The interaction between galaxies and their environment

Trevor PonmanUniversity of Birmingham

Jesper Rasmussen Carnegie Observatories

Environmental effects

Morphology-density relation

Goto et al 2003

Environmental effects

Morphology-density relation

Reduction in HI content

Gavazzi et al 2006

Environmental effects

Morphology-density relation

Reduction in HI content

Suppression of star formation

Haines et al 2007

Cluster formation - simple model Galaxies form by

baryon cooling within dark halos.

These then cluster into groups.

Cluster formation - simple model

Rv

Accretion shock

Galaxies form by baryon cooling within dark halos.

These then cluster into groups.

Galaxy dark halos merge.

Infalling gas is compressed and shocked at Rv.

The result is a virialised group or cluster.

Environmental effects - processes

Gravitational Galaxy-galaxy interactions (mostly tidal) Tidal interaction with cluster potential Galaxy harassment Galaxy merging

Hydrodynamical Ram pressure stripping Viscous stripping Strangulation Thermal evaporation

Concentrate hereon these, and examinethe role of the groupenvironment.

Ram pressure stripping

Classic hydrodynamical gas removal process. Acts on early and late-type galaxies in clusters.

Asymmetric extraplanar HI has been seen in many cluster spirals.

Spectacular tails in the radio or X-ray are also seen (though much rarer).

Wang et al 2004

Chandra emission (blue/green) superimposed on V band image (red).

C153

The active disk-like galaxy C153, moving at v>1500 km s-1 through the complex merging cluster A2125.

Ram pressure stripping

Gunn & Gott (1972) stripping formula

Boselli & Gavazzi 2006

But vgal2 dependence

implies that this won’t work in groups.

Most disk galaxies in nearby clusters (Virgo, A1367 & Coma) can be stripped during core crossing according to this formula.

Coma

Virgo & A1367

Why does stripping in groups matter?

Only ~10% of galaxies are in clusters. Lots more are in groups.

Strong environmental modification is seen in groups.

Morphological transformation begins at large radii in clusters

Studies using large samples of galaxies from 2dF and SDSS suggests that galaxy modification sets in outside the virial radius of clusters, in groups or cluster fringes.

Goto et al 2003

X-ray bright groups show a strong morphology-density relation

Helsdon & Ponman 2003

Galaxy morphology ingroups:

Groups have a very wide range in spiral fraction

X-ray bright groups show a morphology-density relation similar to clusters - but stronger

Dressler clusters

Groups

Groups scaled to 3D density

HI deficiency is seen in groups

Verdes-Montenegro et al studied the HI content of 72 Hickson compact groups, and found them to be significantly HI deficient, relative to HI content predicted by Haynes & Giovanelli (1984).

Verdes-Montenegro et al 2001

HI deficiency:∆ ≡ log MHI,pred. – log MHI,obsThe deficiency was found to be

correlated with the presence of ahot IGM (from Ponman et al 1996).

Why does stripping in groups matter?

Only ~10% of galaxies are in clusters. Lots more are in groups.

Strong environmental modification is seen in groups.

Galaxies may be “preprocessed” in groups, before being incorporated into clusters.

Preprocessing of galaxies in groups

Blue group with ~170 km/s falling into A1367.Tidal disturbance, enhanced star formation, shell

elliptical (likely merger), H trails.Cortese et al 2006

R band H

Why does stripping in groups matter?

Only ~10% of galaxies are in clusters. Lots more are in groups.

Strong environmental modification is seen in groups.

Galaxies may be “preprocessed” in groups, before being incorporated into clusters.

Look for evidence of stripping operating within groups

NGC2276 is a star-forming galaxy in the NGC2300 group

Embedded in T ≈ 107 K group gas

“Tail” of HI+radio continuum emission

55% less HI than typical isolated spirals

High star formation rate (~ 5 M⊙/yr) 1.49 Ghz

Hummel & Beck (1995) Quilis et al. (2000)

Simulation example

Morphology strongly suggests ram pressure stripping. Investigate with Chandra observations.

Ram pressure in groups: NGC Ram pressure in groups: NGC 22762276

Chandra imaging

0.3-2 keV adaptively smoothed images45 ks exposure (D = 37 Mpc).

Diffuse X-ray emission

X-ray analysis

X-ray contours over DSS image

Shock conditions:M ≈ 1.7

vgal ≈ 850 km/s

Thermal pressure

shock

IGMT=0.85 keV

Hot ISMT=0.5 keV

Shocked IGMT=1.1 keV

For vgal ≈ 850 km s-

1,

and nIGM=6x10-4 cm-

3

Results: Role of ram pressureResults: Role of ram pressure

•Gas can be entirely stripped at r>9 kpc•Trigger starburst + outflows•Gas outflows dominated by ram pressure at z>2 kpc•Peel off gas edge of disk via hydro-instabilities Iviscous

stripping)

Effects of ram pressure:

Rasmussen, Ponman & Mulchaey 2006

Implications for galaxy evolutionImplications for galaxy evolution

Current mass-loss rate of gas: ~ 5 M⊙ yr-

1.

Remaining gas should be lost in another 1-2

Gyr.

Star formation will cease “soon”.

End product:

No gas, little star formation.

Larger bulge-to-disc ratio

~ S0 galaxy?

Modelling of stripping in groups

Mayer, Mastropietro & Tran, in prep.

Recent simulations by Kawata & Mulchaey (2007) and Mayer et al (in prep) suggest that hot gas can be stripped from group galaxies.

Results from Vy Tran: 3D SPH simulations of galaxy (dark matter, disk+bulge stars and gas) with vrot=170 km/s, moving through a medium with n=10-3 cm-3, and including cooling and star formation.

vgal=450 km/s

vgal=250 km/s

Isolated galaxy

Galaxy after 500 Myr

Modelling of stripping in groups

Mayer, Mastropietro & Tran, in prep.

Compression of the disk leads to cooling, instability and star formation.

Some of the gas is stripped, and a large fraction is converted into stars.

At present the simulations do not include the effects of feedback from SNe into the gas, so the effect of hot gas production and consequent stripping seen in NGC2276 is not included. gas mass

total baryon mass

Intergalactic gas in groups

So - could ram pressure stripping be responsible for all the HI deficiency seen in groups? Look in detail at the HI-deficient HCGs.

Verdes-Montenegro et al 2001

Intergalactic gas in groups

X-ray plus VLA mapping of hot gas and HI in 8 of the most HI-deficient groups from the Verdes-Montenegro et al study.

Verdes-Montenegro et al 2001

Chandra image of HCG40

Intergalactic gas in groups

• The X-ray properties are found to be very diverse.

• Several, such as HCG30, show no hot IGM.

• Ram pressure stripping cannot be the dominant effect in removing HI in many of these groups.

Rasmussen et al, in prep.

Conclusions Galaxy groups must play a crucial role in removing gas from

galaxies and establishing the morphology-density relation.

Ram pressure stripping can operate in groups, when it acts in conjunction with starburst activity, which may be induced by galaxy interactions.

A study of HI-deficient compact groups shows that ~half have no detectable hot IGM.

Hence, ram pressure stripping cannot be the only mechanism leading to HI loss within groups.

Other mechanisms include: tidal stripping, galaxy winds and gas exhaustion by star formation following removal of any gas reservoir (strangulation).