Molecular hydrogen emission in Cygnus A

We present J , H and K -band spectroscopy of Cygnus A, spanning 1.0–2.4 μm in the rest-frame and hence several rovibrational H₂, H recombination and [Fe  ii ] emission lines. The lines are spatially extended by up to 6 kpc from the nucleus, but their distinct kinematics indicate that the three groups (H, H₂ and [Fe  ii ]) are not wholly produced in the same gas. The broadest line, [Fe  ii ] λ 1.644, exhibits a non-Gaussian profile with a broad base (FWHM≃1040 km s⁻¹), perhaps because of the interaction with the radio source. Extinctions to the line-emitting regions substantially exceed earlier measurements based on optical H recombination lines.
Hard X-rays from the quasar nucleus are likely to dominate the excitation of the H₂ emission. The results of Maloney, Hollenbach & Tielens are thus used to infer the total mass of gas in H₂ v=1–0 S(1)-emitting clouds as a function of radius, for gas densities of 10³ and 10⁵ cm⁻³, and stopping column densities N _H=10²²–10²⁴ cm⁻². Assuming azimuthal symmetry, at least 2.3×10⁸ M_⊙ of such material is present within 5 kpc of the nucleus, if the line-emitting clouds see an unobscured quasar spectrum. Alternatively, if the bulk of the X-ray absorption to the nucleus inferred by Ueno et al. actually arises in a circumnuclear torus, the implied gas mass rises to ∼10¹⁰ M_⊙. The latter plausibly accounts for 10⁹ yr of mass deposition from the cluster cooling flow, for which within this radius.     

Tentative detection of warm intervening gas towards PKS 0548-322 with XMM–Newton

We present the results of a long (∼93 ks) XMM–Newton observation of the bright BL-Lac object  PKS 0548-322 ( z = 0.069)  . Our Reflection Grating Spectrometer (RGS) spectrum shows a single absorption feature at an observed wavelength  λ= 23.33 ± 0.01 Å  , which we interpret as O  vi Kα absorption at   z = 0.058  , i.e. ∼3000 km s⁻¹ from the background object. The observed equivalent width of the absorption line, ∼30 mÅ, coupled with the lack of the corresponding absorption edge in the EPIC pn data, implies a column density of   N _O VI∼ 2 × 10¹⁶ cm⁻²  and turbulence with a Doppler velocity parameter   b > 100 km s⁻¹  . Within the limitations of our RGS spectrum, no O  vii or O  v Kα absorption are detected. Under the assumption of ionization equilibrium by both collisions and the extragalactic background, this is only marginally consistent if the gas temperature is  ∼2.5 × 10⁵ K  , with significantly lower or higher values being excluded by our limits on O  v or O  vii . If confirmed, this would be the first X-ray detection of a large amount of intervening warm absorbing gas through O  vi absorption. The existence of such a high column density absorber, much stronger than any previously detected one in O  vi , would place stringent constraints on the large-scale distribution of baryonic gas in the Universe.     

Compton-thick X-ray absorption in the Seyfert galaxies Tololo 0109383 and ESO 138G1

We present analyses of the ASCA X-ray spectra of two Seyfert galaxies, Tololo 0109383 and ESO 138G1. In both cases, spectral fitting reveals two statistically acceptable continuum models: Compton reflection and partial covering. Both spectra have strong iron K lines, with equivalent widths greater than 1.5 keV. These large equivalent widths are suggestive of heavier obscuration than that directly indicated by the partial-covering models (  210²³ cm^-2),  with the actual column densities being 'Compton-thick' (i.e.   N _H1.510²⁴ cm^-2).  We use the hard X-ray/[O  iii ] flux correlation for Seyferts and data from the literature to provide additional support for this hypothesis. Since Tololo 0109383 is known to have optical type 1 characteristics such as broad Balmer line components and Fe  ii emission, this result marks it as a notable object.     

