Adam Ginsburg's blog

• Anja Feldmeier: The Milky Way Nuclear Star Cluster Beyond 1 pc

  

  Observing the cluster

  • slit-scanning the central cluster with ISAAC
  • foreground removal based on colors
  • included Q-branch H2 lines, stellar CO absorption lines
  • CO -> stellar velocity dispersions
  • flat velocity dispersion profile + fitting profile -> underestimate of BH mass + may affect other galaxies

  Questions

  • Q Fred Lo: Method vs external galaxies. Couldn't understand.
  • Q: What is the error on the anisotropy?
  • A: can fit with different anisotropy
  • Q Lu(?): What happens if you include IR7?
  • A: v disp drops towards center. Problem.
  • Q Soo(?): Does model allow stellar rotation?
  • A: ???

---

• Overview Talk Jessica Lu: Young Stars in the Galactic Center

  

  Why care about cmz young stars?

  • lots of them, compared to elsewhere
  • fate of clusters?
  • IMF, stars affected by environment?
  • interaction with SMBH?

  Nuclear cluster

  • HST image covers CND

  • spectroscopic census of stars with IFUs

  • mass limit ~8-10 msun

  • astrometry great + pm 1 km/s

  • Models of formation + in situ? inward migration? + circular orbits? eccentric?

  • messier now + sharp radial fall-off favors in-situ models + eccentricity rejects in-situ models + "wild dump" of molecular material

  • IMF variation? + claimed very top-heavy mass function

    • rejected by UCLA group

    • fit luminosity function with bayesian model approach
    • modeling required including multiplicity
      • stars brighter than brightest possible unless multiples included
    • salpeter ruled out at 4-sigma significance
      • slope ~1.7

  • Future? + pre-main-sequence should be doable in photometry + astrometry, spectra getting better + can start to measure multiplicity

  Arches cluster

  • membership determined via proper motion measurements + Stolte 2008, Clarkson 2012

  • PMS stars in Arches + disk fraction ~6% + disk evaporation

  • velocity dispersion measurement ~5 km/s + dynamical mass 10^4 msun + compare to photometric mass to get IMF + IMF can't be normal.

    • could be top-heavy or bottom-light

  • Both arches and quintuplet are moving away from Sgr A* in approximately the same direction & amplitude + track orbits backward + Could have common origin (arches & quintuplet)

    • may be in outer x2 orbit, maybe x1/x2 intersection

  • Future: + HST: measure tidal radius + HST: Measure IMF to 1 msun in outer parts

  Ages

  • all seen are young + where are the old? + nonexistent? Or dispersed? + triggering 2-6 Myr ago?

  SMBH interaction

  • most uncertain component

  Young stars around nuclei of other galaxies

  • M31 core cluster + but no gas

  Questions

  I wasn't allowed to ask this question. Apparently I'm invisible to the chair. QUESTION: Can you infer which stars are likely binaries via the IMF modeling? Do spectra velocities agree?

  • Q Evidence for distorted IMF. But you see PDMF. Could it be dynamical?
  • A: in YNC, solid, but not 100%. How do ejection mechanisms scale with mass? Unknown. YNC is the best case. Arches, remains a real concern. Assumption is cluster in equilibrium, may not be true
  • Q Fred Lo: What's the SFR?
  • A: Sgr B2 is forming a massive star cluster today. Continuous as can be. No major evidence for stochasticity.
  • A from the crowd: 3 orders of magnitude scatter.
  • A Gallagher: 0.1-0.01
  • Q Gallagher: How does this compare to M82F? Velocity dispersion too small for luminosity. 40-60 Myr old. Bottom-light IMF
  • A: super star cluster similar
  • Q: When you consider binaries or multiples, are the evolutionary tracks for binaries? Maybe all clusters same age.
  • A: Didn't have them yet, so singles.
  • Q: Correlation between stars and molecular gas?
  • A: no.
  • Q J Ostriker: Given age of clusters, expect 1000x more of them in the galaxy. IF you take number of clusters divided by age, do you get too much mass? If current rate is typical, was there 1000x more in the past?
  • A: IF you add them up, no...? Is SFR now consistent with past? not constant SFR Lower and higher.

