arXiv subjects

L. Binette

Publications and source records attributed to L. Binette.

52 records · Page 3Linked to original sources

arXiv2002

Probing the absorbing haloes around two high-redshift radio galaxies with VLT-UVES

We present VLT-UVES echelle spectroscopy of the HI and CIV absorption in the spatially-extended Lyman-alpha emission around two high-redshift radio galaxies 0200+015 (z=2.23) and 0943-242 (z=2.92). The absorbers in 0943-242 exhibit little additional structure compared with previous low-resolution spectroscopy and the main absorber is still consistent with HI column density of ~10^19 cm^-2. This is consistent with a picture in which the absorbing gas has low density and low metallicity and is distributed in a smooth absorbing shell located beyond the emission-line gas. However, the main absorbers in 0200+015 are very different. The previous single absorber fit of HI column density \~10^19 cm^-2, now splits into two ~4 x 10^14 cm^-2 absorbers which extend more than 15kpc to obscure additional Ly-alpha emission coincident with a radio lobe in these high-resolution observations. Although consistent with the shell-like distribution for the absorption systems, 0200+015 requires a much higher metal enrichment than 0943-242. The metallicity, inferred from the CIV absorption, is considerably lower in 0943-242 than in 0200+015. We explain these differences with an evolutionary scenario based on the size of the radio source. In both sources the HI absorption gas originates from either a gas-rich merger or pristine cluster gas which cools and collapses towards the centre of the dark matter halo. The higher metallicity in the larger radio source (0200+015) may be a result of a starburst driven superwind (concurrent with the triggering of the radio emission) which has engulfed the outer halo in this older source.

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arXiv2002

The spectral index of the ionizing continuum of quasars

The ionizing spectral energy distribution of quasars exhibits a steepening of the distribution short-ward of 1200 A. The change of the power-law index from approximately -1 to -2 so far has been interpreted as being intrinsic to quasars. We study an alternative interpretation, in which a tenuous absorption screen is responsible for the change of index. We find that the most successful function of HI with z requires that the ionization fraction be independent of the metagalactic background radiation, that is frozen in. This implies that the putative component should be of sufficiently low density that it could not recombine (while the background is subsiding) and/or cool from a high temperature state.

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arXiv2002

Are the Narrow Line Regions in Active Galaxies Dusty and Radiation Pressure Dominated?

The remarkable similarity between emission spectra of narrow line regions (NLR) in Seyfert Galaxies has long presented a mystery. In photoionization models, this similarity implies that the ionization parameter is nearly always the same, about U ~ 0.01. Here we present dusty, radiation-pressure dominated photoionization models that can provide natural physical insight into this problem. In these models, dust and the radiation pressure acting on it provide the controlling factor in moderating the density, excitation and surface brightness of photoionized NLR structures. Additionally, photoelectric heating by the dust is important in determining the temperature structure of the models. These models can also explain the coexistence of the low-, intermediate- and coronal ionization zones within a single self-consistent physical structure. The radiation pressure acting on dust may also be capable of driving the fast (~3000 km/s) outflows such as are seen in the HST observations of NGC 1068.

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arXiv2001

Can stellar winds account for temperature fluctuations in H II regions? The case of NGC 2363

We compare the rate of kinetic energy injected by stellar winds into the extragalactic H II region NGC 2363 to the luminosity needed to feed the observed temperature fluctuations. The kinetic luminosity associated to the winds is estimated by means of two different evolutionary synthesis codes, one of which takes into account the statistical fluctuations expected in the Initial Mass Function. We find that, even in the most favorable conditions considered by our model, such luminosity is much smaller than the luminosity needed to account for the observed temperature fluctuations. The assumptions underlying our study are emphasized as possible sources of uncertainty affecting our results.

