The peculiar galaxy NGC 520 (Arp 157) is often interpreted as an interacting pair of galaxies. The identification of the two bulges and overall morphology of the two galaxies has long been a puzzle which the authors attempt to solve here. New longslit optical spectroscopy and near-infrared images of NGC 520 are presented. These data suggest that the northwest peak is the bulge of one of two galaxies in the system. The other larger bulge is clearly evident in the K band image in the middle of the dust lane. The stellar radial velocity profile in the central 10 seconds of the larger bulge is consistent with counterrotation seen in the molecular gas component. This kinematic subsystem could be the remains of a merged gas-rich irregular

Balcells, M.

Stanford, S. A.

English

NASA Technical Reports Server

- 1  
THE KINEh4ATICS AND MQWHOLOGY of NGC 520 ONE, TWO, or THREE 
GALAXIES? 
$.A. Stanford and M. Balcells 
University of Wisconsin 
ABSTRACT 
The identification of the two bulges and overall morphology of the two galaxies has long been a 
puzzle which we attempt to solve in this paper. New longslit optical spectroscopy and near- 
infrared images of NGC 520 are presented. These data suggest that the northwest peak is the 
bulge of one of two galaxies in the system. The other larger bulge is clearly evident in the K 
band image in the middle of the dust lane. The stellar radial velocity profile in the central 10" of 
the larger bulge is consistent with counterrotation seen in the molecular gas component. This 
kinematic subsystem could be the remains of a merged gas-rich irregular. 
The peculiar galaxy NGC 520 (Arp 157) is often interpreted as an interacting pair of galaxies. 
I. INTRODUCTION 
Understanding the peculiar system NGC 520 (Arp 157) has been the goal of several workers 
using observations from many wavelength regions. One of the most basic questions about the 
system has been if the system is one disturbed galaxy or two interacting galaxies. From single 
dish H I mapping, Thuan and Wadiak (198% hereafter TW) concluded that only one galaxy is 
present in the system. Stockton and Bertola (1980; hereafter SB) studied the optical emission 
line kinematics and concluded that there was evidence for two colliding galaxies in NGC 520. 
An optical image of the system may be interpreted as a collision of two disk galaxies seen 
crossed and nearly edge-on. For ease of discussion, these disk-like structures shall be refered to 
as the east-west and southeast-northwest galaxies. The deep (-4.5 hours) photograph of NGC 
520 in SB shows a long tidal tail apparently pulled from the southeast-northwest galaxy. This 
tail stretches south from the northwestern end of NGC 520, curving to the east and then north 
around the main body up to a distance of -32 kpc (&=75 km s-1 Mpc-1). Here we present new 
optical longslit spectroscopy and infrared imaging of NGC 520. These data suggest the location 
of two bulges within NGC 520. The presence of a counterrotating core in the larger bulge 
suggested by CO observations is consistent with the observed stellar kinematics in the central 
10" * 
11. OBSERVATIONS 
Longslit optical spectra were obtained with the GoldCam spectrograph on the 2.1 m telescope 
at =NO in December 1988 with spectral coverage from 3880 to 4872 A and a dispersion of 
1.24 A pixel-1. The slit measured 2" by 4' with a spatial resolution of 0.78" pixel-1. Two slit 
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https://ntrs.nasa.gov/search.jsp?R=19910007595 2020-03-19T19:49:42+00:00Z
positions were observed at PA=87' and at PA=1250. Each night F, G and K type standard stars 
were observed to be used as velocity templates in the fourier quotient and cross-correlation 
analyses (hereafter FQ and XC, respectively; Sargent et al. 1977). Reductions were perforrned 
with IRAF1 using standard procedures. Each aperture extracted from the two slit positions was 
the sum of three columns, which corresponds to a width of 2.34". Near-infrared JHK 
broadband images were obtained using IRIM on the 1.3 m telescope at KPNO on 1 1 December 
1988 UT. The spatial resolpion was 1.35" pixel-1 and the seeing was estimated to be 1.5-2" at 
the time of the observations. Exposures of the blank sky in each band at several positions near 
NGC 520 before and after the object images were used for sky-subtraction at each wavelength, 
