Talks & Ideas (~ 2016)

[bottom] [suzho conference]


Journal: Fielding+16 The Impact of Star Formation Feedback on the Circumgalactic Medium (by prof. Guo)

  • In cosmological study, $r_{200}$ is defined as the radius, within which the average dark matter density is 200 times $\rho_{crit}$:
    It is controversial that it is appropriate for representing the edge of the galaxy (or halo).
  • small halo ($<10^{11} M_{\odot}$), virial shock is unstable and break the halo.
  • Main point —> See Fig. 11


Colloq.:Formation of Galactic Black Hole Low-Mass X-ray Binaries (Xiangdong Li @ Nanjing Univ.)

  • The vast majority of XRBs are transient LMXBs, cycling between periods of quiescence and outbursts (Terarenko+16).
  • Population of LMXBs would be expected to $10^{2} - 10^{4}$
  • $P_{orb,LMXB} < 12 h$
  • At the birth of LMXBs, the final stage of supergiants has a size of $10^{3} R_{\odot}$, which is much larger than current orbital separation ($a=10^{11} cm$)
    • The formation of BH LMXBs requires
      • an extreme initial mass ratio
      • a very wide initial orbital separation
      • enormous mass loss and angular momentum loss
      • survival from the SN explosion
  • Common Envelope Evolution : if accreted material to secondary star is huge, the Kelvin-Helmholtz time scale becomes very long that cannot not manage the thermal equilibrium inside the star with added material. Then, it makes common envelope around the secondary star (van den Heuvel 83).
  • Do LMXBs evolves from IMXBs? : Possible, but not likely since it evolves into very-wide LMXBs (mass transfer from low-mass companion to high-mass BH) or compact LMXBs (angular momentum loss) (Justham+06).
  • Failed supernova when the mass of progenitor star is more massive than 40 $M_{\odot}$ (Fryer 99)
  • 2-5 $M_{\odot}$ mass gap between neutron star and BH (Wyrzykowski+16)
  • Failed SN model can explain the formation of LMXBs.


Journal: Raining on black holes and massive galaxies: the top-down multiphase condensation model (By Maochun)

  • Hitomi turbulence: $\sigma_{v}\simeq160 km/s$, which is very small compared to velocity dispersion of cluster
    • turbulence is not that important.


Colloq.: Acceleration of High-Energy Particles in Supernova Remnants (Siming Liu @ Purple Mountain Observatory)

  • While Cosmic Rays (CRs) dominated by nuclei (proton), there are also electrons, positrons and antiprotons.
  • Age: $10^{7}$ year, Power: $10^{41}$ erg/s, Maximum Energy: $3\times10^{20}$ eV
  • CR distribution is almost isotropic due to the interaction with magnetic filed of ISM.
  • SN shock wave can accelerate the cosmic-ray upto TeV: observation of TACO SNR- edge part (by hard X-ray) —> featureless X-ray.
  • 10% of SNR's Energy goes into CRs.
  • Diffusive stock acceleration may dominate for young SNRs (1K year), stochastic acceleration in the shock downstream may also play an important role.
  • #red| leptonic accelerated CRs breaks in ~TeV and hardronic CRs break in ~100 TeV, but still some PeV CRs are detected. Why?#


Group: AGN accretion disk and outflow revealed by $H_{2}O$ Megamaser disks (By Feng Gao)

  • Microwave Amplification by Stimulated Emission of Radiation
    • $H_{2}0$: Star Formation region, evolved stars, extragalactic nucleus, jets


  • King & Pounds 2015 Annual Review


Journal: Weinberger+16 Simulating galaxy formation with black hole driven thermal and kinetic feedback (By Prof. Yuan)

Journal: Zubovas+16 The small observed scale of AGNdriven outflows, and insideout disc quenching (By Fuguo)

  • Liu et al.
    • Gas-rich environment: Positive FB on star formation (reduced dynamical scale due to compression induces star formation)
    • Gas-poor environment: Negative FB on star formation (shock heating is dominant against the cooling)


Journal: Sadowski+15 Photon-conserving Comptonization in simulation of accretion disks around black holes (by Weixiao)

  • difference between Einstein-Boson vs. Plank

Globus and Levinson 16 Collimation of Magnetic Jet by Disk Wind (By Can)

  • Poynting-fux dominated jet: Magnetic energy to Kinetic energy
  • $50 r_{s} < z< 10^{5} r_{s}$ the jet becomes collimated to parabolic structure.