NGC 3147: a 'true' type 2 Seyfert galaxy without the broad-line region

We report on simultaneous optical and X-ray observations of the Seyfert galaxy, NGC 3147. The XMM–Newton spectrum shows that the source is unabsorbed in the X-rays  ( N _H < 5 × 10²⁰ cm⁻²)  . On the other hand, no broad lines are present in the optical spectrum. The origin of this optical/X-rays misclassification (with respect to the Unification Model) cannot be attributed to variability, since the observations in the two bands are simultaneous. Moreover, a Compton-thick nature of the object can be rejected on the basis of the low-equivalent width of the iron Kα line (≃130 eV) and the large ratio between the 2–10 keV and the [O  iii ] fluxes. It seems therefore inescapable to conclude that NGC 3147 intrinsically lacks the Broad-Line Region, making it the first 'true' type 2 Seyfert galaxy.     

A spectroscopic study of IRAS F10214 + 4724

The z  = 2.286  IRAS galaxy F10214 + 4724 remains one of the most luminous galaxies in the Universe, despite its gravitational lens magnification. We present optical and near-infrared spectra of F10214 + 4724, with clear evidence for three distinct components: lines of width ∼ 1000 km s⁻¹ from a Seyfert 2 nucleus; ≲ 200 km s⁻¹ lines which are likely to be associated with star formation; and a broad (∼ 4000 km s⁻¹) C  III ] 1909-Å emission line which is blueshifted by ∼ 1000 km s⁻¹ with respect to the Seyfert 2 lines. Our study of the Seyfert 2 component leads to several new results. (i) From the double-peaked structure in the Lyα line, and the lack of Lyβ, we argue that the Lyα photons have emerged through a neutral column of N _H ∼ 2.5 × 10²⁵ m⁻², possibly located within the AGN narrow-line region, as proposed for several high-redshift radio galaxies. (ii) The resonant O  VI 1032, 1036-Å doublet (previously identified as Lyβ) is in an optically thick (1:1) ratio. At face value this implies an extreme density ( n _e ∼ 10¹⁷ m⁻³) more typical of broad-line region clouds. However, we attribute this instead to the damping wings of Lyβ from the resonant absorption. (iii) A tentative detection of He  II 1086 suggests little extinction in the rest frame ultraviolet.     

Global velocity field and bubbles in the blue compact dwarf galaxy Mrk 86

We have studied the velocity field of the blue compact dwarf galaxy Mrk 86 (NGC 2537) using data provided by 14 long-slit optical spectra obtained in 10 different orientations and positions. This kinematical information is complemented with narrow-band ([O  iii ]5007 Å and H α ) and broad-band ( B , V , Gunn r and K ) imaging. The analysis of the galaxy global velocity field suggests that the ionized gas could be distributed in a rotating inclined disc, with projected central angular velocity of Ω=34 km s⁻¹ kpc⁻¹. The comparison between the stellar, H  i and modelled dark matter density profile indicates that the total mass within its optical radius is dominated by the stellar component. Peculiarities observed in its velocity field can be explained by irregularities in the ionized gas distribution or local motions induced by star formation.
Kinematical evidences for two expanding bubbles, Mrk 86–B and Mrk 86–C, are given. They show expanding velocities of 34 and 17 km s⁻¹, H α luminosities of 3×10³⁸ and 1.7×10³⁹ erg s⁻¹, and physical radii of 374 and 120 pc, respectively. The change in the [S  ii ]/H α , [N  ii ]/H α , [O  ii ]/[O  iii ] and [O  iii ]/H β line ratios with the distance to the bubble precursor suggests a diminution in the ionization parameter and, in the case of Mrk 86–B, an enhancement of the shock-excited gas emission. The optical–near-infrared colours of the bubble precursors are characteristic of low‐metallicity star‐forming regions (∼0.2 Z_⊙) with burst strengths of about 1 per cent in mass.     