---

• General Discussion (day 2)

  Farhad discussion points:

  • Fraction of dense molecular gas in CMZ?
  • Volume filling factor of dense gas?
  • Abundances in CMZ?
  • Lada 2012 plot: SFR vs Molecular Gas Mass. Density is crucial
    • Steve: Not volume, column density. Assumptions about 3rd dimension. All gas above Lada's column density threshold.
    • Gallagher: exgal community correlates surface density with SFR. Variations on local scales
  • Aalto: HCN as dense gas tracer in ULIRGs - problem.
  • Steve: comparison to centers of other galaxies... rings have low volume filling factor, not pancakes but rings
    • unlikely that column density uniformly distributed along line of sight; likely to be clumped
  • Bally: comment on face-on views. Mostly filaments. Filaments tend to be end-on on positive side, face-on on negative side. Could Brick be an end-on filament? + Gallagher: +1
  • Mills: ALMA data can answer that with excitation. NH3 thought of as high-density tracer, but not in CMZ. + plug by me
  • Juergen Ott: Interferometer and single-dish, 10% filling factor
  • Gallagher: Density limit on SF from star clusters. 10^6-ish, high SFE. Star clusters may imply upper bound
  • Farhad: Mass of molecular clouds from dust emission. Mass of low and high density gas + Rathborne: Opacity, chemistry very tough
  • Aalto: "Medusa merger" large-scale, very high efficiency. Timescales are a problem: no high-density gas tracers seen, but plenty of star formation is seen
  • Mills: How do you trace this highest density gas, then? + Aalto: H2CO, Ao et al. Weak chemical variation. + Rathborne: "Million dollar question". H2CS follows continuum better than other tracers
  • Dust normally considered better tracer of mass. Do we really know dust to gas ratio? Factor of 2 at best
  • Farhad: Question raised yesterday... asymmetry of SFR/mass How do we explain distribution
    • Steve: How measure SFR?
    • Farhad: YSOs counting
    • Bally: You sure they're YSOs?
    • Farhad: Yes, best we can do, age ~few 10^5 yrs. Maybe YSOs obscured? If not, interesting issues
    • Steve: Could YSOs have moved after they formed? Dynamical time...
  • What's the extinciton at 24um? Selection effect? + Farhad: Could be selection. Sgr B2 makes things worst. + Not clear 24um sources come from same volume
  • Dust is the best measure for mass. Only free parameter is temperature. KS not very fundamental.

---

• Rei Enokiya: Large scale and high sensitivity multi line CO surveys toward the Galactic Center

  

  NANTEN2 CO survey

  • 12CO 1-0, 3' beam, 0.43 K
  • high latitude clouds with vlsr -200 to +150 km/s + 24um filaments traced by CO
  • Double Helix nebula (Morris, Uchida, Do 2006) + Torional alfven wave? + polarized lobe from radio arc? + CO detected
  • Other molecular columns tie back to Sgr A*
  • "Magnetic Tower" model + accretion around BH + CND creates B-field, spins it up vertically + 25 km/s outflow speed

  Questions

  • Q McClure-Griffiths: can you distinguish between magnetic and stellar wind models?
  • A: No,not really
  • Q Bally: Do you worry that the LSR velocities are close to local gas and spiral arms? Could foreground confuse? Linewidths consistent with CMZ?
  • A: Star-counting extinction distance estimate gives 8±2 kpc.
  • Q: What evidence for jet?
  • A: no direct evidence.
  • Q Farhad: Could it be infalling rather than outflowing? Maybe hard to tell.
  • A: ...

