Q&A Flashcards
(17 cards)
What are SMBHs?
Almost every massive galaxy contain a SMBH at its center. Their masses range from hundreds of thousands to billions of times the mass of the Sun.
How do we detect SMBHs?
Measure the acceleration of a star in orbit around an unseen object.
What does Lambda-CDM mean?
Lambda is the dark energy, CDM stands for cold dark matter: non-baryonic, low-velocity, dissipation- and collisionless.
Are there other feedback mechanisms at work in NGC 6240, other than from AGNs?
Yes, there is also star-formation feedback. Distinguishing the two is often difficult, which is why this mode has yet to be fully proven observationally.
What’s the difference between the H2 1-0 S(1) line and the H2 0-0 S(8) line?
The S(1) line is a transition where both the rotational and vibrational energy changes. It requires temperatures of around 2000 K. The S(8) transition changes only the rotational energy. Its upper energy level is much higher than S(1), and require temperatures of 3-4000 K for significant population. The S(1) line traces shocks, fluorescence, or X-ray heated regions, while the S(8) line traces shocks or more extreme regions.
What’s the difference between the two Paschen lines?
They are both part of the Paschen series of the hydrogen recombination lines. They occur when free electerns recombine with protons and cascade down energy levels. The Palpha is the transition from n=4 to 3, and the Pbeta is from n=5 to 3. They both trace ionized gas and can be used to estimate star formation.
What’s the difference between the two [Fe II] lines?
They both trace partially ionized gas, often excited by shocks or X-rays. They originate from the same upper energy level which makes them usable for extinction diagnostics.
Why is the 1.7 micron band more reliable than the others?
The 1.0 micron band is quite noisy, but still has enough stellar features for pPXF to work with. This is not the case for the 2.9 micron band, where there are not enough stellar features present to obtain reliable results with pPXF.
How do you decide when 2 vs 3 Gaussians are needed for the emission line fit?
The minimization function is Powells method, a conjugate direction method, that favors initial conditions fairly close to the solution. This means that initializing the amplitude of the rotating component high, and one of the non-rotating amplitudes to zero, will favor zero amplitude rather than decreasing the rotating amplitude too much.
Why do you apply the astrometry corrections?
The VLBI is more precise than JWST. Also, the radio cores identified with VLBI correspond directly to the SMBHs.
Doesn’t gas-rich mergers trigger inflows toward the central BHs?
Yes, they do, but they occur on smaller scales and often in cooler, denser gas. My dataset and wavelength range is more sensitive to hot shock-excited outflows than cold inflow structures.
Do you find evidence of inflows?
No. Most of the kinematic signatures, such as broad line components, velocity shifts, and spatial extension are more consistent with outflowing gas. If inflows are present, it’s either too weak, obscured, or kinematically indistinguishable from turbulence or rotation.
Why do you use the C I ALMA data to compare your maps?
Because they have higher angular resolution than the CO data from Cicone et al. (2018), so they are easier to compare to the JWST data
Is the assumption that stars are tracing virial/rotation valid?
Yes. The stellar velocity field shows a clear rotation pattern across both nuclei. Engel et al. (2010) found that less than 10 % of the stellar population contributes to a high-dispersion component, suggesting most of the stars remain dynamically relaxed and rotation-supported, even during the merger.
Is the assumption that there is some gas coupled with the stars reasonable?
Yes — for three key reasons:
- The progenitor galaxies were gas-rich spirals, so it’s reasonable that some circumnuclear molecular gas survives in rotating structures.
- ALMA Band 7 continuum and CO(2–1) observations (Medling et al. 2019) show molecular gas concentrated within 40 pc of each AGN—overlapping the stellar rotation field.
- Therefore, assuming some gas follows the stars allows us to separate rotationally supported gas from kinematically distinct (e.g., outflowing) gas in the fits.
Explain collisional excitation of H2
Caused by fast-moving gas compressing and heating molecular clouds.
Produces thermalized populations of rotational and vibrational levels.
Expected in AGN-driven outflows, supernova shocks, and merger turbulence.
Explain fluorescence
Fluorescence or UV pumping is caused by far-UV photons that excite H2 to electronic states, which decay into vibrational levels. It dominates in photodissociation regions (PDRs).
