Highligh in Physics 2005

1

• Congresso del Dipartimento di Fisica
• Highlights in Physics 2005
• 1114 October 2005, Dipartimento di Fisica,
  Università di Milano
• 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 PadovaKapteyn
  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 z0.04-0.07. 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 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
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 galaxy are summarized in Table 1. The HI
density distribution and kinematics of NGC 4559
are not symmetric on the two sides. The
lopsidedness in the density distribution can be
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
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
symmetric this is a surprising result of the
combined effect of lopsidedness in kinematics and
in density distribution and suggests that
lopsidedness is probably more common in galaxies
than previously thought. Extra-planar gas - The
HI position-velocity diagram (Fig.2b) shows that
the HI velocity profiles are asymmetric with
respect to the peak. There are wings in the
velocity profiles away from the rotation velocity
(white dots). Broad low-level extensions are
visible toward the systemic velocity and, close
to the galaxy centre, there are traces of
emission at forbidden velocities. Models of a
thin disk with a Gaussian velocity profile do not
exhibit such wings. We call the gas responsible
for these wings anomalous gas (see Fig.2d).
Most likely this anomalous gas does not coexist
with the normal gas in the region close to the
equatorial plane.
a
b
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
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 phenomena in terms
of accretion from the intergalactic medium is not
ruled out. In fact, the above arguments, which do
suggest a link 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. 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.
Radio continuum
Optical (DSS)
Column density contours
Velocity field
c
b
Receding side
b
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).
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
d
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 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)
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 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. Local
dispersion relation - We consider tightly wound
linear density perturbations of the form
?1i?1iexpi(-?tm? ?kdr) under the WKB
ordering m/(rk)O(?), with the epicyclic
expansion parameter defined as ?ch/(rkh). The
obtained dispersion relation in dimensionless
form is n4-2hn3-A(2)n22hA(1)nA(0)0 with the
relevant coefficients defined as
A(n)A(n)(kQh,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 and b are relative density
and temperature ratios.
k
k
n
Fig.3 - The dispersion relation for varying h. In
each frame the real (solid lines) and imaginary
(dashed lines) part of n are given as a function
of the dimensionless wavenumber k. The other
parameters have been set to the following values
a0.1, b0.1, QhQcrit1.14.
The two new parameters (with respect to
preceding studies 4) are defined as d?c/?h and
hm(d-1)(?h/?h). Note that h vanishes for
axisymmetric perturbations (m0) even when the
two components are in relative motion (d?0). For
d1 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 aa/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 hltlt1 and
hO(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)n22hA(1)nA(0)0 so that the
growth rate of the instability can be derived
analytically. In the more general case, hO(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)lt0 , i.e. when ?hlt?plt?c (where nRRen
and ?p ?/m is the pattern speed).
3 Cava, A. 2004, Stabilità di dischi
autogravitanti a due componenti oltre
linstabilità 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 gravity-related 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 z0.04-0.07 covering both hemispheres
, which is generating an unprecedented database
of optical and near-IR 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
z0.07,H070) 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).
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 V22. 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
V20.5.
A1795
Fig. 5 - Comparison of different galaxy surveys
(from 5).
A376
A1795
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.
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).
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.
See also the site http//web.pd.astro.it/wings
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