Kupy galaxKupy galaxií – lekce IIií – lekce II

Pavel Jáchym

single dominant

cD

dominant binary(Coma)

linear array of galaxies

flattened

singlecore ofgalaxies

irregular

Clusters – overviewClusters – overview Classification

◦ concentration (compact – open)◦ distribution of brightest members◦ presence or absence of a cD galaxy◦ sub-clustering◦ morphology of dominant galaxies

◦ Rood & Sastry classification:

Compact groupsCompact groupsHickson (1982)

◦ consist of 4-7 galaxies within an area of only few 100 kpc diameter

◦ typical spacing 20-40 kpc◦ more Sp galaxies than expected ◦ very short lifetimes against merging

◦ Stephan’s Quintet, Seyfert’s Sextet (see Figs.)◦ M/L ~150 – 500

large DM halos around individual galaxies or a common halo encompassing the whole

group

Cosmological simulationsCosmological simulationscan serve as a powerful cosmological probe of the

nature of mysterious dark matter and dark energyCosmological simulations

◦85% of DM, 10% of hot gas and 2-5% of stars◦complicated astrophysics problem involving

nonlinear collapse merging of dark matter radiative cooling of gas star formation chemical enrichment of the intergalactic medium by

supernovae and energy feedback.

Cosmological simulationsCosmological simulationsRich and complex structure of the gas density and temperature

distributions ◦ such as strong and highly aspherical accretion shocks surrounding the

cluster and ◦ turbulent gas motions within the cluster

The cluster gas is also enriched with heavy elements ("metals"), as the metal-enriched gas is stripped off from galaxies when they orbit within the cluster.

Local structuresLocal structures Supergalactic plane

◦ sheet-like structure that contains the Local supercluster, the Coma supercluster, the Pisces-Cetus supercluster, and the Shapley concentration

◦ it separates two giant voids – the Northern and the Southern Local supervoids◦ is the reference plane for the system of supergalactic coordinates

Supergalactic coordinates – MW in the center, plane (x,y) coincides with the supergal. plane, axis y points to the Virgo cluster

Local groupLocal group Cumulative luminosity

function of local group galaxies is consistent with a Schechter function:

Virial radiusVirial radius the characteristic or virial radius (Rv) of a cluster

◦ defined from the theory of structure collapse in an expanding universe as radius within which the mean density of the cluster is 200 times the critical density of the Universe

with Hubble constant H=H(z) radius of a sphere, centered on the cluster, within which virial equil.

Holds the gas is heated by the gravitational infall to temperatures close to the

virial temperature

which ranges in clusters from 1 to 15 keV. The total X-ray luminosities range from about 1043 erg s-1 to 1046 erg s-1

G

H

8

3 2

crit

v

p

R

GMmkT ~

ICM – temperature profileICM – temperature profile

XMM-Newton◦ profiles show a clear decline beyond

≈ 0.2 R200

◦ there is no evidence of profile evolution with redshift out to

z ≈ 0.3In the center the temperature

falls to typically a third or a half of the temperature in the outskirts of the cluster

ICM – metallicity ICM – metallicity

The mean metallicity profile shows a peak in the center, and gently declines out to 0.2 R180

Beyond 0.2 R180 the metallicity is ≈ 0.2 solar and flat

No evidence of profile evolution from z = 0.1 to z = 0.3

LLxx-T relation-T relation

analytic and numerical simulations of cluster formation => total X-ray luminosity LLxx~T~T22

◦ L is dominated by thermal bremsstrahlung L ~ n2 T1/2Rvir

3, ◦ mean gas density n ~ M/Rvir

3 is constant and

◦ T = M/Rvir

however, observations show Lshow Lx x ~ T~ T33

◦ gas has additional heating?