Seminar::Zhaoming Gan Jets and Bubbles

  • disk wind affect ICM?
  • magnetic tower jets
    • Jets are known to be produced in Kinetic mode, in which ADAF advection is dominated in the accretion disk. The jet can be form from this ADAF (rotating is not that important)? How Toroidal magnetic gradient grow in this ADAF model. MAD?
  • Evidence of AGN Feedback (Fabian 12 ARAA, Kormendy & Ho 13 ARAA)
  • Questions for jet dynamics
    • energy conversion in jet
    • kinetic energy eventually dominated?
    • why jet usually seems stable?
    • In-situ particle acceleration?
    • Jet feedback? How and Where?
    • What could we tell from observed jet morphology?
  • Hydrodynamics Jet (Reynolds+02), Hydrojet + magnetic field (O'Neill+, Mendygral+12), Magnetic Tower Jet (Lynden-Bell 96, Li+06, Nkamura+06)
    • Pure Hydrojet is hard to produce radio-lobe structures !!
  • Jet-ICM interaction (Where the Jet Energy go?!!)
    • Shock heating by Jet is 10~30% of Jet Energy
    • Turbulence 4% of Jet Energy
    • direct mixture -> should not work from that bubble features are not broken.
    • thermal conduction, cosmic ray…
    • the fraction of remains in the jet itself.
    • huge fraction of jet energy should go ICM??
      • Thermal E
      • Kinetic E
      • Potential E


Journal: Gonzalez & Zartisky 05Intracluster Light in Nearby Galaxy Cluser: Rel. to the Halos of Brightest Cluster Galaxies (By Yaping)

  • The origin of the halo stars?
  • the evolution of dark matter halo. NFW? or need to modification?


Colloq.::Cosmic Supermassive Black Hole Growth in the 7 Ms CDF-S (Bin Luo @ Nanjing University)

  • Optically, infrared, and radio-selected AGNs almost always show strong X-ray emissions.
    • Accretion-disk corona
    • X-ray is penetrating well (reduce absorption bias): particularly hard one (10-100 keV) more penetrating over high column density.
  • AGNs in Deep X-ray Surveys
  • Annual Review about SMBH Growth along cosmic time (Brandt & Alexander 15)
  • UV, optical, and IR emission dominated by host, while X-ray is dominated by the region close to the SMBH.
  • Highly Vriable X-ray Source —> Tidal disrupt event (Luo+14)


Seminar:Hot Gas Issues: Cooling and Composition (William Mathews @ UC Santa Cruz)

  • PART1: How hot gas cools in galaxy groups & clusters
  • PART2:high oxygen abundance in hot circumgalactic gas
  • Create model equal. hot gas atmospheres in an NFW halo L* galaxy: $M_{h}^{*} = 10^{12.2} M_{\odot}$
  • OVI absorption line: highest ionization level in absorption
  • $x_{5}(T) = n_{O+5}/n_{O}$ peaks sharply at $T=3\times10^{5} K$ (Gnat & Stemberg 07)
  • HSE derivation from NFW profile
    • NFW is collisionless dark matter particle? why dealing like gas?
    • entropy profile ($\log{s} = T/ n^{2/3}$) indicates how feedback is working in the CGM through X-ray observation)
  • Metals come from the galaxy or it is IGM material?
  • If radiate, it contract and move in together to keep virial Temperature ($10^{6} K$) rather then dropping the temperature.
  • general L* galaxy: virial radius $r_{200} = 240 kpc$
  • very small leftover of Oxygen possibly because of galactic outflow (Zahid+11)
  • Apparently, CGM gas entered the L* halo with ~solar abundance requires primordial top-heavy IMF.
  • Perseus cluster virial radius $r_{200} = 1300 kpc$
  • SNcc feedback energy problem
    • total SNcc energy = $10^{51} \times N_{SNcc} = 0.9\times10^{59} ergs$
    • potential energy with density profile from non-feedback profile is $> 1.4\times10^{59} ergs$ —> too large for SNcc
    • $M_{bh} = 4.6\times10^{6} M_{\odot}$ (Milky way) —> $E* = 0.1M_{bh} c^{2}=7\times10^{59} ergs$
    • $M_bh$ in passive galaxies is 40 times larger than that in star-forming galaxies (Reines & Volonteri+13)
      —> $E_{passive} = 280\times10^{59} ergs$
      —> AGN feedback play significant role for releasing energy.


Colloq.:Reconstructing the Initial Conditions to Simulate the Formation of the Local Universe (Houjun Mo @ Tsinghua/UMass Amherst)

  • Particle Mesh code to find best fit of fourier mode $\delta$ for matching $\rho_{\rm mod} = \rho_{\rm constraint}$
  • reconstructing current density field from simulation —> reconstructing initial density field only by gravitational constrain (only using Halo) —> SIMULATION to study history
  • Quench fraction does not only depend on the stellar Mass but also depend on the environment of the galaxy (cluster, filament, sheet, void).
  • $M_{*}/M_{h}$ has a peak at $M_{h}$ of $10^{12} M_{\odot}$ (Lim et al 2016)
    • Implying that Star formation is most efficient at the galaxy with $M_{h}$ of $10^{12} M_{\odot}$


Journal: Dugan+16, Feedback by AGN Jets and Wide-Angle Winds on a Galactic Scale (By Fulai)

  • RAMSES code
  • No gravity (external & self-gravity)
  • SF negative feedback within the central cavity, triggering star formation in ring.