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arXiv2001

Shocks and dust survival in nearby active galaxies: implications for the alignment effect

One of the most popular explanations for the so-called alignment effect in high redshift (z>0.7) radio galaxies is the scattering by dust of the hidden quasar light. As shown by De Young (1998) a problem with the dust scattering model is that the short destruction time-scale for dust grains means that they will not survive the passage of the radio jet. We investigate the survival of dust in the extended ionised gas of nearby active galaxies with jet/gas interactions. We discuss the implications on the alignment effect of high redshift (>0.7) radio galaxies. We conclude that although shocks are likely to destroy dust grains in regions of interaction, dust might survive in enough quantities to scatter light from the active nucleus and produce alignment between scattered light and the radio structures. We propose an observational test to investigate the existence of dust in shocked regions based on the sensitivity of calcium to depletion onto dust grains.

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arXiv2000

The Narrow Line Region of Narrow-Line Seyfert 1 Galaxies

This work studies the optical emission line properties and physical conditions of the narrow line region (NLR) of seven narrow-line Seyfert 1 galaxies (NLS1). Our results show that the flux carried out by the narrow component of H-beta is, on average, 50% of the total line flux. As a result, the [OIII] 5007/H-beta ratio emitted in the NLR varies from 1 to 5, instead of the universally adopted value of 10. This has strong implications for the required spectral energy distribution that ionizes the NLR gas. Photoionization models that consider a NLR composed of a combination of matter-bounded and ionization-bounded clouds are successful at explaining the low [OIII] 5007/H-beta ratio and the weakness of low-ionization lines of NLS1s. Variation of the relative proportion of these two type of clouds nicely reproduce the dispersion of narrow line ratios found among the NLS1 sample. Assuming similar physical model parameters of both NLS1s and the normal Seyfert 1 galaxy NGC 5548, we show that the observed differences of emission line ratios between these two groups can be explained in terms of the shape of the input ionizing continuum. Narrow emission line ratios of NLS1s are better reproduced by a steep power-law continuum in the EUV -- soft X-ray region, with spectral index alpha ~ -2. Flatter spectral indices (alpha ~ -1.5) match the observed line ratios of NGC 5548 but are unable to provide a good match to the NLS1 ratios. This result is consistent with ROSAT observations of NLS1s, which show that these objects are characterized by steeper power-law indices than those of Sy1 galaxies with strong broad optical lines.

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arXiv2000

A vestige low metallicity gas shell surrounding the radio galaxy 0943-242 at z=2.92

Observations are presented showing the doublet CIV 1550 absorption lines superimposed on the CIV emission in the radio galaxy 0943-242. Within the errors, the redshift of the absorption system that has a column density of N_CIV = 10^{14.5 +- 0.1} cm-2 coincides with that of the deep Ly-alpha absorption trough observed by Rottgering et al. (1995). The gas seen in absorption has a resolved spatial extent of at least 13 kpc (the size of the extended emission line region). We first model the absorption and emission gas as co-spatial components with the same metallicity and degree of excitation. Using the information provided by the emission and absorption line ratios of CIV and Ly-alpha, we find that the observed quantities are incompatible with photoionization or collisional ionization of cloudlets with uniform properties. We therefore reject the possibility that the absorption and emission phases are co-spatial and favour the explanation that the absorption gas has low metallicity and is located further away from the host galaxy (than the emission line gas). The estimated low metallicity for the absorption gas in 0943-242 (Z \~ 1% solar) and its proposed location -outer halo outside the radio cocoon- suggest that its existence preceeds the observed AGN phase and is a vestige of the initial starburst at the onset of formation of the parent galaxy.

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arXiv2000

Energy implications of temperature fluctuations in photoionized plasma

We quantify the energy radiated through all the collisionally excited lines in a photoionized nebula which is permeated by temperature fluctuations. We assume that these correspond to hot spots which are the results of an unknown heating process distinct from the photoelectric heating. We consider all the effects of using a higher mean temperature (as compared to the equilibrium temperature) due to the fluctuations not only on each emission line but also on the ionization state of the gas. If this yet unknown process was to radiate a fixed amount of energy, we find that the fluctuations should correlate with metallicity Z when it exceeds 0.7 solar. The excess energy radiated in the lines as a result of the fluctuations is found to scale proportionally to their amplitude t^2. When referred to the total energy absorbed through photoionization, the excess energy is comparable in magnitude to t^2.

