On the origin of the galactic ridge recombination lines

Radio recombination lines are known to be observable at positions along the galactic ridge which are free of discrete continuum sources. Based on the results of a recent survey of H272α lines it is shown that most of the observed galactic ridge recombination lines can be explained as emission from outer low-density envelopes of normal Hn regions. The distribution of low-density ionized gas and discrete HII regions as a function of the distance from the galactic centre is also derived.     

The Origin of the Diffuse Hα in Spirals

Using high-quality H_α 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 H_α 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. This revised version was published online in July 2006 with corrections to the Cover Date.     

Optical integral field spectroscopy of the extended line emission around six radio-loud quasars

We present integral field spectroscopy of a small sample of radio-loud quasars at intermediate redshift (0.26< z <0.60), most of which are associated with large radio sources. All have oxygen line emission extended over tens of kpc, and these nebulae display a diverse range in both morphology and kinematic behaviour. Two quasars show 'plumes' of extended line emission, two show a clumpy structure and a further one shows a smooth distribution. There is no clear pattern with regard to the distribution of the ionized gas in relation to the radio source axis; the extended emission-line regions are found both parallel and perpendicular – and also totally unrelated to – the radio axis. The velocity structure of the ionized gas ranges from essentially static to apparent smooth rotation, and in two cases shows a clear association with the radio source. Given the disparity in properties, the nebulae all show a surprisingly similar ionization state, as measured by the extended lines of [O  ii ] λ 3727 and [O  iii ] λ 5007. Assuming the gas is ionized by at least the nearby quasar nucleus, we use the emission line ratios to infer a pressure in the ionized gas; in all cases we find it to be at high pressure, suggesting confinement by an external (probably intracluster) medium.     

GMRT and VLA observations at 49 cm and 20 cm of the HII region near <Emphasis Type="Italic">l</Emphasis> = 24.8°, <Emphasis Type="Italic">b</Emphasis> = 0.1°

We report multi-frequency radio continuum and hydrogen radio recombination line observations of HII regions near l = 24.8°, b = 0.1° using the Giant Metrewave Radio Telescope (GMRT) at 1.28 GHz (n = 172), 0.61 GHz (n = 220) and the Very Large Array (VLA) at 1.42 GHz (n = 166). The region consists of a large number of resolved HII regions and a few compact HII regions as seen in our continuum maps, many of which have associated infrared (IR) point sources. The largest HII region at l = 24.83° and b = 0.1° is a few arcmins in size and has a shell-type morphology. It is a massive HII region enclosing ∼550 M_⊙ with a linear size of 7 pc and an rms electron density of ∼110 cm⁻³ at a kinematic distance of 6 kpc. The required ionization can be provided by a single star of spectral type O5.5. We also report detection of hydrogen recombination lines from the HII region at l = 24.83° and b = 0.1° at all observed frequencies near V _lsr = 100 km s⁻¹. We model the observed integrated line flux density as arising in the diffuse HII region and find that the best fitting model has an electron density comparable to that derived from the continuum. We also report detection of hydrogen recombination lines from two other HII regions in the field.     

Physical parameters of the Orion Bar photodissociation region from radio recombination line observations at 8 mm

