Multicomponent simulation of emission of low-metallicity H II regions

Using multicomponent photoionization simulation, we investigated the impact of bubble-like structures around starbursts inside the low-metallicity H II regions on the ionization spectrum shape and emission line forming. Radial distribution of density values and other physical parameters of bubble-like structures were taken from Weaver et al. (Weaver et al., 1977, p. 377). The first and second inner components of such models describe the free expansion zone of superwind from the central starburst region and rarefied hot gas of the cavern thermalized by inverse shock wave, respectively. The gas density and electron temperature distributions into these components are obtained from the solution of the system of equations of continuity and energy transfer, including heat conductivity. The third component is a thin shell of high density gas formed from the gas surrounding a bubble and made by direct shock wind wave. The gas density in this component was obtained from isobaric condition at contact discontinuity between the second and third components. Input spectra of the ionizing radiation were obtained from the starburst evolutional models. The evolution grid of the multicomponent low-metallicity photoionization models with free parameters determining physical conditions inside the bubble-like structure was calculated. The impact of bubble-like structure on the change of ionization spectrum shape and the formation of fluxes of important emission lines in low-metallicity case was analyzed in detail.     

Modeling of H II region radiation surrounding the starburst knot taking into account the evolution of structures formed by the superwind

The method for the multicomponent photoionization modeling of the H II region radiation surrounding the starburst knot is presented. The internal structure of the H II region has been determined using the evolutionary model of the superwind bubble from the starburst center. Models of Chevalier and Clegg (1985) and Weaver et al. (1977) have been used to determine the radial distribution of the gas density, the velocity of gas layers, and the temperature in the region of the superwind free expansion and in the cavity, respectively. The chemical content of the internal components of the bubble has been set by the results of the modeling of the evolutionary population synthesis. External components of our models describe a high-density, thin layer of gas formed by the shock wave of stellar superwind from the surrounding gas and a typical H II region, respectively. Input model parameters have been taken from the precalculated evolutionary starburst models based on three types of evolutionary tracks. Evolutionary grids of multicomponent low-metallicity models are calculated. A comparative analysis of the results of their calculation with the observational data has been carried out.     

A Study of the Emission-Line Spectra and the Stellar Populations of Starburst Galaxies

Based on optical–NIR spectra, we discuss the nebular properties and stellar populations of starburst nuclei. Starbursts are found to have higher electron densities and higher excitations than HII nuclei. The emission lines have been used to estimate the nebular oxygen abundances, ionization parameter and radiation softness parameter. From a study of the H_α emission-line equivalent widths and the CaII triplet absorption line equivalent widths, we infer the ages and stellar content of the starburst regions. Most of the nuclei show evidence for a composite population – a young, ionizing population co-existing with evolved, non-ionizing stars of about 5–7 Myr, which are evolving towards the peak of RSG distribution. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Origin of Infrared Emission from the Infrared Luminous Galaxy NGC 4418

We present a study of the origin of infrared (IR) emission in the optically normal, infrared luminous galaxy NGC 4418. By decomposing the stellar absorption features and continua in the range of 3600-8000 A from the Sloan Digital Sky Survey into a set of simple stellar populations, we derive the stellar properties for the nuclear region of NGC 4418. We compare the observed infrared luminosity with the one derived from the starburst model, and find that star-forming activity contributes only 7% to the total IR emission, that as the IR emission region is spatially very compact, the most possible source for the greater part of the IR emission is a deeply embedded AGN, though an AGN component is found to be unnecessary for fitting the optical spectrum.     

Mapping the roots of the galactic outflow in NGC1569

The exact nature of the interaction between hot, fast-flowing star-cluster winds and the surrounding clumpy interstellar medium (ISM) in starburst galaxies has very few observational constraints. Besides furthering our knowledge of ISM dynamics, detailed observations of ionised gas at the very roots of large-scale outflows are required to place limits on the current generation of high-resolution galactic wind models. To this end, we conduct a detailed investigation of the ionised gas environment surrounding the young star clusters in the starburst galaxy NGC1569. Using high spatial and spectral-resolution Gemini/GMOS integral-field unit observations, we accurately characterise the line-profile shapes of the optical nebular emission lines and find a ubiquitous broad (～300 km?s^?1) component underlying a bright narrower component. By mapping the properties of the individual line components, we find correlations that suggest that the broad component results from powerful cluster wind–gas clump interactions. We propose a model to explain the properties of the line components and the general turbulent state of the ISM.     