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arXiv1999

UV rest frame spectroscopy of four high redshift (z>2) active galaxies

We present UV spectra of a small sample of high redshift active galaxies: the hyperluminous, gravitationally lensed system SMM J02399-0136 (z=2.8) and three powerful radio galaxies (z~2.5). Extended continuum and emission lines are detected in all objects. The three radio galaxies present typical spectra with dominant Lya and weaker CIV1550, HeII1640 and CIII]1909. The spectrum of SMM J02399-0136 is strikingly different, showing relatively weak Lya and HeII and strong NV1240 relative to the C lines. We find that the weakness of HeII can be explained if the intermediate density narrow line region (n~10^{5-6} cm^-3 dominates the emission line spectrum, rather than the more extended low density gas (n<100 cm^-3). The line ratios of MRC2025-218 suggest that this is also the case in this object. MRC2025-218 and SMM J02399-0136 show the largest NV/HeII and NV/CIV values found in high redshift radio galaxies. The failure of solar abundance models to explain these line ratios and studies of metal abundances in high redshift quasars and radio galaxies, suggest that nitrogen is overabundant in the ionized gas of these objects. Alternative possibilities which we discuss include NV emission from the broad line region and differential amplification by a gravitational lens. We report the discovery of a PCygni profile in the CIV line in the spectrum of MRC2025-218. We detect also CII1334.5 in absorption and PCygni profiles for the lines SiIV1393.8,1402.8. We do not detect any purely photospheric stellar lines. The nature of the absorption features is not clear. It could be due to stars or an associated absorption line system.

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arXiv1999

The nature of the extreme kinematics in the extended gas of high redshift radio galaxies

We present UV rest frame spectra of 3 powerful narrow line radio galaxies and the hyperluminous type 2 active galaxy SMM02399-0136, all at high redshift (z>2). We find high velocities (FWHM>1000, km/s) in the extended gas of all objects. A natural explanation is the interaction between the radio jet and the ambient gas, that drives shocks into the gas and accelerates the clouds. However, the existence of high velocities in regions where such interactions are not taking place implies that other processes can play a role. We discuss here several possible mechanisms.

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arXiv1998

Evidence of Fermi acceleration of Lyman alpha in the Radio Galaxy 1243+036

The high redshift radiogalaxy 1243+036 (z=3.6) presents an asymmetric Lyman alpha profile of FWHM 1550 km/s as measured by van Ojik et al. We propose that the blue asymmetry in the Lyman alpha profile is not due to narrow absorption dips but consists of narrow emission peaks. We interpret the blueshifted peaks near -1130, -850 and -550 km/s (relative to the peak of full profile) as being the result of Fermi acceleration of Lyman alpha produced by jet-induced star formation in the wake of a 300 km/s shock. This shock would be caused by the deflection of the radio-jet at the observed position of the radio bend which also coincides spatially with the excess Lyman alpha emission reported by van Ojik et al

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arXiv1998

Radiative Acceleration of Coronal Gas in Seyfert Nuclei

The line ratios from coronal gas in Seyferts can be successfully fitted with photoionized clouds of high densities and low volume filling factor. The ionization parameter implied is sufficiently high that models must consider the effect of radiation pressure from the active nucleus. In spite of the nucleus gravitational force, radiation pressure is sufficiently strong to compress and radially accelerate the internally stratified gas clouds provided these contain small amounts of dust (~10% of solar neighborhood value). This radial acceleration could explain the blueshift of the coronal lines relative to systemic velocity without the need of invoking an ambient `pushing' wind. Embedded dust has the interesting effect of making the photoionized clouds marginally ionization bounded instead of matter bounded.

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arXiv1997

The Temperature of Extended Gas in Active Galaxies -- Evidence for Matter-Bounded Clouds

