Hα Surface Brightness Research Articles

A density-gradient model for the H II galaxy I ZW 18

view Abstract Citations (15) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Density-Gradient Model for the H II Galaxy I ZW 18 Campbell, Alison Abstract A model of the low-abundance H II galaxy I Zw 18 is presented in which the electron density decreases with radius, and the central density is high enough to cause partial collisional deexcitation of [O III] λ5007. The model predicts emission-line ratios F_line_/F_Hβ_ that are in good agreement those observed and simultaneously reproduce the electron temperature T_e_(O^2+^) estimated from the ratio Flambda4363_/Flambda4959,5007_, a problem unsolved by constant-density models. Under such conditions, suppression of λ5007 makes Flambda4363_/Flambda4959,5007_ an inaccurate estimator of T_e_(O^2+^), and hence of oxygen abundance, O/H. The best-fit density-gradient model has log O/H = -4.68, ~70% higher than the value inferred from the observed line ratios. The best-fit constant-density model, constructed for comparison, fits most line ratios well, but underestimates the strength of Flambda4363_ and hence produces a T_e_(O^2+^) which is 2500 K too low. Relative to that of oxygen, the nitrogen abundance of both density- gradient and constant-density models is ~50% higher than in most low- metallity galaxies, while the abundances of carbon, sulphur, and neon are approximately normal. Both models overestimate the strength of He I λ5876 for reasonable helium abundances. The Hα surface brightness--radius relation of the density- gradient model is similar to that of a constant-density model at radii greater than the limit imposed by seeing. The observed surface brightness profile changes with position angle and is somewhat different from the profiles of both models, indicating that the true density distribution of I Zw 18 is more complicated than either a constant or a simple power law with radius. The density-gradient model provides a slightly better fit to the data. I Zw l8 is one of a small class of low-abundance H II galaxies and giant H II regions whose emission-line ratios are not well fitted by a simple Stromgren sphere model; the success of the density-gradient model suggests that it may be possible to explain the properties of this small class of objects in the same way. If so, their abundances are higher than suggested by their observed Flambda4383_/Flambda4959.5007. The inclusion of such galaxies would lead to an underestimate of the true slope of the oxygen abundance---helium mass fraction relation, and hence to an overestimate of the primordial helium abundance. Publication: The Astrophysical Journal Pub Date: October 1990 DOI: 10.1086/169246 Bibcode: 1990ApJ...362..100C Keywords: Electron Density (Concentration); Electron Energy; Galactic Structure; H Ii Regions; Oxygen Ions; Astronomical Models; Density Distribution; Emission Spectra; H Alpha Line; Line Spectra; Astrophysics; GALAXIES: ABUNDANCES; GALAXIES: INDIVIDUAL ALPHANUMERIC: I ZW 18; GALAXIES: STRUCTURE; NEBULAE: H II REGIONS full text sources ADS | data products SIMBAD (5) NED (1)

The distribution of the interstellar medium in the Scd galaxy NGC 6946. III - Azimuthal variations in the molecular and atomic components

view Abstract Citations (33) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Distribution of the Interstellar Medium in the Scd Galaxy NGC 6946. III. Azimuthal Variations in the Molecular and Atomic Components Tacconi, Linda J. ; Young, Judith S. Abstract We present here a comparison of the azimuthal variations in the H_2_ and H I surface densities with those in B- and I-band images (Elmegreen and Elmegreen, private communication) and Hα fluxes (Bonnarel, Boulesteix and Marcelin) to gain insight into how the presence of a spiral potential affects the gas surface densities and the massive star formation efficiencies in NGC 6946. In regions of well-defined spiral structure in the northeast quadrant of the galaxy, we find enhancements in the H_2_ surface densities, Hα surface brightnesses, and the μ(Hα)/{simga}(H_2)_ ratio on the spiral arms. In the southwest part of the galaxy where the spiral structure is poorly defined by the optical images, enhancements in the H_2_ surface density are not as well correlated with enhancements in the Hα flux or the μ(Hα)/σ(H_2_) ratio. We interpret the Hα/H_2_ ratio as a measure of the massive star formation efficiency and infer that this efficiency is greater on the spiral arms than in the interarm regions of NGC 6946. Radial increases in the spiral arm amplitudes, as derived in the B and I bands, are evident. Correspondingly, large radial increases in the arm-interarm contrasts of the Hα surface brightness and massive star formation efficiency are also prominent, with smaller increases observed in the H_2_ surface density arm-interarm contrasts. We suggest that the massive star formation efficiency is a sensitive function of spiral arm strength. Publication: The Astrophysical Journal Pub Date: April 1990 DOI: 10.1086/168561 Bibcode: 1990ApJ...352..595T Keywords: Galactic Structure; H Alpha Line; Hydrogen Clouds; Interstellar Matter; Star Formation; Azimuth; Gas Density; Spiral Galaxies; galaxies: individual (NGC 6946) galaxies: interstellar matter; stars: formation; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 6946; GALAXIES: INTERSTELLAR MATTER; STARS: FORMATION full text sources ADS | data products SIMBAD (1) NED (1) Related Materials (2) Part 1: 1986ApJ...308..600T Part 2: 1989ApJS...71..455T

