HRI2010 A radio astronomer’s perspective Ravi Subrahmanyan RRI, Bangalore

Reionization I understand that most of the speakers mean hydrogen reionization when we talk of ‘reionization’; unless we explicitly talk of helium reionization. And most of the speakers (except one?) agree that reionization era ends at z ≈ 6. And extended reionization is preferred today. And so the direct probes of reionization are the CMB – which propagates to us through the reionization era – and 21-cm that either adds to or decrements the CMB. As well as objects – GRBs, QSOs, galaxies of various types – that are detected by many methods embedded in the reionization era. GRBs are the most intense high-redshift backlights that will most likely lead the absorption studies towards higher and higher redshifts.

Post–reionization probes of reionization And we may also constrain the sources and emissivity and state of the gas in the reionization era via population studies of objects, remnants, metallicity, temperatures and ionization state of the IGM at redshifts z < 6 in the ‘post reionization’ era. Sometimes by comparing their properties – luminosity functions and ionization state, for example – across z = 6.

Is there an astronomer who is not probing reionization? Almost anything seems to be a probe of reionization: I heard mentions of the Milky Way and the CMB and primordial black holes! CMB TT, TE and EE spectra Exotic physics in the ‘dark ages’ Redshifted 21-cm radiation GRBs, Lyman Alpha Emittors (LAEs), High redshift galaxies QSO absorption systems Proximity effects HI in nearby galaxies Metal poor stars in the Milky Way

To some astronomy reionization could be a ‘contaminant’! CO from star formation in the reionization epoch may also be an additive contaminant for something beyond: the CMB.

Processes of importance Radiative transfer Magnetic fields Particle cascades Cooling Structure formation Black hole evaporation Cosmic ray transport Something that seemed to me to be missing in this conference was AGNs!

Do we have a broad convergence towards a ‘standard’ reionization model? Do we have a sufficient, minimalist, most-likely model that is basically sufficient to satisfy current constraints? Hierarchical galaxy formation in DM halos – ionizing radiation from the low mass dwarf galaxies at z ~ 15 – building up the ionizing background gradually to complete 50% ionization of the hydrogen gas by z ~ 10 – star-formation in progressively higher mass galaxies – complete reionization by z=6. Thereafter the galaxy build up continues smoothly to match the galaxy evolution observations at lower redshifts. No significant role for Pop IIIs, quasars, AGNs or anything exotic! From my perspective: a rather boring sequence of events!

Observations primarily motivated by unnecessary ‘exotic’ models? If the most likely evolution in the gas is rather slow and reionization is rather extended and the most likely sources of ionization – very low-mass star-forming galaxies in high redshift halos - are rather difficult to detect: Would observations be primarily aiming to exclude unlikely parameter spaces?

No role for AGNs? I did not hear any discussion on the constraints on reionization from the inevitable need to form seeds for the ubiquitous black holes in galaxy centres. Or the inevitable growth in black holes during the reionization era. No mention of ‘inevitable’ AGN feedback – either radiative or kinetic – during the reionization era. Did we miss inviting somebody to this meeting?

Conference Summary: an astronomer’s perspective When Andrea Ferrara lets Donald Rumsfeld take centre stage and have the last word: all is well with the world!

Observational probes of the ‘dark ages’ Fairly exotic sources, physics, cosmology and structure formation were shown to lead to heating and ionization in the dark ages. This leads to a variety of fairly recognizable deviations in the HI spin temperature and in the emission/absorption signatures of redshifted 21 cm from z > 20. The observations that may constrain models with decaying DM, evaporating black holes, isocurvature perturbations, varying fundamental constants and so on may be very low frequency all-sky EoR spectral measurements! Constaints on exotic physics may not require an array on the far side of the Moon!

Observational probes of the ‘twilight zone’ CMB serves as a backlight that probes reionization as it propagates through the reionization era: we might hope to coarsely resolve the redshift evolution in the ionization fraction via the shape of the polarization power spectra: we do hope to derive course resolution band powers from PLANCK. Evolution of the 21-cm power spectrum over the reionization era with LOFAR/MWA/GMRT: ought to tell us the evolution of the ionization fraction. And if the signal-to-noise is good this may also give us a clue to the nature of the ionizing sources. Global EoR spectrum: a probe of the ionization fraction and spin temperature evolution during reionization. GRBs, QSOs, galaxies at z > 6 Will be interesting to see which of these makes the first firm step into the twilight zone! My bet is that it will be PLANCK!

House full! We have seen some fantastic movies this week! Simulations including detailed physics – self regulated reionization, halo bias, cooling by multiple species, stellar evolution – leading to high resolution images of 21-cm spectral line cubes. We have also seen a great deal of successful semi-analytic work in the last few days that did not lead to movies. As an observational astronomer: I see the need for “movies and those who make movies!” Movies do serve as ‘noise’ and ‘error’ free templates for development of ‘matched’ filters that may be ‘tuned’ or ‘trained’ to recognize the signal buried in the ‘noise’. They may be a key to the detection of the EoR. And to aid in the design of the next generation arrays for EoR.

21-cm probe of reionization Will the present generation efforts –LOFAR/MWA/GMRT– will have a firm detection of the 21-cm power spectrum (at z > 6): it was interesting to hear fairly different views from different speakers. Have some of us missed hearing Rumsfeld? Perhaps we do have the raw sensitivity today to make a detection in reasonable time. This is certainly true for the ‘global’ EoR signature. However, we do have to contend with interference and calibration and systematics that may not easily be ‘subtracted’ or ‘marginalized’. But it is great that we do have optimistic members in our community who will in the worst of circumstances make the most of what is possible. I heard the statement more than once this week: “We do our best!”

21-cm power Will the first detection of 21-cm power be a simple measurement of rms fluctuations in image intensities versus redshift? This may have a characteristic profile that might serve as a probe of reionization history. Very much easier to get a clean narrow frequency channel by continuum subtraction using a number of adjacent frequency channels. Such continuum subtraction was demonstrated rather successfully by Bebbington and John Baldwin way back in 1986 using the 6C telescope at 151 MHz (z=8.4)! Will we have this measurement from LOFAR or MWA or GMRT before PLANCK?

High-z bump in UV emissivity? Arguments for an extended reionization era dominated by Pop II star formation in low mass dwarfs. And arguments for a relatively shorter reionization era that has a high-z ‘bump’ in UV emissivity. Any distinct bump usually signifies a new population: motivating observations to identify the new population – Pop III? Which might have a non standard and distinctive 21-cm power spectrum that may be a good motivation for LOFAR/MWA/GMRT.

Correlations & Stacking Cross correlations proposed between 21-cm and QSO absorption spectra; between 21-cm and CMB: excellent ideas! Stacking of 21-cm spectra, of QSO absorption spectra: great ideas and great results! Cross correlations proposed between 21-cm visibilities and 21- cm visibilities: are we forgetting Rumsfeld? Errors localized in real space are best dealt with in real space. Errors localized in Fourier space are best dealt with in Fourier space. Experiments and observations need control samples!

Out of the box! Hyperfine transition of the singly ionized 3 HeII may well be the probe of reionization: Yes, it is 1000 times fainter in brightness temperature compared to 21-cm But it redshifts to where the extragalactic background is lowest – at about 1 GHz – where it is 100 x lower. And we may use telescopes 10 x smaller in physical size: a hundred SHAs instead of a SKA? Caveat (there must be a catch somewhere!): confusion from Galactic and low redshift 21 cm.

Thank you – Jasjeet, Tirth, LOC – for having made this meeting possible and taking care of us all through the week.