Observations of carbon (C), hydrogen and helium (H, He) radio recombination lines (RRLs) at four positions in the Orion Bar photodissociation region (PDR) and toward the center of Orion A have been performed with the RT-22 radio telescope (Pushchino) at 8 mm. The physical parameters of the PDR at these points have been estimated by comparing the carbon RRLs and infrared CII and OI lines. A hydrogen number density in the range 1.2–3.1 × 10⁵ cm^?3 and a mean size of the region along the line of sight (L) in the range 0.006–0.04 pc have been derived. The PDR temperature decreases with increasing distance from the exciting star (θ ¹ C Ori) from 210–230 to 140–150 K (a distance of ≈5′). The data obtained confirm the increase in the PDR size along the line of sight toward the Orion Bar, where, however, L has turned out to be less than the available values in the literature, which can be explained by the presence of clumps in the PDR. A density jump is evident in the Orion Bar region. The PDR zone encompasses the core of the HII region by a thin layer and extends farther, delineating the boundary and the ionization front of the core of the HII region in the Orion Bar and further out the boundary between the halo of the HII region and the molecular cloud. The derived emission measure (EM) toward the Orion Bar has been compared with other C RRL observations. The EM measured from carbon RRLs is EM ≈ 100(±50%) pc cm^?6, imposing constraints on the possible two-component PDR structure. Estimates show that the star θ ¹ C Ori is quite sufficient as a carbon ionization source in the Orion Bar PDR. Some of the data on the ionized hot gas (HII) in this direction have been obtained from H and He RRLs. In particular, the radial velocities (V _lsr) of the HII region are blueshifted with respect to V _lsr of the PDR by 10–17 km s^?1, while the relative ionized helium abundance decreases with increasing distance from the star, indicating that the helium ionization zone is smaller than the ionized hydrogen one.     

The kinematics of the ionized gas in the Circinus galaxy

The TAURUS-2 Fabry–Perot interferometer, mounted on the 3.9-m Anglo-Australian Telescope, has been used to observe the Circinus galaxy. We have mapped the intensity and velocity distribution of the ionized hydrogen in the galaxy using the Balmer series Hα spectral line.
The semiresolved core (observed with a seeing disc of 30 pc) appears amorphous in shape, which is commonly observed in Seyfert 2 galaxies. Its peak coincides with the core position measured in the radio continuum, suggesting that ionized gas surrounds a non-thermal source.
A circumnuclear ring or spiral of radius 220 pc and a rotational velocity of 350 km s⁻¹ (assuming circular motions) surrounds the core. The inclination angle of this feature, i =40°±10°, is less than that of the previously observed radio continuum disc. The velocity channel maps obtained for the Hα ring show that the kinematics resemble those of a rotating ring and the intensity displays a complex structure indicative of several, unresolved, H II regions. We believe the ring to represent a circumnuclear starburst.
Our Hα data also show the presence of the previously detected [O III ] ionization cone to the north-west of the core, measuring more than 400 pc in length. We suggest that the ionization cone lies in a different plane from that of the starburst ring and is directed away from us. Several kinematic components of the core are derived and we calculate an outflow velocity in excess of 150–200 km s⁻¹ for gas above the core of Circinus. We also present evidence for inflowing ionized gas at the centre of Circinus.
The correlation of the Hα and radio continuum features is discussed, as well as the possible presence of a starburst-driven superwind in the Circinus galaxy.     

High resolution fabry-perot and multi-pupil spectral observations of the genuine blue compact dwarf galaxy IZW18

We present the observations of the BCDG IZW18 performed with a Fabry-Perot interferometer at the CFH 3.6 m telescope and with a multi-pupil spectrograph at the SAO (Russia) 6 m telescope. The morphological structure of the galaxy in emission lines and in continuum, the velocity field of the ionized gas, and [OIII]/H ratio distribution along the NW component have been investigated. Besides the NW and SE HII components, we find a population of small HII regions. Continuum maps show that the peaks of the stellar light distribution are displaced with respect to the emission lines maxima. the velocity field shows peculiar motions superposed on an approximately regular background implying solid body rotation. Emission line profiles exhibit an asymmetric structure, except for the NW compact component. The [OIII]/H ratio decreases from the center of the NW component to its edge with the gradient of 1.86 kpc^–1.Published in Astrofizika, Vol. 38, No. 4, pp. 602–615, October–December, 1995.     

