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 GalaxiesCompact Groups of Galaxies

Ideal laboratory of interactions, weak and strongIdeal laboratory of interactions, weak and strong

Nearby counterparts of high-redshift multipletsNearby 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)=4.8 109 MO

M(HI)=1.33 109 MO V=6000 km/s

M(HI)=1.6 109 MO V=5700 km/s

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 kinematicsMotivation for studying internal kinematics

Various environments sampledVarious environments sampled Compact groups = a mixed-bag

In an attempt to learn something In an attempt to learn something about the about the nature and the physics of the compact nature and the physics of the compact groups it is important to classify them groups it is important to classify them according to their evolutionary stages.according to their evolutionary stages.

Groups with different evolutionary stages, sizes, HI content, X-ray properties, …

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:

• Merging groups

• Strongly interacting

• Weakly interacting

• Non-Groups

• Single Irregular Galaxies

• Evolutionary stages of the groups

HCG 31

• Searches for tidal dwarf galaxy candidates

• 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

Johnson & Conti, 2000

HGC 31

Continuum beyond H H

HGC 31

R > 50 000

1 px = 0.405 arcsecFSR = 155 km/s = 48 cxspectral 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 thatmost have peculiar rotation curves + dynamical properties ofthe galaxies do not correlate with any galaxy/group parameter.

2) Rubin et al. (1991) studied 32 galaxies – found that 2/3 havepeculiar rotation curves. They found a large offset of the TFrelation with respect to the field relation in the sense that galaxies in compact groups have “too low velocities for theirluminosities or, luminosities which are overbright for theirrotation velocities”. HCGs have low M/L compared to fieldby 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 TFWe 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 mimica slit through our data in order to recover the rotation curvesobtained 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 TFrelation are mostly derived in the inner parts of the galaxy, theagreement 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 distributionsof 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 Conclusions4) 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!