Cooling flow clustersCooling flow clusters ICM in the centres of galaxy clusters should be rapidly cooling at the rate

of tens to thousands of solar masses per year this should happen as the ICM is quickly losing its energy by the emission

of X-rays X-ray brightness of the ICM is proportional to the square of its density,

which rises steeply towards the centres of many clusters typical timescale for the ICM to cool is relatively short, less than a billion

years. As material in the centre of the cluster cools out, the pressure of the overlying ICM should cause more material to flow inwards (the cooling flow)

Cooling flowsCooling flows it is currently thought that the very large amounts of expected cooling are in

reality much smaller, as there is little evidence for cool X-ray emitting gas in many of these systems = the cooling flow problem

theories for why there is little evidence of cooling include◦ heating by the central AGN possibly via sound waves (seen in the Perseus and Virgo

clusters)◦ thermal conduction of heat from the outer parts of clusters◦ cosmic ray heating◦ hiding cool gas by absorbing material◦ mixing of cool gas with hotter material

The problem appears to be widespread, from the most massive clusters to the centers of individual elliptical galaxies

Perseus cluster:

Distribution of galaxiesDistribution of galaxies

Galaxies of all types gather along filaments and in galaxy groups and clusters

Elliptical and S0 occur preferably in regions with high galactic densities

Spiral and irregular in less dense regionsMorphology-density relation

Cluster\galaxy type

E S0 Sp

Regular 35% 45% 20%

Intermediate 20% 50% 30%

Irregular 15% 35% 50%

Field 10% 20% 70%

S0 galaxies: smooth feature-less disks, larger bulges than in Sp, stars are old and red as in E

E

Sp+Irr

S0

Morphology-density relationMorphology-density relation

Dressler 1980:

Virgo cluster: Fornax cluster:

Effects of environmentEffects of environment only about 1% of galaxies are isolated most are found in groups, ~5% in rich clusters

◦ => environment may play an important role in dense clusters

◦ relative distances between galaxies << rel.dist. between stars in galaxies◦ up to 100 galaxies/Mpc^3, i.e. a mean distance of 150 to 300 kpc.

Tidal interactions◦ merging◦ galaxy harassment tidal stripping galaxies may lose◦ galaxies vs. cluster potential material

Ram pressure stripping, starvation Viscous stripping, thermal evaporation Pre-processing of galaxies in groups

Tidal effects – dynamical frictionTidal effects – dynamical friction As a massive galaxy moves through a “sea” of stars, gas, (and the dark halo),

it causes a wake behind it increasing the mass density behind it This increase in density causes the galaxy to slow and lose kinetic energy The galaxy will eventually fall in and merge with it’s companion

◦ C … depends on structure of both galaxies◦ M … mass of galaxy falling in◦ v … velocity of galaxy falling in◦ ρ ... density of stars (surrounding material)

◦ the slower the galaxy’s speed, the stronger the dynamical force, the more intense the interaction

◦ the more massive the object, the greater the effect

2M

22

dyn v

MGCf

Galaxy interactionsGalaxy interactionsTerminology: Major merger – two similar mass galaxies, gives rise to tidal tails Minor merger – a satellite (dwarf) galaxy merging with a larger massive galaxy,

makes bridges, also tidal stripping Retrograde – galaxy is rotating in opposite direction of velocity of “intruder” Direct – galaxy is rotating in same direction as velocity of “intruder” Impact radius – distance between center of galaxy and intruder Inclination angle between galaxy and intruder Viewing angle – our line of sight to the merger

In 1940s, Holmberg predicted the giant tides developed in the interaction and merger of the galaxies

◦ Simulations on an analogue computer – two systems of 74 movable lamps whose intensity decreases with the square of the distance simulated the stars

Observations of long filaments around interacting galaxies (flux tubes of the magnetic field?, explosions in galaxy centers?)

““Galactic bridges and tails”Galactic bridges and tails” Tommre & Toomre (1972) – their numerical simulations established that

gravitational interaction with another galaxy could be the origin of the filamentary structures◦ encounters on parabolic orbits◦ disks of test particles◦ all self-gravity of the disk neglected

The Antennae

The Mice

Galactic tides – observation examplesGalactic tides – observation examples Disrupted spiral galaxy Arp 188 with a long tail

featuring massive, bright blue star clusters seen by HST:◦ probably a more compact intruder galaxy crossed in

front of Arp 188. During the close encounter, tidal forces drew out the galaxy’s stars, gas, and dust forming the spectacular tail.

