Download The kinematics of Galaxies in Compact Groups

Compact Groups: kinematics and
star formation
Claudia Mendes de Oliveira
U. Sao Paulo, Brazil
Collaborators: P. Amram, C. Balkowski,
H. Plana, E. Cypriano, L. Sodré Jr.
Chania, Aug 2004
Compact Groups of Galaxies

Ideal laboratory of interactions, weak and strong

Nearby counterparts of high-redshift multiplets
H92
Intergalactic HII regions
• Natural place to look for
new objects formed in
galaxy collisions:
compact groups
high galaxy density;
low velocity dispersion
(e.g., Iglesias-Páramo &
Vílchez 2001; Gallagher
et al. 2001)
• Here: discovery of IHII
in Stephan’s Quintet
HCG 92 - HI
(VLA-Williams et al 1998, 1999)
M(HI)=0.5 109 MO
M(HI)=1.33 109 MO V=6000 km/s
M(HI)=1.6 109 MO V=5700 km/s
M(HI)=4.8 109 MO
M(HI)=1.6 109 MO V=6600 km/s
Observations
• Gemini North
• r’ and g’ GMOS imaging with seeing of
~0.75”
• multislit spectroscopy: masks 1” + R400
grating -> 8A resolution, 4000-8000A
coverage
multislit spectroscopy:
• Detection of 4 HII
regions on the HI tail
(Williams et al. 2002)
• projected distance to the
closest bright member
galaxy: > 25kpc
Intergalactic HII regions
(IHII)
• IHII radial velocities
are within ±20 km/s of
the HI velocity:
physical association of
the knots with the
cloud
metallicities
• N2 calibrator (Denicoló, Terlevich & Terlevich 2002)
[NII]λ6584/Hα
<12+log(O/H)>=8.6
(Sun: 8.8±0.1)
• object c: 8.64 ±0.25
using [OIII]/Hβ (Edmunds & Pagel 1984): 8.4 ±0.2
→ the IHII have been formed from pre-enriched material
ages:
• Starburst99 (Leitherer et al. 1999):
solar
metallicity, Salpeter IMF, instantaneous burst
• ages from EW(Hα): 3.2 – 5.6 Myr
mean: 4.6 Myr
mass:
• mass from the emission rate of ionizing fotons, Q(H0) ,
estimated from L(Hα):
(2.9±1.4) x 104 Msun
• expected number of O stars: ~20 to ~200
• mass from the luminosity of the stellar component
R-band luminosities corrected for line-emission: (4.5±2.8) x
104 Msun
• Lower limits because of internal extinction
region c: (Hα/Hβ)=4.4 → AR ~ 1 mag
masses might be at least twice these values
other examples of similar phenomena
• Gerhard et al. (2002): Virgo cluster
M~103 Msun
dproj~18 kpc to NGC4388
• Ryan-Weber et al. (2004) and Oosterloo et
al. (2004) – 10 other examples
• Summary of the IHII
characteristics:
1- compact (except for region
b) but resolved
2- high metallicities (~60%
solar)
3- masses of the order of 104
Msun
4- ages ranging from 3.2 to
5.6 Myr
5- located far from bright
galaxies
6- coincide in location and
velocity with the HI tail
What is the origin of the IHII in
Stephan’s Quintet?
• were they removed from one of the galaxies?
typical ejection velocities ~200-300 km/s
Teject > 108 yr
Ages: a few Myr
• probably they were formed where they are
observed
origins…
→ tidal interactions?
→ interactions with a
hot intergalactic
medium?
→ …?
A few questions…
• role of the IHII in the
pollution of the
intergalactic medium?
• how common the IHII
are?
Kinematics of
member galaxies in
compact groups
Motivation for studying internal kinematics
Various environments sampled
Compact groups = a mixed-bag
Groups with different evolutionary stages,
sizes, HI content, X-ray properties, …
In an attempt to learn something about the
nature and the physics of the compact
groups it is important to classify them
according to their evolutionary stages.
Seyfert's sextet
Kinematics of Galaxies in Compact Groups.
31 groups (104 galaxies)
observed at ESO or CFHT
with a Fabry-Perot System
Data used to study:
Evolutionary stages of
the groups
•
• Merging groups
HCG 31
•
Strongly interacting
•
Weakly interacting
• Searches for tidal dwarf galaxy
•
Non-Groups
candidates
•
Single Irregular Galaxies
•
Tully-Fisher relation for
the group galaxies.
Fabry-Perot Observations


Integral Field Spectroscopy: 
–Large FOV
–Spatial sampling : 0.5’’ à 1’’
(higher than in HI)
–Spectral sampling : ~ 10 km.s-1
(emission line = higher resolution than stellar absorption lines)
HCG 31
A merging group
HGC 31
Johnson & Conti, 2000
HGC 31
Continuum beyond H
H
R > 50 000
1 px = 0.405 arcsec
FSR = 155 km/s
= 48 cx
spectral sampling
= 3.3 km/s
Tully-Fisher
relation for compact
group galaxies.
Two previous studies on the internal kinematics
of compact group galaxies
1) Nishiura et al. (2000) studied 30 galaxies and found that
most have peculiar rotation curves + dynamical properties of
the galaxies do not correlate with any galaxy/group parameter.
2) Rubin et al. (1991) studied 32 galaxies – found that 2/3 have
peculiar rotation curves. They found a large offset of the TF
relation with respect to the field relation in the sense that
galaxies in compact groups have “too low velocities for their
luminosities or, luminosities which are overbright for their
rotation velocities”. HCGs have low M/L compared to field
by 30% - this could be explained if they have smaller dark
halos than their field counterparts.
WE REVISIT THIS PROBLEM – 25 galaxies (13 in common)
Rotation curves obtained from 2d velocity fields.
DATA:
Velocity fields for 29 galaxies Four were too peculiar for TF
We used Vmax instead of Vflat (to be able to use more gal.)
Control samples: Courteau (1997) and Verheijen (2001)
Comparisons with previous studies: The 2D velocity maps
obtained with a Fabry-Perot instrument allowed us to mimic
a slit through our data in order to recover the rotation curves
obtained from long slit spectroscopy.
Lesson:
The velocity fields of compact group galaxies are,
in several cases, significantly affected by noncircular
motions, local asymmetries and misalignments between
the kinematic and
stellar axes. A fine tuning of map parameters (center,
inclination
and position angle of the velocity fields) and an
average of the
velocities over a large sector of the galaxy are needed
to derive
reliable rotation curves and representative values of
Vmax.
Common mass-to-size relations. But: parameters for the TF
relation are mostly derived in the inner parts of the galaxy, the
agreement does not tell us much about the DM (outer halo).
Our observations show that the galaxy halos have not been
significantly stripped inside R25. Nevertheless, mass models using
a common halo for the groups added to the individual star distributions
of each galaxy should be performed to test this scenario.
Asymmetric rotation curves
• H91a and H96a present strong asymmetries
• These are brightest galaxies in M51 like systems
Main Conclusions
1) H31 and H92 contain several IHII, probably
formed in HI clouds that were ejected from
the group members
2) The IHII have metallicities close to solar,
ages of a few Myrs and masses of a few 104
solar masses
3) H31 has a merger in its center
Main Conclusions
4) Compact group galaxies follow the TF
Larger scatter
5) A fine tunning of the parameters and an
average of the velocities over a large sector of
the galaxies are necessary to obtain meaninful
RC's.
6) Outliers are most probably overbright for
their velocities (one has double kinematic
component)
7) Care should be taken when obtaining
rotation curves of galaxies in dense
environments with long slit.
EFHARISTO!