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Congresso del Dipartimento di Fisica
Highlights in Physics 2005
11–14 October 2005, Dipartimento di Fisica, Università di Milano
OAPd
Structure and dynamics of nearby spiral galaxies
C.V. Barbieri*, &, G. Bertin*, D. Bettoni§, R. Boomsma+, A. Cava*,§, F. Fraternali#,
T. Oosterloo$, and R. Sancisi&,+
*Dipartimento di Fisica, Università di Milano
& INAF – Osservatorio di Bologna
§ INAF – Osservatorio di Padova
+Kapteyn Astronomical Institute,Groningen, Olanda
#University of Oxford, Oxford, UK
$ ASTRON -Dwingeloo
Abstract
Recently, the in-depth study of nearby galaxies has led to the surprising discovery that the HI distribution is not confined, as previously thought, to the thin equatorial plane of the disk, but may extend well out of plane, forming a slower-rotating halo-like structure. This finding has important
consequences on our picture of how spiral galaxies form and evolve. More specifically, such extra-planar gas may be the result of a galactic fountain circulation or be indicative of ongoing accretion from the external environment; similar scenarios had been invoked to interpret the phenomenon of the
high velocity clouds in our Galaxy. By a deep 21cm investigation of NGC 4559, we have found [1] one more example of this phenomenon, which adds to the very small number of firm cases of extra-planar gas identified so far. From the dynamical point of view, finding new properties in the structure
of galaxies raises new problems of equilibrium and stability. On a related theme, we have recently addressed the question of the stability of disks characterized by the presence of two components in relative motion [2]. To get a statistically significant picture of the properties of galaxies, the deep study
of individual galaxies should be accompanied by dedicated extensive surveys. One project which may bring important information on the dynamics of normal galaxies is WINGS (Wide-field Imaging Nearby Galaxy-cluster Survey; [3]), a wide-field spectro-photometric survey of a complete all-sky
X-ray selected sample of 78 clusters in the redshift range z=0.04-0.07. WINGS is the deepest (M_v~-14), best resolution (1'') survey of a complete sample of nearby cluster galaxies to date. Redshifts, line indices and equivalent widths of the main absorption and emission features are measured and
provide cluster membership, star formation rates and histories, and metallicity estimates. An extensive observational program is planned to complete the spectral range from the NIR to the UV.
====================================================
Observations - We present 21-cm observations of the spiral galaxy NGC 4559, made with the Westerbork Synthesis Radio Telescope, that Parameter
NGC4559
reveal the presence of an extensive system of extra-planar gas in this galaxy and use them to investigate the structure and kinematics of the ------------------------------------------------------------------------------------------Morphological type
Scd
disc and of the extra-planar gas and the properties of the dark matter halo [1]. The main optical and radio parameters that characterize the Luminosity class
II
h
m
s
Optical centre (α,δ J2000)
12 35 57 .6 +27°57’31”.4
galaxy are summarized in Table 1.
Kinematical centre (α,δ J2000) 12h35m58s.7s ±7” +27°57’32”±5s”
The HI density distribution and kinematics of NGC 4559 are not symmetric on the two sides. The lopsidedness in the density distribution can be Distance (Mpc)
9.7 (1’=2.8 kpc)
1.06 x 1010
SUN)
seen in Fig.2a. The disk is more extended in the approaching side (S-E) and is warped in this direction. The galaxy is lopsided also in its LLB (L
2.53 x 1010
K (LSUN)
a
1.9
kinematics (see Fig.1b and the velocity field in Fig.2a). In spite of these asymmetries, the global HI line profile shown in Fig.1a is highly Disk scale length (kpc)
R
15.82
symmetric: this is a surprising result of the combined effect of lopsidedness in kinematics and in density distribution and suggests that R25ho
16.24
-1
Systemic velocity (km s )
810 ± 4
lopsidedness is probably more common in galaxies than previously thought.
Mean HI inclination angle (deg)
67.2 ±0.6
-37.0 ±1.4
Extra-planar gas - The HI position-velocity diagram (Fig.2b) shows that the HI velocity profiles are asymmetric with respect to the peak. There Mean position (deg)
====================================================
are “wings” in the velocity profiles away from the rotation velocity (white dots). Broad low-level extensions are visible toward the systemic Table 1. Optical and radio parameters for NGC4559 (from [1]).
velocity and, close to the galaxy centre, there are traces of emission at “forbidden” velocities. Models of a thin disk with a Gaussian velocity Fig.1 - (On the right) Global HI line profile (top panel) and rotation
profile do not exhibit such wings. We call the gas responsible for these wings “anomalous” gas (see Fig.2d). Most likely this anomalous gas curves obtained for the two sides of NGC 4559 (bottom panel). The
b
continuous line shows the curve obtained for the whole galaxy.
does not coexist with the normal gas in the region close to the equatorial plane.
The spatial distribution of the anomalous gas is quite smooth and homogeneous, suggesting that it is not the result of a single recent local event (such as the capture of a lump of gas). Moreover, the kinematics of this
component is regular and closely follows that of the thin disk. We investigated the possible lopsidedness of the extra-planar gas. The extra-planar gas seems more abundant on the approaching side; on this side, the
disk appears to be brighter than the receding one, possibly because of a higher star formation activity. Moreover, this is the side where the rotation curve rises more steeply.
