Do Field Ellipticals Exist?
V.J. Martínez (Astronomical Observatory,
University of Valencia)
R.M. Smith (Cardiff University)
What is an elliptical?
e.g. NGC 1399 – the central galaxy in the Fornax cluster
Elliptical galaxies –
• have surface brightness profiles different from that for
spirals
• preferentially lie in regions of high galactic density – the
morphology-density relation (Dressler 1980)
• mostly have little rotation, whilst some show peculiar
kinematics
• have light that is dominated by an old, red, stellar
population with little evidence of recent star
formation/merger activity
• are typically surrounded by a large population of globular
clusters
-- elliptical galaxies are primarily formed during the early
stages of the evolution of the Universe
But....
Very difficult to disentangle galaxy properties from those of
cluster, e.g. Grillmair et al. 1994 for NGC 1399.
Nature or nurture for ellipticals?
– what is intrinsic and what is due to environment?
Take the elliptical away from dense environment – field
ellipticals
Dressler
Dressler1980
1980
Several studies of elliptical galaxies in low-density regions
Silva and Bothun (1998) – little evidence for recent star
formation/merger activity
Kuntschner et al. (2002), Treu et al. (1999,2001,2002) –
strong evidence for recent activity
Major problem - what is a field elliptical?
Or what is the field?
Redshift surveys are notoriously incomplete at the faint/lSB
end (e.g. Driver and de Propris 2003)
Zaritsky et al. (1993,1997) used criteria for isolated spiral
galaxies that appeared to work well.
Magnitude difference > 0.7mag for projected distances of
1Mpc – haloes do not interact
Magnitude difference > 2.2mag for projected distance of
500kpc – true isolation criterion
The Local Group would fail these isolation criteria because
of M31
Where do we find the data?
2 possible sources – NED and LEDA
We used LEDA – more than one million galaxies from a variety of
sources
Need to apply other selection criteria too –
1500km/s < v < 10000km/s
M < -19 – must not go into dwarf regime with all its uncertainties
|b| > 25 degrees
t < -4 – difficult due to possible misclassification due to dust or gas
Driver
Driverand
andDe
De
Propis
Propis2003
2003
Sample not complete so cannot undertake any statistical studies
940 ellipticals satisfy the latter criteria
Used projected separations only – much stricter than other criteria
but ensures galaxies are truly isolated
Applying the Zaritsky criteria only 32 field ellipticals found
Misclassification? – if contain dust or gas?
Smith,
Smith,Martinez,
Martinez,and
andGraham
Graham2004,
2004,ApJ,
ApJ,in
inpress
press
How does the percentage vary with the selection criteria?
Varied the inner cut-off radius between 100kpc and 1Mpc –
power law drop off
Varied the absolute magnitude limit of the primary –
similar distribution but higher percentage isolated –
3 possible causes • Incompleteness
• Lack of background subtraction will lead to steep LF
( α= -2.5)
• Inherent property of bright ellipticals? (c.f. cD and BM I
clusters)
Smith,
Smith,Martinez,
Martinez,and
andGraham
Graham2004,
2004,ApJ,
ApJ,in
inpress,
press,astro-ph/0311599
astro-ph/0311599
The satellite population
Previous studies concentrated on the primary galaxy –
morphology-density relation indicates environment may
play a big part
Surrounding population gives clues to formation history of
galaxy
CDM suggests that galaxies should be surrounded by halo of
dwarfs
Are these field ellipticals surrounded by a halo of much
fainter galaxies?
Use technique similar to Holmberg (1969) and Lorrimer
(1994)
Count galaxies in the neighbourhood of the primaries using
APM scans
Excess should be at redshift of central galaxy
Applied redshift limit of 6500km/s and –16.8 < M < -14.6 for
surrounding galaxies
10 out of the 32 parents had APM data suitable for this study
Stack the results to get a mean profile
Smith,
Smith,Martinez,
Martinez,and
andGraham
Graham2004,
2004,ApJ,
ApJ,in
inpress,
press,astro-ph/0311599
astro-ph/0311599
Excess of dwarf galaxies seen within 500kpc
Necessary to subtract background (difficult) – estimated as
that at about 500kpc
Gradient of –0.6 +/- 0.2 – similar to slope found by Lorrimer
et al.
Slope of luminosity function = -1.8
Agrees with Morgan et al. for isolated spirals and some
estimates for poor groups but major disagreement with
Local Group and the field where slope=-1.2
Lorrimer
Lorrimeret
etal.
al.
1994
1994
Which are true satellites?
Currently trying to undertake for ellipticals what Zaritsky et
al. did for spirals.
Use multi-object spectrograph on WHT to measure redshifts
for all galaxies within 500 kpc of parent.
Jan 2002 (1 night) completely wiped out
Jan 2003 (3 nights) successful apart from instrument
problems: 7 parents
Jun 2003 (1 night): successful: 3 parents
7 galaxies observed, one in a poor group (but still satisfies
criteria)
From 6 galaxies we find 23 satellites – approx. 4 per galaxy,
c.f. Spirals average 1 per galaxy
Supports imaging results that Es have more dwarfs than
spirals
Poor group galaxy has 27 satellites
Satellite list is already available at
http://www.uv.es/martinez/
So what does the velocity dispersion look like?
Have to worry about interlopers
Poor group galaxy has very broad dispersion, typical of a
cluster
Other galaxies have uncertain dispersion – maybe one
Gaussian typical of a cluster, or one narrow Gaussian and one
broader (but not currently centred on 0).
If latter, dispersion appears broader (more massive??)
Need more data (about 100 satellites)
Zaritsky
Zaritskyet
etalal
1997
1997
Conclusions:
1. Have produced a catalogue of definitely isolated elliptical
galaxies
2. These galaxies are surrounded, in the mean, by a halo of
dwarf galaxies
3. There is a strong hint that the LF of the dwarfs is steep
and consistent with the haloes of rich clusters
4. The velocity dispersion of these dwarfs is somewhat
uncertain but hint that the mass of these Es is greater than
spirals
5. More imaging and spectroscopic data is needed