Models ofHii regions from recombination line observations

In order to study the ionized gas with low electron density the H159 and H200 radio recombination lines (v = 1.62 GHz) atl = 30°5 andl = 31°0 in the galactic plane, observed with the NRAO 43 m radiotelescope, are analyzed. The profiles show the LTE/ intensity ratio for the more distant component of the profiles ( _LSR = 100 km s^–1) (Cersosimo and Onello, 1991).To derive the electron density and temperature of the emitting gas a new interpretation of the radio recombination lines is made. We suppose that the emission originates in a superposition of ionized gas layers with different densities along the line of sight. The number of layers in the model is equal to the numbers of different order transitions observed. By solving an equations set, the contribution of different components can be calculated. The method is used to estimate the electron density and temperature of the gas. Eleven models of two non-LTE components are considered. The photon flux required to ionize the regions are calculated and the results are compared with previous observations obtained in the region at 3 cm (H85, H87, H88 lines) (Lockman, 1989).Our results suggest that the necessary photon flux to ionize the extended envelopes of the regions is at least one order of magnitude larger than that needed for ionizing the core of the regions.Member of the Carrera del Investigador Científico y Tecnológico del Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) from Argentina.Also Instituto Argentino de Radioastronomia.     

Low density ionized gas in the inner galaxy — Interpretation of recombination line observations at 325 MHz

The recent survey of H 272α recombination line (324.99 MHz) in the direction of 34 Hn regions, 12 SNRs and 6 regions of continuum minimum (‘blank’ regions) in the galactic plane is used to derive the properties of diffuse ionized gas in the inner Galaxy. The intensity of radio recombination lines at high frequencies is dominated by spontaneous emission in high-density gas and that at low frequencies (325 MHz) by stimulated emission in low-density gas. We have used this property to obtain the electron density in the gas in the direction of blank regions and SNRs, by combining the H 272 α measurements (preceeding paper) with the published data at higher frequencies. Further, we have imposed constraints on the electron temperature and pathlength through this gas using the observed high-frequency continuum emission, average interstellar electron density and geometry of the line-emitting regions. The derived properties of the gas are (i) electron density 0.5–6 cm^-3, (ii) electron temperature 3000–8000 K and (iii) emission measures 500–3000 pc cm^-6 The corresponding pathlengths are 50–200 pc. As the derived sizes of the low-density regions are small compared to the pathlength through the Galaxy, the low-frequency recombination lines cannot be considered as coming from a widely distributed component of the interstellar medium. The Hn regions studied in the above survey cannot themselves produce the H 272α lines detected towards them because of pressure broadening, optical depth, and beam dilution. However, the agreement in velocity of these lines with those seen at higher frequencies suggests that the low-frequency recombination lines arise in low-density envelopes of the Hn regions. Assuming that the temperature of the envelopes are similar to those of the cores and invoking geometrical considerations we find that these envelopes should have electron densities in the range 1–10 cm^-3 and linear sizes of 30–300 pc in order to produce the observed H 272α lines.     

A disturbed molecular disc in the Circinus galaxy

We have used the Swedish ESO Submillimeter Telescope to observe the molecular gas in the Circinus galaxy using the CO(1 → 0) transition as a tracer. The central region and major axis have been mapped and several other points were also observed. The gas in the galaxy is concentrated towards the nucleus, the peak being coincident with the radio/optical core. The inclination of the molecular galactic disc is more comparable to that of the radio continuum than to that of the large-scale H  i emission. Evidence for an anomalous spur structure pointing radially away from the galactic centre is presented, and may indicate a causal link between it and similar features seen in optical lines and radio continuum. Our data suggest the presence of a central molecular ring or disc with radius 300 ± 50 pc and a rotation velocity of about 200 km s⁻¹ (assuming i  = 73°). The dynamical mass of the nucleus is estimated to be no greater than 3.9 × 10⁹ M_⊙. Assuming that the distribution of gas varies smoothly in the outer regions, we calculate the mass of molecular gas in the galaxy to be at least M _mol = 1.1 × 10⁹ M_⊙, and the star-forming efficiency to be 11 ± 2 L_⊙ M_⊙⁻¹. These results imply that Circinus is undergoing a massive central starburst which may be, at least partially, responsible for its extended minor axis emission seen in several wavebands.     