The presence of the forbidden gas is not reproduced by our two-component model probably because we have not considered vertical
Radio continuum
Optical (DSS)
motions.
The problem of the origin - The origin of the extra-planar gas is not known. The main question is whether it is caused by processes
that take place in the galactic disk, such as a “galactic fountain”, or it originates from infall or accretion of extragalactic, primordial gas.
With this study we have collected new evidence for a close relation between the extra-planar gas and the star formation activity.
However, in spite of many arguments apparently favouring an internal origin of the extra-planar gas, an interpretation of the observed
Column density contours
Velocity field
phenomena in terms of accretion from the intergalactic medium is not ruled out. In fact, the above arguments, which do suggest a link
b
c
between the extra-planar gas and star formation in the disk, could be reversed to conclude that the higher star formation rate might
be the end result of accretion or infall rather than the cause at the origin of extra-planar gas.
b
The extra-planar gas of NGC 4559 is similar to that found in NGC 891, UGC 7321 and NGC 2403 [2]. This suggests that the
anomalous gas may be a common feature in spiral galaxies, missed earlier because of insufficient sensitivity of the
previous observations.
a
Fig.2 - (a) Different view of NGC 4559. Optical image,
radio contour levels, column density and isovelocity
contour. (b) HI position-velocity diagram along the major
axis (white dots show the projected rotation curve). (c)
Rotation curves for the cold disk and for the anomalous
gas. (d) Total HI image and intensity-weighted mean
velocity for the anomalous gas. (all figures from [1]).
d
[1] Barbieri, C.V., Fraternali, F., Oosterloo, T., Bertin, G., Boomsma, R., Sancisi, R. 2005, “Extra-planar gas in the spiral galaxy NGC
4559”, Astronomy and Astrophysics, 439, 947
[2] Fraternali, F.,van Moorsel, G., Sancisi, R., Oosterloo, T. 2002, “Deep H I Survey of the Spiral Galaxy NGC 2403 “, The
Astronomical Journal , 123, 3124
n
n
Asymmetric-drift instability - On a related theme, we have addressed the problem of studying the stability of a self-gravitating fluid disk, made of two components
d=1.2 h=0
d=1.2 h=1
characterized by different effective thermal speeds cc and ch for the cold and the hot components (gas and stars). We showed that the small relative motion between the two
components associated with the so-called asymmetric drift can be the origin of instability for suitable non-axisymmetric perturbations. The result is obtained by examining the
properties of a local, linear dispersion relation for tightly wound density waves in such a two-component model [3].
The natural limit of the case, in which the relative drift between the two components is ignored, is recovered. Dynamically, the instability is similar to (although gentler than)
k
k
that known to affect counter-rotating disks. However, in contrast to the instability induced by counter-rotation, which is a relatively rare phenomenon, the mechanism
d=1.2 h=3
d=1.2 h=2
discussed here is likely to be rather common in nature.
In a rotating self-gravitating axisymmetric fluid disk at equilibrium the radial gravitational force is balanced by rotation, with a contribution from the pressure gradient.
For cool disks, the pressure gradient is small and generally neglected. In contrast with previous analyses that focused on the Jeans instability we allow for the presence of
relative motion between the two components, so that in general c≠ h.
k
k
Local dispersion relation - We consider tightly wound linear density perturbations of the form 1i=’1iexp[i(-t+m+ k·dr)] under the WKB ordering m/(r|k|)=O(), with the
- The dispersion relation for varying h. In each frame the real
epicyclic expansion parameter defined as =ch/(rkh). The obtained dispersion relation in dimensionless form is : n4-2hn3-A(2)n2+2hA(1)n+A(0)=0 with the relevant coefficients Fig.3
(solid lines) and imaginary (dashed lines) part of n are given as a
(n)
(n)
defined as A =A (|k|;Qh,a,b,d,h). Here k and n are the dimensionless wavenumber and Doppler-shifted frequency,Qh is the well known axisymmetric stability parameter,a function of the dimensionless wavenumber k. The other parameters
have been set to the following values: a=0.1, b=0.1, Qh=Qcrit≈1.14.
and b are relative density and temperature ratios.
The two new parameters (with respect to preceding studies [4]) are defined as d=c/h and h=m(d-1)(h/kh). Note that h vanishes for axisymmetric perturbations (m=0) even when the two components are in
relative motion (d≠0). For d=1 the two components are corotating, while for d=-1 they are counter-rotating. The one-component dispersion relation is recovered by taking the limit b→1, d→0 and by letting the density
ratioa become vanishingly small. We find also that, by defining a’=a/d2 and b'=b/d2, the stability condition for axisymmetric disturbances is the same as for the standard Jeans instability in two-component disks.