Optical photometry and spectroscopy of the Type Ibn supernova SN 2006jc until the onset of dust formation

We present optical UBVRI photometric and spectroscopic data of the Type Ibn supernova SN 2006jc, until the onset of the dust-forming phase. The optical spectrum shows a blue continuum and is dominated by the presence of moderately narrow (velocity ∼2500 km s⁻¹) He  i emission lines superimposed over a relatively weak supernova spectrum. The helium lines are produced in a pre-existing He-rich circumstellar shell. The observed helium line fluxes indicate the circumstellar shell is dense, with a density of  ∼10⁹–10¹⁰ cm⁻³  . The helium mass in this shell is estimated to be  ≲0.07 M_⊙  . The optical light curves show a clear signature of dust formation, indicated by a sharp decrease in the magnitudes around day 50, accompanied by a reddening of the colours. The evolution of the optical light curves during the early phase and that of the uvoir bolometric light curve at all phases is reasonably similar to normal Ib/c supernovae.     

The anomalous properties of Markarian 1460

We present and discuss optical, near-infrared and H  i measurements of the galaxy Markarian 1460 at a distance of 19 Mpc in the Ursa Major Cluster. This low-luminosity ( M _B =−14) galaxy is unusual because (i) it is blue ( B − R =0.8) and has the spectrum of an H  ii galaxy, (ii) it has a light profile that is smooth and well fitted by an r ^1/4 and not an exponential function at all radii larger than the seeing, and (iii) it has an observed central brightness of about μ _B =20 mag arcsec⁻² , intermediate between those of elliptical galaxies (on the bright μ _B side) and normal low-luminosity dwarf irregular (on the low μ _B side) galaxies. No other known galaxy exhibits all these properties in conjunction. On morphological grounds this galaxy looks like a normal distant luminous elliptical galaxy, since the Fundamental Plane tells us that higher luminosity normal elliptical galaxies tend to have lower surface-brightnesses. Markarian 1460 has 2×10⁷ M_⊙ of H  i and a ratio M (H  i )/ L _B of 0.2, which is low compared to the typical values for star-forming dwarf galaxies. From the high surface-brightness and r ^1/4 profile, we infer that the baryonic component of Markarian 1460 has become self-gravitating through dissipative processes. From the colours, radio continuum, H  i and optical emission line properties, and yet smooth texture, we infer that Markarian 1460 has had significant star formation as recently as ∼1 Gyr ago but not today.     

HST/STIS observations of the high-velocity interstellar cloud HVC 291.2−41.2+80: a warm, mainly ionized high-velocity cloud

We present intermediate-resolution HST /STIS spectra of a high-velocity interstellar cloud ( v _LSR=+80 km s⁻¹) towards DI 1388, a young star in the Magellanic Bridge located between the Small and Large Magellanic Clouds. The STIS data have a signal-to-noise ratio (S/N) of 20–45 and a spectral resolution of about 6.5 km s⁻¹ (FWHM). The high-velocity cloud absorption is observed in the lines of C  ii , O  i , Si  ii , Si  iii , Si  iv and S  iii . Limits can be placed on the amount of S  ii and Fe  ii absorption that is present. An analysis of the relative abundances derived from the observed species, particularly C  ii and O  i , suggests that this high-velocity gas is warm ( T _k∼10³–10⁴ K) and predominantly ionized. This hypothesis is supported by the presence of absorption produced by highly ionized species, such as Si  iv . This sightline also intercepts two other high-velocity clouds that produce weak absorption features at v _LSR=+113 and +130 km s⁻¹ in the STIS spectra.     

Hyperfine splitting of [Al vi] 3.66 μm and the Al isotopic ratio in NGC 6302

The core of planetary nebula NGC 6302 is filled with high-excitation photoionized gas at low expansion velocities. It represents a unique astrophysical situation in which to search for hyperfine structure (HFS) in coronal emission lines from highly ionized species. HFS is otherwise blended by thermal or velocity broadening. Spectra containing  [Al  vi ] 3.66 μm ³P₂←³P₁  , obtained with Phoenix on Gemini South at resolving powers of up to 75 000, resolve the line into five hyperfine components separated by 20–60 km s⁻¹ as a result of the coupling of the   I = 5/2  nuclear spin of ²⁷Al with the total electronic angular momentum J . The isotope ²⁶Al has a different nuclear spin of   I = 5  , and a different HFS, which allows us to place a 3σ upper limit on the ²⁶Al/²⁷Al abundance ratio of 1/33. We measure the HFS magnetic dipole coupling constants for [Al  vi ], and provide the first estimates of the electric quadrupole HFS coupling constants obtained through astronomical observations of an atomic transition.     

