University of Groningen. Stellar kinematics in disk galaxies Gerssen, Joris

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1 University of Groningen Stellar kineatics in disk galaxies Gerssen, Joris IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fro it. Please check the docuent version below. Docuent Version Publisher's PDF, also known as Version of record Publication date: 2000 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Gerssen, J. (2000). Stellar kineatics in disk galaxies s.n. Copyright Other than for strictly personal use, it is not peritted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Coons). Take-down policy If you believe that this docuent breaches copyright please contact us providing details, and we will reove access to the work iediately and investigate your clai. Downloaded fro the University of Groningen/UMCG research database (Pure): For technical reasons the nuber of authors shown on this cover page is liited to 10 axiu. Download date:

2 6 Dark halos in S0 galaxies: NGC 5866 J. Gerssen, A.J. Roanowsky, N. G Douglas, K. Kuijken, M. R. Merrifield, A. Mathieu Due to the lack of suitable tracers the kineatics at large radii in S0 galaxies are poorly constrained. Only planetary nebulae are able to trace the stellar kineatics at large distances fro the centre in these systes. In this paper we present the stellar kineatics of the S0 galaxy NGC 5866 obtained fro planetary nebulae observations. A self-consistent odel is calculated fro stellar absorption line kineatics near one scale length. The observed planetary nebulae kineatics are consistent with the velocities predicted by the odel. THE S0 class of galaxies or lenticulars shares with spiral galaxies the sae basic coponents such as disks, bulges and bars. However, it is still unclear whether these siilarities also point to a single foration scenario. Since present-day galaxies are not fully relaxed dynaical entities, especially at large radii where relaxation ties are the longest, the kineatical properties of a galaxy will partially reflect the echanis by which the galaxy fored. A kineatic investigation of the different types of galaxies will thus provide a key to copare their foration histories. In spiral galaxies the kineatics can norally be derived out to large radii fro HI gas. The flatness of the easured rotation curves is usually interpretated as evidence for the presence of dark atter halos. If lenticular galaxies are fored by the sae process as spiral galaxies we would expect the to have dark halo coponents as well. Unfortunately, lenticular galaxies in general lack an extensive gaseous coponent, and the existence of a halo therefore has to be inferred in another way. Soe 20 percent of all S0 galaxies do actually contain detectable HI but the derived rotation curves are often of low quality due to the low colun densities (van Driel & van Woerden 1991). The results nonetheless see to indicate that the rotation curves reain flat in these systes. Polar ring galaxies provide another easure of the circular speed at large radii (Reshentikov & Cobes 1994), but it is unclear whether these systes are representative 63

3 64 CHAPTER 6 DARK HALOS IN S0 GALAXIES: NGC 5866 of S0 galaxies in general, and globular cluster systes do not trace the kineatics of the relaxed old stellar population in a galaxy. Fortunately, planetary nebulae have proven to be well-suited kineatic tracers to probe the outer regions of both elliptical galaxies (Hui 1993) and spiral galaxies (Douglas et al 2000, hereafter paper 1). Planetary nebulae (hereafter PNe) are bright eission-line objects which radiate their flux in just a few lines, ost notably the [OIII] line at 5007Å which accounts for about 15 percent of the total flux. The PNe distribution in a galaxy follows that of the underlying stellar light iplying that PNe are present at radii where techniques relying on integrated stellar light are unavailable and that PNe adequately trace the stellar kineatics. In paper 1 we presented a technique based on slitless spectroscopy with which we can siultaneously detect PNe and easure their radial velocities. With this technique we iage the galaxy through a narrow band (50 Å) filter centered on [OIII] and disperse the light using a grating. The grating will displace the PNe (point sources) by an aount proportional to their radial velocities. The galaxy will subsequently be iaged for a second tie but with the dispersion direction reversed (by rotating the spectrograph over 180 degrees). Each PNe will thus be iaged twice and the separation between the two ebers of a pair is a direct easure for its radial velocity. In this paper we present the results that we have obtained on the PNe population in the S0 galaxy NGC 5866 and copare these results to stellar absorption lines observations. We then construct a odel for this galaxy and fit this odel to the stellar absorption line data covering the central scale length, and test whether the odel predictions can explain the observed PNe kineatics at several scale length, this will be our null hypothesis. 6.1 Observations We have observed the S0 galaxy NGC 5866 using the ISIS spectrograph on the Willia Herschel Telescope in April 1997 and July This galaxy is seen nearly edge-on, evident fro its proinent dust lane although on a long exposure its appearance is ore siilar to an E6/E7 galaxy. The stellar light is doinated by a large bulge. In fact, dynaical odelling by Pignatelli & Galletta (1999) suggests a disk-to-bulge ass ratio of Table 6.1 lists soe other properties of this galaxy. TABLE 6.1 Paraeters of NGC 5866 Hubble type Y S0.3 Scalelength in 28 arcsec Effective radius 33 arcsec Distance 13 Mpc Inclination p 90 degrees Angular size 4.7 x 1.9 arcin The unvignetted field of view of ISIS in a slitless configuration is four by one arcin. On the first run in April 1997 we observed the galaxy along the ajor axis (position angle 308j ) and centered on the nucleus. The total integration tie was 6.5 hours. The second observing run in June 1998 provided the 180j rotated position angle (i.e. along position angle