Silsbee & Tremaine 16, Lidov-Kozai in merging BH (By Bin)

  • three body (N-body) simulation
  • merger rate $6.1/yr/Gpc^{3}$
  • Spin-orbit coupling: de Sitter precession
  • Spin-Spin coupling: Lense-Thirring precession
  • Spin effect on the GW signal: spin components aligned or anti-aligned with L will modified the direction of L, where L is the orbital angular momentum vector.


Colloq.: Pulsars and Fast Radio Busts(FRBs): Recent Developments (Richard Manchester @ CSIRO Astronomy and Space Science)

  • millisecond pulsars (MPs) are old pulsars that have been recycled by accretion of matter from a binary companion.
    • while half portion of millisecond pulsars are binary system which is reasonable in a sense that angular momentum transported from the companion makes the neutron star fast. However, how the lest of single pulsars can have a fast rotation without that boosting process?
  • indicator of pulsar age: $\tau_{c} = P/2\dot{P}$ in $P-\dot{P}$ diagram
    • MPs has small $\dot{P}$
  • NE2001 (Cordes & Lazio 2002) - electron distribution model for dispersion measure (estimate the distance to pulsar)
  • New electron distribution model - (Yao, Manchester & Wang, 2016)
    • for Galaxy, Magellanic Clouds and IGM
  • Is it physical property different between repeating FRBs and non-repeating FRBs?
  • The possible reason of the lack of detection of gravitational waves (GWs) in merging SMBHs is interaction of stellar/gas at late time
    —> less energy to GWs.


Seminar: Exploring Galactic Gas Flows During Cosmic Afternoon (David Koo @ UC-Santa Cruz)

  • DEEPwinds project: Gas flow in galaxies project in CANDELS field
  • outflowing cool galactic winds are ubiquitous. (bi-conical shape like M82)
  • Properties of Galactic outflows (Li, Bryan & Ostriker 2016)
    • Multi-phase
    • Velocities of outflows are several 100 km/s
      • comparable to the escape velocity of the Dark Matter halo of their host galaxies
    • Mass of outflows are 1% to 10x the star formation rate
  • stellar feedback (SN, winds & radiation from hot stars) is the driver of outflows & active SMBH
  • Outflows limit galaxy masses (Springel & Hernquist 03)
  • Som infall return at the edges of disk ("galactic fountains") (Genel+15)
  • galaxy contain less metals & enriched the inter-galactic medium (IGM) (Li, Bryan & Ostriker 16)
  • star-formation rate vs. Time since Big Bang (cosmic afternoon z= 1.5~0.5)
  • Use UV light source from Galaxy's own stars rather than Quasars
  • why stacking the data improves the quality of the spectrum?
  • mass out flow on average ~$20 M_{\odot} yr^{-1}$
  • mergers are not required for strong winds (only 3/118 had merger-like morphologies (HST))
  • Wind strength is proportion to SFR rather than sSFR (specific SFR: SFR/Mass)
  • Found inflows from 6 edge on galaxies: the evidence of replenishing gas
    • The inflow gas was detected MgII, which indicates meta-rich.
      —> the inflowing gas is likely the gas falling-back (fountain) rather than inflowing of low-metal IGM gas.


Colloquium: What determines Star Formation Rates? (Neal J. Evans II @ UT-Austin)

  • SF rate increase until z~2, and decreases after then.
  • around 10 $M_{sun}/pc^{-2}$ in total gas mass: transition from HI to H2
    • beyond the threshold, SF rate no longer being in linear relationship with Surface density
    • still SF rate is in linear relationship with molecular surface density.
  • Star formation efficiency: SFE = SFR/X (where X is something like mass of clouds)
    • [SFE] = inverse of depletion time ($Myr^{-1}$)
  • Does SFR of a cloud depend on free-fall time of the cloud? (Evans et al. 2014)
    • No correlation !!
  • Kennicutt-Schimt (KS) relation does not apply to molecular clouds in smaller scale, only work for large scale (>10 kpc).
  • Trace Star forming disk plane with radio recombination lines


Journal Club: Sadowski 2016 Thin accretion disks are stabilized by a strong magnetic field (by Defu)

  • radiation pressure dominated thin disk —> SSD is thermally unstable
  • Thermal instability can be suppressed by magnetic field
  • radiation transport: diffusion dominates advection.
    • Hard to believe, since Corona is generally optically thin.

