A&A 396, 73-81 (2002)
DOI: 10.1051/0004-6361:20021333
F. Wernli 1 - E. Emsellem 1 - Y. Copin 2
1 - Centre de Recherche Astronomique de Lyon, 9 Av. Charles André,
69561 Saint-Genis Laval Cedex, France
2 -
Institut de physique nucléaire de Lyon, 4 rue Enrico Fermi, 69622 Villeurbanne Cedex, France
Received 28 March 2002 / Accepted 10 September 2002
Abstract
We present adaptive optics assisted OASIS integral field
spectrography of the S0 galaxy NGC 4621. Two-dimensional stellar kinematical
maps (mean velocity and dispersion) reveal the presence of a 60 pc
diameter counter-rotating core (CRC), the smallest observed to date. The
OASIS data also suggests that the kinematic center of the CRC is slightly
offset from the center of the outer isophotes. This seems to be confirmed by
archival HST/STIS data. We also present the HST/WFPC2 V-I colour map,
which exhibits a central elongated red structure, also slightly off-centered
in the same direction as the kinematic centre. We then construct an
axisymmetric model of NGC 4621: the two-integral distribution function is
derived using the Multi-Gaussian Expansion and the Hunter &
Qian (1993) formalisms. Although the stellar velocities are
reasonably fitted, including the region of the counter-rotating core,
significant discrepancies between the model and the observations demonstrate
the need for a more general model (e.g. a three-integral model).
Key words: galaxies: elliptical and lenticular, cD - galaxies: evolution - galaxies: individual: NGC 4621 - galaxies: formation - galaxies: kinematics and dynamics - galaxies: nuclei
The fact that early-type galaxies are not necessarily oblate major-axis
rotators has been well known since the end of the 1970s. The number of objects
showing significant minor-axis rotation attained ten in 1988 (Wagner
et al. 1988), and some of these objects later showed evidence for
kinematically decoupled core structures (NGC 4365, NGC 4406:
Bender 1988; NGC 4589: Moellenhoff & Bender 1989; NGC 5982:
Wagner 1990). The most cited formation scenario for such structures
is the hierarchical model, which involves single or multiple merging steps
(see e.g. Cole et al. 1994 and references therein). Observations of
kinematically decoupled cores can therefore be used to constrain the merger
tree of galaxies. However the total fraction of early-type galaxies
containing such a core is poorly known (de Zeeuw & Franx 1991). Such
cores are generally revealed using long-slit measurements with 1
FWHMor worse spatial resolution: this introduces an a priori assumption on the
geometry of the central structures, and prevents the detection of cores with
small apparent sizes. Finally, the rough characteristics of these cores
(fraction in mass, relative age and metallicity) remain very uncertain, as
very few studies exist and are often limited to morphological data (see
Carollo et al. 1997 for 15 KDCs).
Recent studies (e.g. Verolme et al. 2002) show that state-of-the-art models combined with two-dimensional integral-field spectroscopic data are required to constrain precisely the global physical parameters of early-type galaxies, such as the inclination angle or the mass-to-light ratio, as well as the central characteristics for objects exhibiting features such as kinematically decoupled cores (e.g. the core mass of IC 1459, Cappellari et al. 2002). Moreover, integral-field spectroscopy also provides access to line-strength maps: the detailed kinematic information can thus be coupled to the line indices in order to more easily disentangle the core from the host galaxy (Davies et al. 2001).
The SAURON survey (Bacon et al. 2001a; de Zeeuw et al.
2002) was carried out in order to study the integral field
kinematics of a sample of 72 early-type galaxies over a wide field of view (>33
41
). In parallel, to obtain complementary high spatial
resolution data, we designed a program to observe their central parts
(
5
5
)
with the integral field spectrograph
OASIS (Bacon et al. 2000). The aim is to probe the central spatial
morphology and dynamics of a subset of galaxies in the SAURON sample, in
order to link them with the global properties of the host galaxies provided by
SAURON and other wide field studies. The additional use of PUEO
(Rigaut et al. 1998), the Canada France Hawaii Telescope AO bonnette,
with OASIS allows us to reach resolutions of about
(
12 pc
at 10 Mpc) in the red part of the visible when bright guiding sources are
available.