Ultracompact HII regions: the impact of newly formed massive stars on their environment

Summary Ultracompact (UC)HII regions are manifestations of newly formed massive stars that are still embedded in their natal molecular cloud. They are among the brightest and most luminous single objects in the Galaxy at far infrared and radio wavelengths. Recent high spatial resolution studies, particularly at radio wavelengths, have greatly contributed to our understanding of these dynamic objects and the impact they have on their environment. A summary is given of our current understanding of the physical properties, morphologies, dynamics, number and distribution in the Galaxy, and molecular environments of UCHII regions. Recent models of the circumnebular dust imply that the graphite/silicate abundance ratio is about half that of dust in the diffuse interstellar medium. The dust cocoons are large, cool, and optically thick shortward of a few microns. There are apparently between 1700 and 3000 UCHII regions in the Galaxy. This represents 10–20% of the total O star population. There are too many UCHII regions (just counting those studied with the VLA) to be consistent with the short dynamical lifetimes of this very compact stage of evolution. Both the morphologies and the large number can be understood if UC HII regions are bow shocks. Models of stellar wind supported bow shocks are discussed and consequences for the dynamics and morphologies of the ionized and molecular gas are explored.     

A high-resolution radio study of neutral gas in the starburst galaxy NGC 520

We present subarcsec angular resolution observations of the neutral gas in the nearby starburst galaxy NGC 520. The central kpc region of NGC 520 contains an area of significantly enhanced star formation. The radio continuum structure of this region resolves into ∼10 continuum components. By comparing the flux densities of the brightest of these components at 1.4 GHz with published 15-GHz data we infer that these components detected at 1.4 and 1.6 GHz are related to the starburst and are most likely to be collections of several supernova remnants within the beam. None of these components is consistent with emission from an active galactic nuclei. Both neutral hydrogen (H  i ) and hydroxyl (OH) absorption lines are observed against the continuum emission, along with a weak OH maser feature probably related to the star formation activity in this galaxy. Strong H  i absorption  ( N _H∼ 10²² atoms cm⁻²)  traces a velocity gradient of 0.5 km s⁻¹ pc⁻¹ across the central kpc of NGC 520. The H  i absorption velocity structure is consistent with the velocity gradients observed in both the OH absorption and in CO emission observations. The neutral gas velocity structure observed within the central kpc of NGC 520 is attributed to a kpc-scale ring or disc. It is also noted that the velocity gradients observed for these neutral gas components appear to differ with the velocity gradients observed from optical ionized emission lines. This apparent disagreement is discussed and attributed to the extinction of the optical emission from the actual centre of this source hence implying that optical ionized emission lines are only detected from regions with significantly different radii to those sampled by the observations presented here.     

Is C/O enhanced in NGC 253?

We have mapped the nuclear region of the starburst galaxy NGC 253 in the³ P ₁ ³ P ₀ line of neutral carbon using the JCMT. Carbon is widespread across the nuclear region with a similiar distribution to CO as expected. Previous studies of Galactic star-forming regions showed that carbon emission is enhanced in photon-dominated regions (where UV photons impinge upon molecular clouds). Previous observations of other PDR tracers such as ionized carbon and FIR continuum constrain the physical conditions in the PDR gas of NGC 253. The carbon we have observed is far brighter than predicted by theoretical models of PDRs with solar elemental values. This indicates that carbon emission is not a reliable diagnostic of the physical conditions in the nuclear region of a galaxy undergoing a burst of star formation.     

Supersonic turbulence in giant extragalactic HII regions

I discuss in this paper the more likely physical mechanisms that could provide the energy input for the supersonic motions observed in giant extragalactic HII regions, together with preliminary results of an ongoing observational program that aims to study, with with high spatial and spectral resolution, the kinematics of the ionized gas of the regions.     

Gas and dust in the BCD galaxy VII Zw 403 (UGC 6456)

Based on the results of the earlier spectroscopic observations with the 6-m BTA telescope of the SAO RAS we refine the metallicity estimates of the complexes of ionized gas in the VII Zw 403 galaxy. Infrared observations from the Spitzer Space Telescope are used to search for a possible correlation of the mass fraction distribution of the polycyclic aromatic hydrocarbons (PAH) with the distribution of ionized and neutral hydrogen, and with the metallicity of gas in the HII regions of the galaxy.     