---

• Denise Riquelme Vasquez: Disk/Halo Interactions: Molecular Clouds in the Galactic Center

  

  CMZ mol clds

  • SiO emission, large linewidth, ...
  • Giant molecular loops + magnetic buoyancy + Parker instability
  • NANTEN 4m large-scale survey
  • X1 & X2 velocities...
  • NH3 measurements of "connection points" of loops
  • attempt to measure heating mechanisms + turbulence decay + CR
  • High isotopic ratios towards halo
  • High temperatures at interaction points

  Questions

  • Q Farhad: Shocked gas seen everywhere in loop?
  • A: need to observe complete loop to get gradient temperature

---

• Kazushi Sakamoto: The Kinematics and Dynamics of Molecular Gas in Galactic Centers

  

  Driving gas into Galactic centers

  • opposing torques at corotation point drives gas inward
  • observational evidence from nearby galaxies
  • barred galaxies have greater concentration (Sheth et al 2005)
  • observations of CO gas kinematics in M83, NGC 1097 + FCRAO Quarry obs
  • ALMA concentration to M83 core... + high velocity gas near core + southern nucleus has spatially distinct outflow lobes

  Questions

  • Q Fred Lo: Can you distinguish between streamline convergence and cloud collisions? Can you determine what happens when clouds collide?
  • A: Yes, with ALMA.
  • Q Farhad: Physical conditions in NGC 253. Temperature distribution? All warm?
  • A: Our obs don't measure temperature. CO gives 50K on 10's of pc scales, so at least that temperature.
  • Q: Big bubbles in NGC 253. Are there clusters near them?
  • A: Didn't find any in original data.

---

• Betsy Mills: Hot Gas, Masers, and Cloud Collisions: The extreme properties of molecular gas at the heart of the Milky Way Galaxy

  

  VLA Survey

  • DnC at 24-36 GHz

  • Brick NH3 + very high temperatures in brick: 100's of K + optically thin 4-4,5-5,7-7 + brightest, densest clumps have lowest temperatures + 300K gas, but no SF

    • CR heating?
    • Shock heating?

  • 36 GHz methanol maser + shock excited + dozens of point sources + 75 candidates, 29 T_B > 1000K (masing)

  • why so many masers? + large-scale shocks? + 80 km/s velocity gradient across cloud + methanol & ammonia emission dominated by convergence point of velocities

  • class I methanol masers usually show SF

  SF in the Brick?

  • 7 VLA continuum sources + peaks in a more distributed set of continuum emission
  • 25 GHz continuum + most continuum outside of the molecular gas + external ionization

  Rest of the CMZ

  • Pistol & Quintuplet continuum with streaks from the nonthermal filaments + second LBV seen in radio
  • NH3 near Sgr A* + NH3 traces edges of Sgr A East SNR + some SF in 50km/s cloud and 20km/s cloud
  • CND not in HC3N, weakly in ammonia
  • no NH3 or HC3N at brightest submm clump
  • nonthermal 3-3 line

  Sgr B2

  • CH3OH only non-self-absorbed against hot core + shell structure
  • NH3 2-2 maser

  Questions

  • Q Walsh: NH3 3-3 maser. Expected more of those. Were there others?
  • A: yes, there are at least 2 others
  • Q: Temperatures of ammonia. Is ammonia the only tracer that gives high temperature towards the Brick?
  • A: Yes, only one towards brick. 400-600K from H2 from Rodriguez-Fernandez. 400K in Sgr A from NH3 too.
  • Q: Formation pumping?
  • A: yes, possible. Need to form on short timescales.
  • Q: Need hefty CR for that heating. Can you expand on what VLA is good for?
  • A: CRs 10^-14, 10^-15. May happen. Hard to explain 100's K
  • A: VLA gets NH3 transitions for temperatures. ALMA gives density. VLA gives free-free. No overlap in frequency
  • Q: Is the brick a cloud? Is this really a transient colliding flow?
  • A: I don't claim that it's bound.

---

• Jill Rathborne: ALMA’s view of the initial conditions within a massive protocluster

  

  The Brick / Lima Bean

  • deep extinction feature
  • Tdust 20K
  • MALT90 gas gives different T than dust + HNCO usually traces 70K gas at n~10^6 + 0.3 pc resolution

  ALMA 13pt mosaic

  • 1.7", dv=3.5 km/s, 20 microJy/bm
  • column pdf is fairly lognormal + tiny excess on the tail + tail is only location of active SF
  • lognormal is predicted...
  • will form stars (already forming some)

  Gallery of data

  • rgb slice movies
  • cold absorption filaments + huge aspect ratio & line width + only seen in HCO+ + approximately perpendicular to polarization

