arXiv subjects

I. F. Mirabel

Publications and source records attributed to I. F. Mirabel.

At least 127 records · Page 7Linked to original sources

arXiv1999

Smoke in the "Smoke Rings": ISO Observations of Dust in Collisional Ring Galaxies

Collisional ring galaxies probably result from a head-on collision between a compact companion galaxy and a gas-rich disk system. We present a review of the discovery of warm dust in five collisional rings observed by ISO which range in total Far-IR luminosity from 10$^{10}$~ $<$ ~L$_{FIR}$ ~$<$ ~10$^{11}$ L$\odot$. The results show that in most cases, the mid-IR (MIR) flux is often a significant fraction of the total energy budget of star formation activity in these galaxies (at least 10% even in the least powerful cases). We argue that the MIR emission, when combined with optical and radio observations, allows us to build a more complete picture of activity in these collisional systems. Although not as extreme as ULIRGs, these collisional systems provide low-redshift examples of the early effects of galaxy collisions on the ISM and may be relevant to the collisional assembly of galaxy disk components at high redshift.

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arXiv1999

Simultaneous Observations of GRS 1758-258 in 1997 by VLA, IRAM, SEST, RXTE and OSSE: Spectroscopy and Timing

We report the results of our multi-wavelength observations of GRS 1758-258 made in August 1997. The energy bands include radio, millimeter, X-ray, and gamma-ray. The observations enable us to obtain a complete spectrum of the source over an energy range of 2 - 500 keV. The spectrum shows that GRS 1758-258 was in its hard state. It is well fitted by the Sunyaev-Titarchuk (ST) Compton scattering model. The spectrum is also fit by a power law with an exponential cutoff (PLE) plus a soft black-body component. The temperature of the soft component is about 1.2 keV, and the energy flux is less than 1.5% of the total X- and gamma-ray flux. The deduced hydrogen column density is in the range of (0.93 - 2.0) 10^{22} cm^{-2}. No significant iron lines are detected. The radio emission has a flat energy spectrum. The daily radio, X-ray and gamma-ray light curves show that GRS 1758-258 was stable during the observation period, but was highly variable on smaller time scales in X- and gamma-rays. The power density spectra are typical for the low-state, but we find the photon flux for the 5 to 10 keV band to be more variable than that in the other two energy bands (2 - 5 keV and 10 - 40 keV). Harmonically spaced quasi-periodic oscillations (QPOs) are observed in the power spectra. The phase lags between the hard photons and the soft photons have a flat distribution over a wide range of frequencies. A high coherence of about 1.0 (0.01 - 1 Hz) between the hard photons and the soft photons is also obtained in our observations. We compare these results with two variation models. Our millimeter observations did not reveal any conclusive signatures of an interaction between the jet from GRS 1758-258 and the molecular cloud that lies in the direction of GRS 1758-258.

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arXiv1999

Mid-IR imaging of Toomre's Merger Sequence

We present results on mid-IR (5--16micron) spectral imaging of a sequence of interacting galaxies, observed by ISOCAM. The galaxies are part of the well known Toomre's ``merger sequence'' which was defined as a sample of galaxies depicting progressive snapshots in the time evolution of a merging event. To trace the intensity of the radiation field in a starburst, we use the ratio of the 15 to 7 micron flux. Our analysis indicates that this ratio increases as galaxies move from the pre-starburst to the starburst phase and goes again down to ~1 in the post-starburst phase, a value typical of normal star forming regions in galactic disks. Moreover, we find that the variation of this ratio is well correlated with the one of the IRAS 25micron/12micron and 60micron/100micron flux ratios.

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arXiv1999

ISO Images of Starbursts and Active Galaxies

We present some highlights from the mid-infrared (5-16 micron) images of mergers of massive galaxies obtained with the Infrared Space Observatory (ISO). We have observed: 1) ultraluminous infrared nuclei, 2) luminous dust-enshrouded extranuclear starbursts, and 3) active galaxy nuclei (AGNs). In this contribution we discuss the observations of Arp 299, a prototype for very luminous infrared galaxies, the Antennae which is a prototype of mergers, and Centaurus A which is the closest AGN to Earth. From these observations we conclude the following: 1) the most intense starbursts in colliding systems of galaxies and the most massive stars are dust-enshrouded in regions that appear inconspicuous at optical wavelengths, 2) the most intense nuclear infrared sources are a combination of AGN and starburst activity, 3) the hosts of radio loud AGNs that trigger giant double-lobe structures may be symbiotic galaxies composed of barred spirals inside ellipticals.