4 I i 6.2 Reduction j ). The total integration tie was again 6.5 hours. Both 6.5 hours science exposures were actually broken up in several shorter (20 to 30 inute) exposures. The overall iage quality of the second run is soewhat less than on the first run because of the atospheric conditions and telescope pointing probles. ISIS is a double ared spectrograph and we were therefore able to siultaneously record iages through a narrow band filter centered on the H y eission line, the second brightest line in ost PNe spectra. This light has not been dispersed because PNe are generally weaker in H y than in [OIII]. Still, it often proved difficult to find an H y counterpart to a PN detected in [OIII] and the H y data have therefore only been used indirectly in the reduction process. 6.2 Reduction The data have been reduced using IRAF. All the usual reduction steps were applied. Each frae, however has only been corrected for pixel-to-pixel sensitivity variation and not for the narrow band filter profile. Since the location of the PNe on the CCDs depend on both position and radial velocities, a correction for the filter profile cannot be ade because the radial velocity of each PN is initially unknown. Once the PNe are detected and their radial velocities easured the PNe fluxes can be deterined but for the kineatical analysis presented here this is not iportant. The background galactic light is reoved by running a edian filter over the iages. After background subtraction the noise levels will not be constant over the iage but will vary accordingly with the aount of subtracted light. To copensate for this effect we constructed a siga iage (i.e. a frae in which each pixel is a easure of the local one-siga noise) and divided the background subtracted iage by this siga iage. Possible PNe candidates were identified by eye, blinking the two different position angles. This ethod proves to be far ore efficient than using autoated routines based on IRAF scripts. A ore detailed description of the reduction process and the PNe identification can be found in paper 1. We experienced rather large instruental flexure during the observations and all the short science exposures therefore had to be shifted to a coon reference position before they could be cobined. This eans that the absolute radial velocities cannot be deterined to better than the largest shifts of about 5 to 6 pixels (p ± ). This situation is described in ore detail in paper 1. However, the internal kineatics are entirely unaffected since all this uncertainty aounts to is an unknown but equal offset for all PNe. The real accuracy of the derived radial velocities depends on how well the centroiding of the detected PNe and an accuracy of half a pixel is a rather ± can be done. Each pixel easures 24 9 I [%$ J conservative estiate of centroiding accuracy. Another factor ± coes fro cobining the two different position angles resulting in an uncertainty of p 9 on the easured radial velocities. 6.3 Results Soe 60 PNe candidates are seen in the April 1997 data. Cobining the with the soewhat lesser quality June 1998 data resulted in 34 pairs of PNe for which the radial velocities could be derived. Fig. 6.1 shows the location of each planetary nebula overlaid on a contour iage of NGC More PNe are found at positive, than at negative, due to the presence of a nuber of foreground star found at negative,. This stellar continuu light when dispersed will cause long light trails and essentially renders that part of the CCD useless for

5 66 CHAPTER 6 DARK HALOS IN S0 GALAXIES: NGC 5866 FIGURE 6.1 The detected PNe population overlaid on a contour iage of NGC 5866 obtained with the JKT. The PNe arked with a square are blue shifted and the stars are red shifted PNe. The size of each sybol is proportional to the radial velocity of each planetary nebula. The solid line indicates the position of the cut along the & direction which we used to verify whether the PNe do indeed follow the stellar light. the detection of PNe. In the central parts of the galaxy PNe are lacking because there the galactic light is too bright to detect PNe against it and the outer ost radius at which we are able to detect PNe is set by the size of the CCD. To assess whether the PNe follow the stellar light we binned the 24 PNe with positive, along the 1 direction in bins of 10 arcsec width to deterine the nuber density as a function of scale height. The nuber density of the PNe closely follows the stellar light distribution, see fig Thus, the PNe are indeed adequate tracers of the stellar distribution Radial velocities Fro fig. 6.1 it is clear that the PNe population shows rotation about the centre. This is even ore apparent in fig ± The ean velocities, deterined separately for both positive and negative radii are ± and respectively. Taking the ean to represent the systeic velocity ± ± we find The literature values for the heliocentric velocities range fro ± to An average of all the literature values that quote errors yields a V'( q hhi 9, still soewhat off fro the PNe deterined systeic J ã ± velocity. But as entioned earlier the uncertainty on the absolute radial velocity deterination of the PNe is rather large, and is furtherore copounded by the sall nuber statistics involved in deriving the ean velocity. Overplotted in fig. 6.3 is the stellar rotation curve of Fisher (1997) derived fro stel-