  Questions

  • Q Mitch: Any qualtitative feel for the bulk velocity field? Rotation, shear, compression?
  • A: Probably shear.
  • Q: Estimate mach number of shocks?
  • A: yes can but haven't yet.
  • A Steve: M~70
  • Q: Column to volume densities. What range?
  • A: 10^6 cm^-3 from the dust, up to 10^7
  • A: Only one transition of each molecule, haven't done local density estimates
  • Q Lang: Is the absorption filament a continuum feature?
  • A: Doesn't line up with anything in particular.
  • A Betsy: There's a parallel filament.
  • Q Farhad: What is happening at the one site of SF?
  • A: 80 blobs throughout the cloud, sizes ~0.3 pc, masses 10-100 msun. 1 water maser with 85 msun. No YSO correspondence

---

• Steve Longmore: Conversion of Gas into Stars in the Galactic Center

  

  Is SF different in disk?

  • 5-10% of gas in CMZ
  • 5-10% of SF in CMZ
  • Density in CMZ 2 orders of magnitude higher

  Suppressing Mechanisms in the CMZ

  • Stars dominate grav potential + outside "ring", gas is not self-gravitating + Within 100 pc, gas is self-grav
  • Local suppression dominated by turbulence by orders of magnitude
  • Dense gas PDF from Kritsuk + dispersion set by Mach number
  • Threshold for SF in CMZ very high: ~10^8 cm^-3

  Turbulence

  • dissipates in a crossing time + what drives it? + MRI, GI, outflows, Sne all orders of magnitude too low + steady-state gas inflow also too small
  • "acoustic instabilities" large enough + outside inner lindblad resonance, spiral waves driven by grav instabilities
  • Montenegro, Yuan, Elmegreen 1999 model + gas falling into stellar potential + "acoustic instability"

  Quiescent vs Starburst phase

  • "simple idea"

  CMZ in Cosmological Context

  • velocity dispersion vs density? + high redshift clouds and CMZ clouds occupy same parameter space
  • "CMZ is inditinguishable from high-redshift starburst"

  Details of stellar mass assembly

  • Molinari Ring picture: 2 vertical oscillations per orbit
  • map mass density along orbit
  • gas compressed at pericenter
  • PPV structure extraction... + orbital fitting + can't fit Molinari ring + closed orbits not possible: precession + can't fit P-V wiggle + doesn't fit Sgr B2
  • Pericenter passage SPH sim + VERTICAL compression + retrograde, counterrotating gas due to shear
  • dense gas fraction vs orbit... + prediction of density vs orbital position + Plug for Ginsburg et al VLA/GBT H2CO densitometry
  • IMF variation interesting future avenue

  Questions

  • Q: On pc scales, fits with Ken-Schmidt relation
  • A: Depends on geometry. Donut vs pancake geometry.
  • Q: Isn't there gas that is more efficient at forming stars?
  • A: Doesn't fit...
  • Q: KS relation doesn't work for CMZs in other galaxies. How did you measure dispersion for high-Z clouds?
  • A: Some high-z clouds with direct measurements. Otherwise, assume virial equilibrium. Robust no matter how you derive it.
  • Q: Struggling in nearby galaxies, particularly because of outflows.
  • A: best we can do
  • Q: CMZ is similar to high-z objects. GC in pre-starburst phase. So we should have a starburst soon.
  • A: Either gas won't form stars, or will do so in bursty fashion
  • Q: Predicts lots of very dense gas in Galactic center, n~10^8.
  • A: See Jill's talk.
  • Q Wardle: Turbulence damps on vertical crossing time (0.5 Myr). 20km/s linewidth disappear in 0.5 Myr? Do you see dead clouds? Don't travel very far, so should damp.
  • A: [should have answered: it's driven!]

---

• Wednesday Morning talks: G2

  9:00-9:30 am Stefan Gillessen (Germany): Overview Talk Observations of the G2 Cloud at SgrA* (25 + 5 minutes)

  

  Summary of measurements...

