Evan Scannapieco

The Type IA supernova rate

We explore the idea that the Type Ia supernova (SN Ia) rate consists of two components: a prompt piece that is proportional to the star formation rate (SFR), and an extended piece that is proportional to the total stellar mass. We fit the parameters of this model to the local observations by Mannucci and collaborators and then study its impact on three important problems. On cosmic scales, the model reproduces the observed SN Ia rate density below z = 1 and predicts that it will track the measured SFR density at higher redshift, reaching a value of (1-3.5) × 10-4 yr-1 Mpc-3 at z = 2. In galaxy clusters, a large prompt contribution helps explain the Fe content of the intracluster medium. Within the Galaxy, the model reproduces the observed stellar [O/Fe] abundance ratios if we allow a short (≈0.7 Gyr) delay in the prompt component. Ongoing medium-z SN surveys will yield more accurate parameters for our model.

Quasar Feedback: The Missing Link in Structure Formation

We consider the impact of quasar outflows on structure formation. Such outflows are potentially more important than galactic winds, which appear insufficient to produce the level of preheating inferred from X-ray observations of galaxy clusters. At late times, energetic material from the densest objects in the centers of galaxies makes its way into the intergalactic medium (IGM), impacting structures on many scales, much as supernovae impact structures on many scales within the interstellar medium. Using a simple analytical model for the distribution of quasars with redshift, coupled with a one-dimensional Sedov-Taylor model for outflows, we are able to make robust statements about these interactions. As large regions of the IGM are heated above a critical entropy of Scrit ≈ 100 keV cm2, cooling becomes impossible within them, regardless of changes in density. On quasar scales, this has the effect of inhibiting further formation, resulting in their observed falloff in number densities below z ≈ 2. On galaxy scales, quasar feedback fixes the turnover scale in the galaxy luminosity function (L*) as the nonlinear scale at the redshift of strong feedback. The galaxy luminosity function then remains largely fixed after this epoch, consistent with recent observations and in contrast to the strong evolution predicted in more standard galaxy-formation models. Finally, strong quasar feedback explains why the intracluster medium is observed to have been preheated to entropy levels just above Scrit, the minimum excess that would not have been erased by cooling. The presence of such outflows is completely consistent with the observed properties of the Lyα forest at z ~ 2 but is expected to have a substantial and detectable impact on Compton distortions observed in the microwave background and the multiphase properties of the warm-hot (z = 0) circumgalactic medium.

THE DETECTABILITY OF THE FIRST STARS AND THEIR CLUSTER ENRICHMENT SIGNATURES

We conduct a comprehensive investigation of the detectability of the first stars and their enrichment signatures in galaxy clusters. As the initial mass function (IMF) of these Population III stars is unknown and likely to be biased to high masses, we base our study on analytical models that parameterize these uncertainties and allow us to make general statements. We show that the mean metallicity of outflows from Population III objects containing these stars is well above the critical transition metallicity (Zcr ~ 10-4 Z☉) that marks the formation of normal stars. Thus, the fraction of Population III objects formed as a function of redshift is heavily dependent on the distribution of metals and fairly independent of the mean metallicity of the universe, or the precise value of Zcr. Using an analytic model of inhomogeneous structure formation, we study the evolution of Population III objects as a function of the star formation efficiency, IMF, and efficiency of outflow generation. For all models, Population III objects tend to be in the 106.5-107.0 M☉ mass range, just large enough to cool within a Hubble time, but small enough that they are not clustered near areas of previous star formation. Although the mean metallicity exceeds Zcr at z ~ 15 in all models, the peak of Population III star formation occurs at z ~ 10, and such stars continue to form well into the observable range. We discuss the observational properties of these objects, some of which may have already been detected in ongoing surveys of high-redshift Lyα emitters. Finally, we combine our Population III distributions with the yield models of Heger & Woosley to study their impact on the intracluster medium (ICM) in galaxy clusters. We find that Population III stars can contribute no more than 20% of the iron observed in the ICM, but if they form with characteristic masses ~200-260 M☉, their peculiar elemental yields help to reconcile theoretical models with the observed Fe and Si/Fe abundances. However, these stars tend to overproduce S/Fe and can account only for the O/Fe ratio in the inner regions of poor clusters. Additionally, the associated supernova heating falls far short of the observed level of ~1 keV per ICM gas particle. Thus, the properties of the first objects may be best determined by direct observation.