6 6.3 Results 67 FIGURE 6.2 Surface brightness profile along the positive z-cut of fig Overplotted are the nuber of PNe found in bins of 10 arcsec width, assuing refection syetry around the ajor axis. The plotted errors are the Poisson errors. lar absorption line spectra. These data extend far enough to suggest that the stellar rotation curve becoes flat. The outerost stellar data just touches the innerost PNe data, clearly deonstrating that these two different detection techniques copleent each other very well. However, ± ± the PNe exhibit significantly lower rotation than the stars (75 9 versus ). Cobined with the spatial distribution of the PNe this suggests that the observed PNe population is not confined to the disk but is instead ore akin to a bulge or a halo population Velocity dispersions Another advantage of PNe over other tracers of the kineatics is that the PNe can be used to get a direct estiate of the stellar velocity dispersion. Estiates of the stellar velocity dispersion were obtained by binning the PNe into radial and aziuthal bins, Fig Instead of siply projecting the position of each PN onto the ajor axis to obtain their radial positions we took the intersection of the isophote that coincides with a particular PN and the ajor axis as a easure of the radius of that specific PN. The aziuthal velocity dispersions are derived by assuing four-fold syetry on the sky and projecting all PNe onto a single quadrant. Due to the sall nuber of PNe in each bin the uncertainties associated with the are rather large. The velocity dispersion along the radial direction appears to be ore or less constant, unlike the stellar absorption line data which shows a steady decline with radius. Along the aziuthal direction it is ore difficult to discern the behaviour of the velocity dispersion since nearly all PNe are contained in just the two central bins. Here too, the velocity dispersions see to reain constant although there is a hint that they actually drop with increasing

7 68 CHAPTER 6 DARK HALOS IN S0 GALAXIES: NGC 5866 FIGURE 6.3 The radial velocities of the detected PNe as a function of position along the ajor axis of NGC The cross indicates the systeic velocity and the solid line is the stellar rotation curve of Fisher (1997) offset to the PNe systeic velocity. height fro the ajor axis. The constancy of the velocity dispersion, at least along the radial direction, is consistent with the conjecture fro the previous section that the PNe constitute a bulge or halo population and are not part of the disk. The velocity dispersion of the total PNe population easures 4*) + ôw ð zó ± 9. This value is corrected for the slight overestiation that occurs when the dispersion of an enseble of points with non-zero errors is deterined (Danese et al 1980). Coparing this value to the stellar data is not straightforward since the stellar velocity dispersions derived fro absorption line data are not constant. A siple linear extrapolation of Fisher s ajor axis velocity dispersions, however, shows that at the innerost radius of the PNe distribution both velocity dispersion are about equal. 6.4 Modelling In order to interpret the PNe results we have constructed a dynaically self-consistent odel of NGC 5866 fro additional photoetric and stellar kineatical data. The odel is used to test the null hypothesis that the observed PNe kineatics agree with the odel predictions. The first step in the construction of the odel is to derive the luinosity density., - band surface photoetry obtained fro the ING archive is used to deproject the light distribution. Since NGC 5866 is close to edge-on the deprojection is in principle unique. However, real data is noisy aking the deprojection less unabiguous in practise. We used the ethod developed by Roanowsky & Kochanek (1997) to deproject this galaxy. Their ethod has the advantage that there are no hidden constraints built in to produce a unique inversion. The

8 , Discussion Radius (arcsec) FIGURE 6.4 The left panel shows the velocity dispersion of the PNe along the ajor axis obtained by binning the PNe into bins of 20 arcsec width. The right panel shows the aziuthal behaviour of the velocity dispersion in bins of 10 degrees width. The ajor axis is at 90 degrees in this panel. deprojection algorith assues axisyetry and reflection syetry about the ajor axis (i.e. 4-fold syetry.) To calculate velocities we assue a constant ass-to-light ratio, axisyetry and a twointegral dynaical odel. The ass density then follows trivially fro the luinosity density and the potential is obtained using the procedure outlined in Binney 4 6 et al 4 (1990). 6 The solutions to the Jeans equation for a two-integral odel then yield the and? coponents of the velocity dispersion (Satoh 1980). To scale the predicted velocities to the observed ajor and inor axis values we have projected the odel line-of-sight velocities onto the plane of the sky and copared the to the stellar absorption line data of Fisher (1997) in a least square sense. We have not separated out the projected odel velocities in its coponents, /Ð /Ð > S /Ð (e.g. van der Marel 1991). Instead of using the ajor axis data to scale the odel we used a spectru obtained parallel to the ajor axis but offset fro it by 6 arsec to avoid probles with the dust lane. Although the ajor and inor axis data sets were fitted independently they give essentially the sae result. In fig. 6.5 the predicted odel kineatics along both the ajor and the inor axis are overplotted with the PNe kineatics. 6.5 Discussion Clearly, the null hypothesis (ass traces light) cannot be discarded on the basis of this odel and data. The inclusion of a dark atter halo in the odel calculations is therefore not required by the data. However, the coplete absence of a dark halo appears to be unlikely. Treblay et al (1995), for instance, used PNe to probe the dark atter distribution in the SB0 galaxy NGC Although the kineatics of the 68 PNe they easured cannot fully explain the total ass that has been estiated for this syste fro a 200 kpc HI ring, it does show that there is an unseen coponent of atter present.