Cooling flows in clusters of galaxies

Summary X-ray images and spectra of clusters of galaxies show strong evidence for cooling flows. In many clusters, the hot gas in the core is cooling at rates of 100Myr^–1 and greater. Few traces of the cooled gas have been observed, but it probably forms into low-mass stars (perhaps brown dwarf or even Jupiter-mass objects). X-ray surface-brightness profiles show that the cooling gas is highly inhomogeneous. Overdense gas cools rapidly to form cooled clumps distributed throughout the flow, with little of the gas ever reaching the cluster centre. Cooled and cooling clumps are disrupted because of their motion relative to the remainder of the gas, tending to produce small cooled fragments and, ultimately, low-mass stars. Large molecular clouds, which are the sites of massive star formation in our galaxy, do not occur in the outer parts of cooling flows. There is evidence of larger gas clumps and the formation of more massive stars in the central few kpc of some cooling flows. It is argued that cooling flows efficiently form dark matter. This has wider implications for the formation of dark matter in massive galaxies.     

Some peculiarities of four host galaxies,containing Seyfert nuclei

Star-like objects are found in Seyfert galaxies Markarian 290, Markarian 298, NGC 1275, and NGC 7469, being connected with the structure peculiarities of the galaxies. The absolute magnitudes of these objects are –16 ^m M–19 ^m . It has been supposed that these star formations must stimulate the instability in the disk of the galaxy followed by the matter fall toward the centre of the galaxy. The gas inflow toward the centre will allow the recent star formations and Seyfert nuclei generation.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

The Hα Emission of the Spiral Galaxy NGC 7479

We use the catalogue of HII regions obtained from a high quality continuum-subtracted H_α 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 H_α surface density distribution. As found previously for late-type spirals, the disc LF shows clear double-linear behaviour with a break at log L ∼ 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. This revised version was published online in July 2006 with corrections to the Cover Date.     

Radio structure of extragalactic X-ray sources

The radio properties ofUhuru X-ray sources with fairly certain extragalactic identifications are described briefly. Radio to X-ray flux ratios are low for rich clusters of galaxies and high for double radio sources. There is some evidence from the Abell 426 (Perseus) and Abell 1367 clusters that a radio galaxy in a rich cluster may be the centre of extended X-ray emission. Nuclei of galaxies have an enormous range in X-ray luminosity; the known range is from 10³⁰ W for our galaxy to 3×10³⁸ W for 3C 273. Unidentified X-ray sources at high galactic latitudes may include new classes of objects with very low radio to X-ray flux ratios or hard X-ray emission.     

Deep high spectral resolution spectroscopy and chemical composition of ionized nebulae

High spectral resolution spectroscopy has proved to be very useful for the advancement of chemical abundances studies in photoionized nebulae, such as H II regions and planetary nebulae (PNe). Classical analyses make use of the intensity of bright collisionally excited lines (CELs), which have a strong dependence on the electron temperature and density. By using high resolution spectrophotometric data, our group has led the determination of chemical abundances of some heavy element ions, mainly O⁺⁺, O⁺, and C⁺⁺ from faint recombination lines (RLs), allowing us to deblend them from other nearby emission lines or sky features. The importance of these lines is that their emissivity depends weakly on the temperature and density structure of the gas. The unresolved issue in this field is that recombination lines of heavy element ions give abundances that are about 2–3 times higher than those derived from CELs – in H II regions – for the same ion, and can even be a factor of 70 times higher in some PNe. This uncertainty puts into doubt the validity of face values of metallicity that we use as representative not only for ionized nebulae in the Local Universe, but also for star‐forming dwarf and spiral galaxies at different redshifts. Additionally, high‐resolution data can allow us to detect and deblend faint lines of neutron capture element ions in PNe. This information would introduce further restrictions to evolution models of AGBs and would help to quantify the chemical enrichment in s‐elements produced by low and intermediate mass stars. The availability of an échelle spectrograph at the E‐ELT will be of paramount interest to: (a) extend the studies of heavyelement recombination lines to low metallicity objects, (b) to extend abundance determinations of s‐elements to planetary nebulae in the extragalactic domain and to bright Galactic and extragalactic H II regions. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)