On the nature of the X-ray absorption in the Seyfert 2 galaxy NGC 4507

We present results of the ASCA observation of the Seyfert 2 galaxy NGC 4507. The 0.5–10 keV spectrum is rather complex and consists of several components: (i) a hard X-ray power law heavily absorbed by a column density of about 3-10²³ cm⁻², (ii) a narrow Fe Kα line at 6.4 keV, (iii) soft continuum emission well above the extrapolation of the absorbed hard power law and (iv) a narrow emission line at ∼0.9 keV. The line energy, consistent with highly ionized neon (Ne IX ), may indicate that the soft X-ray emission is derived from a combination of resonant scattering and fluorescence in a photoionized gas. Some contribution to the soft X-ray spectrum from thermal emission, as a blend of Fe L lines, by a starburst component in the host galaxy cannot be ruled out with the present data.     

Near-IR spectral evolution of dusty starburst galaxies

We propose a multicomponent analysis of starburst galaxies, based on a model that takes into account the young and evolved stellar components and the gas emission, with their respective extinction, in the frame of a coherent dust distribution pattern. Near-IR signatures are preferentially investigated, in order to penetrate as deep as possible into the dusty starburst cores. We computed the 1.4-2.5 μm spectra of synthetic stellar populations evolving through strong, short timescale bursts of star formation (continuum and lines, R ? 500). The evolution model is specifically sensitive to cool stellar populations (AGB and red supergiant stars). It takes advantage of the stellar library of Lançon & Rocca-Volmerange (1992) [A&ASS, 96, 593], observed with the same instrument (FTS/CFHT) as the analysed galaxy sample, so that the instrumental effects are minimised. The main near-IR observable constraints are the molecular signatures of CO and H₂O and the slope of the continuum, observed over a range exceptionally broad for spectroscopic data. The H - K colour determined from the spectra measures the intrinsic stellar energy distribution but also differential extinction, which is further constrained by optical emission line ratios. Other observational constraints are the near-IR emission lines (Brγ, He I 2.06 μm, [Fe II] 1.64 μm, H₂ 2.12 μm) and the far-IR luminosity. The coherence of the results relies on the interpretation in terms of stellar populations from which all observable properties are derived, so that the link between the various wavelength ranges is secured. The luminosity L_K is used for the absolute calibration.We apply this approach to the typical spectrum of the core of NGC 1614. Consistent solutions for the starburst characteristics (star-formation rate, IMF, burst age, morphology) are found and the role of each observational constraint in deriving satisfactory models is extensively discussed. The acceptable contamination of the K band light by the underlying population amounts ≥ 15% even through a 5 arcsec aperture. The model leads to a limit on the direct absorption of Lyman continuum photons by dust situated inside the ionised areas, which in turn, with standard gas-to-dust ratios, translates into small characteristic sizes for the individual coexisting H II regions of the massive starburst area (clusters containing ∼ 10² ionising stars). We show that room is left for IMFs extending to 120 M_⊙, rather than truncated at ∼ 60 M_⊙ as most conservative studies conclude. High internal velocity dispersions (≥ 20 km s⁻¹) are then needed for the H II regions. An original feature of this work is to base the analysis of near-infrared spectral galaxy observations on a large wavelength range, using models constructed with spectral stellar data observed with the same instrument. However a broader use of this spectral evolution model on other spectral or photometric data samples is possible if the spectral resolution of the model is adapted to observations or if colours are derived from the energy distributions.Catherine J. Cesarsky     

Spectroscopic constraints on outflows from BN-type objects

New high-quality high spectral resolution observations of the HI line emission from massive young stellar objects are described and discussed. It is proposed that two distinct physical components contribute to the observed emission. One of these is an optically-thick high-velocity stellar wind, the other a more slowly moving optically-thin volume of gas that may, in the case of S106IR at least, be caused by mass loading of the stellar wind. This decomposition is shown to resolve a long-standing problem regarding the relative widths of high and low opacity lines.     

