is estimated at around 35 300 K.
Both more observations of confirmed high-metallicity regions and a finer metallicity grid for the evolutionary synthesis models are needed in order to understand the ionizing populations of H  ii regions.     

Supersonic turbulence in giant extragalactic HII regions

I discuss in this paper the more likely physical mechanisms that could provide the energy input for the supersonic motions observed in giant extragalactic HII regions, together with preliminary results of an ongoing observational program that aims to study, with with high spatial and spectral resolution, the kinematics of the ionized gas of the regions.     

Emission stratification in two-component HII regions with stellar wind

To explain the variety of observed optical emission stratification in the shells around Wolf-Rayet stars, we have calculated the nonstationary cooling of a homogeneous gas layer heated to a temperature (0.4–2) × 10⁵ K. We have assumed that the nebula is ionized by its central star and consists of a rarefied gas and a set of clouds with different densities through which adiabatic shock waves produced by the stellar wind propagate. Based on this model, we have determined the sequence in which the emission in Hα and in nebular oxygen lines appears. The Hα emission attributable to the electron-collision excitation of hydrogen atoms is produced earliest on the periphery of nebulae, the [O III] line emission follows next, and, finally, the Hα recombination emission is produced. The results obtained are in good agreement with the observational data.     

Photometry of young starlike objects in the dwarf galaxy NGC 2976— age and metallicity

We present BVI_c photometry of the brightest stars andcompact star clusters in NGC 2976, a dwarf galaxy in the interacting M81/M82 group. Deep CCD images of the galaxy were obtained with the 6m‐Telescope of the Special Astrophysical Observatory (Russia) at arcsec resolution. About 290 young stars and concentrated young clusters were measured. Supplementary data in the ultraviolet are taken from the literature. The extinction to the measured objects is comparatively low, E(B — V) ∼ 0.15 .. 0.20 mag. We estimate the ages of youngest resolved stars and concentrated star clusters to be ∼5 · 10⁶ years. This population is concentrated in a broad stripe facing M81. In the central disk the population is a bit older, about 8 · 10⁶ years, this may be a hint to an outward spreading star formation process. The metallicity of the disk population is estimatedas solar (z ∼ 0.02) from a fitting to Padova theoretical stellar isochrones.     

GMOS spectroscopy of the S0 galaxy NGC 3115

We present Gemini Multi-Object Spectrograph longslit spectroscopy of the isolated S0 galaxy NGC 3115. We have determined kinematical data and Lick/IDS absorption line-strength indices for the major axis out to around 9 kpc and for the minor axis out to around 5 kpc (around 2R _e ). Using stellar population models which include the effects of variable [α/Fe] ratios, we derive metallicities, abundance ratios and ages for the stellar population of NGC 3115. We find that [α/Fe] remains fairly constant with increasing radius at around  [α/Fe]= 0.17  for the major axis but increases rapidly for the minor axis to around  [α/Fe]= 0.3  . We also find that to first order, this behaviour can be explained by a simple spheroid + disc model, where the spheroid has  [α/Fe]= 0.3  and the disc shows close to solar abundance ratios. The disc also appears considerably younger than the spheroid, having an age of around 6 Gyr compared to 12 Gyr for the spheroid. We compare these results to those previously presented for the globular cluster system of NGC 3115.     

The stellar content of a spiral arm of NGC 4321

The effects of the passage of a spiral arm through the disc of the giant Virgo Sc galaxy NGC 4321 are investigated with Hubble Space Telescope WFPC2 images in two colours. Concentrating on a portion of the southern spiral arm of NGC 4321, we have applied a new program to solve for the star formation histories in the arm and interarm regions separately. The observational uncertainties and the variable crowding across the spiral arm are taken into account using the results of artificial star tests. In the interarm regions the data are consistent with a constant star formation rate for the last 50 Myr while the stars in the arm region show a star formation rate four times larger than in the interarm regions in the last 5 Myr.     

Observational appearance of circumstellar ionized regions and ultracompact HII regions

Im Rahmen eines entwickelten theoretischen Modells, das die Dynamik von Protosternhüllen während der Entstehung massereicher Sterne dar-stellt, werden die Hauptentwicklungsstufen von HII-Gebieten bestimmt. Es sollen charakteristische Besonderheiten der Entwicklung von HII-Gebieten um entstehende Sterne mit Leuchtkräften von und besprochen werden.     

