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Pub. Astron. Soc. Pacific, Volume 85, October 1973 the PECULIAR

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Pub. Astron. Soc. Pacific, Volume 85, October 1973 the PECULIAR Pub. Astron. Soc. Pacific, Volume 85, October 1973 THE PECULIAR GALAXY NGC 523 GUIDO CHINCARINI* McDonald Observatory, The University of Texas AND HARRY M. HECKATHORN NASA Johnson Space Center University of Houston Received 27 April Í973, revised 18 June 1973 Electrographic and spectroscopic observations have been obtained of the peculiar galaxy NGC 523. The observations suggest that the pecularities of the system result from a close encounter by two dwarf galaxies, with tidal interaction responsible for the formation of the galactic bridge and tails. Key words: peculiar galaxies — tidal interaction — close encounters — electrography — spec- troscopy Introduction (CI 717-35m exposure, CI 720.3-15m exposure, 1 The galaxy NGC 523 is listed in Vorontsov- and CI 720.2-55° exposure) the slit was oriented across the bright eastern and western knots at Velyaminov's morphological catalogue (MCG 111 6-14-18), Arp's Atlas of Peculiar Galaxies (Arp P.A. 85°. On the fourth (01823-60 exposure) 158), and in Zwicky's Catalogue of Selected Com- the slit was set along the central and eastern pact Galaxies and Post-Eruptive Galaxies (Zw knot at P.A. 80° (see Plate I). The central knot 0122.5 + 3346). The latter describes the system is barely visible at the telescope. as "post-eruptive, blue, three compact knots con- Radial velocities were derived by using a digit- nected by bright bar, fan-shaped jets and matrix, ized two-screw Grant comparator at the NASA total rrip = 13.5". Johnson Space Center. The two best spectro- Often, the existing survey work and morpho- grams were measured independently by both of logical descriptions poorly reflect the dynamics us in order to avoid any bias in matching the of peculiar and/or compact objects — inviting profiles on the oscilloscope (see Figs. 1 and 2). more or less legitimate speculation. By using The scatter of points is mainly due to emulsion electrographic and spectroscopic observations, irregularities and night-sky illumination which we hope to improve our understanding of peculiar deforms the line profiles. The dispersion curve extragalactic objects such as NGC 523. fits the comparison wavelengths better than 30-40 km sec-1 (residuals). The bright western Observations knot (see Plates I and II) turned out to be a Four spectrograms of the galaxy NGC 523 foreground star with a heliocentric radial ve- were obtained by Chincarini and Rood (1972) at locity 5 km sec-1. The eastern knot shows Kitt Peak National Observatory. The Carnegie rather strong [On] λ3727 emission and the K- image-tube spectrograph attached to the 84-inch and Η-absorption lines of Can. On the longer telescope was employed with the best focus set at exposure (see Plate III) faint [On] emission λ = 4000 Â. A wavelength range from 3500 Â and Can absorption lines extend all the way to 6000 Â was obtained at a reciprocal disper- along the bright bar indicating that both the sion of 250 Â mm ~l. On three spectrograms eastern and central knots and the bar are com- posed of stars and gas. However, at the location ♦Visiting Astronomer, Kitt Peak National Observatory, which is operated by the Association of Universities of the central knot there is no evidence of [On] for Research in Astronomy, Inc., under contract with strengthening so that its source is probably the National Science Foundation. limited to the gas of the bar which envelops it. © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System THE PECULIAR GALAXY NGC 523 569 may be a consequence of tidal interaction. The innermost "isodents" are nearly circular and only slightly larger than the seeing diameter. The heliocentric velocity of the system (See Fig. 1) is about 4840 km sec-1.* With a Hubble con- stant of 75 km sec-1 Mpc-1 the system is at a distance of 65 Mpc where 1 arc second corres- ponds to 313 pc. The diameter of the eastern knot is therefore less than 4 kpc and in the range of a dwarf galaxy. It is worth calling attention to the Seyfert sextet whose members have a similar size. The system also shows difíuse matter in well-developed bridges and tails. The central knot is highly elongated in P.A. ~ PLATE I 75°. The apparent major and minor axes are Forty-minute exposure electrograph made in blue light about 3.7 kpc and 1.5 kpc, respectively. Tenta- at the Cassegrain focus of the 82-inch Struve reflector at tively we may assume an inclination of í ^ McDonald Observatory. Slit orientations for the spectra arcsin bla = 25° (where i is the angle between obtained at Kitt Peak National Observatory are indicated. the plane of