The interaction between galaxies and their environment Trevor Ponman University of Birmingham Jesper Rasmussen Carnegie Observatories
Jan 17, 2016
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).