Colloquium: Fast vs. Slow: Galaxy Death at z ~ 2 vs. z ~ 0 (Sandra Faber @UC Santa Cruz)

  • central stellar density within 1 kpc, $\Sigma_{1}$ as an important quenching indicator
    • higher $\Sigma_{1}$ means quenching (Fang+2013)
    • compaction estimated order of violence
  • star formation histories in dark matter halos (Behroozi, Wechsler, and Conroy 2013)
  • Dekel & Birnboim 2006 ; Andrei Kravtsov
    • Switch from cold flows to hot bubbles at $M_{crit} \sim 10^{12} M_{\odot}$
    • Characteristic halo quenching mass = $10^{12} M_{\odot}$
  • cooling flow examples (Abell 1795 ; Perseus A) - Cooling filamentary H-alpha
  • Feedback / heating (Hydra A; Perseus A) - Cavities
  • Centrally concentrated galaxies have higher entropy
    • Interanal density inhomogenities permit mass lumps to trade J and E, which enables entropy increase
    • Colliding gas clouds, if present, heat and radiate, lose energy, sink to center, from new stars —> Dissipation
  • Quenching at z=0 is de to the interplay of three factors:
    • Less gas falls onto halos
    • growing halo mass cools more slowly
    • Growing BH mass heats more strongly
  • fermi bubble: Bubble energetics require $\sim 10^{42} ergs$, 400 times more energy than central starburst can supply (Miller & Bregman 2016)
  • Triggers of wet compaction (Zolotov+15)
  • AGN fraction in the quenching history (Kocveski+2016)


Colloquium: The MACRO Simulated Galaxy Cluster Project: The Final Chapter (Weiguang Cui @ ICRAR/UWA)

  • Cosmological simulation (Simulations of Galaxy Clusters) with Gadget
  • mock image of radio image requires magnetic field in the simulations?
  • what kinds of AGN feedback is included?
    • answer: only injecting thermal energy for this project. They will consider mechanical feedback in future work.
  • mock image of X-ray: PHOX code (Biffi et al. 2012, 2013): bremsstrahlung continuum plus metal emission lines
  • mock image of Optical: Stellar synthesis code
  • black holes inside the cluster —> how dealing with black hole merging?
  • Theoretical approach for checking the dynamical state of the cluster (relaxed or unrelaxed)
    • Virial ratio (virial Theorem)
    • center of mass offset
    • substructure mass fraction
    • velocity dispersion deviation

16.09.26: Journal Club

Randall et al 2015 Rough estimation of shock feedback in NGC5813 (By Xiaodong)

  • $t_{cool} \approx \frac{\frac{5}{2}nkT}{n^2 \Lambda(T)}$

Li Yuan et al. 2015 Cooling, AGN Feedback and Star Formation in Simulated Cool-Core Galaxy Clusters (By Maochun)

  • cooling flow in steady state: 100s-1000s M_sun/year » local observed SFR ~ 10 M_sun/year

16.09.05: Journal Club

Vernaleo & Reynolds 2006 AGN feedback and cooling flows (By Yaping)

  • Cooling flow problem
    • Cluster temperature: 1/2 - 1/3 $T_{vir}$
    • Cooling time is way shorter than hubble time
    • Observed SFR: ~10 $M_{\odot}/yr$ « 100s-1000 $M_{\odot}/yr$ (expected value)
    • lack of cool gas below 102 keV in the X-ray
  • Effective heating required the channels to fill in between powerful outbursts
  • Alternative solution for cooling flow problem.
    • Internal dynamics of jet (density drill effect)
    • slow heavy jet
    • turbulence
    • thermal conduction
    • viscosity
    • magnetic field

Wever & Davis 1967 The angular momentum of the solar wind (By Can)

  • There are multiple critical points in magnetic winds. (Just like sonic point in hydro case)

16.08.30: Journal Club

Exoplanet Formation: Disk-driven migration vs. Lidov-Kozai migration (By Bin)

  • Kozai-Lidov effect
    • The pericenter distance should be close enough e_max -> 1
    • Initially circular orbit-> Kozai effect by a distant perturber-> high eccentricity-> Energy dissipation: Hot Jupitor

Reynolds et al. 2005 Buoyant Radio Lobes in a Viscous Intra-Cluster Medium (By Zhaoming)

  • King's beta model $\rho(r)= \rho_{0} [1 + (r/r_{0})]^{\beta}$
  • Bubbles could not survive during the buoyant motion in inviscid ICM
  • Reynolds number: $Re \equiv = \frac{inertial forces}{viscous forces} = \frac{v L}{\nu}$
  • Spitzer viscosity: molecular level of the viscosity (for thermally fully ionized plasma).
  • viscosity could help to stabilize the flow and keep the integrity of the static bubble, but probably doesn't work for jet-driven bubbles since $v>>1$.

