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.

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.

—' 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 indicated by dots, while those mea- ■■■ sured from the Ca π Κ- and H- absorption lines are indicated by squares and triangles, respectively. 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. In fact, the widths minosity through the filter combination can be neglected. of the [On] emission and the Can absorption

EAST

PLATE III Reproduction of a spectrum of NGC 523 obtained with the Carnegie image-tube spectrograph of the 84-inch telescope at Kitt Peak National Observatory. The βΟ-minute exposure (plate CI 823) was made at P.A. 80° with the slit oriented through the eastern and central knots. Selected lines of the He-Ne-Ar comparison spectrum are marked and the galaxy lines of [Ο π] λ3727, Κ and Η of Ca π are indicated by short bars. lines are practically the same as those of the 50 kpc.* Distances of 30 kpc can be reached in night-sky lines. Turbulent motion both in the 3 X 108 years at velocities of the order of gaseous and stellar components is therefore 100 km sec-1 < 400 km sec-1. A close encounter may explain the main fea- Perhaps peculiar objects such as NGC 523 tures of the system such as the distortions in the will remain open to speculation for some time. two main bodies and the diffusion of matter and Toomre and Toomre (1972), Eneev, Kozlov, and gas into intergalactic space. The bar itself could Sunyaev (1973) have recently given examples of be a consequence of the weak interaction of two encounters illustrating the formation of bridges bodies with different mass. Other explanations and tails as a consequence of tidal interaction. In may face more serious difficulties. We have seen many cases of close encounters the main galaxy that by assuming circular orbits we derive a more captures part of the material and a long-lasting or less reasonable mass. In a way it is unfortunate bridge can be formed. A ratio of about 5:1 in that uncertainties are so many and large that we mass, as in the case of NGC 523, calls for a rather have an acceptable result in almost any case. close encounter due to the weak gravitational Other considerations, however, rule out such a interaction. If this is the case, the amount of possibility. The central object calls for an in- material spread into space by the encounter is clination ^ 25°. The bar, assuming it origi- magnificent. Our electrograph and the 200-inch nated as mass lost from the two objects, would photograph by Arp show that the brighter part in this case be a disk seen edge on and, therefore, of the system extends for about 20 kpc. Matter at a much different inclination. The various is, however, visible over a range of at least 40 kpc- asymmetries of the isophotes and the asym- metrical distribution of matter surrounding the *Dr. A. Toomre expressed the opinion that the wide, main objects would remain without explanation. faint tail in approximately the east-southeast direction is the best indication of the encounter. He also called our From Arp's imagery, and ours, such matter has attention to the similarity between NGC 523 and NGC 520 the form of bridges and tails. In addition, small (Arp 157). knots (with sizes < 1 kpc) are visible in the elec-

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 572 CHINCARINI AND HECKATHORN trograph and Arp's photograph. These are simi- No. NCR 44/012/152. One of us (H.M.H.) lar to what we see on the NW side of the eastern would like to acknowledge the support of a Na- knot, Plates I and II, and probably are Η π tional Research Council research associateship regions. while the camera was being made operational. We are reluctant to refer to such a system as The authors gratefully acknowledge Dr. Rood evidence of explosive phenomena in galaxies. for stimulating discussions and Dr. A. Toomre for Zwicky's description of the object and its very reading the manuscript before publication. Mr. peculiar form may suggest it. We have no evid- David Doss was of great help during the prepara- ence of highly turbulent motion and only a very tion of the Kron camera and often assisted during slight indication of a difference between the mo- the observations. It is a pleasure to thank Drs. tion of the gas ([On]) and stars (Ca n). Further- Abies and Kron who collaborated in making a more, there are filamentary structures but no new photocathode for the electrographic cam- alignment with the knots (see for example the era—our work could not have been possible isodents in the NE region of the eastern knot and otherwise. along the bar, and the direction of distortion in the main body). It seems therefore hard to con- REFERENCES ceive that the whole system was generated by an explosive event which occurred in the eastern Chincarini, G., and Heckathom, H. M. 1973 in Pro- ceedings of the S.P.LE. 17th Annual Meeting, San object. Diego, August 1973 (in press). The development and operation of the Kron Chincarini, G., and Rood, H. J. 1971 (unpublished). 1972, A./. 77, 4. electrographic detector was supported by Eneev, T. M., Kozlov, Ν. N., and Sunyaev, R. A. 1973, McDonald Observatory, The University of Texas, Astr. and Ap. 22, 41. and by the NASA Johnson Space Center Grant Toomre, Α., and Toomre, J. 1972, Ap. J. 178, 623.