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Lecture 17, the Effects of Environment

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Lecture 17, the Effects of Environment Upcoming class schedule Thursday March 15th 2pm AGN evolution (Amy Barger) Monday March 19th Project Presentation (Brad) Thursday March 22nd postponed to make up after spring break.. Spring break March 26-30 then back to normal as of Tuesday 3rd April GALAXIES 626 Lecture 16: The effects of environment The colour-magnitude diagram Red sequence Note the bimodality of galaxy colours blue sequence Sloan DSS data The colour-magnitude diagram Dependence on environment Fraction of red galaxies depends strongly on density. Density-dependence present at all luminosities. Bright and faint galaxies show trend with density Moving galaxies from the blue to the red peak What are the implications for the galaxy transformation mechanism? Blue & red peaks are correlated with density, But Density evolves. Galaxies must move from one to the other How long do they take? If they take too long, then the gap between the peaks will be filled in..... Timescales for Galaxy Transformation How rapid must the blue→red transition be? Two gaussian model always fits the data well ± there is no room for an Red Peak intermediate population. colour evolves rapidly if timescale for star formation to stop is short if transformations occur Blue Peak uniformly in time: need τ<0.5 Gyr Summary · There are distinct populations of red (ªearlyº) and blue (ªlateº) galaxies · The mean colors and spread of the two populations depends only weakly on environment · But the relative abundance of the populations is a strong function of environment. · Blue galaxies move quickly into the red population as their environment changes ± Why? Evolution with redshift How does the dependence on environment change with redshift? GGaallaaxxyy cclluusstteerrss:: z=0 z=0.39 z=0.83 · Local galaxy clusters are dominated by passively evolving galaxies with high formation redshifts · How does the evolution compare with the general field? · Nature or nurture: clusters are built from groups. How do groups evolve? Butcher-Oemler Effect · Concentrated clusters at high redshift may have more blue galaxies than concentrated clusters at low redshift · But blue fraction depends strongly on luminosity and radius so care needs to be taken to evaluate blue fraction at same luminosity limit, and within same (appropriate) radius. n o i t c a r f e u l B Cluster SFR evolution · ªButcher-Oemler effectº also seen in the general field · Is the effect stronger in clusters? Field Nakata et al. (2005)· Based on sparsely- sampled [OII] 2dF Postman, Lubin & Oke 2001 van Dokkum et al. 2000 spectroscopy · Suggests fraction of star- Fisher et al. 1998 Clusters forming galaxies evolves Czoske et al. 2001 relatively weakly in clusters · 0 0.3 1 Redshift Evolution of the red peak n o i t c a n r o f ti y c x a a l fr a y g High density x d la e a R All galaxies g d Low density e MV < -20 R Redshift TTyyiningg ssttaarr ffoorrmmaattioionn ttoo ssttrruuccttuurree ggrorowwtthh Groups Clusters Low redshift groups · Relation between star formation rate and local density Field Field Morphologies: · There are fewer spiral galaxies in groups than in the field, at the same redshift. n o i t · No evidence for more c a r Groups disturbance/irregularities in group f 0 galaxies S / E Field Field n o i t c a r f on l i t a r c i a p fr S l Groups a r i p S Groups Vel. Dispersion (km/s) The connection between star Field formation rate, morphology Groups and environment Distributions are corrected for differences in luminosity function between group and field Elliptical S0 Early spiral Late spiral Like clusters, groups contain passive spirals: disk morphology but low star formation rates Evolution in groups · Use [OII] equivalent width to find fraction of galaxies without significant star formation F S · - most galaxies in groups n o at z~0.4 have significant n f o es i star formation ± in n o ax i t al contrast with local g ac r F groups Groups Group SFR evolution s · Fraction of non-SF galaxies e i x a increases with decreasing l a g redshift SF - n o n · for both groups and field f o n o i · Insensitive to aperture effects t ac Fr Field es i x a l a g F S - n o n f o n o i t c ra F Summary · More star formation in groups at z=0.5 than at z=0 · On average, groups at 0<z<0.5 have less star formation and fewer spiral galaxies than the field. · Passive spiral galaxies are a key component of groups at z=0.0-0.5 Environmental Mechanisms Why should galaxy properties depend on the environment? Collisions / harassment "Strangulation" Ram-pressure (actual effects) Different