100,796 research outputs found
The Cold Gaseous Halo of NGC 891
No full textWe present H I observations of the edge-on galaxy NGC 891. These are among the deepest ever performed on an external galaxy. They reveal a huge gaseous halo, much more extended than seen previously and containing almost 30% of the H I. This H I halo shows structures on various scales. On one side, there is a filament extending (in projection) up to 22 kpc vertically from the disk. Small (M gtrsim 106 Modot) halo clouds, some with forbidden (apparently counterrotating) velocities, are also detected. The overall kinematics of the halo gas is characterized by differential rotation lagging with respect to that of the disk. The lag, more pronounced at small radii, increases with height from the plane. There is evidence that a significant fraction of the halo is due to a galactic fountain. Accretion from intergalactic space may also play a role in building up the halo and providing the low angular momentum material needed to account for the observed rotation lag. The long H I filament and the counterrotating clouds may be direct evidence of such accretion
The HI halo of spiral galaxies
No full textA deep H I survey with the VLA of the spiral galaxy NGC 2403 has revealed the existence of a thick, low density layer of neutral gas surrounding the thin 'cold' disk. This layer has a mean rotation velocity 25-50 km s(-1) lower than that of the disk and a 10-20 km s(-1) inflow towards the centre of the galaxy. In the central parts there are velocity differences from rotation of up to 150 km s(-1). Chandra observations of NGC 2403 show a diffuse, hot X-ray emitting gas component with a temperature of a few 106 K. These results point at galactic fountain type of flows between disk and halo. 'Halo' gas with similar characteristics has also been observed in other spiral galaxies (e.g. NGC 6946, NGC 891). Such gas is probably similar to the IVCs and to some of the HVCs of the Milky Way
Hidden interaction in SBO galaxies
No full textGalaxies, like plants, show a large variety of grafts: an individual of some type connects physically with a neighborhood of same or different type. The effects of these interactions between galaxies have a broad range of morphologies depending, among other quantities, on the distance of the closest approach between systems and the relative size of the two galaxies. A sketch of the possible situations is shown in tabular form. This botanical classification is just indicative, because the effects of interactions can be notable also at relatively large separations, when additional conditions are met, as for example low density of the interacting systems or the presence of intra-cluster gas. In spite of the large variety of encounters and effects, in the literature the same terms are often used to refer to different types of interactions. Analysis indicates that only few of the situations show evident signs of interaction. They appear to be most relevant when the size of the two galaxies is comparable. Bridges and tails, like the well known case of NGC 4038/39, the Antennae, are only observed for a very low percentage of all galaxies (approx. 0.38 percent, Arp and Madore 1977). In most cases of gravitational bond between two galaxies, the effects of interactions are not relevant or evident. For instance, the detection of stellar shells (Malin and Carter 1983), which have been attributed to the accretion of gas stripped from another galaxy or to the capture and disruption of a small stellar system (Quinn 1984), requires particular observing and reduction techniques. Besides these difficulties of detection, time plays an important role in erasing, within a massive galaxy, the effects of interactions with smaller objects. This can happen on a timescale shorter than the Hubble time, so the number of systems now showing signs of interaction suggests lower limits to the true frequency of interactions in the life-time of a stellar system
Kinematics of the ionised gas in the spiral galaxy NGC 2403
No full textWe present a study of the kinematics of the ionised gas in the nearby spiral galaxy NGC 2403 using deep long-slit spectra obtained with the 4.2-m William Herschel Telescope. The data show the presence of a halo component of ionised gas that is rotating more slowly than the gas in the disk. The kinematics of this ionised halo gas is similar to that of the neutral halo gas. On small scales. broad line profiles (up to 300 km s(-1) wide) indicate regions of fast Outflows of ionised gas. We discuss these new results in the context of galactic fountain models
The gaseous haloes of disc galaxies
No full textThe study of gas outside the plane of disc galaxies is crucial to understanding the circulation of material within a galaxy and between galaxies and the intergalactic environment. We present new HI observations of the edge-on galaxy NGC891, which show an extended halo component lagging behind the disc in rotation. We compare these results for NGC891 with other detections of gaseous haloes. Finally, we present a dynamical model for the formation of extra-planar gas
Gaseous Haloes: Linking Galaxies to the IGM
No full textIn recent years evidence has accumulated that nearby spiral galaxies are surrounded by massive haloes of neutral and ionised gas. These gaseous haloes rotate more slowly than the disks and show inflow motions. They are clearly analogous to the High Velocity Clouds of the Milky Way. We show that these haloes cannot be produced by a galactic fountain process (supernova outflows from the disk) where the fountain gas conserves its angular momentum. Making this gas interact with a pre-existing hot corona does not solve the problem. These results point at the need for a substantial accretion of low angular momentum material from the IGM
HI study of the warped spiral galaxy NGC5055: a disk/dark matter halo offset?
No full textWe present a study of the Hi distribution and dynamics of the nearby spiral galaxy NGC 5055 based on observations with the Westerbork Synthesis Radio Telescope. The gaseous disk of NGC5055 extends out to about 40 kpc, equal to 3.5 R-25, and shows a pronounced warp that starts at the end of the bright optical disk ( R-25 = 11.6 kpc). This very extended warp has large-scale symmetry, which along with the rotation period of its outer parts (similar or equal to 1.5 Gyr at 40 kpc), suggests a long-lived phenomenon. The rotation curve rises steeply in the central parts up to the maximum velocity (v(max) similar or equal to 206 km s(-1)). Beyond the bright stellar disk (R-25), it shows a decline of about 25 km s(-1) and then remains flat out to the last measured point. The standard analysis with luminous and dark matter components shows the dynamical importance of the disk. The best fit to the rotation curve is obtained with a "maximum disk". Less satisfactory fits with lighter disks help to set a firm lower limit of 1.4 to the mass-to-light ratio in F band of the disk. Such a "minimum disk" contributes about 60% of the observed maximum rotational velocity. NGC 5055 shows remarkable overall regularity and symmetry. A mild lopsidedness is noticeable, however, both in the distribution and kinematics of the gas. The tilted ring analysis of the velocity field led us to adopt different values for the kinematical centre and for the systemic velocity for the inner and the outer parts of the system. This has produced a remarkable result: the kinematical and geometrical asymmetries disappear, both at the same time. These results point at two different dynamical regimes: an inner region dominated by the stellar disk and an outer one, dominated by a dark matter halo offset with respect to the disk
Dwarfs walking in a row. The filamentary nature of the NGC 3109 association.
No full textWe re-consider the association of dwarf galaxies around NGC 3109, whose known members were NGC 3109, Antlia, Sextans A,
and Sextans B, based on a new updated list of nearby galaxies and the most recent data. We find that the original members of the
NGC 3109 association, together with the recently discovered and adjacent dwarf irregular Leo P, form a very tight and elongated
configuration in space. All these galaxies lie within 100 kpc of a line that is ' 1070 kpc long, from one extreme (NGC 3109) to the
other (Leo P), and they show a gradient in the Local Group standard of rest velocity with a total amplitude of 43 km s
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