Early Enrichment of the Intergalactic Medium and Its Feedback on Galaxy Formation

Supernova-driven outflows from early galaxies may have had a large impact on the kinetic and chemical properties of the intergalactic medium (IGM). We use three-dimensional Monte Carlo cosmological realizations of a simple linear peaks model to track the time evolution of such metal-enriched outflows and their feedback on galaxy formation. We find that at most 30% of the IGM by volume is enriched to values above 10-3 Z☉ in models that include only objects that cool by atomic transitions. The majority of enrichment occurs relatively early (5 z 12) and leads to a mass-averaged cosmological metallicity between 10-3 and 10-1.5 Z☉. The inclusion of Population III objects that cool through H2 line emission has only a minor impact on these results: increasing the mean metallicity and filling factor by at most a factor of 1.4 and moving the dawn of the enrichment epoch to z ≈ 14 at the earliest. Thus, enrichment by outflowing galaxies is likely to have been incomplete and inhomogeneous, biased to the areas near the starbursting galaxies themselves. Models with a 10% star formation efficiency can satisfactorily reproduce the nearly constant (2 ≤ z ≤ 5, Z ≈ 3.5 × 10-4 Z☉) metallicity of the low column density Lyα forest derived by Songaila in 2001, an effect of the decreasing efficiency of metal loss from larger galaxies. Finally, we show that IGM enrichment is intimately tied to the ram-pressure stripping of baryons from neighboring perturbations. This results in the suppression of at least 20% of the dwarf galaxies in the mass range ~3 × 108-3 × 109 M☉ in all models with filling factors greater than 2% and an overall suppression of ~50% of dwarf galaxies in the most observationally favored model.

THERMONUCLEAR .Ia SUPERNOVAE FROM HELIUM SHELL DETONATIONS: EXPLOSION MODELS AND OBSERVABLES

During the early evolution of an AM CVn system, helium is accreted onto the surface of a white dwarf under conditions suitable for unstable thermonuclear ignition. The turbulent motions induced by the convective burning phase in the He envelope become strong enough to influence the propagation of burning fronts and may result in the onset of a detonation. Such an outcome would yield radioactive isotopes and a faint rapidly rising thermonuclear “.Ia” supernova. In this paper, we present hydrodynamic explosion models and observable outcomes of these He shell detonations for a range of initial core and envelope masses. The peak UVOIR bolometric luminosities range by a factor of 10 (from 5×10 41 −5×10 42 erg s −1 ), and the R-band peak varies from MR,peak = −15 to −18. The rise times in all bands are very rapid (< 10 d), but the decline rate is slower in the red than the blue due to a secondary near-IR brightening. The nucleosynthesis primarily yields heavy α-chain elements ( 40 Ca through 56 Ni) and unburnt He. Thus, the spectra around peak light lack signs of intermediate mass elements and are dominated by Ca ii and Ti ii features, with the caveat that our radiative transfer code does not include the non-thermal effects necessary to produce He features. Subject headings: binaries: close— novae, cataclysmic variables— nuclear reactions, nucleosynthesis, abundances— supernovae: general— white dwarfs

The detectability of pair-production supernovae at z ≲ 6

Nonrotating, zero-metallicity stars with initial masses 140 M 260 M☉ are expected to end their lives as pair-production supernovae (PPSNe), in which an electron-positron pair-production instability triggers explosive nuclear burning. Interest in such stars has been rekindled by recent theoretical studies that suggest primordial molecular clouds preferentially form stars with these masses. Since metal enrichment is a local process, the resulting PPSNe could occur over a broad range of redshifts, in pockets of metal-free gas. Using the implicit hydrodynamics code KEPLER, we have calculated a set of PPSN light curves that addresses the theoretical uncertainties and allows us to assess observational strategies for finding these objects at intermediate redshifts. The peak luminosities of typical PPSNe are only slightly greater than those of Type Ia, but they remain bright much longer (~1 yr) and have hydrogen lines. Ongoing supernova searches may soon be able to limit the contribution of these very massive stars to 1% of the total star formation rate density out to z ≈ 2, which already provides useful constraints for theoretical models. The planned Joint Dark Energy Mission satellite will be able to extend these limits out to z ≈ 6.

