---
abstract: 'We present the rotation velocities $V$ and velocity dispersions $\sigma$ along the principal axes of seven elliptical galaxies less luminous than $M_B= -19.5$. These kinematics extend beyond the half-light radii for all systems in this photometrically selected sample. At large radii the kinematics not only confirm that rotation and “diskiness" are important in faint ellipticals, as was previously known, but also demonstrate that in most sample galaxies the stars at large galactocentric distances have $\bigl (V/\sigma\bigr )_{max}\sim 2$, similar to the disks in [*bona-fide*]{} S0 galaxies. Comparing this high degree of ordered stellar motion in all sample galaxies with numerical simulations of dissipationless mergers argues against mergers with mass ratios $\le 3:1$ as an important mechanism in the final shaping of low-luminosity ellipticals, and favors instead the dissipative formation of a disk.'
author:
- 'Hans-Walter Rix$^{1,4}$, Marcella Carollo$^{2,5}$ and Ken Freeman$^3$'
title: Large stellar disks in small elliptical galaxies
---

Astrophysical Journal Letters, March 1999.

$^1$ Steward Observatory\
$^2$ Johns Hopkins University\
$^3$ Mount Stromlo and Sidings Springs Observatory\
$^4$ Alfred P. Sloan Fellow\
$^5$ Hubble Fellow\

[*subject headings*]{}: galaxies: formation - galaxies: evolution - galaxies: structure - galaxies: elliptical and lenticular - galaxies: kinematics and dynamics

Introduction
============

Elliptical galaxies occupy only a small volume in the stable parameter space defined by mass, luminosity, compactness, and rotational support (Kormendy and Djorgovski 1989; de Zeeuw and Franx 1991). This means that their present-day structure is not determined by stability, but by their formation history, specifically by the relative importance and the time ordering of dissipative processes [*vs.*]{} merging. Yet, the actual role of gas dissipation, resulting in gaseous and stellar disks, and of violent relaxation, leading to spheroidal systems with largely random stellar motions, is still under debate for ellipticals of different luminosity classes.

Observationally it has been established that “low-luminosity" ellipticals ($L\ltorder \frac{1}{2} L_*$) show more rotation and photometric diskiness than the brighter systems (e.g., Davies 1983; Bender et al. 1989). Yet, taken alone, these results cannot constrain uniquely their formation history, as there are many paths towards systems with modest rotation. For example, N-body simulations of dissipationless major mergers can produce rotating remnants $(V/\sigma)_{max}\ltorder 1$ with disky isophotes (e.g., Heyl, Hernquist & Spergel 1996; Weil & Hernquist 1996). On the basis of statistics, Rix & White (1990) argued that most of the so-called faint ellipticals are not just “disky” objects, but are rather the face-on counterparts of S0 galaxies, implying that they contain dynamically-cool stellar disks that make up an appreciable fraction of the total mass. If confirmed kinematically, the presence of an outer stellar disk in the majority of these systems would strongly support gas dissipation as the last significant step in their formation history. For individual objects, detailed photometric and kinematic studies have shown indeed that dynamically fragile stellar disks are present in several elliptical galaxies (Rix & White 1992; Scorza & Bender 1995).

In this paper we attempt to move beyond questions of individual misclassification of galaxies, by asking whether there is any significant fraction of “faint elliptical galaxies” that have reached their present state through largely dissipationless merging as the last formation step? To this end we present and discuss kinematic measurements (to $R > R_e$) for a photometrically selected sample of seven bona-fide ellipticals with $M_B \ge
-19.5$.