Star formation rates and the kinematics of gas in the spiral arms of NGC 628

The relations between star formation rates along the spiral arms and the velocities of gas inflow into the arms in the grand‐design galaxy NGC 628 were studied. We found that the radial distribution of the average star formation rate in individual star formation regions in regular spiral arms correlates with the velocity of gas inflow into the spiral arms. Both distributions have maxima at a galactocentric distance of 4.5–5 kpc. There are no correlations between the radial distributions of the average star formation rate in star formation regions in spiral arms and outside spiral arms in the main disc. We also did not find a correlation between the radial distribution of the average star formation rate in star formation regions in spiral arms and the H I column density. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Gas motions in the plane of the spiral galaxy NGC 3631

The velocity field of the nearly face-on galaxy NGC 3631, derived from observations in the H α line and H  i radio line, is analysed to study perturbations related to the spiral structure of the galaxy. We confirm our previous conclusion that the line-of-sight velocity field gives evidence of the wave nature of the observed two-armed spiral structure. Fourier analysis of the observed velocity field is used to determine the location of corotation of the spiral structure of this galaxy, and the radius of corotation R _c is found to be about 42 arcsec, or 3.2 kpc. The vector velocity field of the gas in the plane of the disc is restored, and, taking into account that we previously investigated vertical motions, we now have a full three-dimensional gaseous velocity field of the galaxy. We show clear evidence of the existence of two anticyclonic and four cyclonic vortices near corotation in a frame of reference rotating with the spiral pattern. The centres of the anticyclones lie between the observed spiral arms. The cyclones lie close to the observed spirals, but their centres are shifted from the maxima in brightness.     

Photometry of the low-luminosity spiral galaxy NGC 4136

Multicolor BVRI surface photometry of the low-luminosity (M _V ≈?18^m) spiral galaxy NGC 4136 is presented. The photometric parameters of its components and the color distribution over the galactic disk are estimated. The color indices and the corresponding effective ages are determined for the brightest star-forming regions. The disk-to-dark halo mass ratio is derived from the measured rotation curve of the galaxy. The disk mass dominates within the optical boundaries of the galaxy, so its disk can be considered as a self-gravitating system.     

Magnetic and optical spiral arms in the galaxy NGC 6946

The spiral pattern in the nearby spiral galaxy NGC 6946 has been studied using the wavelet transformation technique, applied to galaxy images in polarized and total non-thermal radio emission at λλ 3.5 and 6.2 cm, in broadband red light, in the λ 21.1 cm H  i line and in the optical Hα line. Well-defined, continuous spiral arms are visible in polarized radio emission and red light, where we can isolate a multi-armed pattern in the range of galactocentric distances 1.5–12 kpc, consisting of four long arms and one short spiral segment. The 'magnetic arms' (visible in polarized radio emission) are localized almost precisely between the optical arms. Each magnetic arm is similar in length and pitch angle to the preceding optical arm (in the sense of galactic rotation) and can be regarded as its phase-shifted image. Even details like a bifurcation of an optical arm have their phase-shifted counterparts in the magnetic arms. The average relative amplitude of the optical spiral arms (the stellar density excess over the azimuthal average) grows with galactocentric radius up to 0.3–0.7 at r ≃5 kpc, decreases by a factor of two at r =5–6 kpc and remains low at 0.2–0.3 in the outer parts of the galaxy. By contrast, the magnetic arms have a constant average relative amplitude (the excess in the regular magnetic field strength over the azimuthal average) of 0.3–0.6 in a wide radial range r =1.5–12 kpc. We briefly discuss implications of our findings for theories of galactic magnetic fields.     

The spectral investigation of seven HII regions in Kazarian galaxies

Spectra from SDSS DR5 are used for a spectral study of seven HII regions in Kazarian galaxies. The abundances of helium and heavy elements, and also the quantity of ionizing stars and the star formation rate in these galaxies, are determined. The oxygen abundance, 12+log(O/H) lies between 7.94 and 8.35. The mean abundance ratios log(S/O), log(Ar/O), and log(Ne/O) are equal to −1.63, −2.37, and −0.78, respectively. For these HII regions, log(N/O) lies between −0.63 and −1.37. On an N/O-O/H diagram they occupy the same area as high excitation HII regions. In all likelihood the ages of the HII regions studied here exceed the 100–300 million years, required for the enrichment in nitrogen by intermediate-mass stars.. The star formation rate is one order as in the HII regions of spiral and irregular galaxies and ranges from 0.05 ÷ 0.81 M_⊙. year⁻¹. __________ Translated from Astrofizika, Vol. 51, No. 1, pp. 75–87 (February 2008).