We report measurements of the electron temperature at about a dozen locations in the extended emission-line regions of five active (Seyfert and radio) galaxies. Temperatures (T_{[OIII]} and T_{[NII]}) have been determined from both the I([OIII]\lambda4363)/I([OIII]\lambda5007) and I([NII]\lambda5755)/I[NII]\lambda6583) ratios. T_{[OIII]} lies in the range (1.0 -- 1.7) \times 10^{4}K. We find a strong trend for T_{[OIII]} to be higher than T_{[NII]}, with the difference typically being \approx 5,000K. Because the critical density for collisional de-excitation of the ^{1}D{_2} level in NII is lower than that of the same level in OIII, the deviations of the measured intensity ratios from those expected for T_{[OIII]} = T_{[NII]} in the low density limit are unlikely to result from collisional de-excitation. The measured values of T_{[OIII]} and the differences between T_{[OIII]} and T_{[NII]} are very similar to those found in Galactic planetary nebulae. It is argued that the dominant form of energy input to the clouds is photoionization, but detailed modelling indicates that the temperature difference is too large to be accounted for in terms of photoionization of ionization-bounded clouds. We propose instead that both matter- and ionization-bounded clouds are present in the extended emission-line regions, with most of the [OIII] emission originating from a hot zone in the matter-bounded clouds and essentially all of the [NII] from the ionization-bounded clouds.

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arXiv1996

Ca depletion and the presence of dust in large scale nebulosities in radiogalaxies (II)

We investigate here the origin of the gas observed in extended emission line regions surrounding AGNs. We use the technique of calcium depletion as a test to prove or disprove the existence of dust in such a gas in order to discriminate between two main theories: (1) a cooling process from a hotter X-ray emitting phase surrounding the galaxy, (2) merging or tidal interaction between two or more components. We have obtained long slit spectroscopy of a sample of objects representative of different galaxy types although our main interest focus on radio galaxies. The spectral range was set to always includethe [CaII]$λλ$7291,7324 doublet. The faintness or absence of such lines is interpreted as due to the depletion of calcium onto the dust grains and, therefore, is a proof of the existence of dust mixed with the gas in the EELRs.

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arXiv1996

The effects of resonance scattering and dust on the UV line spectrum of radio galaxies

In the powerful, high redshift radio galaxies, it is believed that the dominant source of ionization for the interstellar gas is the hard radiation field associated with the active nucleus. The photon source is generally external to the clouds being ionized and so the geometrical perspective from which the gas is observed and the presence and distribution of dust must be properly accounted for in the diagnostic process. In this paper, we examine the formation of the three strong lines: CIV$λ$1549, Ly$α$ and CIII]$λ$1909 which are often observed in the nuclear and extended emission from these sources. We find that the observed trends, in particular the high CIV$λ$1549/Ly$α$ ratio, are often better explained by geometrical (viewing angle) effects than by the presence of large quantities of dust either within or outside the excited clouds. We show that neutral condensations along the line-of-sight, by reflecting photons near the wavelength of Ly$α$, can increase the observed CIV/Ly$α$ ratio. The existence of HI absorption clouds (i.e., mirrors) external to the emission region leads also to the presence of large, diffuse haloes of what appears to be pure, narrow Ly$α$ emission.

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arXiv1995

Ca depletion and the presence of dust in large scale nebulosities in radiogalaxies (I)

We show that the study of the Calcium depletion is a valid an highly sensitive method for investigating the chemical and physical history of the very extended ionized nebulae seen around radio galaxies (EELR), massive ellipticals and `cooling flow' galaxies. By observing the near IR spectrum of nebular regions characterized by low excitation emission lines (LINER-like), we can use the intensity of the [CaII]$λλ7291,7324$Å doublet --relative to other lines, like H$α$-- to infer the amount of Calcium depletion onto dust grains. The presence of dust in these objects --which does not necessarily result in a measurable level of extinction-- would favour a `galactic debris' rather than a `cooling flow' origin for the emitting gas. Before aplying such test to our data, we study four possible alternative mechanisms to dust depletion and which could have explained the absence of the [CaII] lines: a) ionization of Ca$^+$ from its metastable level, b) thermal ionization of Ca$^+$, c) a high ionization parameter and/or a harder ionizing contiuum than usually asummed and d) matter bounded models associated to a hard ionizing continuum. We show that none of these alternative mechanisms explain the absence of the [CaII] lines, except possibly for the highly ionized EELR where a high ionization parameter is required combined with a soft power law. We thus conclude that for the other low excitation emission regions (cooling flows, liners, low excitation EELR), the abscence of the CaII lines {\it must} be due to the depletion of Calcium onto dust grains.

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