Non-axisymmetric perturbations - For the case of non-axisymmetric disturbances we consider two situations: h<<1 and h=O(1). In the first case, where the relative motion between the components is small, we can
approximate the relevant dispersion relation as a quadratic –A(2)n2+2hA(1)n+A(0)~0 so that the growth rate of the instability can be derived analytically. In the more general case, h=O(1), if we take m to be large, the basic
dispersion relation should be modified because we are entering the different regime of open waves. In Fig.3 we illustrate the qualitative behavior of the roots of the dispersion relation at fixed values of d for varying h.
Since, at variance with the case of axisymmetric perturbations, the unstable root has a non-vanishing real part of the frequency, we are in the presence of a convective instability. Finally, we have demonstrated that the
instability develops when nR(nR-h)<0 , i.e. when h<p<c (where nR=Re[n] and p = /m is the pattern speed).
[3] Cava, A. 2004, “Stabilità di dischi autogravitanti a due componenti: oltre l’instabilità di Jeans”, Tesi di Laurea, Università di Milano
[4] Bertin, G., Romeo, A. B. 1988, “Global spiral modes in stellar disks containing gas”, Astronomy and Astrophysics, 195, 105
Motivations - Clusters of galaxies, bound systems of hundreds or thousands of galaxies, are an ideal environment to study galaxy evolution and to learn how this is affected by different physical processes: gravityrelated phenomena, starbursts and star formation, interactions with the intergalactic medium, and feedback from central massive black holes. We are carrying out a photometric and spectroscopic survey of nearby
galaxy clusters to build a local reference sample to be used as a baseline for evolutionary studies. This project consist of making a detailed analysis of the stellar populations and morphological structure for a new, large
sample of galaxies belonging to clusters at low redshift.
This Wide-field Imaging Nearby Galaxy-cluster Survey (WINGS) [5] is a wide-field survey of a complete X-ray selected sample of
77 galaxy clusters at z=0.04-0.07 covering both hemispheres , which is generating an unprecedented database of optical and near-IR
A1795
images, optical spectroscopy, and Halpha narrow-band images over a wide area centred on each cluster.
The survey has been conceived to fill in the lack of a systematic investigation of nearby clusters and their galaxy content. WINGS is the
deepest (MV ~-14), best resolution (1''=1.3kpc at z=0.07,H0=70) survey of a complete sample of galaxies in nearby clusters to date.
Using wide-field detectors, we have been able to sample the whole structure of the clusters. For instance, even if the nominal resolution
(FWHM in kpc) of WINGS is only slightly better than that of the survey by Dressler (1980), its data quality (CCD) is definitively better
and its depth is incomparably better (~6 mag) with respect to the Dressler's survey (see Fig.5).
a) A1795
z~0.058
Fig. 5 - Comparison of different galaxy surveys (from [5]).
A376
N gal
The Photometric Analysis - For each cluster of the sample, by using specially designed automatic tools, we produce a deep
photometric catalog and a surface photometry catalog, relative to a subsample of bright/large enough galaxies of the previous, deep list.
To perform a detailed study of the morphological distribution of the galaxies in the cluster, we use GAsPHOT [6], an automatic
tool for classification. For every cluster we typically detect about 2500 galaxies. They form our “deep galaxy catalogues”. The
completeness of these galaxy catalogues is typically achieved down to V~22. In addition we produce a surface photometry catalogue.
In this catalogue we include the photometric profiles (including ellipticity and isophote position angle) of each object together with the
global parameters extracted from the profiles (total V magnitude, effective radius (re), Sersic index (n)). About 600 galaxies for every
cluster have been processed and classified by GAsPHOT down to V~20.5.
Redshift z
A1795
b) A1795
z~0.12
Fig.4 – An example of spectra of two galaxies in
the field of A1795 (optical image in the top panel).
The first galaxy belongs to the cluster (z0.06), the
second one is from a background cluster (z0.12).
See also the site:
http://web.pd.astro.it/wings
The Spectroscopic Data - A natural follow up of the photometric survey is the long-term spectroscopic program that we are
carrying out using spectra taken with the WHT-WYFFOS and AAT-2dF multifiber spectrographs. The observed spectral range is
38007000 Å with an intermediate resolution of 6 9 Å for measuring redshifts (see Fig. 4), equivalent widths and line indices of
emission and absorption lines.
In Fig.4 we show an example of the spectra obtained with WHT-WYFFOS for two galaxies in the cluster A1795 (upper panel), while
in Fig.6 we present an example of redshift distributions obtained using the package RVSAO in the IRAF environment.
Fig. 6 - Two examples of the distributions of galaxy redshifts
in the clusters A376 and A1795.
Star formation rates and histories, as well as metallicity estimates, will be derived for about 150 galaxies per cluster from the line indices and equivalent
widths measurements, allowing us to explore the link between the spectral properties and the morphological evolution in different density environments and
across a wide range in cluster X-ray luminosities and optical properties.
[5] Bettoni, D., Fasano, G., Pignatelli, E., Poggianti, B., Moles, M., Kiaergaard, P., Varela, J., Couch, W., Dressler, A. 2003, “The WINGS survey: the first results”, IAU Symposium 216, 181
[6] Pignatelli, E., Fasano, G. 2004, “Automatic Galaxy Photometry and Morphology in Wide Fields”, IAU Symposium 216, 82