The Isaac Newton Telescope (INT) Photometric Hα Survey of the Northern Galactic Plane (IPHAS) is a 1800‐deg2 CCD survey of the northern Milky Way spanning the latitude range −5° < b < + 5° and reaching down to r′≃ 20 (10σ). Representative observations and an assessment of point‐source data from IPHAS, now underway, are presented. The data obtained are Wide Field Camera images in the Hα narrow‐band, and Sloan r′ and i′ broad‐band filters. We simulate IPHAS (r′−Hα, r′−i′) point‐source colours using a spectrophotometric library of stellar spectra and available filter transmission profiles: this defines the expected colour properties of (i) solar metallicity stars, without Hα emission, and (ii) emission‐line stars. Comparisons with observations of fields in Aquila show that the simulations of normal star colours reproduce the observations well for all spectral types earlier than M. A further comparison between colours synthesized from long‐slit flux‐calibrated spectra and IPHAS photometry for six objects in a Taurus field confirms the reliability of the pipeline calibration. Spectroscopic follow‐up of a field in Cepheus shows that sources lying above the main stellar locus in the (r′− Hα, r′−i′) plane are confirmed to be emission‐line objects with very few failures. In this same field, examples of Hα deficit objects (a white dwarf and a carbon star) are shown to be readily distinguished by their IPHAS colours. The role IPHAS can play in studies of spatially resolved northern Galactic nebulae is discussed briefly and illustrated by a continuum‐subtracted mosaic image of Shajn 147 (a supernova remnant, 3° in diameter). The final catalogue of IPHAS point sources will contain photometry on about 80 million objects. Used on its own, or in combination with near‐infrared photometric catalogues, IPHAS is a major resource for the study of stellar populations making up the disc of the Milky Way. The eventual yield of new northern emission‐line objects from IPHAS is likely to be an order of magnitude increase on the number already known.

This article reviews observations and models of the diffuse ionized gas that permeates the disk and halo of our Galaxy and others. It was inspired by a series of invited talks presented during an afternoon scientific session of the 65th birthday celebration for Professor Carl Heiles held at Arecibo Observatory in August 2004. This review is in recognition of Carl's long standing interest in and advocacy for studies of the ionized as well as the neutral components of the interstellar medium.Comment: 29 pages, 19 figures; accepted by Reviews of Modern Physic

We have obtained multislit spectroscopic observations from 3700 to 9200 Å with Low Resolution Imaging Spectrometer at the Keck I telescope for 31 H II regions in the disc of the Andromeda galaxy (M31), spanning a range in galactocentric distance from 3.9 to 16.1 kpc. In nine H II regions we measure one or several auroral lines ([O III] [O II] λ7325) from which we determine the electron temperature (T e ) of the gas and derive chemical abundances using the direct T e -based method. We analyse, for the first time in M31, abundance trends with galactocentric radius from the direct method, and find that the Ne/O, Ar/O, N/O and S/O abundance ratios are consistent with a constant value across the M31 disc, while the O/H abundance ratio shows a weak gradient. We have combined our data with all spectroscopic observations of H II regions in M31 available in the literature, yielding a sample of 85 H II regions spanning distances from 3.9 to 24.7 kpc (0.19-1.2 R 25 ) from the galaxy centre. We have tested a number of empirical calibrations of strong emission line ratios. We find that the slope of the oxygen abundance gradient in M31 is −0.023 ± 0.002 dex kpc −1 , and that the central oxygen abundance is in the range 12+log(O/H) 8.71-8.91 dex (i.e. between 1.05 and 1.66 times the solar value, for 12+log(O/H) = 8.69), depending on the calibration adopted. The H II region oxygen abundances are compared with the results from other metallicity indicators (supergiant stars and planetary nebulae). The comparison shows that H II region O/H abundances are systematically ∼0.3 dex below the stellar ones. This discrepancy is discussed in terms of oxygen depletion on to dust grains and possible biases affecting T e -based oxygen abundances at high metallicity.

Aims. This is the first paper of a series that aims to understand the formation and evolution of bars in early-type spirals and their influence in the evolution of the galaxy. Methods. Optical long-slit spectra along the bar major-axis of a sample of 20 galaxies are analyzed. Velocity and velocity dispersion profiles along the bar are presented. Line-strength indices in the bar region are also measured to derive stellar mean-age and metallicity distributions along the bars using stellar population models. Results. We obtain mean ages, metallicities and chemical abundances along the bar of 20 galaxies with morphological types from SB0 to SBbc. The main result is that we find a large variation in age and metallicity along the bar in 45% of our sample. We find three different types of bars according to their metallicity and age distribution along the radius: 1) Bars with negative metallicity gradients. They show a mean young/intermediate population (<2 Gyr), and have amongst the lowest stellar maximum central velocity dispersion of the sample; 2) bars with null metallicity gradients. These galaxies do not show any gradient in their metallicity distribution along the bar and have negative age gradients (i.e. younger populations at the bar end); 3) bars with positive metallicity gradients, i.e. more metal rich at the bar ends. These galaxies are predominantly those with higher velocity dispersion and an older mean population. We found no significant correlation between the age and metallicity distribution, and bar/galaxy parameters such as the AGN presence, size or the bar strength. From the kinematics, we find that all the galaxies show a disk-like central component. Conclusions.The results from the metallicity and age gradients indicate that most galaxies with high central stellar velocity dispersion host bars that could have been formed more than 3 Gyr ago, while galaxies with lower central velocity dispersions show a wider distribution in their population and age gradients. A few bars show characteristics compatible with having been formed less than <2 Gyr ago. However, we do not have a definite answer to explain the observed gradients and these results place strong constrains on models of bar formation and evolution. The distribution of mean stellar population parameters in the bar with respect to σ is similar to that found in bulges, indicating a close link in the evolution of both components. The disk-like central components also show the important role played by bars in the secular evolution of the central structure.

Abstract. In this paper we make a quantitative study of the hypothesis that the diffuse Hα emitted from the discs of spiral galaxies owes its origin to the ionizing photons escaping from H ii regions. The basis of the models is the assumption that a fraction of the Lyman continuum (Lyc) luminosity from the OB stars within each H ii region escapes from the region, leaking into the diffuse gas. A basic input element of any such model is a position and luminosity catalogue in Hα of the H ii regions in the galaxy under examination, down to a low limiting luminosity, and we have previously produced a catalogue of this type for NGC 157. An initial family of models can then be generated in which the Lyc escaping from an H ii region is parametrized in terms of the observed Hα luminosity of the region and the escaping fluxes allowed through the diffuse disc gas. These models can then be refined using a measured map of H i surface density to effect the down-conversion of the Lyc to Hα. For NGC 157 an H i map was available. Although its moderate angular resolution did limit the accuracy with which we could test our models, the predicted diffuse Hα surface brightness distributions from our models were compared with the observed distributions showing that, in general terms, the hypothesis of density bounding for the H ii regions allows us to predict well the spatial distribution of the diffuse ionized gas. In the model yielding the best fit to the data, the regions of lower luminosity lose a constant fraction of their ionizing flux to their surroundings, while for H ii region luminosities above a specific transition value the ionizing escape fraction is a rising function of the Hα luminosity.