Central Regions Of Active Galaxies Research Articles

Origin of the light cosmic ray component below the ankle

The origin and nature of the ultrahigh energy cosmic rays remains a mystery. However, considerable progress has been achieved in past years due to observations performed by the Pierre Auger Observatory and Telescope Array. Above ${10}^{18}\text{ }\text{ }\mathrm{eV}$ the observed energy spectrum presents two features: a hardening of the slope at $\ensuremath{\sim}{10}^{18.6}\text{ }\text{ }\mathrm{eV}$, which is known as the ankle, and a suppression at $\ensuremath{\sim}{10}^{19.6}\text{ }\text{ }\mathrm{eV}$. The composition inferred from the experimental data, interpreted by using the current high energy hadronic interaction models, seems to be light below the ankle, showing a trend to heavier nuclei for increasing values of the primary energy. Also, the anisotropy information is consistent with an extragalactic origin of this light component that would dominate the spectrum below the ankle. Therefore, the models that explain the ankle as the transition from the galactic and extragalactic components are disfavored by present data. Recently, it has been proposed that this light component originates from the photodisintegration of more energetic and heavier nuclei in the source environment. The formation of the ankle can also be explained by this mechanism. In this work, we study in detail this general scenario, but in the context of the central region of active galaxies. In this case, the cosmic rays are accelerated near the supermassive black hole present in the central region of these types of galaxies, and the photodisintegration of heavy nuclei takes place in the radiation field that surrounds the supermassive black hole.

High resolution ALMA observations of dense molecular medium in the central regions of active galaxies

AbstractIn the central regions of active galaxies, dense molecular medium are exposed to various types of radiation and energy injections, such as UV, X-ray, cosmic ray, and shock dissipation. With the rapid progress of chemical models and implementation of new-generation mm/submm interferometry, we are now able to use molecules as powerful diagnostics of the physical and chemical processes in galaxies. Here we give a brief overview on the recent ALMA results to demonstrate how molecules can reveal underlying physical and chemical processes in galaxies. First, new detections of Galactic molecular absorption systems with elevated HCO/H13CO+column density ratios are reported, indicating that these molecular media are irradiated by intense UV fields. Second, we discuss the spatial distributions of various types of shock tracers including HNCO, CH3OH and SiO in NGC 253 and NGC 1068. Lastly, we provide an overview of proposed diagnostic methods of nuclear energy sources using ALMA, with an emphasis on the synergy with sensitive mid-infrared spectroscopy, which will be implemented by JWST and SPICA to disentangle the complex nature of heavily obscured galaxies across the cosmic time.

Molecules in Action: Extragalactic ISM at high resolution

AbstractDuring the last two decade, observations have shown the potential of molecular tracers to get insights into the physical processes taking place in the central regions of active galaxies. However, observations were severely limited by both sensitivity and resolution. This resulted also in a limited sample of bright enough galaxies where molecular species other than carbon monoxide could be observed. Current instruments like ALMA and the upcoming NOEMA are already changing our view of the extragalactic ISM molecular observations. In fact, it is now possible to study the physical properties of individual spatially resolved star forming GMCs in external galaxies, as well as resolving the physical structure of the ISM in the surroundings of AGNs at scales of a few parsecs. Here I quickly review some of the most recent observational studies in the nuclear regions of galaxies which are setting new standards in the ways we can study the extragalactic ISM properties.

A relation between circumnuclear H I, dust, and optical cores in low-power radio galaxies

From new observations and literature data we investigate the presence of HI, dust, and optical cores in the central kiloparsec of low-power radio galaxies. The goal of this pilot study is to identify physical relations between these components, which can help us to study kinematics and feeding mechanisms in future samples of active galaxies. Our results are consistent with neutral gas being associated with dust on sub-kiloparsec scales. Objects that have HI absorption always have significant amounts of dust in their host galaxy. If there is no visible dust in the host galaxy, there is also no HI absorption. The presence of an unresolved optical core correlates with the HI column density, with the core being absent in high column density sources. This work opens a path for studying the kinematics of cold material in the central regions of active galaxies by combining information of HI absorption and molecular lines. Consistent with previous work, we find no evidence for a compact, parsec-scale obscuring torus in low-power radio galaxies.

Image reconstruction in optical interferometry

Since the first multitelescope optical interferometer [1], considerable technological improvements have been achieved. Optical (visible/infrared) interferometers are now widely open to the astronomical community and provide the means to obtain unique information from observed objects at very high angular resolution (submilliarcsecond). There are numerous astrophysical applications, such as stellar surfaces, environment of premain sequence or evolved stars, and central regions of active galaxies. See [2]-[4] for comprehensive reviews about optical interferometry and recent astrophysical results. As interferometers do not directly provide images, reconstruction methods are needed to fully exploit these instruments. This article aims at reviewing image reconstruction algorithms in astronomical interferometry using a general framework to formally describe and compare the different methods. The challenging issues in image reconstruction from interferometric data are introduced in the general framework of inverse problem approach. This framework is then used to describe existing image reconstruction algorithms in radio interferometry and the new methods specifically developed for optical interferometry.