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arXiv1999

Plasma and Warm Dust in the Collisional Ring Galaxy VIIZw466 from VLA and ISO Observations

We present the first mid-infrared (Mid-IR) ($\lambda5-15μ$m) and radio continuum ($λλ$20,~6 and 3.6 cm) observations of the star-forming collisional ring galaxy VII Zw 466 and its host group made with the Infrared Space Observatory and the NRAO Very Large Array. A search was also made for CO line emission in two of the galaxies with the Onsala 20m radio telescope and upper limits were placed on the mass of molecular gas in those galaxies. The ring galaxy is believed to owe its morphology to a slightly off-center collision between an `intruder' galaxy and a disk. An off-center collision is predicted to generate a radially expanding density wave in the disk which should show large azimuthal variations in overdensity, and have observational consequences. The radio continuum emission shows the largest asymmetry, exhibiting a crescent-shaped distribution consistent with either the trapping of cosmic-ray particles in the target disk, or an enhanced supernova rate in the compressed region. On the other hand, the ISO observations (especially those made at $\lambda9.6μ$m) show a more scattered distribution, with emission centers associated with powerful star formation sites distributed more uniformly around the ring. Low-signal to noise observations at $\lambda15.0μ$m show possible emission inside the ring, with little emission directly associated with the \ion{H}{2} regions. The observations emphasize the complex relationship between the generation of radio emission and the development of star formation even in relatively simple and well understood collisional scenarios.

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arXiv1999

Long-Term X-ray Monitoring of 1E 1740.7-2942 and GRS 1758-258

We report on long-term observations of the Galactic-bulge black hole candidates 1E 1740.7-2942 and GRS 1758-258 with the Rossi X-Ray Timing Explorer. 1E 1740.7-2942 has been observed 77 times and GRS 1758-258 has been observed 82 times over the past 1000 days. The flux of each object has varied by no more than a factor of 2.5 during this period, and the indices of the energy spectra have varied by no more than 0.4. The power spectra are similar to other black-hole candidates: flat-topped noise, breaking to a power law. Each object has exhibited a brightening that lasted for several months, and we have a found a time lag between the photon power-law index and the count rate. In both sources, the spectrum is softest during the decline from the brightening. This behavior can be understood in the context of thin-disk and advection-dominated accretion flows coexisting over a wide range of radii, with the implication that both sources have low-mass companions and accrete via Roche-lobe overflow.

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arXiv1999

Feeding the central engine in giant radio galaxies

Giant radio galaxies are thought to be massive ellipticals powered by accretion of interstellar matter onto a supermassive black hole. Interactions with gas rich galaxies may provide the interstellar matter to feed the active galactic nucleus (AGN). To power radio lobes that extend up to distances of hundreds of kiloparsecs, gas has to be funneled from kiloparsec size scales down to the AGN at rates of ~1 Mo yr^-1 during >10^8 years. Therefore, large and massive quasi-stable structures of gas and dust should exist in the deep interior of the giant elliptical hosts of double lobe radio galaxies. Recent mid-infrared observations with ISO revealed for the first time a bisymmetric spiral structure with the dimensions of a small galaxy at the centre of Centaurus A (Mirabel et al. 1999, A&A 341, 667). The spiral was formed out of the tidal debris of accreted gas-rich object(s) and has a dust morphology that is remarkably similar to that found in barred spiral galaxies. The observations of the closest AGN to Earth suggest that the dusty hosts of giant radio galaxies like CenA, are "symbiotic" galaxies composed of a barred spiral inside an elliptical, where the bar serves to funnel gas toward the AGN.

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arXiv1998

Molecular Gas in The Cartwheel Galaxy

We present the first detection of molecular gas in the Cartwheel, the prototype of a collisional ring galaxy formed in the head-on encounter of two galaxies. Until now, only very little atomic gas and no CO had been detected in the centre, where gas is theoretically expected to pile up. Using the Swedish ESO Submm Telescope, we detected both 12CO(1-0) and (2-1) line emission towards the central position. The line ratio and the line widths suggest that the CO(2--1) emission is sub-thermal and that the CO(1-0) emission arises within the central 22'' (13 kpc); it is probably associated with the inner ring and nucleus. We infer a mass of molecular gas (H2) of 1.5 to 6 10^9 M_sol, which is significantly higher than the approx. 10^8 M_sol of atomic gas within that region. The low excitation of the gas, whether it is due to a low temperature or a low density, is consistent with the weak star-forming activity observed in the centre of the Cartwheel.