history... Ram Pressure Stripping Mostly important in clusters where the surrounding gas density is high.... Ram Pressure Stripping Clusters are not empty ± in fact, most baryons are in a diffuse form, the ªintra cluster mediumº M As a galaxy travels through the C I ICM, it aaaaafeeals a force, the ªram pressureº ρ v22πGΣ RΣ R The gash disk will be to radius gR if: Gravitational Force due to ICM restoring force Ram pressure Stripping - simulations M C I Galaxy NGC 4388 Expels Huge Gas Cloud NGC 4388 is a member of the the Virgo Cluster. It is classified as an active galaxy. One hypothesis holds that the gas was stripped away as NGC 4388 made its way through the intergalactic medium of the Virgo Cluster. Galaxy interactions Gravitational interaction can produce major shifts in the gas and stars of the galaxy Major mergers produce massive star forming events but more minor repeated events can gradually remove the gas from a galaxy Galaxy Collisions, Tides and Harassment ·Tidal truncation ·Slow encounter ·Depends on gradient of potential ·Big impact on the dark halo, but not significant for stellar component ·Impulsive heating ·Fast encounter ·Importance increases as relative velocity decreases ·Harassment ·The cumulative effect of repeated encounters Galactic mergers Most dramatic examples: major mergers between galaxies of comparable mass. Large morphological changes as a consequence of the interaction. Observationally and theoretically, find that major mergers are uncommon - perhaps ~1 such merger in the lifetime of the Universe for a large galaxy in the field. Examples of galaxy collisions in the real universe and in a simulation (Moore et al 1995) The Antennae Galaxy Best place for major mergers - small groups of galaxies Galaxies in rich clusters are generally less vulnerable to mergers, despite the very high density of galaxies because the velocities are higher. Galaxy Collisions, Tides and Harassment size x force Perturbation to gradient Time of velocity of star encounter in galaxy 1 Galaxy 1 m GM b Δv =2 r r ¿ b3 V 4G2 M 2 m b ΔE= r 2 V 3b4 V 2 M Perturber, Change of galaxy 2 internal energy of galaxy 1 Minor mergers between galaxies of very different masses are much more common. Example: the Magellanic clouds Bound satellites orbiting within the extended halo of the Milky Way (~50 kpc distance) Eventually will spiral in and merge Sagittarius dwarf galaxy is another satellite which is now in process of merging… • What effect does the merger have on the disk galaxy? • How fast does the spiral-in and merger occur? Dynamical Friction Why does the orbit of a satellite galaxy moving within the halo of another galaxy decay? Stars in one galaxy are scattered by gravitational perturbation of passing galaxy. Stellar distribution around the intruder galaxy becomes asymmetric - higher stellar density downstream than upstream. Gravitational force from stars produces a `frictional’ force which slows the orbital motion. How quickly will the LMC merge with the Milky Way? Simple estimate - dynamical friction time: 200 km/s V V 3 t » » friction ∣dV /dt∣ 4 pG2 Mnm ln L ~3 1010 Solar masses Galactic density at LMC - for flat rotation curve estimate 3 x 10-4 Solar masses / pc3 With these numbers, estimate orbit will decay in ~3 Gyr Close satellite galaxies will merge! Strangulation - removal of the gas halo Removes surrounding gas and so stops infall and star formation Strangulation - removal of the gas halo Summary of Mechanisms Ram-pressure Needs dense ICM and high velocities - clusters Collisions / harassment Mostly in groups where slow galaxy interactions are common "Strangulation" Removal of the gas halo: no more fuel supply Similar to ram-pressure stripping but much easier! Mechanisms Ram-pressure Density too low Needs dense ICM and high velocities - clusters Collisions / harassment Groups are preferred place! "Strangulation" Removal of the gas halo: no more fuel supply Similar to ram-pressure stripping but much easier! Transformation too rapid Mechanisms Collisions / harassment Groups are preferred place! The best mechanism to explain the observational data What next? Look at ± but most effective in galaxy individual galaxies to groups rather than clusters see if they provide evidence to support the ªbig pictureº Summary Clear bimodality of galaxy properties More dense environments... there is less star formation A higher fraction of passive/red galaxies But... Star Forming/blue galaxies have similar properties in all environments (only the fraction changes) ªPassiveº galaxies exist in all environments Galaxy transformation is rapid! Its most likely to happen in galaxy groups rather than in clusters End.
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