NGC 4621, an S0 galaxy (Lauer et al. 1995), with MV=9.6, located in
the Virgo cluster (D= 18.3 Mpc, Tonry et al. 2001), is the first
target of the subsample that was observed with OASIS/PUEO. Its steep
power-law central luminosity profile (
,
Gebhardt et al. 1996) is ideally adapted to serve as a guiding source
for the AO. NGC 4621 is close to edge-on, and its innermost
isodensities reveal a nuclear disk (Sil'Chenko et al. 1997).
In this paper, we present the first results of our OASIS observations of NGC 4621, with the discovery of the smallest counter-rotating stellar core observed to date. The scale of the core is 60 pc, while the previously detected cores have an average radius of around 1 kpc (see the sample of Carollo et al. 1997). Detailed information on the different data sets used in this work, including HST/WFPC2 and STIS observations, are provided in Sect. 2. The corresponding measured photometry and stellar kinematics are presented in Sect. 3. We computed a two-integral distribution function (DF) model of the galaxy, which is presented in Sect. 4. The discussion is carried out in Sect. 5.
Ca-triplet OASIS exposures of NGC 4621 were obtained in January 2000 using
the medium spectral configuration (MR3: 8346-9152 Å) at the f/20 ( PUEO) focus of the CFHT, and a spatial sampling of 0
16. Eight out of
ten 30 min exposures were fully reduced using the dedicated XOasis
package
,
and merged into a single frame (two frames were discarded due to bad seeing
conditions and associated loss of guiding). The resulting 3D
dataset
probes about
(300
300 pc2), with a
resolution of
km s-1. We modeled the PSF of the merged
datacube using the sum of two concentric Gaussians: their parameters were
retrieved (via a least-square procedure) by comparison with the WFPC2/HST
F814W frame as in Bacon et al. (2001b). The resulting FWHM of the merged
datacube is 0
51.
The total S/N (summed over the bandwidth) drops very quickly towards the outer
parts, as it is 65 at the center, 20 at 1
and 10 at 2
on the
major-axis. We thus convolved the datacube with a Gaussian of variable width in
order to obtain a higher S/N over the field (see Fig. 1).
We used a tuned version of the Fourier Correlation Quotient (FCQ; Bender 1990) method to derive the line of sight velocity distributions (LOSVD) at each measured spatial position. We used a single template star (G8III: HD073665), but tested the stability of the kinematical mesurements with different templates, and different continuum subtraction parameters, only resulting in minor differences. The LOSVDs were then fitted using a single Gaussian function, yielding the mean velocity and dispersion maps. We could not obtain reasonable maps of higher order moments, due to the rather low signal to noise ratio (even after the convolution performed on the datacube). The error bars on the velocity and dispersion measurements were computed via a Monte-Carlo algorithm using the measured S/N (see Copin 2000, Ph.D. Thesis).
In order to obtain an accurate three-dimensional mass model of the galaxy we
need both wide-field and high-resolution photometry to probe the visible mass
up to large radii and to correctly sample the central structure (the cusp).
We used a 20
V band image of the galaxy taken at the OHP 2 m
Telescope, kindly provided by R. Michard (Idiart et al. 2002). The central regions were examined with the help of
WFPC2/HST data
retrieved from the archive (F555W and F814W filters,
Faber et al. 1997, #5512). For both bands, three unsaturated frames
were merged and cosmic ray-corrected (330 s and 230 s exposure time for F555W
and F814W filters respectively). We performed photometric calibration using
the VEGAMAG standard, and used the PSFs computed using tinytim
.
We adjusted the levels of the OHP image to the F555W frame taking into account
the different spectral domains and the PSF.
We used long slit data from Bender et al. (1994,
hereafter BSG94; data kindly provided by R. Bender) in order to complement the
OASIS data up to large radii: major and minor axis velocity and dispersion
measurements are available inside 40
(3.5 kpc), with a seeing of
1
8 FWHM (other kinematical - and line-strength - measurements were
published by Sil'chenko 1997, but with a lower spatial resolution).