Emission-line outflows in PKS1549−79: the effects of the early stages of radio-source evolution?

We present new spectroscopic observations of the southern radio galaxy         . Despite the flat-spectrum character of the radio emission from this source, our optical spectra show no sign of the broad permitted lines and non-stellar continuum characteristic of quasar nuclei and broad-line radio galaxies. However, the high-ionization forbidden lines, including [O  iii ] λλ 5007, 4959, are unusually broad for a narrow-line radio galaxy     , and are blueshifted by 600 km s⁻¹ relative to the low-ionization lines such as [O  ii ] λλ 3726,3729. The [O  ii ] lines are also considerably narrower     than the [O  iii ] lines, and have a redshift consistent with that of the recently detected H  i 21-cm absorption-line system. Whereas the kinematics of the [O  iii ] emission lines are consistent with outflow in an inner narrow-line region, the properties of the [O  ii ] emission lines suggest that they are emitted by a more extended and quiescent gaseous component. We argue that, given the radio properties of the source, our line of sight is likely to be lying close to the direction of bulk outflow of the radio jets. In this case it is probable that the quasar nucleus is entirely obscured at optical wavelengths by the material responsible for the H  i absorption-line system. The unusually broad [O  iii ] emission lines suggest that the radio source is intrinsically compact. Overall, our data are consistent the idea that     is a radio source in an early stage of evolution.     

Submillimetre detection of a high-redshift type 2 QSO

We report on the first SCUBA detection of a type 2 QSO at   z = 3.660  in the Chandra Deep Field South. This source is X-ray-absorbed, shows only narrow emission lines in the optical spectrum and is detected in the submillimetre: it is the ideal candidate in an evolution scheme for active galactic nuclei (AGN) (e.g. Fabian 1999 ; Page et al. 2004 ) of an early phase corresponding to the main growth of the host galaxy and formation of the central black hole. The overall photometry (from the radio to the X-ray energy band) of this source is well reproduced by the spectral energy distribution (SED) of NGC 6240, while it is incompatible with the spectrum of a type 1 QSO (3C 273) or a starburst galaxy (Arp 220). Its submillimetre (850 μm) to X-ray (2 keV) spectral slope  (α_SX)  is close to the predicted value for a Compton-thick AGN in which only 1 per cent of the nuclear emission emerges through scattering. Using the observed flux at 850 μm we have derived a star formation rate of  550–680 M_⊙ yr⁻¹  and an estimate of the dust mass   M _dust= 4.2 × 10⁸ M_⊙  .     

Infrared helium–hydrogen line ratios as a measure of stellar effective temperature

We have observed a large sample of compact planetary nebulae in the near-infrared to determine how the 2¹P–2¹S He  i line at 2.058 μm varies as a function of stellar effective temperature, T _eff. The ratio of this line with H  i Br γ at 2.166 μm has often been used as a measure of the highest T _eff present in a stellar cluster, and hence of whether there is a cut-off in the stellar initial mass function at high masses. However, recent photoionization modelling has revealed that the behaviour of this line is more complex than previously anticipated. Our work shows that in most aspects the photoionization models are correct. In particular, we confirm the weakening of the 2¹P–2¹S line as T _eff increases beyond 40 000 K. However, in many cases the model underpredicts the observed ratio when we consider the detailed physical conditions in the individual planetary nebulae. Furthermore, there is evidence that there is still significant 2¹P–2¹S He  i line emission even in the planetary nebulae with very hot     central stars. It is clear from our work that this ratio cannot be considered as a reliable measure of effective temperature on its own.     

