Understanding formation of galaxies from their environments

Yipeng Jing

Shanghai Astronomical Observatory

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

Structure formation

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

JYP & Suto, Y. 2000, ApJ, 529, L69

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)

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

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

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

• Hierarchical formation, galaxies falling into bigger halos, and galaxies mergers

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

Spectroscopic (redshift) survey of 10**6 galaxies

Sloan Digital Sky Survey (SDSS)

Orientation of central galaxies relative to host halos

• Yang X.H., et al. astroph/0601040, MN, 2006

• Kang X., et al. , MN, 2007

Isodensity Surfaces of halos

• Use SPH method to get the density for each particle and form the isodensity surfaces (Jing & Suto 2002)

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

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

Alignment for the whole sample

• f= N(θ) /N_ran(θ) • 24,728 pairs

Dependences on the color

Dependences on group mass

Which satellites contributed ?

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

Jing & Suto 2002

Jing & Suto 2002

Radius RJing & Suto (2002)

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

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

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

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

If some misalignment between the central galaxy and its host halo

• Gaussian distribution with the width – 60 degrees for blue – 30 degrees for red

Dependence on halo mass

• Schematic picture to explain the alignment

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?

Color of centrals and satellites

• To understand– Hot gas stripping– Cold gas and stars stripping by tides– AGN activity

Weinmann et al. 2006

More severe for more massive clusters

But hot gas not stripped immediately!

Fraction of blue galaxies

Astroph/0709.1354; downsizing

Monaco et al. 2006, ApJ

Downsizing requires satellite galaxies to lose a significant amount of stars before merging into the central galaxies

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

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

Clustering properties

Overall comparison for different typesBrinchmann et al. 2004

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

Clustering properties

high/low SFR/M*

Projected cross-correlation function

Clustering properties

As a function of SFR/M*, at different scales

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

Weak dependence on mass of the companion

Dependence on the concentration of star-forming galaxies

Highly concentrated star forming galaxies, as ellipticals

neighbour counts of SF galaxies; <30% have a neighbor at r_p<100 kpc/h

high SFR/M* star forming without a neighbour

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;

Are AGN the products of galaxy mergers?

• Li, C et al. (2008b)

Clustering properties

Overall comparison for different typesBrinchmann et al. 2004L(O III)/M_bh indicator f

or the strength of accretion rate

Matched sample in redshift, stellar mass and 4000 °A break index D4000

Strong star formation of AGN!

but are these stars the same as in the starburst or produced with the black hole accretion ?

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)

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.