which accounted for the subtraction of the dark count in the process. Flatfield images were 
obtained by median summing all the sky images obtained in the course of one night at each 
wavelength at each integration time. Observations were obtained of standard stars taken from 
Elias et al. (1982) to calibrate the photometry. 
111. RESULTS 
Radial velocity profiles of both optical Spectroscopy slit positions were first obtained with 
the XC routine and then velocity dispersions were obtained with the FQ routine. The XC- 
determined velocities were used as input to the FQ program. Because of the low signal to noise 
ratio at most points along the slit, pairs of apertures were added in order to obtain more accurate 
velocity dispersions. Thus, the points in the plotted dispersion profiles represent 4.7" apertures. 
The XC radial velocities for slits 1 and 2 are shown in Figure 1, along with the velocity 
dispersions obtained from the FQ routine. All radial velocities shown in this paper are 
heliocentric. The zeropoint on the abscissa corresponds to the nucleus position of the east-west 
galaxy (henceforth called the primary nucleus), as determined from a K band image (see below). 
A contour plot of the single frame K band image is presented in Figure 2 which covers the 
central 78" x 84" of NGC 520. Using the APPHOT package in IRAF, magnitudes were 
obtained of selected areas of the JHK images using 2.7" apertures as shown in Figure 2. The K 
band image shows one bulge in the middle of the east-west dust lane, and another peak which 
coincides with the optical northwest emission peak. The colors obtained at the larger bulge 
indicate that although there is a significant amount of obscuration and hot dust emission, the 
elongated shape in the central lot is primarily due to stellar light. 
IV. DISCUSSION 
As stated in the introduction, the first question to be answered is whether NGC 520 is a 
single disturbed galaxy, or two interacting galaxies. Perhaps the most interesting and important 
1Image Reduction and Analysis Facility is distributed by the NOAO, which is operated by the Association of 
Universities for Research in Astxonomy, Ine., under contract to the National Science Foundation. 
348 
h c 
E 2300 
.Y 
x z 
Y 
g 2200 
$ 
- 
2100 
-20 0 20 4 0  
Radius (") 
Figure 1. Radial velocity and dispersion profiles of slits 1 (left) and 2 (right). The velocities were 
obtained from the average of all slit 1 exposures, using the XC routine to find the radial velocties and 
the FQ routine to find the velocity dispersions. Except as noted in the text, each point represents a 
three column wide aperture -2.3". The errorbars represent one G errors. The zeropoint in radius is at 
the position of the primary nucleus. The horizontal dashed line in the slit 1 profile represents the 
systemic velocity of the nucleus. 
Figure 2. Contour map of the K band image. The lowest contour represents 19.0 magnitudes arcsec-2 
and the contour interval is 0.2 magnitudes. The scale is 1.35 arcsec pixel-l and the axes are in units of 
pixels. The positions of the small aperture photometry are shown by the circles whose diameter equals 
the apertm size. 
349 
result bearing on this question is seen in the velocity dispersion profile from slit 2. At the 
position of the northwest emission peak there is a sharp rise of 100 km s-1 to a peak value of 240 
km s-1 which quickly drops back to -140 km s-1 to the northwest. We believe that this 
dispersion peak is real because it is seen in the profiles determined separately from the night 3 
and night 4 spectra. That the dispersion peak is due to the presence of a bulge is a natural 
explanation because galactic bulges have high dispersions. The calculated dynamical mass at the 
dispersion peak of -1.5 x 1010 bb is reasonable for a 1.5 kpc diameter region centered at a 
galactic bulge. We suggest that the northwest peak is one of the bulges of the galaxies in NGC 
520. 
Next, we discuss the slit 1 velocity profiles obtained for the east-west galaxy. The velocity 
at the nucleus position r=O is taken to be the systemic velocity. The velocity profde has an 
overall S shape. Over the central 304.5 kpc, the radial velocity points can be fitted fairly well 
by a straight line, indicative of solid body rotation. In the central 15" of the slit 1 velocity 
