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Understanding formation of galaxies from their environments
Yipeng Jing
Shanghai Astronomical Observatory
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A brief overview of structure formation
• A concordance LCDM model emerged;• Structures form from bottom up;• Most basic properties of dark matter halos well u
nderstood now,– Number density approximately by PS; – Internal structure by NFW profile;– Halos are triaxial with larger halos being more elongat
ed;– Halos are pointed along nearby filaments; also pointe
d preferentially to each other;– Halos are slowly rotating with the spin parameter 0.05;
spin parameters are log-normal distributed;– Rotation preferentially along the minor axis of halos
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Structure formation
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Physical processes of galaxy formation
• Gas cooling and disk galaxy formation;• Galaxies falling into bigger halos with halos
merges; ram pressure and tidal stripping may take away hot gas and even cold gas from satellite galaxies;
• Mergers of gaseous galaxies lead to starbursts; • dry mergers are important as well; formation of E
galaxies• Black holes grow with merges and accretion;• Supernova feedback and AGN feedback
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JYP & Suto, Y. 2000, ApJ, 529, L69
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Okamoto et al. 2005, MNRAS, 363,129
Formation of galactic disk depends on the formation of stars and the feedback; much more complicated than the conventional disk formation scenario by Fall and Efstathiou (1980)
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Physical processes of galaxy formation
• Gas cooling and disk galaxy formation;• Galaxies falling into bigger halos with halos
merges; ram pressure and tidal stripping may take away hot gas and cold gas from satellite galaxies;
• Mergers of gaseous galaxies lead to starbursts; • dry mergers are important as well; formation of E
galaxies• Black holes grows with merges and accretion;• Supernova feedback and AGN feedback
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Strangulation: hot gas stripping
Gravitational tidal force can remove cold gas and even part of stellar mass of a satellite galaxy
Wang, H.Y., Jing et al., in preparation
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Physical processes of galaxy formation
• Gas cooling and disk galaxy formation;• Galaxies falling into bigger halos with halos
merges; ram pressure and tidal stripping may take away hot gas and even cold gas from satellite galaxies;
• Mergers of gaseous galaxies lead to starbursts;
• dry mergers are important as well; formation of E galaxies
• Black holes grows with merges and accretion;• Supernova feedback and AGN feedback
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• Hierarchical formation, galaxies falling into bigger halos, and galaxies mergers
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Physical processes of galaxy formation
• Gas cooling and disk galaxy formation;• Galaxies falling into bigger halos with halos
merges; ram pressure and tidal stripping may take away hot gas and even cold gas from satellite galaxies;
• Mergers of gaseous galaxies lead to starbursts; • dry mergers are important as well; formation of E
galaxies• Black holes grows with merges and
accretion;• Supernova feedback and AGN feedback
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Spectroscopic (redshift) survey of 10**6 galaxies
Sloan Digital Sky Survey (SDSS)
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Orientation of central galaxies relative to host halos
• Yang X.H., et al. astroph/0601040, MN, 2006
• Kang X., et al. , MN, 2007
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Isodensity Surfaces of halos
• Use SPH method to get the density for each particle and form the isodensity surfaces (Jing & Suto 2002)
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Why do we do this?
• Understanding disk formation– Relation with the rotation (spin) of the dark
matter halos;– Dynamical evolution;
• Understanding elliptical formation– Major merges
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Observational Sample
• SDSS DR2
• Halo based groups (unique!); selected from SDSS (Yang et al. 2005 MNRAS 356, 1293)
• Useful information– Central and satellites; – Mass of the halos– Color of the group members
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Alignment for the whole sample
• f= N(θ) /N_ran(θ) • 24,728 pairs
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Dependences on the color
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Dependences on group mass
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Which satellites contributed ?