Sample and Observations
=======================

Our seven galaxies are a random sub-sample of the RC3 catalog (de Vaucouleurs et al. 1991), with Hubble types $T\le -4$ and luminosities below $M_B= -19.5$. No prior kinematic information was used, and nothing, save the luminosity cut, should have biased the selection process towards rapidly rotating objects. We added three S0 galaxies ($T =
-2,-3$) for comparison. The spectra were obtained during two different runs (February 11-14, 1997, and September 29 – October 2, 1997) at the KPNO 4-m telescope, using the RC spectrograph with the KPC-24 grating in second order. In February 1997 the detector was T2KB CCD with $2048^2$ pixels of $24\mu$m$^2$, which were binned along the slit to yield $1.38''$/pixel. In the September 1997 run we used a 3k$\times$1k F3KB CCD with $15\mu$m$^2$ pixels binned to $0.86''$/pixel along the slit. With a $2.5''$ wide slit and on-chip binning by two in the spectral direction the effective instrumental resolution was $\sigma_{instr}\approx 50$ km/s. The spectra were centered on 5150$\AA$, covering H$\beta$ $\lambda$4861$\AA$, \[OIII\] $\lambda$5007$\AA$, Mg$_b$ $\lambda$5175$\AA$, Fe $\lambda$5270$\AA$ and Fe $\lambda$5335. Table 1 lists relevant properties of the sample members and the observations, including the adopted major axis position angle and the exposure times. Spectra of several K giants were acquired with the same instrumental setup, and used as kinematic templates. The basic data reduction (bias and dark subtraction, flat-fielding, correction for slit-vignetting, wavelength calibration, airmass and Galactic extinction corrections, correction for instrumental response) was performed using [IRAF]{} and [MIDAS]{} software.

The stellar kinematics were obtained with the template fitting method, described in Rix & White (1992), after the data were binned along the slit to constant signal-to-noise. Then the best fit mean velocity $V$ and dispersion $\sigma$ were determined by $\chi^2$-minimization, along with the best fitting composite stellar template. The resulting error bars are the formal uncertainties based on the known sources of noise. External tests on this and other data sets demonstrate that they are a good approximation to the true uncertainties.

Results and Discussion
======================

Figure 1 shows the major and minor axis kinematics for the sample galaxies. Similar to S0 galaxies, the velocity dispersions in these ellipticals are found to drop from a central peak to near our instrumental resolution at large radii. Most sample members exhibit little - if any - rotation along the minor axis, in contrast to their strong rotation along the major axis. Their kinematics suggest that these systems are nearly axisymmetric and likely have a rather simple dynamical structure. Only NGC1588 shows considerable minor axis rotation over the same radial region where the photometry shows isophotal twisting; this is possibly due to the interaction with its close companion (NGC1589).

In Figure 2 we quantify the degree of rotational support in these galaxies, by plotting $V/\sigma$ as a function of the galactocentric distance along the major axis. As a diagnostic of the dynamical state beyond the effective radius, we adopt the maximum, $(V/\sigma)_{max}$, and the outermost value $(V/\sigma)_{out}$, of each $V/\sigma$ curve \[see Table 1\]. In terms of these quantities, Figure 3 re-states the main result: for four out of seven ellipticals, $(V/\sigma)_{max}\gta2$; two additional ellipticals have $(V/\sigma)_{max}\gta1.5$.

As a benchmark, we compare the observed $(V/\sigma)_{max}$ \[and $(V/\sigma)_{out}$\] values to maximally rotating, oblate “Jeans models.” We consider models where the residual velocities are isotropic in the co-rotating frame. The models are built following the method outlined in Binney, Davies, & Illingworth (1990; see also Carollo & Danziger 1994 for particulars), assuming a Jaffe (1983) law. The curves in Figure 3 represent maximum $V/\sigma$ within $2 R_e$, as a function of projected axis ratio (or inclination). We considered [*intrinsic*]{} axis-ratios of $c/a=0.6$ and $c/a=0.4$ and models with and without dark halos (which are three times more extended and more massive than the luminous component). As Figure 3 illustrates, no model can reproduce values of $(V/\sigma)_{max}\sim 2$ for any viewing angle, unless it has $c/a\ltorder 0.4$! This implies that all sample galaxies are intrinsically [*very*]{} flat. A possible exception is the one distorted galaxy, NGC1588, for which our kinematics are compatible with $c/a<0.6$, if $(V/\sigma)_{out}$ is considered instead of $(V/\sigma)_{max}$. The probability that 6 out of 7 ellipticals are that flat is vanishing ($\approx 2.4 \times 10^{-6}$) on the basis of the expected shape distribution of low-luminosity oblate spheroidals (Tremblay & Merritt 1996).

