# Presentation: 663702

## Radiative Feedback in the Formation of the First ProtogalaxiesJarrett L. Johnson 1            Thomas H. Greif 1,2            Volker Bromm 11 Department of Astronomy, University of Texas at Austin                      2 Institut für Theoretische Astrophysik, Universität Heidelberg

- Radiative Feedback in the Formation of the First ProtogalaxiesJarrett L. Johnson 1            Thomas H. Greif 1,2            Volker Bromm 1**1 ****Department of Astronomy, University of Texas at Austin****                      2 ****Institut f****ü****r Theoretische Astrophysik, Universit****ä****t Heidelberg**

**References**
- Johnson J. L., Greif T. H., Bromm V. 2007, ApJ, in press
-       (astro-ph/0612254)
- Navrátil, P. A., Johnson, J. L., Bromm, V. 2007, IEEE
-       Visualization*, *conditionally accepted
- Springel V., Yoshida N., White S. D. M. 2001, New Astro, 6, 79

**Jarrett L. Johnson**
**Doctoral Candidate**
**University of Texas at Austin**

- _[jljohnson@astro.as.utexas.edu](mailto:jljohnson@astro.as.utexas.edu)_
**(512) 471-3447**

**Summary**
- We follow the thermal, chemical, and dynamical evolution of the primordial gas as it becomes incorporated into a protogalaxy. While the intergalactic medium is, in general, optically thick to Lyman-Werner photons only over physical distances of  tens of kiloparsecs, the high molecule fraction that is built up in relic H II regions and their increasing volume-filling factor render even the local IGM optically thick to LW photons over distances of a few kiloparsecs.  Population II.5 stars, postulated to have masses of the order of 10 solar masses, can likely form from re-collapsing relic H II region gas. We find that the local radiative feedback from Population III stars suppresses the star formation rate only slightly.

**Introduction**** **
- The first galaxies form under the influence of the radiative feedback from the first generations of stars.  This feedback acts to heat and ionize the gas within the H II regions surrounding the first stars, as well as to photodissociate hydrogen molecules within the larger Lyman-Werner bubbles that surround these sources.  However, because the first stars (Pop III), believed to have had masses of the order of 100 MSun, lived for only ~ 3 Myr, the local radiative feedback from these stars is intermittent.  Thus, atoms and molecules are often able to reform, while molecule formation is strongly promoted within the prevalent relic H II regions that the first stars leave behind.  We investigate the complex interaction between the radiation emitted by the first stars and the processes that characterize subsequent star formation.

**Methodology **
- We employ the parallel version of the smoothed particle hydrodynamics (SPH) code GADGET (Springel et al. 2001) to carry out three-dimensional cosmological simulations, in which we self-consistently track the formation of, and radiative feedback from, individual stars in the formation of a protogalaxy.
- Our simulation evolves both the dark matter and baryonic components, initialized according to the CDM model at     *z* = 100.  We adopt cosmological parameters close to those measured by *WMAP* in its first year.  We use a periodic box with a comoving size *L* = 460 kpc *h*-1, with a number of particles *N*SPH = *N*DM = 1283.  As in our previous work, we include an extensive chemical network for deuterium, along with the standard hydrogen and helium chemistry, allowing us to accurately account for the important coolant HD.
- We follow the evolution of the primordial gas until the gas collapses to our resolution limit of *n*res ~ 20 cm-3.  At this point we photoheat the high density gas, from a point source, using rates computed for a 100 MSun primordial star. This allows us to capture the correct density profile for the gas within the minihalo hosting the star.  Using a ray-tracing algorithm, we then compute in detail the H II regions of the star, as well as the regions affected by its molecule-dissociating radiation.  With the thermal and chemical state of the gas in these regions adjusted accordingly, we continue the simulation, stopping to carry out this routine for each of the first eight stars that form in our box.

**Results**
- The high molecule fraction that is built up in relic H II regions and their increasing volume-filling factor render even the local IGM optically thick to LW photons over physical distances of a few kiloparsecs.
- Pop III relic black holes may begin accreting                        efficiently after ~ 60 Myr from the time of their formation, when the photo-heated relic H~II region gas can cool and re-collapse into the 106 MSun minihalo which hosts the black hole.
- Pop II.5 stars, postulated to have masses of the order of 10 MSun , can likely form from re-collapsing relic H II region gas.
- Local radiative feedback from Pop~III stars suppresses the star formation rate only slightly.
- The high optical depth to LW photons through relic H II regions suggests that the first stars may have produced only a weak H2-dissociating background radiation field.

- First H II region and Lyman-Werner (LW) bubble, at redshift       *z* ~ 23.  Clockwise from the top left are the temperature, electron fraction, H2 fraction, and density, plotted in projection.  The H II region extends out to ~ 4 kpc in radius from the central Pop III star, while the LW bubble extends to ~ 5 kpc, within which the molecule fraction is substantially suppressed.  In each panel, the lighter shades signify higher values of the quantity plotted.**   **** **
** ****  **

- Properties of the primordial gas at redshift *z* ~ 18, at the end of the life of the eighth star in our simulation.  The SPH gas particles which have experienced an ionized phase within an H II region are colored in orange, while those that have not are colored in black.  Clockwise from top left are the temperature, free electron fraction, HD fraction, and H2 fraction plotted as functions of gas density.  The relic H II region gas cools largely by adiabatic expansion but, importantly, also by cooling facilitated by the high abundance of H2 and HD molecules, which arises owing to the high electron fraction in this gas.  ** **

- Optical depth to LW photons averaged over two volumes in our box, as a function of redshift *z*.  The diamonds denote the optical depth averaged over the entire cosmological box of comoving length 460 kpc *h*-1, while the plus signs denote the optical depth averaged only over a cube centered on the middle of the box and containing the inner comoving 153 kpc *h*-1 of the box.  The solid line denotes the optical depth to LW photons, averaged over the whole box, that would be expected for the case in which the H2 fraction does not change from the primordial value of          2 x 10-6; for this case, the optical depth changes owing only to cosmic expansion.

- The chemical interplay in relic H II regions.  While all molecules are destroyed in and around active H II regions, within the relic H II regions of extinct Population III stars, the high residual electron fraction catalyzes the formation of an abundance of H2 and HD molecules.  Here the light and dark shades of blue denote regions where the free electron fraction is 5 x 10-3 and 5 x 10-4, respectively, while from lightest to darkest, the shades of green denote regions where the H2 fraction is 10-4, 10-5, and 3 x 10-6.  Note that the regions where the molecule fraction is highest lie within relic H II regions, which thus play an important role in subsequent star formation, allowing molecules to become shielded from photodissociating radiation and altering the cooling properties of the primordial gas.