Nuclear Spirals Research Articles

NIR properties of isolated spirals with and without an AGN

We analyze here the morphological and photometrical properties of a sample of isolated spirals with (18) and without (11) an active nucleus, based on near-infrared imaging in the J and K' bands. No difference is found between the properties of bulges, disks and bars of active and non-active galaxies. Central colors (the innermost 200 pc) of active galaxies are redder that the centers of non-active spirals. Secondary bars are not ubiquitous. Recent numerical simulations seem to be able to account for those active, barred galaxies with no secondary bar, either by means of nuclear spirals or by the direct effect of the primary bar.

Density Waves inside the Inner Lindblad Resonance: Nuclear Spirals in Disk Galaxies

We analyze the formation of grand-design two-arm spiral structure in the nuclear regions of disk galaxies. Such morphology has been recently detected in a number of objects using high-resolution near-infrared observations. Motivated by (1) the observed continuity between the nuclear and kiloparsec-scale spiral structures and (2) low arm-interarm contrast, we apply the density wave theory to explain the basic properties of the spiral nuclear morphology. In particular, we address the mechanism for the formation, maintenance, and detailed shape of nuclear spirals. We find that the latter depends mostly on the shape of the underlying gravitational potential and the sound speed in the gas. Detection of nuclear spiral arms provides diagnostics of mass distribution within the central kiloparsec of disk galaxies. Our results are supported by two-dimensional numerical simulations of gas response to the background gravitational potential of a barred stellar disk. We investigate the parameter space allowed for the formation of nuclear spirals using a new method for constructing a gravitational potential in a barred galaxy, where positions of resonances are prescribed.

ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] Observations of the C[CLC]f[/CLC]A Seyfert 2 Galaxies: The Fueling of Active Galactic Nuclei

We present an investigation of possible fueling mechanisms operating in the inner kiloparsec of Seyfert galaxies. We analyze visible and near-infrared Hubble Space Telescope images of 24 Seyfert 2 galaxies from the CfA Redshift Survey sample. In particular, we are searching for the morphological signatures of dynamical processes responsible for transporting gas from kiloparsec scales into the nucleus. The circum- nuclear regions are very rich in gas and dust, often taking the form of nuclear spiral dust lanes on scales of a few hundred parsecs. While these nuclear spirals are found in 20 of our 24 Seyfert galaxies, we —nd only —ve nuclear bars among the entire sample, strongly reinforcing the conclusions of other investiga- tors that nuclear bars are not the primary means of transporting this material into the nucleus. An esti- mate of the gas density in the nuclear spirals, based on extinction measurements, suggests that the nuclear spiral dust lanes are probably shocks in nuclear gas disks that are not strongly self-gravitating. Since shocks can dissipate energy and angular momentum, these spiral dust lanes may be the channels by which gas from the host galaxy disks is being fed into the central engines.

Nuclear Bar, Star Formation, and Gas Fueling in the Active Galaxy NGC 4303

A combination of Hubble Space Telescope (HST) WFPC2 and NICMOS images are used to investigate the gas/dust and stellar structure inside the central 300 pc of the nearby active galaxy NGC 4303. The NICMOS H-band (F160W) image reveals a bright core and a nuclear elongated bar-like structure of 250 pc in diameter. The bar is centered on the bright core, and its major axis is oriented in proyection along the spin axis of the nuclear gaseous rotating disk recently detected (Colina & Arribas 1999). The V-H (F606W - F160W) image reveals a complex gas/dust distribution with a two-arm spiral structure of about 225 pc in radius. The southwestern arm is traced by young star-forming knots while the northeastern arm is detected by the presence of dust lanes. These spirals do not have a smooth structure but rather they are made of smaller flocculent spirals or filament-like structures. The magnitudes and colors of the star-forming knots are typical of clusters of young stars with masses of 0.5 to 1 x $10^5 M_{solar}, and ages of 5 to 25 million years. The overall structure of the nuclear spirals as well as the size, number and masses of the star-forming knots are explained in the context of a massive gaseous nuclear disk subject to self-gravitational instabilities and to the gravitational field created by the nuclear bar. According to the model, the gaseous disk has a mass of about 5 x 10^7 M_{solar} inside a radius of 400 pc, the bar has a radius of 150 pc and a pattern speed of about 0.5 Myr^{-1}, and the average mass accretion rate into the core (R < 8 pc) is about 0.01 M_{solar}$ yr^{-1} for about 80 Myr.

Metric characteristics of nuclear rings and related features in spiral galaxies

The metric properties of nuclear rings, pseudorings, spirals, and bars are discussed in the context of resonance theory and are used to gauge the range of sizes for such features. We find that the nuclear rings and pseudorings of strongly barred galaxies (types SB and SAB) have a wide range in linear diameter, from about 0.4 kpc in ESO 566-24 to 3.8 kpc in ESO 565-11. The mean diameter is about 1.1 kpc for a sample of 20 objects. We suggest that there are clear analogues of the nuclear rings of strongly barred spirals in both weakly barred (SAB and SAB) and nonbarred (SA) spirals. The size ratios of nuclear rings with respect to outer rings and pseudorings, as well as morphological characteristics, are best explained if outer rings and pseudorings are linked to the outer Lindblad resonance, and nuclear rings, pseudorings, and spirals are linked to the inner Lindblad resonance

Collimated Bipolar Outflow and the Formation of Nuclear Spirals; Possible Role of Magnetic Fields

The existence of non–steady phenomena, namely activity in the form of radial motions (outflow) of matter from the nuclei of galaxies is well established at present. Active Galactic Nuclei (AGN) constitute a topic of great interest and are intensively studied by all existing observational techniques. Conventionally objects classified as AGN span a range from quasars, radio galaxies to Seyferts 1 and 2. It appears, however, that there exist galaxies which exhibit somewhat milder activity which does not qualify their inclusion in the AGN group. The designation of MAGN (M for mildly) was suggested in the past (Pismis, 1986) to cover the less energetic nuclei. It may be reasonable to consider that active nuclei form a sequence, the difference along it being due to the energetics of the nuclei, from the most active quasars and radio galaxies down to the mildest ones like M31 or our Galaxy. The phenomenon underlying the activity may thus be universal, subject to the intrinsic energetics of the nuclei (Pismis, 1987).

Formation of Bi-Polar Spiral Features in Galactic Nuclei

In earlier communications we have called attention to the existence of very tight spiral formations in the nuclei of some galaxies, particularly of the early type barred spirals, proposing a mechanism for their formation (Pişmiş and Moreno 1984). These kiloparsec-scale structures are loosely termed “rings” but their overall morphology suggests strongly that the image is a tightly wound double spiral usually delineated by knots of emitting material, “hot-spots”. A striking characteristic of these spirals is their very high luminosity compared to the outer regions of the galaxy. 26 galaxies with “nuclear” spirals are listed by Buta, thesis. A systematic search with short exposure high resolution imagery may reveal many more nuclear spirals. NGC 4736 (Sb) though not a barred spiral has a well defined inner spiral delineated by very luminous H II regions.