Kupy galax ií – lekce II

Kupygalaxií – lekce II Pavel Jáchym

Clusters – 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: linear array of galaxies flattened single dominantcD dominant binary(Coma)‏ irregular singlecore ofgalaxies

Compact groups • Hickson (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 simulations • can serve as a powerful cosmological probe of the nature of mysterious dark matter and dark energy • Cosmological 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 simulations • Rich 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 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 group • Cumulative luminosity function of local group galaxies is consistent with a Schechter function:

Virial 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

ICM – 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 ≈ 0.3 • In the center the temperature falls to typically a third or a half of the temperature in the outskirts of the cluster

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

Lx-T relation • analytic and numerical simulations of cluster formation => total X-ray luminosity Lx~T2 • L is dominated by thermal bremsstrahlung L ~ n2 T1/2Rvir3, • mean gas density n ~ M/Rvir3 is constant and • T = M/Rvir • however, observations show Lx ~ T3 • gas has additional heating?

Cooling 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 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 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 regions • Morphology-density relation S0 galaxies: smooth feature-less disks, larger bulges than in Sp, stars are old and red as in E

Morphology-density relation • Dressler 1980: • Virgo cluster: Fornax cluster: Sp+Irr E S0

Effects 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 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

Galaxy interactions Terminology: • 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” • 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 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: Arp 188 The Antennae – interacting galaxies NGC 4038 – 4039 The Mice – interacting galaxies NGC 4676

Tidal 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 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

Mergers • 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 light • Cumulative 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 potential • Produces distorted galaxies with enhanced star formation rate • Low vs. high surface galaxies • Intracluster (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) • 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: whereρICM … ICM density, vg … relative velocity galaxy-ICM, ΣICM … ISM surface density, Φ(r,z) … total galaxy potential

RPS, cont. • NGC 4522 (in Virgo cluster) normal stellar disk + truncated HI disk

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

Galaxies 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 act • NGC 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 …

HI tails • In central region of Virgo cluster • 110 x 25 kpc • Material from NGC 4388 • Gas can remain neutral for about 108 yr

X-ray trails • E.g. galaxy C153 in cluster A2125

Viscous stripping • Nulsen 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 RPS • may dominate in edge-on motion of galaxies • mass loss rates can be comparable to RPS rates

Starvation/strangulation • Estimate: • typical galaxyof 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

Thermalevaporation • Galaxies are surrounded by the hot ICM • effects of heat conduction and consequent evaporation of the ISM in contact with the hot ICM • At 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 environments • From large surveys like 2dF and SDSS • hundreds of thousand galaxies allow comparison between different environments • Colors, 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. • 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

dE+dS0 all types E+S0 Sp+Irr Virgo 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

Gas content of late-types • HI-deficiency • Galaxies closer to cluster center have smaller HI disks (with normal central surface density) • Fraction of deficient galaxies is correlated with the X-ray luminosity • Solanes 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 gasnot affected (?) • RPS enhances the star formation rate in the inner disk by a factor of ~2

def > 0.3 HI-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:

Kupy galax ií – lekce II