We finally reduced unpublished HST/STIS major-axis data (Green, ID #8018; G750M grating, Ca-Triplet) in order to examine the central kinematics of NGC 4621 at high spatial resolution. Two exposures with a total of 72 min were available. The calibrated data were retrieved via the STScI data archival system (calstis pipeline). Appropriate flat field exposures were retrieved to correct for the fringing ( mkfringeflat/defringe IRAF routines, Goudfrooij & Christensen 1998), critical at these wavelengths, as well as five K0-III kinematical template star exposures (HR7615, Green, #7566). Further rejection of cosmic rays was then performed on the NGC 4621 individual exposures, before recentring and merging. The stellar kinematics were extracted using the same FCQ and fitting methods as for the OASIS data. As for the OASIS datacubes, the S/N of the STIS data was found sufficient only to derive the mean stellar velocity and dispersion within the central arcsecond.
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Figure 1: OASIS stellar kinematic data of NGC 4621. Top left: mean velocity field. The zero-velocity curve has been emphasized with a white thick line. Top right: velocity dispersion. White contours are reconstructed isophotes. The data have been smoothed to improve the signal to noise: the black circles represent the beam sizes (FWHM). Below the cuts along the major and the minor axes are shown along with the best fitting two-integral model (dashed lines, see Sect. 4). |
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Figure 2: STIS mean velocity (left) and dispersion (right) profiles along the major axis. The dashed line corresponds to the best fitting two-integral model (see Sect. 4). The velocity dispersion is clearly underestimated by the present model, which hints for the need of an additional central dark mass, a more general treatment (three-integral model) or both. |
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To avoid any confusion due to the complexity in both the photometry and the
kinematics of NGC 4621, we will use the following convention: the photometric
major axis of the galaxy (as measured by HST) defines the x-axis, the
negative values being in the SE quadrant, referred to as the left side of the
galaxy. The center (0,0) is defined as the center of the HST isophotes of
NGC 4621 within 1
,
excluding the central 0
1, the isophotes of
which exhibit a significant asymmetry (see next section). The centering and
rotation procedures of the HST frames have been performed using an algorithm
which minimizes the standard deviation of the recentered and rotated frame
subtracted by its flipped counterpart. This leads to accuracies of 0.1 degrees
and 0
005. All figures in this paper share a common orientation, as shown
in Fig. 3.
The OASIS stellar mean velocity and dispersion maps are presented in
Fig. 1. The datacubes have been centered and rotated in order to
match the HST data, using the center and angle provided by the PSF fitting
procedure mentioned in Sect. 2. The velocity field reveals a
clear counter-rotating core (CRC). The position of the zero-velocity curve is
used to measure the center and the size of the CRC: it is 1
7 (150 pc) in
diameter, and off-centered by
towards the SE. The total
velocity amplitude observed within the central 1
along the major axis
is 100 km s-1, while the peak-to-peak velocity amplitude of the CRC's is only
35 km s-1. The stellar velocity dispersion map peaks at
km s-1.
The dispersion map however exhibits high frequency substructures in the
central 0
5 (Fig. 1), the minor-axis dispersion profile even
having a local minimum at the centre. Considering the amplitude of these
structures and their spatial scales (comparable to the local FWHM), we should
wait for data with better signal to noise ratio to discuss these features.
The BSG94 data show no significant minor-axis rotation, and a maximum velocity
of 140 km s-1 at 30
(see BSG94 or Fig. 7). Besides
a relatively weak velocity amplitude in the innermost 4
(<60 km s-1), these data do not show any hint for the existence of the
counter-rotating core. We will use the BSG94 kinematics to constrain the
dynamical model outside the OASIS field of view.
The STIS velocities and dispersions are shown in Fig. 2. In
these data the size of the CRC is 0
7 (
60 pc) and its velocity
center is located at
(
4.5 pc). We have carefully
checked the centering of the STIS data with respect to
the reference WFPC2 images, and found that this offset is robust.