The tidal action allows the formation of four arms if the two companions are disk galaxies. When their masses are comparable, the two internal spiral arms join up to form a bridge that disappears quickly, the two external spiral arms are drawn into two antennae:

The Antennae – interacting galaxies NGC 4038 – 4039

The Mice – interacting galaxies NGC 4676

Arp 188

Tidal actionTidal action The tidal force experienced by an object of diameter d in interaction with a mass M at a

distance D: the parts closest to M are more attracted than those away. The order of magnitude of the force: Ftide ~ GMd/D3

If the distance between two galaxies is greater than their individual radii, the main term in the tidal forces varies as cos2θ in the plane of the galaxy◦ There exist two poles of perturbation rotating with an angular velocity Ω => formation of two spiral

arms in the galaxy In the case where the companion galaxy’s orbit lies in an inclined plane, the azimuthal

dependence of the tidal force is no longer bisymmetric but contains the Fourier term m=1 => excitation of oscillations observed in warps

The principal effects are purely kinematic, which explains the success of the simple restricted three-body simulations◦ Especially the selfgravity of the gas is negligible. It is far more perturbed than the stellar component

due to its small velocity dispersion and its greater extension into the external regions The tidal interaction can be very violent and collisions between clouds in the spiral arms give

rise to starbursts There exists a certain correlation between the presence of bars and companions: interacting

spirals yield a greater fraction of barred galaxies than field galaxies Formation of filaments and rings

Galaxy mergersGalaxy mergers Major mergers of galaxies generally lead to elliptical-like remnants, with some

irregular structures in the outer regions Depending on the orbital geometry of the merger, the remnant can either be

prolate or oblate. In general, mergers of two equal-mass disks lead to rounder remnants if the spins of

the merging progenitors are more tilted relative to the orbital angular momentum. Highly flattened remnants can be produced in prograte and retrograde encounters Mergers affect both stellar and gaseous content Dynamically cool stellar disks warm up Compression of gas => shocks => star formation Formation of bars Nuclear inflows – nuclear star burst, AGN Some material ends up in long tails and bridges (see Toomre &Toomre 1972) Most of material stays bound

MergersMergers Slow interactions important in galaxy groups

◦ smaller velocity dispersions than in clusters Long-living tidal tails in clusters destroyed by

potential of the cluster

The Milky Way is warped by the passage of the Magellanic Clouds

The Magellanic Clouds may eventually merge with the Milky Way.

Tidal dwarfs

Galaxy harassment, IC lightGalaxy harassment, IC lightCumulative effect of frequent close high-velocity encounters

◦ once per Gyr, ◦ relative velocity of ~ 1500 km/s◦ Impact parameter of ~ 50 kpc

+ tidal effect of cluster potentialProduces distorted galaxies with enhanced star formation rateLow vs. high surface galaxiesIntracluster (IC) light

◦ forms as galaxies collide and interact gravitationally within the cluster◦ gravitational forces strip stars out of their parent galaxy => diffuse web of

faint ICL throughout the cluster

Ram pressure stripping (RPS)Ram pressure stripping (RPS) Momentum transfer process Gunn & Gott (1972) assumed that after the formation of a galaxy cluster, the

remaining gas is thermalized via shock heating to virial temperature (~ 107K) As spiral galaxies move through this hot plasma at densities of ~ 10-3 cm-3, the

ISM in disks can be partly or totally removed by the ram pressure of the ICM Gunn & Gott (1972) predict that the ISM is removed from the disk if the ram

pressure exceeds local gravitational restoring force:

where ρICM … ICM density, vg … relative velocity galaxy-ICM, ΣICM … ISM surface density, Φ(r,z) … total galaxy potential

,

),( v ISM

max

2gICM

z

zr

RPS, cont.RPS, cont.NGC 4522 (in Virgo cluster)

normal stellar disk + truncated HI disk

RPS, cont.RPS, cont.NGC 4569 (in Virgo cluster)

Galaxies caught in the actGalaxies caught in the act Truncated gas disks One-sided off-plane gas Tail Bowshock on the opposite side e.g. NGC 4522

◦ rot.vel.~130 km/s, ~ 1Mpc from M87, los velocity ~ 1300 km/s

◦ stellar disk undisturbed◦ Hα truncated to 3 kpc (-> even molecular gas is

stripped?)◦ HI similar to Hα

◦ RPS to low? bulk motions and density enhancements due

to subcluster merging?