  • Mass of BH known "better than your own mass"

  • SINFONI data

  • ALMA cycle 0 data misses G2 b/c of redshift

  • date, location of periapse uncertain by ~months, factor of 2 in radii + radius ~100 AU + optically thin + ionized + 3 earth masses + density 10^5

  • PaA detected! redshifted into visible band

  • PV-diagram evolution with SINFONI

  • Some gas has passed pericenter + pericenter is prolonged - 1 yr

  • no x-ray heating detected yet

  • rotation measure still constant

  • wiki page for project idea: MPE wiki

  • Where does it come from? + wind interaction creating clumps?

    • S91, IRS16SW have winds that could do it

    • infalling jet material?
    • form not at apocenter, but halfway in at high velocity

  • Proplyd? + predicted rising flux; this didn't happen

  • T Tauri star?

  • Star inside? + so far, evidence says no + gas is not presently bound

  • Sgr A* flares nightly

  Questions

  • Q: current velocity inconsistent with current model
  • A: Maybe internal velocity has changed. Some gas on other side. Maybe things have changed now that there is no longer a single source
  • Q Farhard: Estimate of density of sheared gas when on opposite side from the BrG?
  • A: Haven't done that yet. Evolution is mostly radial (line-of-sight)
  • Q Bally: Will the trailing gas continue to fall in? What is it?
  • A: It is there, orbital motion slow, but close to overall BrG emission and confused with ambient. 20 years to infall
  • Q: Are B-fields dynamically important?
  • A: Talk tomorrow - various scenarios, possible that magnetized. Don't think we will see directly.
  • Q: Predicting X-ray flares. Can you determine how much mass infalls?
  • A: Not an easily invertible problem.
  • Q Fred Lo: Infall of cloud to 1000 schwarzchild radii. Can you say anything about interior?
  • A: Can't see anything yet. Should theoretically be "sea" of NS, BH around SMBH. Maybe will intersect?
  • Q Mitch: If star in center, what limits on wind speed?
  • A: First observations were unresolved. Rather low wind speed, 50-100 km/s

  9:30-9:50 am Leo Meyer (USA): Keck Observations of G2 and SgrA (15 + 5 minutes)

  

  Agree on some things:

  • red source
  • evolving FWHM
  • tidal interaction
  • eccentric orbit

  Uncertainties:

  • How much gas is there?
  • compact gas cloud? How survived in the environment?
  • Or is it a star?
  • Will it have observable impact on Sgr A*'s emission

  2013 data

  • doesn't change anything
  • BrG for radial vel and proper motion + bias in L' phot
  • so far purely keplerian orbit + why would a gas cloud follow keplerian?
  • slight changes to orbital parameters, mostly because of reference frame

  G2 tail

  • PV diagram criticism: doesn't show spatial association
  • tail not on the orbit
  • claim that there is unassociated, independent gas

  Blueshifted emission

  • wrong location?
  • Residual of SO2?
  • may not be what is seen in SINFONI data

  G2 not unique

  • ~5 sources clearly seen
  • [but G2 is the only one with high velocity]

  Sgr A* brightness

  • currently as faint as ever seen
  • new "hidden markov" model
  • Sgr A* has only one state

  Questions

  • Q: Can you predict rate of G2-like interactions?
  • A: No, but they don't explain flaring. 1-2 every 20 years?
  • Q: New inclination angle closer to 90. Maybe more extended in radial direction.
  • Q: Spectrum of G1?
  • A: not ready to show yet
  • Q: Caution about position-velocity diagram. If disrupted, shouldn't be keplerian in all pieces.
  • A: it's the slit width in the PV diagram that's a problem
  • Q: State transitions - can you incorporate multiwavelength data?
  • A: "very general" model. Needs to be calibrated for other wavelengths

  9:50-10:10 am Andreas Eckart (Germany): The infrared K-band identification of the DSO/G2 source from VLT and Keck data (15 + 5 minutes)

  

  Proper motions of "dusty sources"

  • K-band detection of G2 cloud
  • prior to 2008, confused with S63

  Possibilities for "DSO" = G2

  • externally heated + expect extended dust emission + not observed
  • internally heated + expect compact K-band (observed) + requires internal heating source: star
  • massive star possible
  • accretion of material?
  • "Tail emission" part of background?
  • interacting disk models suggest tail is part of interacting disks
  • X3, X7 clouds...
  • Where from? + compact clouds in CND + clouds collide -> high eccentricity infall + compression -> star formation + "beads" of stars falling in...