At STIS resolution the major-axis velocities of the CRC reach
75 km s-1with respect to the systemic. The maximum velocity
dispersion is
km s-1, at
.
These values are consistent with the OASIS data considering the
higher spatial resolution of the STIS data.
Both F555W and F814W images reveal a peculiar structure in the core. Indeed,
the luminosity peak is offset from the center of the outer isophotes by
0
01 towards the east. The photometric peak is located in the upper
left quadrant (Fig. 3). This feature does not seem to be an
artefact, as it is clearly visible on each individual HST frame, and in both
bands, and cannot be attributed to centering uncertainties (0
005).
The two HST frames were then PSF-crossconvolved and divided to obtain
a V - I colour image (Fig. 4). It reveals a central gradient (V-Iincreases towards the center) with a y-axis elongated structure
centered at (-0
02, 0
01) (see dashed line on
Fig. 3). The isochromes have slightly higher ellipticities than
the isophotes (
at 2
5 for V-I, vs. 0.4 for the V frame).
In this Section, we present the methods used to model the photometry and
kinematics of the central region of NGC 4621. The available data suggest that
the very central region (100 pc) of NGC 4621 slightly departs from
axisymmetry. The CRC seems off-centered in both the OASIS and STIS data.
However, the off-centering is only 4 pc. At the resolution of the BSG94
data, this is obviously not resolved. Moreover, the position angle measured
in the OHP photometry does not vary more than 2 degrees within 15
.
Axisymmetry is therefore a reasonable approximation for a first modelling.
Axisymmetric, two-integral models then have the advantage to be semi-analytical.
We first derived simple Jeans models to roughly
constrain the input parameters (inclination, mass-to-light ratio). We then
used the Hunter & Qian (1993) formalism to compute the two-integral
distribution function f of NGC 4621, as a function of energy (E) and of the
vertical component of angular momentum (Lz). The present modelling is only
intended to provide a first view at the dynamics of NGC 4621, so we
decided not to include a central dark component: this issue will be examined
in detail in a forthcoming paper.
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Figure 3:
Inner isophotes of NGC 4621 (WFPC2 F555W, solid contours, step of
0.2 mag). The center (0,0) of the galaxy is defined as the center of the
outer isophotes (outside 0
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Figure 4:
V-I map (WFPC2, F555W and F814W filters). Left: convolved with
a gaussian of
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Figure 5: Top panels: multi Gaussian Expansion fit (thick contours) of NGC 4621 superimposed on the V band isophotes (thin contours). Top left: OHP V-band photometry. Top right : HST/WFPC2 F555W band image (isophote step of 0.4 mag/arcsec2). Notice the nuclear disc in the HST data. Bottom left panel: NGC 4621 light profiles along r2=x2/a2+y2/b2, where a and b are the semi major and minor axes of fitted ellipse respectively. Crosses correspond to the WFPC2 image, circles to the deconvolved WFPC1 data from Byun et al. (1996). The dashed and dotted curves correspond to the convolved and deconvolved MGE models respectively. Bottom right panel: ellipticity profile, with Michard's (solid bold line) and WFPC2 (solid hairline) data, along with the MGE model (dotted line). |
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q | |
1 | 4.502 ![]() |
0.040 | 0.860 |
2 | 7.713 ![]() |
0.112 | 0.610 |
3 | 4.792 ![]() |
0.201 | 0.941 |
4 | 1.161 ![]() |
0.438 | 0.344 |
5 | 1.475 ![]() |
0.516 | 0.919 |
6 | 4.638 ![]() |
1.036 | 0.325 |
7 | 6.317 ![]() |
1.280 | 0.872 |
8 | 3.357 ![]() |
2.486 | 0.275 |
9 | 2.700 ![]() |
3.211 | 0.658 |
10 | 1.326 ![]() |
5.698 | 0.817 |
11 | 6.313 ![]() |
6.926 | 0.377 |
12 | 6.417 ![]() |
12.468 | 0.639 |
13 | 3.295 ![]() |
25.674 | 0.627 |
14 | 8.208 ![]() |
57.091 | 0.633 |
15 | 1.517 ![]() |
128.782 | 1.000 |
The first step in modelling NGC 4621 was to build a luminosity model which properly reproduced the observed photometry. We used the Multi Gaussian Expansion (MGE) formalism proposed by Monnet et al. (1992) and Emsellem et al. (1994), which expresses the surface brightness as a sum of two-dimensional Gaussians. Assuming the spatial luminosity is also a sum of (three-dimensional) Gaussians, given the choice of viewing angles, and using an MGE model for the PSF, we could then deconvolve and deproject the MGE model uniquely and analytically.
