The first major science programme for the PN.S will be
to study the kinematics in a large sample of bright, round elliptical
galaxies. With a high abundance of observable PNe per galaxy, and
with presumably simpler dynamics than in more flattened ellipticals,
these systems should be the most immediately fruitful for dynamical
analysis. On the basis of previous experience and calculations (see
below), we expect to obtain 100-400 PN velocities per galaxy,
extending to 5-15 Reff (see figure below); these data will be
combined with available stellar absorption line kinematical data from
the inner parts. With ~ 100 velocities, we will be able to
measure the rotational structure of a galaxy, and make a rough
estimate of the total halo mass. With ~ 250 velocities, using
our existing orbit-modelling methods (Romanowsky & Kochanek 2000), we
can measure the total mass of the galaxy to ~ 20% and quantify
the anisotropy of the orbits (the parameter) to ~ 30%,
both representing unprecedented degrees of accuracy.
A sample of 433 planetary nebulae (PNe) overlaid on the stellar isophotes
of NGC 5128,
where Reff 5 kpc.
The PNe marked with circles have negative relative velocities, and
the PNe with crosses have positive velocities.
marks the position of the rotation axis.
With our proposed observations, we will be able to obtain data of
comparable quality for a much more representative sample of
From Hui et al. (1995).
(Click on figure to enlarge.)
- Survey sample
Our survey sample of 12 galaxies consists of all ellipticals that are
bright (B <= 12.5), round (E2 or rounder), in the observable
declination and velocity range (cz < 2500 km s-1), and that are
calculated to yield >= 100 PN detections in two nights' dark-time
observing with the PN.S. (Our exposure times are calculated to yield
>= 250 PNe for the four brightest galaxies, and ~ 100 PNe for
the other 8.) This sample represents a wide variety of environments
and kinematical and morphological types within the roundness and
brightness constraints (see figure below), and includes 5 galaxies to be
observed by SAURON. With these data, we will gain an unprecedented
knowledge of the presence, amount, and distribution of dark matter in
a large sample of ellipticals, as well as of the angular momentum in
their outer parts.
A full listing of our sample can be found
Coverage in parameter phase-space of our galaxy sample.
The parameters include absolute magnitude MB,
central velocity dispersion c,
rotational importance (v/)*,
isophote shape C4 (< 0 for boxiness, > 0 for diskiness),
luminosity concentration Tphot,
and environmental density (roughly indicated
The filled squares show galaxies expected to yield >= 250 PNe,
and the open squares show those with ~ 100 PNe.
(Click on figure to enlarge.)
- Numbers of PNe
We have calculated the number of PNe we expect
to observe for each galaxy, based on such factors as distance,
luminosity, surface brightness, seeing, and integration time;
these predictions are informed by our observational experience with
counter-dispersed imaging techniques.
As an example, below we present the calculations for a few of our
The following information is used for a rough calculation.
The bright-end cutoff of the PNLF, as measured in the
[O III] 5007Å line, is defined as
M*5007=-4.48 (Ciardullo et al. 1989),
where the magnitudes are defined as
m5007 = -2.5 log F5007 -13.74.
Individual galaxies contain
2.5 = 20-100 PNe
brighter than m*5007+2.5 for every
109 L of B-band
light, depending on the galaxy's luminosity and colour (Hui et al. 1993; Ciardullo 1995).
We take an overall system efficiency on the WHT of 18% into each
and an atmospheric extinction of 0.2 mag.
A dark sky
background of V=21.4 (40Å bandwidth) with 1" seeing produces
Thus, one could see objects at SNR=6
down to roughly m5007=28.2 in 8 hours of integration.
For a more accurate (and pessimistic) calculation,
we integrate along a curve of detection completeness vs. magnitude
(see simulations page),
and also take further factors into account,
such as the fraction of the galaxy visible in the field-of-view,
background light from the galaxy,.
|NGC||Distance D (Mpc)||Cutoff magnitude
m*5007 (+2.5)||Luminosity MB
||Specific density of PNe 2.5
||# PNe to m*5007+2.5
||Flux from m*5007+2.5 PN (counts/sec)
||Integration time to get S/N=6 for m*5007+2.5 PN (hrs)
||More accurate # PNe in 8 hrs
||More accurate # PNe in 12 hrs
- Velocity measurements
With a filter bandpass of 40Å, we can accomodate
velocities within ± 1200 km/s
of the filter's central wavelength
(which is set to be near the Virgo Cluster velocity of 1200 km/s).
This will allow us to acquire nearly all the PNe
for even the highest-dispersion galaxies.
In the sample performance table,
the PNe were assumed to be located to an accuracy of 0.5 pixel.
Simulations show that 0.2 pixel can be expected.
values as limits,
radial velocities will be obtained with a precision of between
15 and 40 km/s.
Note that although contamination by HII regions can be a major issue
for PNLF-distance determinations
(less true for gas-poor ellipticals),
this is much less of a concern for the dynamics --
the HII regions will be tracing the same gravitational potential,
with a somewhat different spatial distribution.
- Future science ideas
- Measure rotation in outer parts of edge-on spirals to compare
to HI rotation curve (rule out any effects of magnetic fields)
last modified by AJR, 14 January 2002