rotation and the line of sight). The North is at the top, orientation as for Plate IL flattening points to the presence of rotational velocity. Indeed, visual inspection of the spectra, At the same location the Can lines are rather and perhaps the Can velocity curve (Fig. 1), strong and show some tilt. indicates rotation for both components; [On] An electrograph of the galaxy, Plate I, was shows some differential velocity as well but with obtained in blue light (Sll photocathode and a different slope on the eastern edge of the east- BG12 -f GG13 filter combination) at McDonald em knot. (The extent of the knots perpendicular Observatory by the authors using the 82-inch to the dispersion direction is only slighdy larger Struve reflector, with a Kron electrographic cam- than the seeing image.) Rotation of one compon- era manufactured by Myron Lewis, later kindly ent is observed in other cases, as for example the modified by Kron. Details on our development group Zw 0036.3 + 0646 (Chincarini and Rood work with the camera will be given elsewhere 1971), in which a compact object shows higher (Chincarini and Heckathom 1973). The scale rotational velocity than the other members. at the Cassegrain focus of the 82-inch is 7.4 arc However, we do not have yet enough data and seconds mm ~1 and a reduction by a factor of two analysis on the statistical distribution of spins in was introduced by thé Kron camera. During the groups. Tidal interaction may result in loss of 40-minute exposure, the seeing diameter was angular momentum and perhaps the presence 1.8 arc seconds to the half-intensity point. Iso- of rotational velocity indicates a dynamically density tracings (see Plate II) were obtained at young and unstable system. Ν AS A-JSC using a Joyce-Loebl micro densitom- We can hardly consider the system as a pair eter with a Tech-Opps isodensitracer mod- moving in circular orbits. For the sake of com- ification. The scanning aperture size was 25 pleteness, however, let us do so and compute microns (0.37 arc seconds) square and the density the mass from a difference in radial velocity of difference between successive symbols is 0.069 100 km sec 1, Half the mass of the system is 9 density units. F = mf{i, Ω) = 7.2 X 10 3Wo. The mass of the 10 Discussion system is therefore ^ 1.4 X 10 3Ko under the assumption of circular orbits. The bright knot on the eastern side of the system is clearly distorted, showing isophotes elongated in the directions P.A. = 15°, 125°, 270° such that ♦Chincarini and Rood (1972) give 4731 km sec-1 a major axis cannot be defined. The asymmetry due tô a different weighting of the measured velocities between eastern and western isodensity contours along the bar. © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 570 CHINCARINI AND HECKATHORN —' r Fig. 1 — Rotation curve of ··· ··· NGC 523 measured from plate CI 823 at P.A. 80°. Velocities • [Oll] 3727 ■ Ce II 3933 measured from the [On] ▲ Coll 3968 λ3727 emission line are indi- cated by dots, while those mea- ■■■ sured from the Ca π Κ- and H- absorption lines are indicated by squares and triangles, res- pectively. The more uncertain AA▲ ■ ·;.νΛ . A AA Äaaa" A· " " ■ values are indicated by smaller ■■ ■ ■ A ■ ■ ■ A symbols. The positions of the eastern and central knots are shown by arrows. 30 20 10 0 DISTANCE FROM EASTERN KNOT (SECONDS OF ARC) 5000 'o· 4900 ·· • ·· » 4800 )ι ! 4700 i [on] 3727 ■ Ca II 393^ tCa II 3968 20 10 0 10 PLATE II DISTANCE FROM EASTERN KNOT (SECONDS OF ARC) Isodensity tracing of a 40-minute-exposure electrograph Fig. 2 — Rotation curve of NGC 523 measured from made in blue light at the Cassegrain focus of the 82-inch plate CI 720 at P.A. 85°. The symbols are identical to Struve reflector at McDonald Observatory. those in Figure 1. ence was estimated from a microdensitometer scan along the bar as Am ^. With the area defined by the resolution limit Γ Tracings along the spectra of the two knots (1.8 arc seconds diameter) the intensity ratio and the region which is between them show very between the eastern and central objects similar energy distributions and spectroscopic is 415:1 (Am = 1^55) in the blue.* The maxi- features. We are inclined to assume a sim- mum intensity ratio measured at the very center ilar composition and 3W/L ratio. Thus the of each knot is 4.57:1 (Am = 1^5). The dif- mass ratio of the two objects is about 5:1. The ference is due to the distribution of light, with brightness of the eastern knot is attributed to a the eastern knot being more compact than the higher central star density. central knot. The integrated magnitude differ- We can only give an upper limit for the turbu- *The contribution of the [On] emission to the lu- lent motion of gas and stars.
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