The procedure was performed stepwise. We began fitting the wide field V-band
image. We then subtracted the outer gaussian components from the high
resolution WFPC2 image, and fitted the residual image (central 15
). In
this step, we had to exclude the innermost 0
4, to avoid convergence
problems probably due to the slightly asymmetric central feature
(Fig. 3). We finally fitted the very central arcsecond.
Gathering these three parts, we thus obtained a 15 Gaussian components model,
with the same center and PA, the parameters of which are given in
Table 1. The goodness of the fit is illustrated in
Fig. 5.
We then made use of the Hunter & Qian (1993) formalism to build the
two-integral distribution function of the galaxy using the best fit value for
the mass-to-light ratio of
found from simple
Jeans models. We used a default value of
,
which produced a
marginally better fit.
The DF is divided into two parts, which are respectively even and odd in
Lz. The even part (
)
is uniquely determined by the
input MGE mass model. This involves the calculation of a path integral in the
complex plane, as described by Hunter & Qian (1993). The odd part of
the distribution function (
)
is then parametrized, and
adjusted to fit the kinematical data. We chose the parametrization proposed by
van der Marel et al. (1994), and modified it to account for the CRC. The
original parametrization corresponds to Eq. (1), and we
additionally allowed
to be function of
(Fig. 6). The analytical form of this
function is the same
as the one in Eq. (2), with an additional variable change. We can
adjust the energy
above which the stars begin to
counter-rotate, as well as the smoothness of the transition (
:
abrupt transition, a=1: smooth transition, see Fig. 6).
The best fit model reproduces the BSG94 velocity profiles reasonably well
(Fig. 7) with values of
ranging from 8 outside
10
to -2 in the central part. The higher resolution OASIS
(Figs. 8 and 1), and STIS
(Fig. 2) velocity measurements, revealing the counter-rotating
core, are also well fitted by this two-integral model. The best fitting model
uses a core with a diameter of 1
1 (
), and an
abrupt transition (a=100 i.e. almost all stars having
are counter-rotating, see bold line in
Fig. 6). A rough estimate of the CRC mass can be made by selecting
stars counter-rotating in the central part. This is performed by integrating
the DF weighted by a function which is 0 for
and
Lz > 0 and 1 otherwise. The total mass of NGC 4621, which is given by the
mass-to-light ratio and the deprojected MGE-model is
.
The total mass of the CRC is
,
yielding a mass fraction of 0.12%.
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Figure 6:
Parametrization of ![]() ![]() ![]() ![]() ![]() ![]() |
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The dispersion profiles are well reproduced by the model outside the central
few arcseconds. The central values of the dispersion predicted by the
models are however systematically too low compared to the BSG94 observations.
This is confirmed by the OASIS and STIS dispersion values: the model
predicts a central dispersion of 220 km s-1, to be compared with
the central observed STIS dispersion of
km s-1.
We were finally unable to fit the higher order moments, even at large radii.
The h3 values predicted by the model
are thus systematically too high, by a factor of almost two.
This discrepancy could not be solved even by changing the parameters
of the odd part of the DF.
These two discrepancies do indicate that we need a more general dynamical
model for NGC 4621. First, we should remove the constraint imposed
by the two-integral model by allowing a third integral of motion.
There may then still be the need for an additional central dark mass
to explain the observed dispersion values. Such a model will be examined
in a forthcoming paper (Wernli et al., in preparation).