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Summary for the observation
• Satellites align with the major axis of the centrals, in contrast with the classic Holmberg(1969) effect;
• The effect stronger for red centrals/satellites; vanishes for blue centrals; have chance to have our Milky Way
• Stronger for richer systems;• Stronger for satellites at smaller halo-
centric distance
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Jing & Suto 2002
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Jing & Suto 2002
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Radius RJing & Suto (2002)
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Semi-analytical modeling of galaxy formation based on N-body simulations
• Physical processes: heating, cooling, star formation and feedback, chemical evolution, dust extinction, SSP, galaxy mergers and morphology transformation; (quite complete compared with previous works)
• Subhalos well resolved; Galaxy mergers are dealt with much better than previous works;
• Cooling time scale is longer than standard; flat faint end of LF;
• Cut off cooling in massive halos with AGN formation and feedback
• Kang X., YPJ, H.J.Mo, G. Boerner (2005) • Kang, Jing, Silk, 2006
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Predictions from Semi-analytical model + Numerical Simulation
• Difficulty to predict the orientation of the central galaxies– Spiral galaxies: may
not be related to halo spin from recent simulations
– Ellipticals: detailed simulation of mergers
• Useful constraints from the observation
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Assumption on the orietation of the central galaxy
• Central galaxy aligns perfectly with the dark matter within r_vir or within 0.3 r_vir
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Predictions from Semi-analytical model + Numerical Simulation
• Difficulty to predict the orientation of the central galaxies– Spiral galaxies: may
not be related to halo spin from recent simulations
– Ellipticals: detailed simulation of mergers
• Useful constraints from the observation
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If some misalignment between the central galaxy and its host halo
• Gaussian distribution with the width – 60 degrees for blue – 30 degrees for red
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Dependence on halo mass
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• Schematic picture to explain the alignment
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Conclusions from the modeling
• The alignment effect is explained if– the red central has some mis-alignment with t
he host halo(Gaussian width 30degrees)– the blue central has more (60 degrees)
• Color and halo mass dependences explained;
• Important Implications: Is the disk of spirals determined by the spin of the host? Intrinsic alignment for weak lensing?
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Color of centrals and satellites
• To understand– Hot gas stripping– Cold gas and stars stripping by tides– AGN activity
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Weinmann et al. 2006
More severe for more massive clusters
But hot gas not stripped immediately!
Fraction of blue galaxies
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Astroph/0709.1354; downsizing
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Monaco et al. 2006, ApJ
Downsizing requires satellite galaxies to lose a significant amount of stars before merging into the central galaxies
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A few points for the future work
• Hot gas stripped not immediately after falling into the host; need more work to quantify this;
• Stars of satellites must be stripped out by tides; existence of the IC stars;
• In order to keep the central galaxies red, blue components of satellites must be removed
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Interaction-induced star formation enhancement (Li et al. 2008a)
• Sample selection– SDSS DR4; 400,000 galaxies r<17.7– Use emission line diagram to select star-forming
galaxies r<17.6– Use SFR/M*, specific star formation rate as the
star formation strength
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Clustering properties
Overall comparison for different typesBrinchmann et al. 2004
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Methods
• cross correlation function with spectroscopic sample of all galaxies ; neighbour counts
• Enhancement function with reference to galaxies in a photometric sample to limiting magnitude 19; other limits18.5 and 19.5 also used, to study the effect of companion’s mass;
• Morphology --- sign of interaction
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Clustering properties
high/low SFR/M*
Projected cross-correlation function
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Clustering properties
As a function of SFR/M*, at different scales
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Interaction-induced enhancement function
dependence on mass of the SF galaxy
Average boot of SFR/M* as a function of the distance to the nearest neighbor in r<19 but r-r_sfg<1.4
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Weak dependence on mass of the companion
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Dependence on the concentration of star-forming galaxies
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Highly concentrated star forming galaxies, as ellipticals
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neighbour counts of SF galaxies; <30% have a neighbor at r_p<100 kpc/h
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high SFR/M* star forming without a neighbour
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Summary
• SF galaxies have more close neighbors• High SF galaxies are small in small halos with cold
gas; low SF galaxies are bigger in larger halos without gas
• little dependence found on mass of the companion; • Interaction increases SF with decrease of the scaled
separation; • Strong star forming galaxies are more concentrated,
consistent with the merging scenario• High SF galaxies do not necessarily have close
neighbors, but many are post mergers;
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Are AGN the products of galaxy mergers?
• Li, C et al. (2008b)
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Clustering properties
Overall comparison for different typesBrinchmann et al. 2004L(O III)/M_bh indicator f
or the strength of accretion rate
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Matched sample in redshift, stellar mass and 4000 °A break index D4000
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Strong star formation of AGN!
but are these stars the same as in the starburst or produced with the black hole accretion ?
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Conclusion
• no evidence that enhanced AGN activity is also connected with interactions;
• Open questions– are young stars produced with accretion?– Are AGN post-merger events?
• Our results consistent with the picture:merger, starburst, AGN (with or without young
stars formed)
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Final Remarks
• The observations have provided important clues to the important processes of galaxy formation, but the interpretation is far from definite;
• Detailed theoretical modeling, especially numerical simulations, are needed.