Microwave polarization in the direction of galaxy clusters induced by the CMB quadrupole anisotropy

Electron scattering induces a polarization in the cosmic microwave background (CMB) signal measured in the direction of a galaxy cluster owing to the presence of a quadrupole component in the CMB temperature distribution. Measuring the polarization towards distant clusters provides the unique opportunity to observe the evolution of the CMB quadrupole at moderate redshifts, z ∼0.5–3. We demonstrate that for the local cluster population the polarization degree will depend on the cluster celestial position. There are two extended regions in the sky, which are opposite to each other, where the polarization is maximal, ∼0.1( τ /0.02) μK in the Rayleigh–Jeans part of the CMB spectrum ( τ being the Thomson optical depth across the cluster). This value exceeds the polarization introduced by the cluster transverse peculiar motion if v _t<1300 km s⁻¹. One can hope to detect this small signal by measuring a large number of clusters, thereby effectively removing the systematic contribution from other polarization components produced in clusters. These polarization effects, which are of the order of ( v _t c )² τ , ( v _t c ) τ ² and ( kT _e m _e c ²) τ ², as well as the polarization owing to the CMB quadrupole, were previously given by Sunyaev and Zel'dovich for the Rayleigh–Jeans part of the spectrum. We fully confirm their earlier results and present exact frequency dependences for all these effects. The polarization degree is considerably higher in the Wien region.     

ASCA observations of deep ROSAT fields — III. The discovery of an obscured Type 2 AGN at z = 0.67

We report on the discovery of a narrow-emission-line object at z  = 0.672 detected in a deep ASCA survey. The object, AXJ 0341.4–4453, has a flux in the 2–10 keV band of 1.1 ± 0.27 × 10⁻¹³ erg s⁻¹ cm⁻², corresponding to a luminosity of 1.8 × 10⁴⁴ erg s⁻¹ ( q ₀ = 0.5, H ₀ = 50 km s⁻¹ Mpc⁻¹). It is also marginally detected in the ROSAT 0.5–2 keV band with a flux 5.8 × 10⁻¹⁵ erg s⁻¹ cm⁻². Both the ASCA data alone and the combined ROSAT/ASCA data show a very hard X-ray spectrum, consistent with either a flat power law (α < 0.1) or photoelectric absorption with a column of n _H > 4 × 10²² cm⁻² (α = 1). The optical spectrum shows the high-ionization, narrow emission lines typical of a Seyfert 2 galaxy. We suggest that this object may be typical of the hard sources required to explain the remainder of the X-ray background at hard energies.     

Formation of interstellar clouds: Parker instability with phase transitions

We numerically follow the nonlinear evolution of the Parker instability in the presence of phase transitions from a warm to a cold H  i interstellar medium in two spatial dimensions. The nonlinear evolution of the system favours modes that allow the magnetic field lines to cross the galactic plane. Cold H  i clouds form with typical masses  ≃10⁵ M_⊙  , mean densities  ≃20 cm⁻³  , mean magnetic-field strengths  ≃4.3 μG  (rms field strengths  ≃6.4 μG  ), mass-to-flux ratios  ≃0.1–0.3  relative to critical, temperatures  ≃50 K  , (two-dimensional) turbulent velocity dispersions  ≃1.6 km s⁻¹  and separations  ≃500 pc  , in agreement with observations. The maximum density and magnetic-field strength are  ≃10³ cm⁻³  and  ≃20 μG  , respectively. Approximately 60 per cent of all H  i mass is in the warm neutral medium. The cold neutral medium is arranged into sheet-like structures both perpendicular and parallel to the galactic plane, but it is also found almost everywhere in the galactic plane, with the density being highest in valleys of the magnetic field lines. 'Cloudlets' also form whose physical properties are in quantitative agreement with those observed for such objects by Heiles. The nonlinear phase of the evolution takes ≲30 Myr, so that, if the instability is triggered by a nonlinear perturbation such as a spiral density shock wave, interstellar clouds can form within a time suggested by observations.     