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Figure 7:
Best fit two-integral DF model. Left: major axis. Right: Minor
axis. Inclination is 90 degrees, no black hole and
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Figure 8: Best fit two-integral DF model for the comparison with OASIS. Only the velocity map is shown: central dispersion values are too low due to the absense of a central dark mass. Grey levels range from -50 to +50 km s-1. |
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In this paper, we report the discovery of a 60 pc diameter
counter-rotating core using new OASIS integral field spectroscopy, a result
confirmed with archival STIS long-slit data. A structure at this scale has
never been observed before, except in M 31, which exhibits a 20 pc co-rotating
decoupled core (Bacon et al. 2001b and references therein). Typical sizes
of kinematically decoupled cores are at the order of 1 kpc (See e.g. the sample
of Carollo et al. 1997). The computed mass fraction of 0.12%
(
)
is on the low side, as compared for instance to
the CRC of IC 1459, which is 0.5% for a core size of 3 kpc (Cappellari
et al. 2002), but for which the estimation is of higher accuracy. As a
comparison, the black hole mass from the
relation for
IC 1459 is
(Merritt & Ferrarese 2001b)
and
for NGC 4621 (Merritt & Ferrarese
2001a). The mass fraction estimate provided here in the case of
NGC 4621 can only be indicative: the DF separation is inaccurate and
difficult, and to get a precise result we would need an analysis in integral
space.
Both individual OASIS and STIS data sets suggest that the CRC of NGC 4621 is slightly off-centered with respect to the center of the outer isophotes. This offset seems consistent with the weak asymmetry detected in the WFPC2/HST V and I frames and the V-I colour map (Fig. 3). The central structure elongated along the minor axis detected in the V-I colour map represents an increase of V-I=0.03 which could either be due to an intrinsic stellar population gradient, or to dust. In the latter case, this would correspond to AV=0.06 (assuming RV=3.1 and RI=1.86 from models of the Galaxy, Rieke & Lebofsky 1985). Note that we do not detect any high frequency structure in the individual WFPC2 V and I frames.
The two-integral models reasonably reproduce the observed velocity profiles in the outer parts, as well as in the region of the CRC (OASIS and STIS). These axisymmetric models do obviously not take into account the observed off-centering. The predicted central value of the velocity dispersion is significantly too low by 50 km s-1. We were also unable to simultaneously fit higher order Gauss-Hermite moments: e.g. the predicted h3 is systematically too high (see Fig. 7). This demonstrates that we need more general dynamical models in order to properly fit the observed kinematics. We are therefore in the process of constructing three-integral Schwarzschild models, with the possibility to include a central dark mass. This may solve the discrepancies mentioned above.
If one assumes a merger-scenario for the CRC's origin, the off-centering, if confirmed, could be either due to the fact that the merging process is still on-going, or may be the result of a stable mode (see e.g. M 31 in Bacon et al. 2001b). The short dynamical timescale at the radius of the CRC (1 Myr) seems to favour the latter hypothesis. Further discussions regarding this issue must however wait for additional spectroscopy. A detailed study at high spatial resolution of the stellar populations in the central arcsecond would certainly help in this context. Existing high resolution STIS spectra at bluer wavelengths can be used for this purpose.
Stellar kinematics at HST resolution is available for only a handful of early-type galaxies. M 31 is one of the rare examples where it is possible to have sufficient spatial resolution to measure kinematical features at parsec scales, such as its 20 pc nucleus (Bacon et al. 2001b and references therein). The OASIS data presented in this paper demonstrate that it is possible to study substructures with a characteristic size of 60 pc in galaxies at the distance of the Virgo cluster. The presence of the CRC in NGC 4621 raises new questions about the dynamical status of the centers of early-type galaxies. What is the fraction of early-type galaxies that have such substructures? Two-dimensional spectroscopy at high spatial resolution is clearly needed to simultaneously study the dynamical and chemical contents of these cores, and to eventually understand their origin.
Acknowledgements
We would like to thank Raymond Michard and Ralf Bender for the data they provided, Tim de Zeeuw for a careful reading of the manuscript and his suggestions, and the referee Dr M. Carollo for her thorough and constructive report.