The star formation law in galactic disks

view Abstract Citations (1625) References (105) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Star Formation Law in Galactic Disks Kennicutt, Robert C., Jr. Abstract Measurements of the distribution of Hα emission in galaxies have been combined with published H I and CO data, in order to reassess the dependence of the massive star formation rate (SFR) on the density and dynamics of the interstellar gas. The disk-averaged Hα surface brightness is correlated with the mean atomic and total gas surface densities, but is only weakly correlated with the mean molecular gas density inferred from CO emission. Radial profiles of gas and Hα emission in 15 galaxies have been used to define the relationship between the SFR and gas surface density. In dense regions the SFR and total gas density are well represented by a Schmidt power-law relation (R = a{SIGMA}^N^_g_), with index N = 1.3 +/- 0.3, and nearly the same value for the zero point a, at least for the galaxies in our sample. This Schmidt law breaks down, however, at densities below a critical threshold value. Massive star formation is completely suppressed at surface densities well below the threshold, while at densities near the the threshold value the slope of the SFR-density relation is much steeper than a normal Schmidt law. The observed threshold densities vary by an order of magnitude among the galaxies in our sample and appear to be largely independent of whether the predominant gas phase is atomic or molecular. The star formation threshold appears to be associated with the onset of large-scale gravitational instabilities in the gas disks. A simple single-fluid (Toomre) disk stability model predicts threshold densities and radii which are in excellent agreement with the observations. The same analysis demonstrates that the radial distribution of gas in late-type galaxies closely follows the threshold surface density, confirming a hypothesis originally proposed by Quirk, and indicating the importance of star formation thresholds in virtually all parts of the disk. The combination of a nearly linear star formation law at high densities with a strongly nonlinear SFR-density law near the threshold redresses many of the previous observational difficulties with the Schmidt law and may provide physical insights into such diverse phenomena as nuclear and disk starbursts, the suppression of star formation in gas-rich S0 galaxies and low surface brightness galaxies, star formation in spiral arms, the observation of radial cutoffs to stellar disks, and the star formation histories of disk galaxies. Publication: The Astrophysical Journal Pub Date: September 1989 DOI: 10.1086/167834 Bibcode: 1989ApJ...344..685K Keywords: Galactic Evolution; H Alpha Line; H Ii Regions; Interstellar Gas; Star Formation Rate; Emission Spectra; Gas Dynamics; Radial Distribution; Spiral Galaxies; Astrophysics; GALAXIES: EVOLUTION; GALAXIES: STELLAR CONTENT; STARS: FORMATION full text sources ADS | data products NED (34) SIMBAD (33)

Observations of comet Halley at Hα and 6300 Å

A small aperture telescope has been used with the Fabry‐Perot interferometer at the Arecibo Observatory to make high spectral resolution measurements of Comet Halley emissions at 6562.72 Å (Hα) and 6300.30 Å (O¹D). Observations in March, 1986, are characterized by a highly structured spectral signature at Hα. The Hα spectra imply non‐isotropic outflow of hydrogen atoms following dissociation of the parent species. The Hα surface brightness for a 5′.9 field of view centered on the comet head was 59 ± 13 rayleighs(R) on March 12, 1986, decreasing to 25 ± 6R on March 23 (after correction for atmospheric extinction). Calculation of atomic hydrogen production rates and outflow velocities is not straightforward due to the non‐isotropic nature of the atomic hydrogen outflow. The O(¹D) emission at 6300.3 Å was accompanied by a feature at 6300.8 Å that we attribute to NH2. The brightness of the O(¹D) emission for a 6′ field of view was 260 ± 50 R on March 15 and 17. This brightness decreased by a factor of about 2 when the comet was viewed with a 5′.9 field of view centered 6′ off the comet nucleus (sunward or tailward). The width of the O(¹D) emission line was 3.9 ± 1.5 kms−1 on March 15, and 7.4 ± 2.2 km s−1 on March 17. The O(¹D) spectral line profiles on March 17 were skewed to the red side of line center.

H-alpha scans of the intergalactic H I cloud in Leo

A high-sensitivity search has been made for Hα emission from the large, intergalactic H I cloud discovered by Schneider et al. (1983). The authors are able to place a conservative upper limit of 1.7×10<SUP>-7</SUP>ergs cm<SUP>-2</SUP>s<SUP>-1</SUP>sr<SUP>-1</SUP> on the Hα surface brightness of the cloud, which corresponds to an emission measure through the cloud of &lt;2 cm<SUP>-6</SUP>pc. The data also provide evidence (at about the 4σ level of significance) for the actual detection of Hα emission at 7×10<SUP>-8</SUP>ergs cm<SUP>-2</SUP>s<SUP>-1</SUP>sr<SUP>-1</SUP>.

Distribution of Dust in the Orion Nebula

The distribution of the dust is found by comparing 3-cm brightness temperatures, Hα surface brightness, and Hβ surface brightness. The dust is probably mixed with the gas. The Balmer radiation originates in front of the dark bay.