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arXiv1998

A barred spiral at the centre of the giant elliptical radio galaxy Centaurus A

We report observations at mid-infrared and sub-millimeter wavelengths of Centaurus A (CenA, NGC 5128), the giant elliptical galaxy that harbors the closest radio loud Active Galactic Nucleus (AGN) to Earth. The dust emission from the deep interior of CenA reveals a bisymmetric structure with a diameter of 5' (5 kpc), centred at the AGN. This structure is remarkably similar to that of a barred spiral galaxy, with the bar lying in a plane that is tilted ~18 degrees from the line of sight. The true nature of the distribution of dust in the inner regions of CenA is noticeably displaced from the more chaotic and widespread optical obscuration. The barred spiral is a quasi-stable structure formed at the center of the giant elliptical from the tidal debris of a gas-rich object(s) accreted in the past 10^9 years. The total size and mass of interstellar gas in the barred spiral at the center of CenA is comparable to the small Local Group spiral galaxy Messier 33. The observation of this remarkable structure opens the more general question on whether the dusty hosts of giant radio galaxies like CenA, are "symbiotic" galaxies composed of a barred spiral inside an elliptical, where the bar serves to funnel gas toward the AGN.

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arXiv1998

Dust in the Wheel: The Cartwheel Galaxy in the Mid-IR

We present mid-infrared images at $6.7 μm$ and $15 μm$ of ``The Cartwheel'' (AM~0035-33), the prototypical collisional ring galaxy. The observations, taken with ISOCAM, reveal the distribution of hot dust in the galaxy and its two companions in the north-east. The intensity of the mid-IR emission from the outer star forming ring of the Cartwheel shows considerable azimuthal variation and peaks at the most active H$α$ region of the ring. The 15 $μm$ to 6.7 $μm$ flux ratio of 5.2 is the highest among all the galaxies of our sample. A surprising result of our observations is the discovery of significant emission from the inner regions of the galaxy, including the inner ring, spokes and nucleus, where previously only low level H$α$ emission had been reported. At 6.7 $μm$, this emission is stronger than the one from the outer star forming ring, and at 15 $μm$, it represents 40% of the emission from the outer ring. The H$α$ to mid-IR flux ratios from the inner regions are consistent with the heating of grains from weak star formation activity.

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arXiv1998

Search for the optical and infrared counterpart of GRS 1758-258

We report the results of a deep search for the optical and near infrared counterpart of the microquasar source GRS1758-258. At least two possible candidate counterparts of the binary star companion have been recognized on the basis of astrometric coincidence to within 1". Our photometric study shows that the brightest of them would be consistent with a K-type giant star, while the weakest one would be a main sequence F companion. Follow up spectroscopic observations in the near infrared H and K-bands have failed so far to provide evidence for emission lines that may support an unambiguous identification. However, the proximity of these two sources to the sub-arcsec VLA radio position of GRS 1758-258 makes them deserving further attention in the future.

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arXiv1998

Repeated Relativistic Ejections in GRS 1915+105

In 1994 February-August we observed with the VLA four ejection events of radio emitting clouds from the high energy source GRS 1915+105. These events are all consistent with anti-parallel ejections of twin pairs of clouds moving away from the compact source at $\sim$ 0.92 of the speed of light and angles of $\sim$ 70$^{\circ}$ with respect to the line of sight. The flux ratios and time evolution of the expelled clouds are consistent with actual motions of the radiating matter rather than with the simple propagation of pulses in a medium moving at slower velocities. The large kinetic power of the sudden, short, and rather discontinuous ejections exceeds by more than an order of magnitude the maximum steady photon luminosity of the source, suggesting that in GRS 1915+105 a radiation acceleration mechanism of the ejecta is unlikely. As in other galactic and extragalactic radio sources, the decrease in flux density as a function of angular separation from the central source shows a steepening with distance. Additional ejection events have been observed in 1995 and 1997 and we compare them with the 1994 events.