  Questions

  • Q: Extension of 0.2" in BrG. 20mas from Gillessen?
  • A: Extended emission is from the tail
  • Q: Movie - how possible to form stars at the close approach because of the short timescale of SF
  • A: Orbital compression
  • Q: Compressing gas - does that inhibit other SF? Prevent continuous SF?
  • A: Short event... this model ongoing sf at lower level.... IRS13N young objects
  • Q: Jupiter-like object?
  • A: couple Lsun - 20-30 Lsun, points towards more massive object than a Jupiter
  • Q Bally: Gas densities in simulation?
  • A: intial density of clump n~10^5. Look at poster
  • Q Ghez comment: analysis of images, not real data...
  • A: Data downloaded from Keck archive
  • Ghez: see anna bailey's poster, which shows technique for getting limits. We don't see G2.
  • A: We see 19th magnitude "DSO"

  10:10-10:30 am Darryl Haggard (USA): G2 as a Probe of Sgr A*'s Hot Environs: A View from Chandra and JVLA (15 + 5 minutes)

  

  Predicted possible x-ray emission

  • multi-PI Chandra campaign
  • X-ray view of CMZ: magnetar next to Sgr A* + confusion, overlapping PSFs + multi-hit issues (pile-up)
  • no hints of G2 in X-ray
  • one crazy bright flare
  • magnetar can be used to probe LOS to Sgr A*
  • simultaneous Chandra/VLA

  Questions

  • Q: VLA obs over past year. No evidence for flux density of Sgr A*. No bow shock emission.
  • Q: Column density estimates. Associated with flare? Associated with Sgr A*? LOS?
  • A: Conflation
  • Q: Can you explain emission with single age (?)
  • A: Separate fits are statistically equivalent

  10:30-11:00 am Coffee Break

  11:00-11:25 am Gunther Witzel (USA): IR observations and variability of SgrA* (20 + 5 minutes)

  NIR var Sgr A*

  • NIR spectral index...
  • Power law distribution of brightness...
  • time behavior, structure function of spectral indices
  • Break in PSD + 150, 500 minute break points
  • simultaneous observations in L and H band + time-resolved measurements of spectral index + Very constant spectral index across all simultaneous measurements

  Questions

  • Q: Can you combine VLT and Keck data to extend a night?
  • A: I've tried that....
  • Q: Red curve below black, black below red on some plots. What's happening to the spectral index?
  • A: spectral index variations are significant. Small compared to... other publications
  • C: Amazing how constant spectral index is

  11:25-11:45 pm Banafsheh Shahzamanian (Germany): NIR polarized observations of Sagittarius A* (15 + 5 minutes)

  NACO polarization measurements

  • Sgr A* highly polarized
  • Polarized flux density has lognormal + power-law distr.
  • polarization and total flux density (weakly) anticorrelated + adding noise flattens correlation + confirms real... >3-sigma detection
  • theory: depolarization at high flux comes from relativistic boosting

  Questions

  • Q: How does anticorrelation fit with the explanation of high-flux -> high pol?

  • A: Highly polarized flux is from bright flares at high total flux densities

    [I didn't understand]

  • Q Fred Lo: Any idea what radius flares are coming from?
  • A: 6 Schwarzchild radii in the model
  • Q: What constraints?
  • A: (from audience) variability timescale
  • Q: Is there a preferred vector position angle?
  • A: ...?
  • Q Sacha: Anticorrelation between "degree of correlation" and "flux" translates between polarized flux and total emission. We have interpreted as different components of an outflow, bright and high degree of polarization and faint with low degree of poln. Is this consistent?
  • Q: 40-50% of time, flux is noise-dominated. Could there be noise-dominated state bias?
  • A: We have tested by adding noise
  • Q: Would individual flares track up and down the anticorrelation? Or is it only for large # of flares?
  • A: only for composite.
  • Q Mitch: What kind of correlation do you see related to position angle?
  • A: Not included in our analysis