9 [ [ [ [ [ J 70 CHAPTER 6 DARK HALOS IN S0 GALAXIES: NGC 5866 FIGURE 6.5 The odel predicted stellar kineatics and the stellar absorption line data of Fisher (1997) used to scale the odels. The solid line is the scaled ajor axis prediction while the dashed line depicts the inor axis. The triangles show the PNe kineatics using the sae radial bins as in fig One possible explanation could be that the dark halo already doinates at the radii of the stellar absorption line data. The odel will then not be self-consistent but the PNe kineatics and the stellar kineatics should both follow the odel predictions. Qualitatively the PNe data should fall soewhere between the ajor and inor axis odel predictions since the detected PNe population is not confined to the disk, fig The average aziuthal angle of all the PNe suggests that the PNe kineatics should follow the ajor axis prediction ore closely that the inor prediction, however the actual behaviour of the PNe sees to indicate the opposite., Prugniel & Heraudeau 1998) is about 15. Although this value is higher than what is typically found in disk galaxies, where the ratios are usually in the range one to five, it falls right in the iddle of the range (10 to 20) of elliptical galaxies (Lauer 1985.) S0 galaxies constitute an interediate class of galaxies in between spiral galaxies and elliptical galaxies, However, Burstein (1999) points out that the distinction between (giant) elliptical galaxies and S0 galaxies is probably blurred, iplying that S0 galaxies are probably ore closely related to elliptical galaxies that to spiral galaxies. However, Eselle et al (1999) find a The ass-to-light ratio of the odel (converted to the S -band using S L, of about 6.5 for the S0 galaxy NGC The stellar light in NGC 5866 is doinated by the large bulge and Bottea (1999) shows that the values of bulges are typically soewhat higher than what is found for disks. The derived value for NGC 5866, although a little large, is therefore not inconsistent with the values found in other galaxies. Clearly, the results presented here need to be reexained carefully, since it sees unlikely that the S0 galaxy NGC 5866 is truly without a dark halo. Observations of ore PNe will provide a better constraint on the halo kineatics. The dedicated Planetary Nebulae Spec- i s

10 6.5 Discussion 71 trograph (Arnaboldi et al 1999) with its large field-of-view and high throughput will shortly ake it possible to effectively detect additional PNe. A single S0 galaxy, however, cannot yield any statistical inforation about the halos properties of these systes, and a larger saple of S0 galaxies is therefore essential. A coparison of the properties derived fro such a saple with the properties of spiral galaxies will establish whether or not both systes have the sae foration echaniss. References Arnaboldi M., Capaccioli M., Douglas N.G. et al 1999, Proceedings of the SAIT syposiu, Naples 1999 Binney J., Davies R.L., Illingworth G.D., 1990, ApJ, 361, 78 Bottea R., 1999, A&A, 348, 77 Burstein D., astro-ph/ Danese L., De Zotti G., di Tullio G., 1980, A&A, 82, 322 Douglas N.G., Gerssen J., Kuijken K., Merrifield M.R., 2000, MNRAS in press (paper 1) Eselle E., Dejonghe H., Bacon R., MNRAS, 303, 495 Fisher D., 1997, AJ, 113, 950 Hui X., 1993, PASP, 105, 1011 Lauer T. R., 1985, ApJ, 292, 104 Pignatelli E., Galletta G., 1999, A&A, 349, 369 Prugniel Ph., Heraudeau Ph., 1998, A&AS, 128, 299 Reshentikov V.P., Cobes F., 1994, A&A, 291, 57 Roanowsky A. J., Kochanek C. S., 1997, MNRAS, 287, 35 Satoh C., 1980, PASJ, 32, 41 Treblay B., Merritt D., Willias T.B., 1995, ApJ, 443, L5 van Driel W., van Woerden H., 1991 A&A, 243, 71 van der Marel R. P., 1991, MNRAS, 253, 710

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