Dissipationless simulations of equal mass galaxy mergers mostly produce remnants with $(V/\sigma)_{max} \ltorder 1$ inside $\sim 2R_e$ (Heyl, Hernquist and Spergel, 1996). In addition, for most spin-orbit geometries major mergers also produce significant kinematic misalignment, which would be reflected in minor axis rotation. Neither property is consistent with our sample galaxies, ruling out formation through mergers of nearly equal mass. Unequal mass mergers ($\sim 3:1$) can produce remnants with rather disk-like shapes and kinematics (Barnes, 1996; Bekki 1998; Barnes 1998). Barnes (1998) finds remnants to be close to axisymmetric with little minor axis rotation, in this respect consistent with our sample properties. He quantifies the rotational support of the merger remnants by the parameter $\lambda'$, the total angular momentum of the most tightly bound half of the stars, normalized by the value for perfect spin alignement. For Barnes’ eight 3:1 mergers $\lambda'_{sim}=0.38$ with a scatter of $ 0.04$. As $\lambda'_{sim}$ is not observable, we attempt to construct an analogous quantity $\langle\lambda'\rangle_{obs}$ for our sample members (see Table 1), by estimating the azimuthal velocity, normalized by the local circular velocity and averaged over the inner 50% of the light. For this estimate we had to make the following assumptions: (i) the overall potential is logarithmic; (ii) the velocity dispersions are isotropic; (iii) the galaxy is axisymmetric with an intrinsic axis ratio of $0.4$ (or $0.6$; see above); (iv) the circular velocity is estimated as $v_c\approx \sqrt{v_\phi^2+2 \sigma^2}$, which is both correct for non-rotating systems in logarithmic potentials and in the “asymmetric drift" limit for $\rho_*\propto r^{-2}$; (v) the stars are on average 30$^\circ$ from the mid-plane. With this, we find $\langle\lambda'\rangle_{obs}=0.55$ with a scatter of $0.06$ (see Table 1): every observed sample galaxy has considerably more ordered motions than any of the 3:1 mergers. We checked that plausible changes in the above model assumptions will alter not $\langle\lambda'\rangle_{obs}$ by $\langle\lambda'\rangle_{obs} - \lambda'_{sim}$.

The degree of streaming motion found in our sample galaxies is nearly that of the dominant disks of S0 galaxies \[Fisher 1997 and the $(V/\sigma)_{max}$ values for the S0s of our sample\]. Without exception the sample members appear even more rotationally supported than simulated unequal mass (3:1) mergers (Barnes 1998). This strongly suggests that the dissipative formation of a massive and extended stellar disk has been the last major step in building these galaxies. This inference does not preclude, however, that for some fraction this disk has been heated considerably by subsequent gravitational interactions.

Our data, therefore allow us to push the long-known result that [*rotation is important in low-luminosity ellipticals*]{} (Davies 1982) one step further: most of these “elliptical" galaxies in an effectively luminosity-limited sample, actually contain stellar disks at large radii, comparable in mass and size to S0s. These kinematic results, combined with earlier photometric evidence ( e.g., Rix & White 1990 and references therein), now put this idea on solid observational grounds. Perhaps these results imply that at smaller mass scales the epoch of mergers ended before the epoch of star-formation.

HWR is supported by the Alfred P. Sloan Foundation. CMC is supported by NASA through the grant HF-1079.01-96a awarded by the Space Telescope Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract NAS 5-26555.