16.08.22: Journal Club

King et al. 2013, NGC 7213: the lowest mass Seyfert with distinctive disk-jet coupling (By Fuguo)

  • low mass SMBH
  • accretion mode is changed -> the photon index is changed
  • time lag bet. $L_x$ & $L_{Radio}$
    • 10 $R_{grav}$ -> jet base
  • softer as the $L_x$ is brighter -> two phase accretion ?!
    • two phase accretion: the presence of cold clumps embedded in the hot accretion flow.
    • soft X-ray can be generated from the cooling of the clumps while hard X-ray generally comes from the high energy electrons.
    • Therefore, the presence of the cold clumps makes softer in the spectrum for higher luminosity (indicating more accretion).
    • This relationship also can be seen in (geometrically) thin disk + coronae (SSD disk).
  • positive & flat correlation bet. $L_{x}$ & $L_{Radio}$
  • $L_x$ come from the hot accretion disk at Low luminosity AGN
  • $L_x$ come possibly from the hot corona at high luminosity AGN
    • However, the latter has not been proved by RMHD: always weak of $L_x$ in Corona

Miller et al. 2015 Powerful Disk Winds (By Defu)

  • the upper wind launching point: $r\sim10^3 GM/c^2$ (cf. the location of the BAL wind is about $r\sim (0.5-1) 10^3 GM/c^2$)
    • related to ionization parameter
  • wind is equatorial
  • The wind driven mechanism in Soft state and SSD accretion disk
    • Thermally driven wind? no, thin disk
    • Line driven? T is too high
    • Magnetic driven? possible.


Querejeta, M. et al., 2016. AGN feedback in the nucleus of M51. Available at: [Accessed July 4, 2016].

  • Inclined Jet in M51


Colloquium: Tidal Disruption Events as a probe of Super-Eddington Accretion (Lixin Jane Dai @ Maryland Univ.)

  • TDE: Super Eddington (key question)
    • Debris return / fallback rate
      • dm/dt = dm/de * de/dt (dm debris mass falling back to black hole) ==> $dm/dt \propto -5/3$
  • disk formation:
    • ways of debris circularization - Nozzle shock (Guillochon+ 14)
    • compare $t_{circularization}$ vs $t_{viscosity}$ & $t_{fallback}$
  • observational signature
    • X-ray reverberation observation
  • TDE source- only star? any possibility for gaseous cloud?
    possible. But time scale must be different.
  • What prevent super-eddington accretion from TDE?
    • inefficient collision in disk
    • mis-aligned orbit.
  • Super-Eddington Accretion (McKinney,Dai,Avara 2015 ; Sadowski&Narayan 2016)
    • Accretion rate $\dot{M} > \dot{M}_{Edd} = L_{edd}/\eta \,c^2$
    • Accretion disk: pressure supported, geometrically and optically( why?) thick. —> MAD accretion
    • Outflow: thick
    • Radiation: Beamed
  • HARMRAD: GRRMHD simulation, MI scheme


Colloquium: How do Black Holes Grow Across the Cosmic History? (Ezequiel Treister @ Universidad Catolica, Chile)

  • Obscured accretion
  • extragalactic X-ray Background
  • How the background X-ray can be distinguished from the Milkyway X-ray (XRBs)?
  • X-ray spectrum of unobscured AGN is softer than that of obscured AGN.
  • Photoelectric absorption affect mostly low energy emission (makes it harder X-ray)
  • Compton Thick AGN: obscured by $N_{H}>20^{24} cm^{-2}$
  • Compton thin BH accretion happens in:
    • Moderate luminosity AGN
    • z~0.5-1
    • obscured sources
  • Possibly, at least nearby MW, the fraction of Compton Thick AGN is about 20~30% of entire AGN population.
  • BH Growth (Alexander & Hickox 2011; Treister et al. 2012)
    • merger vs. secular
    • High luminosity AGN relates with mergers (Treister et al. 2012)
    • 90% of the AGN by number triggered by secular processes.
  • Obscured quasars are the product of the merger of two massive gas-rich galaxies. After a time (96 Myr) it will be cleared out.


Seminar: Multi-wavelengh observation of quiescent stellar mass black holes (James Miller-Jones @ Curtin Univ.)