Caught in the actCaught in the actNGC 4569

◦ Highly HI-def.◦ shows central starburst◦ soft X-ray emission at one side◦ HI arm◦ ~500 kpc from center, 1100

km/s

NGC 4402◦ also dust is stripped

Caught in the act …Caught in the act …

HI tailsHI tails

In central region of Virgo cluster110 x 25 kpcMaterial from NGC 4388Gas can remain neutral for about 108 yr

X-ray trailsX-ray trails

E.g. galaxy C153 in cluster A2125

Viscous strippingViscous strippingNulsen 1982

◦ outer layers of a spherical galaxy travelling through the hot ICM experience a viscosity momentum transfer that is sufficient to drag out some gas at rates depending on the character of the flow (turbulent: drag force ~ ram pressure force)

occur simultaneously with RPSmay dominate in edge-on motion of galaxiesmass loss rates can be comparable to RPS rates

Starvation/strangulationStarvation/strangulationEstimate:

◦ typical galaxy of mISM ~ 2 109 Msol and SFR ~ 2 Msol/yr consumes its gas within ~1 Gyr

◦ even if stars returned half of the consumed material back to ISM, the gas would be exhausted after few Gyr =>

Larson et al. (1980): galaxies are surrounded by reservoirs of gas => gas supply

The reservoirs however can be stripped quite easily◦ => starvation (strangulation)

Bekki et al. (2002): about 80% of the halo gas can be stripped during few Gyr even from galaxies in cluster outskirts

ThermalThermal evaporationevaporationGalaxies are surrounded by the hot ICMeffects of heat conduction and consequent evaporation of

the ISM in contact with the hot ICMAt the interface between the hot ICM and cold ISM the

temperature of the ISM steeply rises and the gas evaporates and is not retained by the gravitational field

Thermal evaporation depends on the ICM temperature and on the magnetic field, and to a lesser extent on the density.

analytical estimates of the time-scales of the evaporation in clusters, subclusters, and groups◦ about 1 - 3 Gyr, 2 - 7 Gyr, and 10 Gyr, respectively

Galaxies in different environmentsGalaxies in different environments

From large surveys like 2dF and SDSS◦hundreds of thousand galaxies allow comparison between

different environmentsColors, morphology, and star formation rates

◦In dense environments blue, late-type, and star forming galaxies are less common than in low-density environments

◦Color-density relation◦Morphology-density relation◦Star formation-density relation

Galaxies in different environments, cont.Galaxies in different environments, cont.In the nearby universe (z<0.1)

◦ Sparse regions (ngal<1 Mpc-2 or R>Rvir) Values converge towards the field population

◦ Intermediate regions (ngal=1–6 Mpc-2 or R=1–0.3 Rvir) Fraction of late-type’s decreases Galaxies with strong SF vanish

◦ High-density regions (ngal>6 Mpc-2 or R<0.3 Rvir) Early-type fraction increases

With increasing z◦ Butcher-Oemler effect = increase

of the fraction of blue galaxies in clusters with z

Virgo clusterVirgo cluster

Virgo cluster◦ Distribution of galaxies of

all types follow that of X-ray

◦ Late-types are more extended

◦ Early-types more concentrated

◦ 52% of bright spirals have truncated Hα disks

all types

Sp+IrrE+S0

dE+dS0

Gas content of late-typesGas content of late-typesHI-deficiencyGalaxies closer to cluster center have smaller HI disks (with normal

central surface density)Fraction of deficient galaxies is correlated with the X-ray

luminositySolanes et al. (2001): 1900 galaxies in 18 nearby clusters

◦ In 2/3 of the clusters the galaxies show HI deficiency◦ Fraction of gas-poor galaxies increases towards the centre◦ Gas-poor galaxies tend to be on more radial orbits

Molecular gas not affected (?)RPS enhances the star formation rate in the inner disk by a factor

of ~2

HI-deficiencyHI-deficiency

expected HI mass corresponds to an isolated galaxy of the same morphological type and optical diameter

Virgo core galaxies:◦ average deficiency of ~ 2.6

Distribution of HI-deficient Virgo galaxies:

expectedHI,

observedHI,logdefM

M=

def > 0.3