Bender, R., Surma, P., Doebereiner, S., Moellenhoff, C., Madejsky, R., 1989, A&A, 217, 35 Barnes, J. 1996 in “Formation of the Galactic Halo ... Inside Out", eds. H. Morrison and A. Sarajedini, ASP Conference Series, p. 415. Barnes, J. 1998 in “Galaxies: Interactions and Induced Star Formation", Kennicutt, R., Schweizer, F and Barnes, J., Saas Fe Advanced Course 26, Springer, §7. Bekki, K. 1998, ApJ, 502, L133. Binney, J.J., Davies, R.L., Illingworth, G.D., 1990, ApJ, 361, 78 Carollo, C.M., Danziger, I.J., 1994, MNRAS, 270, 743 de Zeeuw, P. T. and Franx, M., 1991, ARA&A, 29, 239 Davies, R.L., Efstathiou, G., Fall, S.M., Illingworth, G., Schechter, P., 1983, ApJ, 266, 41 de Vaucouleurs, G., de Vaucouleurs A., Corwin H. G. Jr., Buta R. J., Paturel G., Fouqu[é]{} P., 1991, Third Reference Catalog of Bright Galaxies, Springer (RC3) Fisher, D., 1997, AJ, 113, 950 Heyl, J.S., Hernquist, L., Spergel, D.N., 1996, ApJ, 463, 69 Jaffe, W., 1983, MNRAS, 202, 995 Kormendy, J. and Djorgovski, G., 1989, ARA&A, 27, 235. Rix, H-W., White, S.D.M., 1990, ApJ, 362, 52 Rix, H-W., White, S.D.M., 1992, MNRAS, 254, 389 Scorza, C., Bender, R., 1995, A&A, 293, 20 Tremblay, B., Merritt, D., 1996, AJ, 111, 2243 Weil, M.L., Hernquist, L., 1996, ApJ, 460, 101

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  --------- ------ ----------- -------------- ------- ---------------- ------------ ------- --------------- ------- -------------------- -------------------- -----------------------------
  Name      $T$     V$_{hel}$       $D$        $M_B$       $R_e$        $\epsilon$    Run    T$_{maj/min}$   $PA$    $(V/\sigma)_{out}$   $(V/\sigma)_{max}$   $ \langle{\lambda'}\rangle$
                     (km/s)        (Mpc)       (mag)       (arcs)                               (secs)       (deg)                                            
  NGC386    -5        5707           76        -19.2   11.8$\ddagger$      0.13      Sep97    18000/8100       5            1.0                  1.0                   0.47 (0.43)
  NGC1588   -4.6      3397      19$^\dagger$   -17.7        12.8           0.30      Sep97    10800/3400      35            1.05                 1.4                   0.58(0.54)
  NGC1603   -5?       5038           67        -19.5        12.2           0.30      Sep97    18000/2100      50            1.5                  1.5                   0.50 (0.46)
  NGC2592   -5        1925           26        -18.9   10.5$\ddagger$      0.17      Feb97     5400/5400      53            3.0                  3.0                   0.65 (0.62)
  NGC2699   -5        1660           22        -18.1   9.0$\ddagger$       0.07      Feb97     5400/5400      45            2.1                  2.1                   0.63 (0.60)
  NGC2778   -5        1991           27        -18.8        15.7           0.28      Feb97     5400/3600      40            2.1                  2.4                   0.54 (0.50)
  NGC3605   -5         581      14$^\dagger$   -17.6        21.2           0.33      Feb97     4400/5400      22            1.8                  1.8                   0.47 (0.43)
  NGC2577   -3        2068           28        -19.0   24.3$\ddagger$      0.38      Feb97     5400/3600      105           3.0                  3.8                   0.71 (0.68)
  NGC3156   -2         980      15$^\dagger$   -17.9        14.4           0.42      Feb97     5400/5400      47            2.4                  3.0                   0.60 (0.57)
  NGC7617   -2        4176           56        -19.1        10.6           0.35      Sep97    18000/12560     30            1.5                  1.5                   0.62 (0.58)
  --------- ------ ----------- -------------- ------- ---------------- ------------ ------- --------------- ------- -------------------- -------------------- -----------------------------

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