Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userSpectroscopy of Outlying H II Regions in Spiral Galaxies: Abundances and Radial Gradients
135
0
0.0
(
0
)
Authors
L. van Zee
Ask ChatGPT about the research
No Arabic abstract
We present the results of low dispersion optical spectroscopy of 186 H II regions spanning a range of radius in 13 spiral galaxies. Abundances for several elements (oxygen, nitrogen, neon, sulfur, and argon) were determined for 185 of the H II regions. As expected, low metallicities were found for the outlying H II regions of these spiral galaxies. Radial abundance gradients were derived for the 11 primary galaxies; similar to results for other spiral galaxies, the derived abundance gradients are typically -0.04 to -0.07 dex/kpc.
rate research
Read More
We have compared Monte Carlo photoionization models of H II regions with a uniform density distribution with models with the same central stars and chemical compositions but with 3-D hierarchical clumps. We compare the abundances of He, N, O, Ne, and S obtained from emission line strengths and [O III] and [N II] temperatures to those in our models. We consider stellar temperatures in the range 37.5 -- 45kK and ionizing luminosities from 10^{48} to 10^{51} photons/s. Clumped models have different ionic abundances than uniform. For hot stars, He^0/He^+ is 2 -- 3%, much larger than with uniform models. This amount of He I is independent of metallicity and so impacts the determination of the primordial abundance of He. The total abundances of O, Ne, and S obtained by the usual methods of analysis, using T([OIII) for high stages of ionization and T([NII]) for low, are about as accurate for clumped models as for uniform and within about 20% of the true values. If T([OIII]) is used for analyzing all ions, the derived (O/H) is 40 to 60% too large for cool stars but is good for hot stars. Uniform models have similar errors, so the clumping does not change the accuracy of abundance analysis. The physical causes of the ionic abundance errors are present in real nebulae. In clumped models, helium ionizing radiation from zones of high ionization (low He^0 and low UV opacity) can penetrate nearby regions near the edge of the ionized zone. This effect allows He^0 to absorb more stellar photons than in uniform or radially symmetrical geometries. In turn, these absorptions compete with O+, etc., for those energetic stellar photons.
We examine the relation between breaks in the surface brightness profiles and radial abundance gradients within the optical radius in the discs of 134 spiral galaxies from the CALIFA survey. The distribution of the radial abundance (in logarithmic scale) in each galaxy was fitted by simple and broken linear relations. The surface brightness profile was fitted assuming pure and broken exponents for the disc. We find that the maximum absolute difference between the abundances in a disc given by broken and pure linear relations is less than 0.05 dex in the majority of our galaxies and exceeds the scatter in abundances for 26 out of 134 galaxies considered. The scatter in abundances around the broken linear relation is close (within a few percent) to that around the pure linear relation. The breaks in the surface brightness profiles are more prominent. The scatter around the broken exponent in a number of galaxies is lower by a factor of two or more than that around the pure exponent. The shapes of the abundance gradients and surface brightness profiles within the optical radius in a galaxy may be different. A pure exponential surface brightness profile may be accompanied by a broken abundance gradient and vise versa. There is no correlation between the break radii of the abundance gradients and surface brightness profiles. Thus, a break in the surface brightness profile does not need to be accompanied by a break in the abundance gradient.
We present observations of an H$alpha$ emitting knot in the thick disk of NGC 4013, demonstrating it is an H II region surrounding a cluster of young hot stars $z = 860$ pc above the plane of this edge-on spiral galaxy. With LBT/MODS spectroscopy we show this H II region has an H$alpha$ luminosity $sim 4$ - 7 times that of the Orion nebula, with an implied ionizing photon production rate $log Q_0 gtrsim 49.4$ (photons s$^{-1}$). HST/WFPC2 imaging reveals an associated blue continuum source with $M_{V} = -8.21pm0.24$. Together these properties demonstrate the H II region is powered by a young cluster of stars formed {em in situ} in the thick disk with an ionizing photon flux equivalent to $sim$6 O7 V stars. If we assume $approx6$ other extraplanar halpha -emitting knots are H II regions, the total thick disk star formation rate of gc 4013 is $sim 5 times 10^{-4}$ M$_odot$ yr$^{-1}$. The star formation likely occurs in the dense clouds of the interstellar thick disk seen in optical images of dust extinction and CO emission.
We present optical broad- and narrow-band imaging of a sample of a dozen barred galaxies. These images are analyzed in conjunction with our previously published near-infrared imaging of their central regions and with literature values for, e.g., bar strengths and the total star formation activity of the galaxies. We present B, I and H alpha images, and radial profiles derived from these, to infer geometric and dynamical parameters of the structural components of the galaxies, such as bar lengths, bar ellipticities, and location of star formation and dust. We find that the more centrally concentrated the H alpha emission in a galaxy is, i.e., the higher the fraction of star formation originating in the circumnuclear region, the higher the overall star formation rate, as measured from far-infrared flux ratios. Stronger bars host smaller nuclear rings, but the strength of the bar does not correlate with either the intrinsic ellipticity of the ring or the offset between the position angles of the bar and the ring. We interpret these results in comparison with modelling of gas inflow in the circumnuclear region, and show that they were theoretically expected. We confirm observationally, and for the first time, the anti-correlation predicted from theory and modelling between the degree of curvature of the bar dust lanes and the strength of the bar, where stronger bars have straighter dust lanes.
Based on the ISO spectral catalogue of compact HII regions by Peeters et al. (2001), we present a first analysis of the hydrogen recombination and atomic fine-structure lines originated in the ionized gas. The sample consists of 34 HII regions located at galactocentric distances between Rgal = 0 and 15 kpc. The SWS HI recombination lines between 2 and 8 mum are used to estimate the extinction law at these wavelengths for 14 HII regions. An extinction in the K band between 0 and $sim$ 3 mag. has been derived. The fine-structure lines of N, O, Ne, S and Ar are detected in most of the sources. Most of these elements are observed in two different ionization stages probing a range in ionization potential up to 41 eV. The ISO data, by itself or combined with radio data taken from the literature, is used to derive the elemental abundances relative to hydrogen. The present data thus allow us to describe for each source its elemental abundance, its state of ionization and to constrain the properties of the ionizing star(s).
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