Feeding Versus Feedback in AGNs from Near-Infrared IFU Observations: The Case of Mrk 1066

Previous studies of the central region of active galaxies show that the molecular and ionized gas have distinct kinematics and flux distributions, with the former dominated by quiescent kinematics characteristic of rotation in the galactic plane and the latter with more disturbed kinematics and apparently extending to larger galactic latitudes. These results suggest that the molecular gas can be a tracer of the feeding of the AGN and the ionized gas a tracer of its feedback (e.g., Riffel et al. 2009, 2008, 2006; Storchi-Bergmann et al. 2009a, b). In the present study we use Gemini NIFS integral field observations of the inner 700×700 pc2 of the Seyfert galaxy Mrk 1066 at a spatial resolution of ~ 35 pc to investigate if the above scenario is also valid for this galaxy.

Cosmic rays in central regions of active galaxies

We consider the acceleration of charged particles by the Fermi mechanism on magnetic field irregularities in active galactic regions. The relativistic particles are shown to be accelerated most efficiently, while the acceleration of nonrelativistic particles by this mechanism is possible only in highly nonuniform galactic nuclei, that is, in nuclei with strong turbulization. The conditions for the acceleration of charged particles in active galactic nuclei at various stages of their evolution are investigated.

X-ray and Gamma-ray properties of AGN: Results from XMM-Newton, Chandra and INTEGRAL

The X-ray observatories XMM-Newton and Chandra provided a wealth of exciting new results on active galaxies, and many more are expected to emerge soon from the Gamma-ray mission INTEGRAL. Chandra delivered X-ray images of outstanding detail, reaching subarcsecond spatial resolution for the first time in X-ray astronomy. XMM-Newton with its high sensitivity provided X-ray spectra of unprecedented signal/noise. Spectra are resolved in great detail with the spectrometers aboard XMM-Newton and Chandra for the first time. INTEGRAL is dedicated to imaging and spectroscopy in the X-ray and Gamma-ray regime up to 3 MeV with significantly improved sensitivity and resolution as compared to previous Gamma-ray missions. The X-ray and Gamma-ray imaging and spectral observations are greatly improving our understanding of the physical processes in the central region of active galaxies. Here, I give a review of recent X-ray and Gamma-ray results on Active Galactic Nuclei.

Stellar Population Synthesis in AGN Host Galaxies

We are interested in the stellar content in the central region of active galaxies. In this aim, we perform stellar population synthesis of the integrated starlight in the Optical and NIR using a mathematical method (Pelat, 1997, MNRAS 284, 365) dedicated to the determination of the best solution (among the many local solutions) in the stellar population synthesis problem which is a complex inverse problem. A new development of the code allows the determination of the error zone around the solution, expected from the observational uncertainties. In the next sections, we present this very powerful method and results obtained from a sample of AGN.

Reprocessing of radiation by multi-phase gas in low-luminosity accretion flows

We discuss the role that magnetic fields in low-luminosity accretion flows can play in creating and maintaining a multi-phase medium, and show that small clouds or filaments of dense cold gas can dramatically reprocess the ‘primary’ radiation from tori. In particular, radio emission is suppressed by free-free absorption, and an extra (weak) component appears at optical wavelengths. This is expected to be a common process in various environments in the central regions of active galaxies, such as broad-line regions, accretion disc coronae and jets.

Submillimeter Astronomy Reaches New Heights

LLANO DE CHAJNANTOR, CHILE-- High in the Chilean Andes, over 5000 meters above sea level, astronomers are planning the world9s loftiest observatory: a 10-kilometer-wide array of dishes to peer into the universe9s cold recesses and open a sharp eye on celestial features ranging from surface markings on Pluto to extrasolar planets to the dusty central regions of active galaxies. Lying in one of the driest deserts on Earth and perched above 50% of the atmosphere, this plateau is the next best place to outer space for submillimeter studies. The array is being planned by a nascent collaboration between the European Southern Observatory, the U.S. National Radio Astronomy Observatory, and possibly Japanese astronomers as well.