UV emission lines in passively evolving galaxies can reveal the progenitors of type Ia supernovae

The question of which progenitor channel can reproduce the observed rate of Type Ia supernovae (SNe Ia) remains unresolved, the two leading models being the so-called single and double degenerate scenarios. The former implies a large population of accreting, nuclear-burning white dwarfs with photospheric temperatures T～10⁵–10⁶ K during some part of their accretion history. Recently, we demonstrated that a population of accreting white dwarfs large enough to reproduce the observed SN Ia rate would contribute significantly to the ionizing radiation expected from the stellar population in early-type galaxies, now commonly observed to host spatially extended regions of neutral and ionized gas. From our photoionization calculations, we show that one can constrain the contribution of the single degenerate channel to the SN Ia rate in early-type galaxies from upper limits on the luminosity of a number of emission lines characteristic of ionization by high-temperature sources. Detection (or strong upper limits on) He II 1640 Å and [C II] 1335 Å, expected to be overluminous in these galaxies if the single-degenerate channel holds true, can strongly constrain the total luminosity of nuclear-burning white dwarfs in these populations. In the near-UV, our photoionization calculations demonstrate that the EW of the [O II] 3727 doublet and the [Ne III] 3869/[O II] 3727 ratio can also provide a powerful diagnostic, particularly in post-starburst galaxies. Together with the He II 3203 Å (5 → 3) recombination line, these lines present an excellent opportunity for strongly constraining the population of accreting, nuclear-burning white dwarfs, and in general the available ionizing continuum, at relatively short delay-times (time from initial starburst).     

Galactic winds and circulation of the interstellar medium in dwarf galaxies

We study, through 2D hydrodynamical simulations, the feedback of a starburst on the ISM of typical gas-rich dwarf galaxies. The main goal is to address the circulation of the ISM and metals following the starburst. We assume a single-phase rotating ISM in equilibrium in the galactic potential generated by a stellar disc and a spherical dark halo. The starburst is assumed to occur in a small volume in the centre of the galaxy, and it generates a mechanical power of 3.8×10³⁹ or 3.8×10⁴⁰ erg s⁻¹ for 30 Myr. We find, in accordance with previous investigations, that the galactic wind is not very effective in removing the ISM. The metal-rich stellar ejecta, however, can be efficiently expelled from the galaxy and dispersed in the intergalactic medium.
Moreover, we find that the central region of the galaxy is always replenished with cold and dense gas a few 100 million years after the starburst, achieving the requisite for a new star formation event in ≈0.5–1 Gyr. The hydrodynamical evolution of galactic winds is thus consistent with the episodic star formation regime suggested by many chemical evolution studies.
We also discuss the X-ray emission of these galaxies and find that the observable (emission-averaged) abundance of the hot gas underestimates the real one if thermal conduction is effective. This could explain the very low hot-gas metallicities estimated in starburst galaxies.     

Circumnuclear Star Forming Activity in NGC 3982

We present a study of the nearby Seyfert galaxy NGC 3982 using optical,infrared and X-ray data acquired by SDSS,Spitzer and Chandra.Our main results are as follows:(1) A simple stellar population synthesis on the nuclear and circumnuclear SDSS spectra gives unambiguous evidence of young stellar components in both the nuclear and circumnuclear regions.(2) The Spitzer Infrared Spectrograph (IRS) spectrum of the central region (～3") shows a power-law continuum,a silicate emission feature at 9.7 μm,and significant PAH emission features at 7.7,8.6,11.3 and 12.7/zm,suggesting the coexistence of AGN and starburst activities in the central region of NGC 3982.(3) We estimate the star formation rate (SFR) of the circumnuclear (～5"-20") region from the Ha luminosity to be for the active nucleus of NGC 3982 from radio to X-ray,and obtain a bolometric luminosity of Lbol=4.5×1042 erg s-1,corresponding to an Eddington ratio (Lbol/LEdd) of 0.014.The HST image of NGC 3982 shows a nuclear mini-spiral between the circumnuclear starforming region and the nucleus,which could be the channel through which gas is transported to the supermassive black hole from the circumnuclear star-forming region.     

The stellar wind as a key to the understanding of the spectral activity of IN Com

We present long-term spectral observations (R = 20000) of IN Com in the region of the H_α, H_β, and He I 5876 lines. One distinguishing characteristic of the stellar spectrum is the presence in the H_α line of an extended two-component emission with limits up to ±400 km/s. Emission parameters show the rotation modulation with the stellar rotation period and a significant variability on the long-term scale. Similar emissions are also observed in the H_β and He I 5876 lines. Our results allow us to conclude that observational emission profiles are formed in an optically thin hot gas. This is a result of the presence of a circumstellar gas disk around IN Com. Its size does not exceed several stellar radii. The material for the disk is supported by the stellar wind from IN Com. The detected variability of H_α-emission parameters shows a clear connection with the photopolarimetric activity of the star. This fact allows us to associate the long-term spectral variability with cycles of stellar activity of IN Com.     

