Maite Rozas

POPULATIONS OF HIGH-LUMINOSITY DENSITY-BOUNDED H II REGIONS IN SPIRAL GALAXIES: EVIDENCE AND IMPLICATIONS

In this paper we present evidence that the H II regions of high luminosity in disk galaxies may be density bounded, so that a significant fraction of the ionizing photons emitted by their exciting OB stars escape from the regions. The key piece of evidence is the presence, in the Ha luminosity functions (LFs) of the populations of H iI regions, of glitches, local sharp peaks at an apparently invariant luminosity, defined as the Stromgren luminosity Lstr), LH(sub alpha) = Lstr = 10(sup 38.6) (+/- 10(sup 0.1)) erg/ s (no other peaks are found in any of the LFs) accompanying a steepening of slope for LH(sub alpha) greater than Lstr This behavior is readily explicable via a physical model whose basic premises are: (a) the transition at LH(sub alpha) = Lstr marks a change from essentially ionization bounding at low luminosities to density bounding at higher values, (b) for this to occur the law relating stellar mass in massive star-forming clouds to the mass of the placental cloud must be such that the ionizing photon flux produced within the cloud is a function which rises more steeply than the mass of the cloud. Supporting evidence for the hypothesis of this transition is also presented: measurements of the central surface brightnesses of H II regions for LH(sub alpha) less than Lstr are proportional to L(sup 1/3, sub H(sub alpha)), expected for ionization bounding, but show a sharp trend to a steeper dependence for LH(sub alpha) greater than Lstr, and the observed relation between the internal turbulence velocity parameter, sigma, and the luminosity, L, at high luminosities, can be well explained if these regions are density bounded. If confirmed, the density-bounding hypothesis would have a number of interesting implications. It would imply that the density-bounded regions were the main sources of the photons which ionize the diffuse gas in disk galaxies. Our estimates, based on the hypothesis, indicate that these regions emit sufficient Lyman continuum not only to ionize the diffuse medium, but to cause a typical spiral to emit significant ionizing flux into the intergalactic medium. The low scatter observed in Lstr, less than 0.1 mag rms in the still quite small sample measured to date, is an invitation to widen the data base, and to calibrate against primary standards, with the aim of obtaining a precise, approx. 10(exp 5) solar luminosity widely distributed standard candle.

Kinematics of Ionized and Molecular Hydrogen in the Core of M100

We present high angular and velocity resolution two-dimensional kinematic observations in the spectral lines of Hα and CO J = 1 → 0 of the circumnuclear starburst region in the barred spiral galaxy M100, and compare them with kinematics derived from our previously published numerical modeling. The Hα data, fully sampled and at subarcsecond resolution, show a rotation curve that is rapidly rising in the central ~140 pc, and stays roughly constant, at the main disk value, further out. Noncircular motions are studied from the Hα and CO data by detailed consideration of the velocity fields, residual velocity fields after subtraction of the rotation curve, and sets of position-velocity diagrams. These motions are interpreted as the kinematic signatures of gas streaming along the inner part of the bar, and of density wave streaming motions across a two-armed minispiral. Comparison with a two-dimensional velocity field and rotation curve derived from our 1995 dynamical model shows good qualitative and quantitative agreement for the circular and noncircular kinematic components. Both morphology and kinematics of this region require the presence of a double inner Lindblad resonance in order to explain the observed twisting of the near-infrared isophotes and the gas velocity field. These are compatible with the presence of a global density wave driven by the moderately strong stellar bar in this galaxy. We review recent observational and modeling results on the circumnuclear region in M100, and discuss the implications for bar structure and gas dynamics in the core of M100 and other disk galaxies.

NGC 3521: Stellar Counter-Rotation Induced by a Bar Component

The spiral galaxy NGC 3521 exhibits apparently normal kinematic properties of gas and stars along its major axis. However, the analysis of the LOSVD reveals strong asymmetries. A decomposition of the LOSVD data with a two-Gaussian component model shows two counter-rotating stellar components. The observed kinematic decoupling is interpreted as a projection effect induced by the presence of a bar component seen almost end on. The bar produces locally a greater concentration of retrograde stellar orbits but this does not relate to a specific counter-rotating population. The signatures of the bar are identified in the velocity field derived from long-slit spectra obtained along the major, minor and 45° intermediate axes and from R-band surface photometry.

Density Bounding of Giant HII Regions and the Ionisation of the Diffuse Interstellar Medium

Three different types of evidence are presented in favour of the hypothesis that the HII regions in disk galaxies with Halpha luminosities greater than a critical value of 10^38.6 erg s^-1 are density-bounded, and that the escaping Lyman continuum photons from these are the principal ionising agents for the diffuse ISM in disk galaxies. This has important implications for the ionisation of the intergalactic medium, and for computed star formation rates in spirals.

The Hα Emission of the Spiral Galaxy NGC 7479

AbstractWe use the catalogue of HII regions obtained from a high quality continuum-subtracted Hα image of the grand-design spiral galaxy NGC 7479, to construct the luminosity function (LF) for the HII regions(over 1000) of the whole galaxy. Although its slope is within the published range for spirals of the same morphological type, the unusually strong star formation along the intense bar of NGC 7479 prompted us to analyse separately the HII regions in the bar and in the disc. We have calculated the physical properties of a group of HII regions in the bar and in the disc selected for their regular shapes and absence of blending. We have obtained galaxy-wide relations for the HII region set: diameter distribution function and also the global Hα surface density distribution. As found previously for late-type spirals, the disc LF shows clear double-linear behaviour with a break at log L

The Internal Kinematics of Complete Sets of HII Regions in Disc Galaxies

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SABIO: new system for telescope full scheduling and queue operation to be implemented at the Observatorio de El Teide

∼ 38.6 (in erg -1). The bar LF is less regular. This reflects a physical difference between the bar and the disc in the properties of their populations of regions.

Hα Luminosity Function Of H II Regions: Preliminary Calibration of a Powerful Standard Candle

Using high-quality Hα images of five spiral galaxies, we have studied the luminosity and distribution of the emission from diffuse ionized gas (DIG). The estimated DIG luminosities account for 25–60%of the total Hα emission in each galaxy and analysis of the distribution has shown that the DIG is highly correlated geometrically with the most luminous HII regions of the galaxies. The power required to ionize the DIG is very high. The mean ionization rates per unit surface area of a galaxy disc are of the order of 107 cm-2 s-1. Lyman continuum photons (Lyc) from OB asociations are the most probable sources of this ionization. Here we propose a specific model for these sources: we show that the Lyman photon flux that leaks out of the density-bounded HII regions of the galaxies is more than enough to ionize the measured DIG in the five galaxies analysed.