Jerusalem WS lecture notes: 06. The First Galaxies and Reionisation

By A. Loeb, with a short, but brilliant skit on adventure and creativity in science, slides uploaded here.

• spectrum of the sky:
• dip: Ly\alpha photons keep the gas coupled to its kinetic temperature
• most of the comoving volume of the Universe is at high redshifts:
• SDSS LRGs: 0.001 of the observable volume of the Universe (Loeb &Wyithe 2008)
• What is the best time to be a cosmologist? At early time, Hubble scale grows, but due to expansion the comoving distance starts to shrink: objects enter and exit the horizon. The optimal cosmic epoch for precision cosmology was at z = 10 (http://arxiv.org/abs/1203.2622)
• http://prl.aps.org/abstract/PRL/v106/i17/e171302
• LF at various redshifts -- DM haloes mass slope
• H, He cooling curve: no excitation at lower energies (T < 10^4), H_2 becomes important at lower T
• Pop. 3.1 stars -- the very first stars, ~10^8 Myr after BB, SF conditions were differented, higher T: ~200K
• Pop 3.2 stars: UV photons ionize surrounding gas --> much more efficient cooling by forming H_2, HD, HD radiative cooling --> gravitational instabilities
• galaxies much more clustered on scales ~100 comoving Mpc than at present -- not many regions exceed the collapse threshold: bias, numerical bottleneck (need large dynamic range)
• at higher redshifts (~8) you need to produce more photons per baryons to keep the IGM reionized
• Pop3 observables:
  • stellar remnants:
  • Pair instability supernovae: best-understood SNe, Cooke et al. 2012. Lasts for years: long duration, luminosity comparable to SN II. 160-240 M_sol progenitors: if you start with a few hundredths M_sol progenitor, winds don't reduce the mass enough to avoid the PI SN. Collisionally-formed stars.
  • Long GRBs: seen to the edge of the obs. Universe, jets are drilling a hole in the star, followed by SN. Cosmological stretch of time: longer observation time
• Ly\alpha forest: probing ISM by QSO spectra
• Spin transition: Harward connection: horn antenna, snowballs, spin temperature, 21 cm tomography, cosmological evolution of the 21cm signal, X-ray heating (stellar mass BHs? X-ray binaries? AGNs?), LOS anisotropy: Kaiser effects, Pritchard & Loeb 2011, 'CMB is old', EDGES experiment: reionisation was not abrupt, EOR signal
• extension to other lines: emission (CO, C, C II) by groups of galaxies: unresolved galaxy survey

Reionisation, healing masked arrays

This morning Benedetta Ciarli (MPA) gave an interesting talk about cosmic reionisation, modelling they're doing and observations with LOFAR, where they might glimpse the 21 cm line forest.
Two take-home messages were:

• Helium fraction must be included in reionisation simulations
• _If_ (apparently, it's a big if) LOFAR finds a high-redshift radio-loud quasar, they might see the absorbtion features along the line of sight, and obtain better constraints on reionisation parameters.

Later on I was solving a problem that preoccupies me these weeks ahead of the thesis comittee meeting: automated photometry of CALIFA (actually, any SDSS) galaxies, using growth curve analysis for flux extraction. The field stars and other irrelevant objects were already masked by N., but the holes left in the image arrays have to be interpolated, and that's not a trivial task, apparently. Python's interp2d fails on arrays this big (standard SDSS frames, I won't do manual cutouts), besides, I'm not sure it is suitable for this goal, as well as all similar python/numpy routines.

My current approach is finding contiguous bad (=masked) regions in the image array, forming smaller sub-arrays and filling the masked regions within them with the mean of a circular aperture around. That's obviously a naive approach to take -- once I get it running, I'll be able to do more realistic interpolation, e.g. using the galaxy's Sersic index.

It was an interesting detour through numpy array masking, motion detection, image processing and acquainting myself with the slice object. I kind of enjoy diving into those gory details instead of using some nice high level routine -- I'm reinventing the wheel, obviously, but that's more scientific than using black box methods.

Finding contiguous regions within an array, apparently, is not that trivial as I first expected, but the solution I used was really helpful -- stackoverflow rocks as usual.