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arXiv1998

The dark side of star formation in the Antennae galaxies

We compare mid-infrared images of the Antennae galaxies (NGC 4038/39) from the Infrared Space Observatory, with optical images from the Hubble Space Telescope. The mid-infrared observations show that the most intense starburst in this colliding system of galaxies takes place in an off-nucleus reg ion that is inconspicuous at optical wavelengths. The analyses of the mid-infrar ed spectra indicate that the most massive stars are being formed in an optically obscured knot of 50 pc radius, which produces about 15% of the total luminosity from the Antennae galaxies between 12.5 microns and 18 microns. The mid-infrared observations reported here demonstrate that the interpretation of star formation properties in colliding/merging systems based on visible wavelengths alone can be profoundly biased due to dust obscuration. The multiwavelength view of this nearby prototype merging system suggests caution in deriving scenarios of early evolution of high redshift galaxies using only observations in the narrow rest-frame ultraviolet wavelength range.

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arXiv1997

Multi-Wavelength Monitoring of GRS 1915+105

Since its discovery in 1992, the superluminal X-ray transient GRS 1915+105 has been extensively observed in an attempt to understand its behaviour. We present here first results from a multi-wavelength campaign undertaken from July to September 1996. This study includes X-ray data from the RXTE All Sky Monitor and BATSE, two-frequency data from the Nancay radio telescope, and infrared photometry from the 1.8m Perkins telescope at Lowell Observatory. The first long-term well-sampled IR light curve of GRS 1915+105 is presented herein and is consistent with the interpretation of this source as a long-period binary. We compare the various light curves, searching for correlations in the behaviour of the source at differing wavelengths and for possible periodicities.

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arXiv1997

Accretion instabilities and jet formation in GRS 1915+105

We report simultaneous observations in the X-ray, infrared, and radio wavelengths of the galactic superluminal source GRS 1915+105. During episodes of rapid disappearance and follow up replenishment of the inner accretion disk evidenced by the X-ray oscillating flux, we observe the ejection of relativistic plasma clouds in the form of synchrotron flares at infrared and radio wavelengths. The expelled clouds contain very energetic particles with Lorentz factors of about 1000, or more. These ejections can be viewed as small-scale analogs of the more massive ejecta with relativistic bulk motions that have been previously observed in GRS 1915+105.

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arXiv1997

The hard X-ray source GRS 1734-292: a Seyfert 1 galaxy behind the Galactic Center

The radio, infrared and optical counterparts of the hard X-ray source GRS 1734-292 (l{II}=358.9 deg, b{II}=+1.4 deg) are reported. The optical spectrum exhibits broad (>1000 km s^-1) H alpha emission at a redshift of 0.0214 +/- 0.0005. The radio counterpart is a double-sided synchrotron jet of 5 arcsec, which at a distance of 87 Mpc corresponds to a size of 2 kpc. The multiwavelength observations of GRS 1734-292 indicate that this X-ray source is a Seyfert 1 galaxy behind 6 +/- 1 magnitudes of visual absorption.

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arXiv1997

Galactic Black Hole Binaries: Multifrequency Connections

We review the recent multifrequency studies of galactic black hole binaries, aiming at revealing the underlying emission processes and physical properties in these systems. The optical and infrared observations are important for determining their system parameters, such as the companion star type, orbital period and separation, inclination angle and the black hole mass. The radio observations are useful for studying high energy electron acceleration process, jet formation and transport. X-ray observations can be used to probe the inner accretion disk region in order to understand the fundamental physics of the accretion disk in the strongest gravitational field and the properties of the black hole. Future higher sensitivity and better resolution instrumentation will be needed to answer the many fundamental questions that have arisen.

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arXiv1997

Infrared Jet of the Galactic Superluminal Source GRS 1915+105

A near-infrared jet emanating from the galactic superluminal source GRS 1915+105, discovered at $λ$ = 2.2 micrometer by Sams et al. (1996) on 18-21 July 1995, was not seen later anymore, neither in the Mirabel et al. (1996) images, taken on 4 August 1995 and that we analyze here, nor by Eikenberry & Fazio (1997) on 16-17 October 1995. Therefore, the radiative lifetime of the electrons which have created this emission is $τ$ < 17 days. The detection, and the later non-detection, of this infrared jet is consistent with a synchrotron emission, provided that the magnetic field in the ejected clouds is B > 160 mG, and that the Lorentz factor of the internal motions of the electrons in the jet is $γ< 10^{4}$. The lower limit for the magnetic field suggests a magneto-hydrodynamic mechanism for the acceleration and collimation of the ejecta.

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