The X-ray Emission of NGC 1068

This paper summarises the X-ray properties of NGC 1068 from the observers perspective and reports new observations with the ROSAT HRI. Below ? 2 keV, the spectrum is steep and probably represents thermal emission from gas with temperature kT ? 0.1 - 0.6 keV. Above ? 2 keV, the spectrum is much flatter and may be described by a power-law with energy index α ? 0.3. Images with the ROSAT HRI reveal that about half the X-ray flux in the 0.1 - 2.4 keV band is extended on scales > 5″ (360 pc). Recent ROSAT PSPC observations of starburst galaxies show integrated soft X-ray spectra which are very similar to that of NGC 1068 below 2 keV. The spatially extended, steep, soft X-ray emission of NGC 1068 probably originates through thermal emission from a hot wind driven by the disk starburst, the Seyfert nucleus or a combination of the two. On the other hand, the hard emission above 2 keV is almost certainly dominated by the Seyfert nucleus.     

Gemini GMOS/integral field unit spectroscopy of NGC 1569 – II. Mapping the roots of the galactic outflow

We present a set of four Gemini-North Multi-Object Spectrograph/integral field unit (IFU) observations of the central disturbed regions of the dwarf irregular starburst galaxy NGC 1569, surrounding the well-known superstar clusters A and B. This continues on directly from a companion paper, in which we describe the data reduction and analysis techniques employed and present the analysis of one of the IFU pointings. By decomposing the emission-line profiles across the IFU fields, we map out the properties of each individual component identified and identify a number of relationships and correlations that allow us to investigate in detail the state of the ionized interstellar medium (ISM). Our observations support and expand on the main findings from the analysis of the first IFU position, where we conclude that a broad (≲400 km s⁻¹) component underlying the bright nebular emission lines is produced in a turbulent mixing layer on the surface of cool gas knots, set up by the impact of the fast-flowing cluster winds. We discuss the kinematic, electron-density and excitation maps of each region in detail and compare our results to previous studies. Our analysis reveals a very complex environment with many overlapping and superimposed components, including dissolving gas knots, rapidly expanding shocked shells and embedded ionizing sources, but no evidence for organized bulk motions. We conclude that the four IFU positions presented here lie well within the starburst region where energy is injected, and, from the lack of substantial ordered gas flows, within the quasi-hydrostatic zone of the wind interior to the sonic point. The net outflow occurs at radii beyond 100–200 pc, but our data imply that mass-loading of the hot ISM is active even at the roots of the wind.     

A spectroscopic study of the peculiar galaxy UGC 5600

We present our observations of the galaxy UGS 5600 with a long-slit spectrograph (UAGS) and a multipupil field spectrograph (MPFS) attached to the 6-m Special Astrophysical Observatory telescope. Radial-velocity fields of the stellar and gaseous components were constructed for the central region and inner ring of the galaxy. We proved the existence of two nearly orthogonal kinematic subsystems and conclude that UGC 5600 is a galaxy with an inner polar ring. In the circumnuclear region, we detected noncircular stellar motions and suspected the existence of a minibar. The emission lines are shown to originate in H II regions. We estimated the metallicity from the intensity ratio of the [N II]λ6583 and Hα lines to be nearly solar, which rules out the possibility that the polar ring was produced by the accretion of gas from a dwarf companion.     

Large-scale variability in the profiles of Hα and Hβ in the spectrum of the Herbig B8e star MWC 419 and a model interpretation of it

Spectroscopic data taken with a moderate resolution spectrograph in the region of the Hα and Hβ lines are presented for the Herbig B8e star MWC 419. The spectroscopic observations were accompanied by broad band BVR photometric measurements. The observations reveal a variability in the line profiles that is typical of Herbig Ae/Be stars with signs of a strong stellar wind. The greatest changes are observed in the region of the absorption components of the line profiles, which convert the profile from a type P CygII to P CygIII, as well as in the intensities of the central emission components. A model technique is used for quantitative interpretation of this variability and it shows that the P Cyg profile conversion of the absorption component can be explained in terms of a stellar wind model in which its distribution over latitude varies on a time scale of a few days. __________ Translated from Astrofizika, Vol. 50, No. 2, pp. 259–280 (May 2007).     

