Almudena Zurita

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.

Fabry-Perot observations of the ionized gas in the spiral galaxy NGC 6951

We present two dimensional kinematic observations at high angular and velocity resolution measured via the H emission line in the active barred spiral NGC 6951, obtained using the TAURUS II Fabry-Perot system on the 4.2 m William Herschel Telescope. From the radial velocity map we produced the rotation curve, which has a rapidly rising inner portion, and then remains remarkably flat to large radii. Subtracting a two-dimensional projected model of this rotation from the observed map yields a residual velocity map which shows signicant non-circular motions, above all in the circumnuclear zone, where they reach projected values of 45 km s 1 .T he kinematic and morphological properties of this zone, a narrow well-dened annular region, point to inflow of gas resulting from perturbation in the stream-lined gas flow due to the bar, as predicted in dynamical models. The overall gas kinematics leads us to infer the presence of an inner disc within the main galactic disc, whose presence is marked by two inner Lindblad resonances at 180 pc and 900 pc from the centre respectively. Our map of the H velocity dispersion shows characteristic values of 20 km s 1 for the Hii regions, and an obvious correlation between higher values and the brightest Hii regions. The value rises to >100 km s 1 as the nucleus is approached.

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

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

{\text{H}}_\alpha

The ionized intergalactic gas: baryonic dark matter.

∼ 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.

A comprehensive observational study of the young barred galaxy NGC 3359

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.

Global properties of the population of Hii regions in NGC 7479 from photometric Hα imaging

We propose the use of the H II region luminosity function (LF) as a refined powerful standard candle for extragalactic distances. We have found evidence of a change in the properties of H II regions in spirals at a fixed luminosity in Hα which we term the Stromgren luminosity (LHα = 3.39 1038 erg s-1). The key piece of evidence is the presence in the Hα H II region LF of a change in gradient and a local sharp peak. The invariance and high luminosity of the change measured in the LF was proposed in previous studies as a possible secondary standard candle, but it is calibrated here for the first time using the LFs of NGC 925, NGC 4535 and M100, whose Cepheid distances have been determined in the HST Extragalactic Distance Scale Key Project. Using this method we derive a value of H 0 of 73±7 km s-1 Mpc-1.

The ionized gas in the spiral galaxy NGC 3359 - II. Kinematics

The diffuse Hα emitted from the discs of spirals is of order 50% of their total Hα emission (Ferguson et al., 1996; Zurita et al., 2000), requiring strong energy input. Energy of stellar winds or SNe falls short by two orders of magnitude, white dwarf Lyc by one order. Sciama’s (e.g. 1990) decaying neutrino theory offers an explanation, but clear evidence against it exists (Bowyer et al., 1999). OB stars are probable candidates, but are embedded in H II regions which absorb their Lyc flux; any escaping flux must travel up to kpc in H I to ionize the diffuse gas (DIG). We have evidence that a major fraction of the Lyc escapes, and have modelled its propagation through the DIG.