  11:45-12:05 pm Kazunori Akiyama (Japan): Long-term monitoring of Sgr A* at 43GHz with VERA and KVN+VERA (15 + 5 minutes)

  Sgr A* with VLBI

  • larger at lower frequencies
  • variability correlated with spectral index
  • single component for 3 years + size not correlated with radio flux
  • flares come from component emerging from optically thick region?
  • Flares with G2? + maybe detected bowshock?
  • VERA shows consistent major axis size
  • KVN + VERA: comparable to VLBA

  Questions

  • Q: Variation of intrinsic size with flux... difficult measurement to make. VLA+ALMA observations of radio flares requires relativistic...
  • Q: HET hasn't passed 10% of material (??)
  • Q: Size doesn't increase with flux. But that's interstellar scattering size. Could intrinsic size have changed by 10-15% without being detected?
  • A: ???
  • Q Fred Lo: intrinsic size measured. Any elongation/ellipticity?
  • A: Just a 1-dimensional measurement

  12:05-12:25 pm Salomé Dibi (Netherlands): Exploring plasma behavior around Sgr A*

  Sgr A* in context...

  • only flaring AGN
  • discussion of particle distrbution related to SED & state
  • accretion flow injecting electrons into plasma
  • nonthermal synchrotron with cooling break is best distribution

  Questions

  • Q Broderick: Should be able to produce both observed features of flares...
  • A: Tried to reproduce spectral index. Flaring in infrared could provide strong constraints
  • Q: Spectral index should evolve in your model, but we heard that it is not changing
  • C: Radio spectrum has gone even lower... falls on power law, useful for modeling. Makes deviation even more severe.
  • C: spectral index measurements - flares measured with spectral indices don't have X-ray counterparts. Maybe X-ray bright flares may have non-constant spectral indices.
  • C: Fit within 2 R_G, so radio is not relevant (optically thick)
  • Q Ostriker: 1-zone model. What is the geometry?
  • Event close to BH.

  12:25-12:45 pm Jim Moran (USA): History of High Resolution Imaging of SgrA* (15+5 min)

  History

  • 1951 Cygnus A: resolved double
  • looking for compact structure in HII regions with 35km Green Bank interferometer
  • simple observation -> 0.1" size, 10^7K T_B
  • OVRO group found HII regions, but searching for BH, Balick & Brown searching for HII and found SMBH + Fred Lo 3.7cm found Sgr A*, T_B ~ 10^8K, <0.02"
  • Jodrell Bank -> size ~ freq^-2
  • Fish et al: early 1.3mm VLBI + consistent with Gaussian or annulus
  • annular radii not at all constrained by short baselines

  Questions

  • C Fred Lo: Explanation of series of events. Obstacle for detecting structure of Sgr A* is scattering.
  • If you can characterize the Gaussian interstellar scattering, you could in principle deconvolve
  • C Farhad: Variability of Sgr A* is a problem. 230 GHz has high, hourly variability
  • Goal should be testing models on 10min timescales

  Discussion Section

  • We agree on some things: + source is infalling + L-band source + Br-G detected
  • Controversy:
    • K-band detection is controversial. Can we sort that out? If it's there, will it change interpretation a lot?
  • K-band source would be compatible with hot dust. H-band is the real discriminator
  • What is the source we detect? There are K-band sources, but their natures are unclear
  • Extinction makes it extra hard
  • Ghez: What is G2? Very similar to S-star orbits. Orbital characteristics of binary exchange. Perhaps just a close binary, soon to form an S-star? Tidally disrupt atmosphere, but be a star underneath
    • Problem: star not that big. If it's a star, very cool star dominated by dust, difficult to detect.
  • Closest approach is no closer than S-stars + orbit also matches disk
  • Dusty, looks like an S-star + No, it is a GAS source, not a dust source. Gas is directly detected
  • As it travels by Sgr A*, do I care if it has anything inside?
  • Ghez: forces us to consider role of binaries at cmz
  • Comment: wait and see what happens + What if nothing happens? Will that resolve the issue?
  • Are we just seeing weather in Sgr A*? + eh, maybe?
  • Are we agreeing on emission mechanism?
    • continuously variable source. No quiescent vs flaring state in IR; more prominent in X-ray

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