Extended line emission around seven radio-loud quasars at redshift z ∼ 2

We present near-infrared spectra of seven radio-loud quasars with a median redshift of 2.1, five of which were previously known to have Ly α nebulae. Extended [O  iii ] λ 5007 and H α emission are evident around six objects, at the level of a few times 10⁻¹⁶ erg cm⁻² arcsec⁻² s⁻¹ within ≃2 arcsec of the nucleus (≡16 kpc in the adopted cosmology). Nuclear [O  ii ] λ 3727 is detected in three of the five quasars studied at this wavelength and clearly extended in one of them.
The extended [O  iii ] tends to be brighter on the side of the nucleus with the stronger, jet-like radio emission, indicating at least that the extranuclear gas is distributed anisotropically. It is also typically redshifted by several hundred km s⁻¹ from the nuclear [O  iii ], perhaps because of the latter being blueshifted from the host galaxy's systemic velocity. Alternatively, the velocity shifts could be due to infall (which is suggested by linewidths ∼1000 km s⁻¹ FWHM) in combination with a suitable dust geometry. Ly α /H α ratios well below the case B value suggest that some dust is present.
Photoionization modelling of the [O  iii ]/[O  ii ] ratios in the extended gas suggests that its pressure is around or less than a few times 10⁷ cm⁻³ K; any confining intracluster medium is thus likely to host a strong cooling flow. A comparison with lower redshift work suggests that there has been little evolution in the nuclear emission-line properties of radio-loud quasars between redshifts 1 and 2.     

[Al ii] in the ultraviolet spectrum of the symbiotic star RR Telescopii

An inspection of a GHRS/ HST spectrum of the symbiotic star RR Telescopii reveals the presence of the [Al  ii ] 3s²¹S – 3s3p ³P₂ line at a vacuum wavelength of 2661.06±0.08 Å, 8.89±0.08 Å away from the Al  ii ] 3s²¹S – 3s3p ³P₁ intercombination transition at 2669.95 Å, in good agreement with the theoretical prediction of Δ λ =8.80 Å. We also find that the Al  ii ] line profile is asymmetric, showing a strong low-density component with a weak high-density wing, redshifted by 30 km s⁻¹, in agreement with the findings of Schild & Schmid, which were based on optical observations. Our measurement of the emission-line ratio R I (2661.06 Å)/ I (2669.95 Å)=0.027±0.003 implies log  N _e=5.8±0.2, in good agreement with the densities found from other ions, such as Si  iii . These results provide strong evidence that we have detected the [Al  ii ] line, the first time (to our knowledge) that this feature has been reliably identified in an astrophysical or laboratory spectrum.     

Infrared spectroscopy of the Be/X-ray transient A0535+26

We present infrared spectroscopy of the Be/X-ray binary HDE 245770/A0535+26 obtained over the period 1992–1995. The spectra show significant variability, reflecting changes in the circumstellar environment during this time. A reduction in the flux observed in the Paschen series lines between 1993 December and 1994 September correlates with a similar reduction in both the strength of Hα and the optical continuum emission, which can be attributed to a reduction in the emission measure of the disc. A turnover between optically thin and thick emission is seen for both Paschen and Brackett series lines, and allows an estimate of the disc density as ∼10¹² cm⁻³. Echelle spectroscopy reveals strong similarities between the He I 1.008, 2.058 μm, Hα and Paschen series line profiles, suggesting their formation in a similar (and asymmetric) region of the disc. In contrast, the line profile of He I 6678 Å indicates that it is formed at smaller radii than the other transitions.     

Spectroscopy of the optical Einstein ring 0047 – 2808

We present optical and near-infrared spectroscopic observations of the optical Einstein ring 0047 – 2808. We detect both [O III ] lines λλ4959, 5007 near ∼ 2.3 μm, confirming the redshift of the lensed source as z  = 3.595. The Lyα line is redshifted relative to the [O III ] line by 140 ± 20 km s⁻¹. Similar velocity shifts have been seen in nearby starburst galaxies. The [O III ] line is very narrow, 130 km s⁻¹ FWHM. If the ring is the image of the centre of a galaxy, the one-dimensional stellar velocity dispersion σ = 55 km s⁻¹ is considerably smaller than the value predicted by Baugh et al. for the somewhat brighter Lyman-break galaxies. The Lyα line is significantly broader than the [O III ] line, probably due to resonant scattering. The stellar central velocity dispersion of the early-type deflector galaxy at z  = 0.485 is 250 ± 30 km s⁻¹. This value is in good agreement both with the value predicted from the radius of the Einstein ring (and a singular isothermal sphere model for the deflector), and with the value estimated from the D _n −σ relation.