Non-thermal emission in Wolf–Rayet stars: are massive companions required?

We examine the radio spectral indices of 23 Wolf–Rayet (WR) stars to identify the nature of their radio emission. We identify nine systems as non-thermal emitters. In seven of these systems the non-thermal emission dominates the radio spectrum, while in the remaining two it is of comparable strength to the thermal, stellar wind emission, giving 'composite' spectra. Among these nine systems, seven have known spectroscopic or visual binary companions. The companions are all massive O or early B-type stars, strongly supporting a connection between the appearance of non-thermal emission in WR stars and the presence of a massive companion. In three of these binaries, the origin of non-thermal emission in a wind-collision region between the stars has been well established in earlier work. The binary systems that exhibit only thermal emission are all short‐period systems where a wind-collision zone is deep within the opaque region of the stellar wind of the WR star. To detect non-thermal emission in these systems requires optically thin lines of sight to the wind-collision region.     

Models of forbidden line emission profiles from axisymmetric stellar winds

A number of strong infrared forbidden lines have been observed in several evolved Wolf–Rayet (WR) star winds, and these are important for deriving metal abundances and testing stellar evolution models. In addition, because these optically thin lines form at large radius in the wind, their resolved profiles carry an imprint of the asymptotic structure of the wind flow. This work presents model forbidden line profile shapes formed in axisymmetric winds. It is well known that an optically thin emission line formed in a spherical wind expanding at constant velocity yields a flat-topped emission profile shape. Simulated forbidden lines are produced for a model stellar wind with an axisymmetric density distribution that treats the latitudinal ionization self-consistently and examines the influence of the ion stage on the profile shape. The resulting line profiles are symmetric about line centre. Within a given atomic species, profile shapes can vary between centrally peaked, doubly peaked, and approximately flat-topped in appearance depending on the ion stage (relative to the dominant ion) and viewing inclination. Although application to WR star winds is emphasized, the concepts are also relevant to other classes of hot stars such as luminous blue variables and Be/B[e] stars.     

Star formation in starburst galaxies from near-IR spectroscopy and evolutionary population synthesis

We show that the combined study of the near-IR Stellar Energy Distribution (SED) and emission lines in IR luminous starburst galaxies, using evolutionary population synthesis, gives constraints both on the star formation (SF) parameters and on the relative spatial distribution of the components (burst stars, evolved stars, H II regions, dust). We also note that the He I λ 2.06/Br _γ ratio, used to constrain the stellar upper mass limit, is sensitive to a variety of other factors that should be accounted for.     

Dust in the disk winds from young stars as a source of the circumstellar extinction

We consider the problem of dust grain survival in the disk winds from T Tauri and Herbig Ae stars. For our analysis, we have chosen a disk wind model in which the gas component of the wind is heated through ambipolar diffusion to a temperature of ～10⁴ K. We show that the heating of dust grains through their collisions with gas atoms is inefficient compared to their heating by stellar radiation and, hence, the grains survive even in the hot wind component. As a result, the disk wind can be opaque to the ultraviolet and optical stellar radiation and is capable of absorbing an appreciable fraction of it. Calculations show that the fraction of the wind-absorbed radiation for T Tauri stars can be from 20 to 40% of the total stellar luminosity at an accretion rate ? _a = 10^?8-10^?6 M _⊙yr^?1. This means that the disk winds from T Tauri stars can play the same role as the puffed-up inner rim in current accretion disk models. In Herbig Ae stars, the inner layers of the disk wind (r ≤ 0.5 AU) are dust-free, since the dust in this region sublimates under the effect of stellar radiation. Therefore, the fraction of the radiation absorbed by the disk wind in this case is considerably smaller and can be comparable to the effect from the puffed-up inner rim only at an accretion rate of the order of or higher than 10^?6 M _⊙yr^?1. Since the disk wind is structurally inhomogeneous, its optical depth toward the observer can be variable, which should be reflected in the photometric activity of young stars. For the same reason, moving shadows from gas and dust streams with a spiral-like shape can be observed in high-angular-resolution circumstellar disk images.