  • Sketch of Quasar vs. Microquasar (Mirabel & Rodriguez 1998)
  • Accretion-jection coupling (Corbel et al. 2013)
    $L_{radio}\propto L_{Xray}^{0.7}$
  • BHs is much fainter than NSs at same $P_{orb}$: taken as the evidence of BH
  • 3 Power output channels: Radiation(X-rays), Black hole growth, Outflows(winds/jets) (Fender et al. 2003)
    * X-ray emission (Plotkin et al. 2013):
    • weak thermal emission from truncated disk
    • Comptonisation of disk photons by corona
    • Synchrotron self-compton from jet base
    • Synchrotron from jet
  • BHs in Globular Clusters: Retention of BH in GCs (Morscher et al. 2014)
    • nearby the cluster center of M22
    • BHs in GCs are likely more massive
      • low metallicity: less wind
      • no common envelop stage
    • Flat-spectrum radio source w/ X-rays: M62
    • 300 X-ray sources in 46 Tucanae
  • C IV but no He II lines: accretion signature


  • magneto-centrifugal Blanford-Payne mechanism vs. magnetic pressure-triven tower outflow (Stepanovs & Fendt 2016)

—> Figure in the Book "The Physics of Accretion onto Black Holes"


Junior meeting: A model for gas dynamics and chemical evolution of the Fornax Dwarf Spheroidal galaxy (Zhen Yuan @ JiaTong Univ.)

  • MW galaxy: merging / dSph galaxy: secular evolution
  • Dark matter halo grows as the galaxy evolves and saturates at some point (~ 5 Gyr)
  • pre-satellite phase / satellite phase
  • larger systems get homogeneous more quickly & more metal enriched.


  • Scale of the Universe


LIGO (Laser Interferometer Gravitational-wave Observatory) Press Conference


Group meeting by Dr. Fuguo Xie

  • The soft X-ray luminosity is in anticorreation with the Temperature: $L_{soft} \sim T^{-4}$


Colloquium: Studying Galaxy Population in the Big Data Era (Yingjie Peng @ KIAA/Peking Univ.)

  • Large survey: zCOSMOS (ESO 8m VLT) - very deep and narrow survey (pencil survey)
  • star formation quenching (star-forming galaxy -> passive red galaxy)
    • environment-quenching (nurture) - internal quenching
      • strangulation, Ram pressure striping, tidal effects, harassment, mergers
    • mass-quenching (nature) - external quenching
      • AGN feedback, Star Formation feedback
  • SFR of star-forming galaxy does not depend on environment.
  • Stellar metallicity is key parameter that tells the history of the universe.
    • quenching by sudden gas removal: star formation stops abruptly
    • strangulation: star formation continues and used up the gas
      -> stellar metallicity for quiescent (passive) galaxies is higher than for star-forming galaxies.
      -> stellar metallicity enhancement in a "closed-box" model
  • Local quiescent galaxies with $M_{star}<10^{11} M_{\odot}$ are primarily quenched as a consequence of "strangulation"
  • quenching by galactic outflow is minor.
  • sSFR(t) for the gas-regulator model
  • varying the efficiency of AGN Feedback (Booth & Schaye 2009)
    —> SFR is insensitive to the feedback efficiency.


Colloquium: Cosmology and Astrophysics with Galaxy Clusters from the Numerical Perspective (Erwin Lau @ Yale Univ.)

  • cluster mass could be underestmiated (Plank 2015 data) (biased by the HSE assumption)
    • M_cluster = M_therm (+ M_random + M_bulk + M_accel)
  • specific entropy $s= \log K$ where $P=K\,\rho^{5/3}$ for monoatomic gas
  • Omega500 Simulation Project (High-resolution N-body+Gasdynamics Cosmological Simulation w/ Adaptive Refinement Tree (ART) code
    • 500 Mpc simulation domain
  • $\left<n^{2}\right> \geq \left<n\right>^{2}$ in case that gas clumpness is large
  • Gas clumps & filaments observation (by Chandra (highest angular resol.) or XMM (high sensitivity))
  • Two temperature: proton(high: contains much more Thermal E) & electron(low) (because coulomb time scale is larger than the age of the universe).

why $T_{i} > T_{e}$ when the coulomb time scale is longer than dynamical time scale?
Shock waves in settings such as the solar wind, the interstellar medium, and galaxy clusters are collisionless, in that the shock transition occurs on a scale comparable to the proton gyroradius, which is orders of magnitude smaller than the collisonal mean free path. In this case, one might expect that a collisionless shock thermalizes a fraction of the kinetic energy of each incoming particle, leading to mass-proportional temperatures. The actual situation is more complex.


Group meeting

  • Magnetically Arrested Disk (MAD)
    • Formation of MAD (how the magnetic field can be enhanced near the black hole?)
      The frozen-in magnetic flux is dragged into the center by accreting gas, and this makes the magnetic pressure increased inevitably. The increased magnetic field subsequently reduce the inflow gas velocity, implying that the frozen-in magnetic field is denser and denser effectively at the very center. (Narayan et al. 2003)
    • The black hole "wants" the plasma and it "does not want" the magnetic field. (Punsly 2001 (book))
      The survival of the field by the breaking up the accretion flows and its EM sources into a current rings around black hole (Figure below). And he said it is closely related to the physics of the magnetic reconnection


Colloquium: Extreme Exoplanetary System (Dong Lai @ Cornell Univ.)