profile, there is a hint of a series of velocity reversals. The error bars are too large for us to be 
confident in the reality of the reversals. However, they trace a pattern which is highly symmetric 
about the nucleus position and velocity. Moreover, this pattern is seen in the profiles obtained 
from the separate spectra of night 1 and night 3, lending credence to the reality of the reversals. 
Complicating the interpretation of the apparent bumps is that slit 1 may lie at an angle to the major 
axis of the east-west galaxy, and that the stellar light from the central 15" contains contributions 
from both bulge and disk stars. 
The kinematics of the primary nucleus has also k e n  investigated in the CO line (Sanders et 
al. 1988). The CO linecenter V0(CO)=2261 km s-1 agrees with the systemic V,(stellar)=2275 
km s-1 found in the slit 1 velocity profde at the position of the primary nucleus. More interesting 
is that the CO profile over the central 6" i$ in countemtation with respect to the overall decline in 
velocity in the east to west direction seen by SB and by us. The elongated morphology and the 
velocity profde of the CO emission shows that the molecular gas probably lies in a rapidly- 
rotating disk. The counterrotating molecular disk gives support to believing the reversals seen in 
our stellar radial velocity profde. Such velocity reversals have been observed in other galaxies 
(Konnendy 1984; Jednejewski and Schechter 1988; Balcells and Stanford 1989). The reversals 
have been described as countemtation due to a cannibalized dwarf elliptical galaxy (Balcells and 
Quinn 1989) or to a young stellar disk formed out of an ingested irregular galaxy. There is one 
important problem with the countemtation interpretation. SB found a monotonic decline in the 
optical emission line velocities over the central 6 at the primary nucleus, indicating the ionized 
gas does not also show counterrotation. No f m  conclusion can be drawn on the possible 
velocity reversals until btter longslit spectra can be obtained along the east-west galaxy. 
350 
At the primary bulge, Young, Kleinmann and Allen (1988) find apparently strong Ha 
emission veiled by dust. The Ha data argue in favor of current star formation within the central 
20l of the primary bulge. The morphology of this region suggests an explanation of the star 
formation. At the primary nucleus, the K band image shows the same elongated structure seen in 
the GO source. The ratio of the axes at r=8” is about 2:l in the K band light. We speculate that 
massive stars recently formed out of the CO disk, and have become red supergiants which 
dominate the K band light in the central 6 .  Hence, instead of the spherical shape seen in the R 
band contours at radii outside of the 12” diameter CO source, an elongated shape is seen in the 
central 6” because the dominant stars formed out of a nuclear disk. Furthermore, if we firmly 
believed that the primary contained a merged, counterrotating system, then we could further 
speculate that the kinematics of a counterrotating gas disk would be likely to cause a burst of star 
formation by producing a higher than normal cloud-cloud collision rate. 
We thank the IAU for providing a travel grant to enable SAS to attend IAU Colloquium 124. 
SAS acknowledges support from NASA grant NAS5-25451, and MB from a subcontract to 
NASA contract NAS7 -9 1 8. 
Balcells, M.C., and Quinn, P. 1989, Ap.J., submitted. 
Balcells, M.C., and Stanford, S.A. 1989, Ap J., submitted. 
Elias, J.H., Frogel, J.H., Matthews, K., and Neugebauer, 6. 1982, A.J., 87, 1029. 
Jedrezejewski, R., and Schechter, P.L. 1988, ApJ .  Lett., 327, L55. 
Kennicutt, R. 1983, Ap.J., 272, 45. 
Kormendy, J. 1984, Ap.J., 287, 577. 
Sanders, D.B., Scoville, N.Z., Sargent, A.I., and Soifer, B.T. 1988, Ap.J. Lett., 324, L55. 
Sargent, W.L.W, Schechter, P.L., Boksenberg, A., and Shortridge, K 1977, Ap J., 212,324. 
Stanford, S.A. and Balcells, M. 1990, Ap.J., in press. 
Stanford, S.A. 1989, Ap.J., submitted. 
Stockton, A., and Bertola, F. 1980, Ap.J., 235, 37. 
Young, J.S, Kleinmann, S.G., and Allen, L.E. 1988, Ap.J. Lett., 334, L63. 
Thuan, T.X. and Wadiak, E.J 1983, Ap.J., 252, 125. 
REFERENCES 
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The kinematics and morphology of NGC 520: One, two, or three galaxies

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