Circumnuclear Star Formation in Active Galaxies

We study the gas kinematics and chemical abundances in the inner regions of six galaxies with active galactic nuclei. The galaxies were selected for having star-forming regions in rings or spirals within a few kiloparsecs of the nucleus. The goal of the project is to search for any connections between the gas-dynamics, the chemical abundances, and the nuclear activity. Narrowband images have been obtained to map out the ionized gas. Medium-dispersion long-slit spectroscopy at several positions over the nuclear region has been used to obtain the gaseous velocity field and distribution of excitation. The H ii regions in the rings are located near the turnover point of the rotation curves, suggesting association with the inner Lindblad resonance. The nuclear ring in the LINER NGC 1326 may be located between the two inner Lindblad resonances, and the inner rings in the Seyfert 2 galaxy NGC 3081 and the LINER/ Seyfert 1 NGC 7213 may be associated with the Ω – κ/4 resonance. A warm ionized medium (WIM) is found to pervade the inner region in all galaxies. There is a correlation between the [N II]/Hα and [S ii]/ Hα ratios and the FWHM of the [N ii] λ6584 emission line in this WIM, indicating that shocks contribute to the ionization of the gas. We show that contamination by this diffuse emission increases the measured [N ii]/Hα and [S ii]/Hα ratios at H ii regions close to the nuclei and thus may account, in part, for the systematically higher [N ii]/Hα and [S ii]/Hα found in the present work and by previous authors for near-nuclear H ii regions compared with H ii regions farther out in the disk of the galaxy. In the present paper, we correct for this contamination by interpolating the contribution of the WIM at the H ii regions and subtracting it off. Oxygen and nitrogen abundances are obtained for 18 H ii regions and are extrapolated to obtain the abundance of the nuclear gas, which is found to range from solar to 2 times solar for oxygen and from 2 to 4 times solar for nitrogen. Such values are similar to those observed in nonactive galaxies with the same luminosity and morphological type and do not support the idea that the central regions of active galaxies have undergone unusual chemical processing.

Mechanism of formation of a dipole magnetic field in the central regions of active galaxies

A model of the formation of large-scale magnetic fields of dipole configuration in the central regions (r ≈ 100 pc) of active galaxies is studied. It is assumed that these regions contain a rapidly rotating, highly ionized gas (Ω ≈ 5·10−15 sec, Ne ≈ 103 cm−3). Ionized matter escapes from the center of the region with a velocity of several hundred km/sec and is entrained by the rotation of the surrounding medium. Biermann's "battery" effect [L. Biermann, Z. Naturforsch., 5a, 65 (1950)] operates under such conditions, and circular electric currents are formed in the medium, which amplify the dipole magnetic fields. During the active phase of a galaxy, about 108 years, the magnetic field strength at the boundary of this region may reach 10−4–10−3 G.

Synchrotron Radiation from Energetic Electrons Emitted by AGN: A Probe for Magnetic Fields in External Galaxies

Shock acceleration of protons in the central region of active galaxies can energize them to Lorentz factors as high as 108 (Sikora et al., 1987), and these can subsequently produce a host of other relativistic particles including pions, e+e– pairs and neutrons. The luminosities of each species are expected to be of the same order of magnitude. Rapid decay of the pions leads to the secondary production of photons and pairs with energies of around 109 − 1011 MeV. The electrons and positrons can escape the compact central region and interact with the microwave background forming a pair cascade, and can also emit synchrotron radiation in the magnetic field. The neutrons do not interact with the field, and a significant fraction of them can escape the central region of a galaxy (Kirk and Mastichiadis, 1989). They can travel until they decay, producing protons and electrons in outer regions of the galaxy. Their decay time of γnτn gives a typical length for decay of about 1 kpc for the most energetic neutrons. The synchrotron radiation of these decay product electrons is examined in Baring (1989, in preparation), and it produces definite signatures of galactic magnetic fields. Magnetic fields of 1μG imply synchrotron emission in the X-ray and soft gamma-ray range for maximum Lorentz factors of γe = 1010, with a continuum extending down to much lower energies. It is observed that cooler neutrons deposit electrons at smaller radii, and these electrons are cooler (in a decay γe ~ γn). Hence the radiation would be cooler at smaller radii. This provides a diagnostic for the magnetic field: estimates of the field strength are possible from cut-offs that are expected in spectra from galactic halos. The injection of energetic electrons via neutron decay is found to yield a sharp cut-off in the injection distribution at γm = γe ~ r/τnc at radius r. Below this, no electrons are injected since they are produced in decays at smaller radii. This implies a low energy cutoff of ωm = γ2mBmec2 in the spectrum at given radius. Typically for r = 10 pc and a field of 1μG, the cutoff is at 10−8mec2 in the far infra-red. At larger radii, this low energy cutoff rapidly increases to X-ray energies. This cut-off provides a good way to measure the magnetic field strength and obtain its spatial dependence. In practice the situation is complicated by the superposition of different regions within the galaxy along the line of sight. Ways in which the observations could be deconvolved are discussed in Baring (1989, in preparation). A real possibility exists for detecting these signatures of energetic neutron emission from central regions of nearby galaxies and using the electron synchrotron spectra to spatially map galactic fields.

Cloud Disruption and Formation

Star formation is responsible at least for a part of radiation coming from central regions of active galaxies. Violent bursts of star formation yield rapidly evolving massive stars, which transform in metal-rich enviroments into so called Warmers / Terlevich et al., 1987 / changing nebular spectra to Seyfert or Liner type.