1. Misaligned Hot Jupiters & 2. Circumbinary Planets

  • orbital period puzzle (orbital period of most planets is 10 days ~~ too short !!)
    * Kepler compact Planetary System
  • orbital is highly eccentric
  • Planets around binary stars
  • Spin-Orbit Misalignment Puzzle (by analysis transit pattern)
    cf.) all major planets lie in the same plane (within 2deg), which is inclined to the Sun's equator by 7 deg.
  • Question for Hot Jupiter
    • How did they migrate to <0.05 AU?
    • How did their orbits get misaligned with host star?
  • Possible Channel
    • Disk-driven migration
    • Planet-Planet interactions
    • "Lidov-Kozai + Tide" migration induced by a companion star (3 body dynamics)
      • Eccentricity and inclination oscillation induced if inclination angle > 40 deg.
      • population of hot jupiter comparing between single star and binary system? Kozai requires binary system.
  • star is spinning (3-30days) —> oblate —> precess in the gravitational influence by the orbiting planet.


Science Talk: Compact Objects (Dong Lai @ Cornell Univ.)

  • general relativity: mass as well as pressure contribute to gravity (unlike to classical: only mass contributes to gravity)
    (GR - E relates w/ mass, but E also relates w/ particle random motion by pressure)
    measure mass limit of neutron star by oppenheimer limit


Colloquium: New developments of Fast Radio Bursts (Ue-Li Pen @ CITA)

  • Green Bank Telescope to observe 21 cm.
  • Fast Radio Bursts (short wavelength: faster (early), long wavelength: slower (later) in radio) -> dispersion
    Masui et al. 2015, Nature 528, 523
    • flux change: scintillation (electric field interfere the light: just like star twinkle but not planets (due to larger size))
      —> can measure the size of the FRBs (micro arc second)
    • all pulsar can scintillate because of size under micro arc second
    • nothing else (e.g. quasar) does not scintillate.
  • typical FRBs is located at z~1
  • Candates
    • cataclysmic: exploding hawking black holes, merging neutron stars, blitzars (fast rotating ns -> bh)
    • repeating: magnetar flares, planet-neutron star, supergiant pulse
    • local: flare stars, microwave ovens
  • Faraday rotation (Rotation Measure)-> measure magnetic field


Colloquium: Turbulent Amplification of Magnetic Fields at Shocks in ICM and ISM (Suoqing Ji @ UCSB)

  • Bullet cluster - colliding dark matter (clear evidence against MOND (Modified Newtonian Dynamics), which expects that all dominant gravity sources are located within the gas. This cluster shows that most dominant gravity sources are located two regions (blue) observed by gravitational lens effect, on the other hand, the collisional gas materials undergo shocks in central area (red).
  • The comparison of Cluster shock relics and SN shocks in the context of shocks & B-field
    • Sausage Radio Relic - enhanced synchrotron by shock front (B-field parallel to shock front) : weak shock
    • SN shock - B-field is radial direction. : Strong shock
  • B-field enhancement mechanism
    • Bell instability from cosmic ray streaming
    • Richtmyer-Meshkov instability
    • enhanced density
  • enhanced B-field can be transferred to other energy form (thermal or radiation)?
  • Strong shock - equi-partition bet. B-field and Thermal E.
  • Turbulent dynamo is important in SN but not in cluster.

Very energetic gamma rays are easily knocked off by visible light, the light from galaxies and stars that fills intergalactic space, he said. Lower-energy gamma rays mainly interact with ultraviolet light and X-rays. There are lots of sources of visible photons — stars and galaxies — but not as many X-ray sources that would bother the lower-energy gamma rays.

Many of the prominent discoveries in astrophysics have been serendipitous. The first discovery of a planet in recorded history was made by chance while William Herschel was surveying bright stars in the sky. Satellites with the intention of monitoring nuclear bomb detonations on Earth found the first signals of cosmic gamma ray bursts. And, to conclude this far-from-exhaustive list, the afterglow of the Big Bang was accidentally discovered by two employees of Bell Labs using a device meant to track the company’s satellites (Penzias and Wilson ended up winning the Nobel Prize for this find, unintentionally “scooping” the discovery from physicist Bob Dicke who predicted this phenomenon).

  • AGN unified model
    Type 1: Broad line + Narrow line
    Type 2: Only Narrow line

Colloquium: The mass and angular momentum distribution of cosmological simulated massive Early-Type Galaxies(ETGs) to large radii (Xufen Wu @ USTC)

  • two model
    • elliptical galaxies are fully assembled at high z -> no significant structure evolv.
    • major merger: ellipticals are very massive at z~2, few major ETGs observed; dark mass reduces the mass growth. —> problem
  • suggested alternative two-phase scenario :
    • massive ETGs grow initially through rapid star formation fuelled by infall of cold gas at Z>2
    • galaxies grow gradually through minor mergers, accreting old stars formed in outer halos —> efficient size evolution
  • two-phase scenario for ETG formation by (Naab et al. 2009)
  • HSE equil. -> X-ray emission: mass can be underestimated (from Prof. Shen)
  • ellipticals has smaller dark matter halo than spirals (some science paper; from Prof. Shen)
  • v_rms declines at outer region the slope depends on the galactic properties like size, mass?
  • more massive galaxies have less angular momentum
    <- due to the random direction of major merger galaxies and thermalize after merger.
Volonteri et al. (2007) explored a number of scenarios for the formation of ellipticals and spirals and showed that it is plausible for the nuclear BHs in spirals to have lower spins than those in ellipticals. This suggests that the spin paradigm may explain the radio loud/quiet dichotomy.

  • why?? Major merger makes spin of BH faster??

Astrophysics Symposium (2015.12.04-06, Suzhou)


The galactic food chain in the Virgo Cluster of Galaxies

Colloq. by Erig Peng (KIAA/Peking)

[ $\alpha/Fe$ ] indicates how the stars form in the history of galactic evolution:
rapid, radical or slowly.

  • high value: more radical and fast like burst
  • low value: slowly formed.

Because Fe lines mostly come from the SNI while $\alpha$ particles come from massive & early type stars~!


Fermi Bubble

In 2010, astronomers discovered something phenomenal in the center of our Milky Way. Two huge bubbles of γ-ray emission–now known as the Fermi Bubbles


We don’t know the answer yet, but most astronomers think it falls into one of two main categories: jets or star formation.

  • Our SMBH was once active. (The legacy of Jets)
  • Lots of star formation


05.16.2012: skim journal

Startling superflares

A superflare on a Sun-like star is a brightening that has an evergy of from $10^{33}$ to more than $10^{39}$ erg and last from minutes to days.

Superflares occur on single, middle-aged stars that are rotating slowly and are powered by the fusion of hydrogen in their core. Such stars, which are technically known as main-sequence stars of spectral type F8 to G8, include the closest known 'twins' of our Sun. The similarity of superflares to solar flares suggests that superflares arise from magnetic effects. However, the now-default model for these events involves a magnetic field that connects the star to an orbiting 'hot Jupiter' — a planet that has a mass comparable to, or larger than, that of Jupiter but that is much closer to its host star than Jupiter is to the Sun.


One idea to explain the superflares observed by Maehara et al.2 invokes the presence of intense magnetic fields that connect the star with a Jupiter-like planet in very close orbit around the star. The magnetic-field lines will become twisted and amplified by the orbital motion of the planet, and at some time the lines will be strained and twisted to the point of breaking. The broken lines will accelerate particles to very high energy and release this energy in an explosive event, similar to what happens in ordinary solar flares seen on the Sun.

03.22.2012: colloquium

Dr. Evgenya Shkolnik (Lowell Observatory)

Tides and Magnetic Fields in Hot Jupiter System

The methods of exo-planet detection:

  • radial velocity
  • transit
  • direct imaging
  • micro lensing
  • pulsar timing

What is Hot Jupiter??
-> Mp > 0.3 MJ & a < 0.1 AU

Star-Planet Interaction (SPI) -> On/Off (still don't know why, but observationally inferred)

stellar surface activity in the form of active spots may be induced by the giant planet (here is Hot Jupiter)
—> activity depends on
* magnetic field of star
* magnetic field of planet
* orbital distance w.r.t. Alfven radius of hot star

12.15.2011: colloquium

Dr. Martha L. Boyer (STScI; Space Telescope Science Institute)

- Dust Production in the local group -

Candidate for dust formation:

  • Super Novae (0.001~0.05M$_\odot$)
  • Massive Stars (0.001~0.01M$_\odot$)
  • Dust Growth in ISM
  • Asymptotic Giant Branch(AGB) (10-11~10-7M$_\odot$)

She focused more on last subject, and showed some preliminary results on
investigating the AGBs in SMC & LMC.


11.08.2011: colloquium

Dr. Samir Salim (Univ. of Indiana)

- Young Stars in Old Galaxies - Star Formation in E/S0s -

15% of Early Type Galaxies (ETGs) have Star Formation. (~ 1M_sun/yr)
He mentioned that UV can be observed from S0 galaxies even though
they don't have any Halpha emission.

UV can be generated from

  • star formation
  • Horizontal Branch (old star): UV appears more diffuse
  • AGN continume: point source

20% of S0 shows ring-like structure possibly originated by

  • collision between two galaxies (like ring-galaxy)
  • resonance by bar: happens @ Corotation radius or outer Lindblad Resonance

—> smooth, regular rings, and long last

LSB: Low Surface Brightness Galaxy
What is the origin of giant LSB??