Measurement of a Peak in the Cosmic Microwave Background Power Spectrum from the North American Test Flight of Boomerang

We describe a measurement of the angular power spectrum of anisotropies in the cosmic microwave background (CMB) at scales of 0&fdg;3 to 5 degrees from the North American test flight of the Boomerang experiment. Boomerang is a balloon-borne telescope with a bolometric receiver designed to map CMB anisotropies on a long-duration balloon flight. During a 6 hr test flight of a prototype system in 1997, we mapped more than 200 deg(2) at high Galactic latitudes in two bands centered at 90 and 150 GHz with a resolution of 26&arcmin; and 16&farcm;5 FWHM, respectively. Analysis of the maps gives a power spectrum with a peak at angular scales of 1 degrees with an amplitude 70 µK(CMB).

On Type Ia Supernovae From The Collisions of Two White Dwarfs

ABSTRACT We explore collisions between two white dwarfs as a pathway for making Type IaSupernovae (SNIa). White dwarf number densities in globular clusters allow 10 100redshift . 1 collisions per year, and observations by (Chomiuk et al.2008) of globularclusters in the nearby S0 galaxy NGC 7457 have detected what is likely to be a SNIaremnant. We carry out simulations of the collision between two 0.6M white dwarfsat various impact parameters and mass resolutions. For impact parameters less thanhalf the radius of the white dwarf, we nd such collisions produce ˇ 0.4 M of 56 Ni,making such events potential candidates for underluminous SNIa or a new class oftransients between Novae and SNIa.Key words: hydrodynamics { nuclear reactions, nucleosynthesis, abundances {(stars:) white dwarfs { (stars:) supernovae: general. 1 INTRODUCTIONType Ia supernovae (henceforth SNIa) play a key role inastrophysics as premier distance indicators for cosmology(Phillips 1993; Riess et al.1998; Perlmutter et al.1999), asdirect probes of low-mass star formation rates at cosmologi-cal distances (Scannapieco et al.2005; Mannucci et al.2006;Maoz 2008) and as signi cant contributors to iron-groupelements in the cosmos (Wheeler et al.1989; Timmes etal.1995; Feltzing et al.2001; Strigari 2006). Our current un-derstanding is that there are two major progenitor systemsfor these events. The rst possibility, the single-degeneratescenario, consists of a carbon-oxygen white dwarf in a bi-nary system evolving to the stage of central ignition by massoverow from a low-mass stellar companion (Whelan & Iben1973; Nomoto 1982; Hillebrandt & Niemeyer 2000). The sec-ond possibility, the double-degenerate scenario, consists ofthe merger of two white dwarfs in a binary system (Iben& Tutukov 1984; Webbink 1984; Yoon et al.2007). It is un-known at what relative frequency both of these channelsoperate (Livio 2000; Maoz 2008).Collisions between two white dwarfs, are likely to hap-pen less frequently than binary mergers. However, as dis-cussed in Timmes (2009) and Rosswog et al.(2009), theywill occur in globular clusters where the stellar densities areextremely high. For a typical globular cluster velocity dis-persion of ˇ5-10 km s

AGN Feedback Causes Downsizing

We study the impact of outflows driven by active galactic nuclei (AGNs) on galaxy formation. Outflows move into the surrounding intergalactic medium (IGM) and heat it sufficiently to prevent it from condensing onto galaxies. In the dense, high-redshift IGM, such feedback requires a highly energetic outflow, driven by a large AGN. However, in the more tenuous low-redshift IGM, equivalently strong feedback can be achieved by less energetic winds (and thus smaller galaxies). Using a simple analytic model, we show that this leads to the antihierarchical quenching of star formation in large galaxies, consistent with current observations. At redshifts prior to the formation of large AGNs, galaxy formation is hierarchical and follows the growth of dark matter halos. The transition between the two regimes lies at the z ≈ 2 peak of AGN activity..

The Impact of Small-Scale Structure on Cosmological Ionization Fronts and Reionization

The propagation of cosmological ionization fronts during the reionization of the universe is strongly influenced by small-scale gas inhomogeneities due to structure formation. These inhomogeneities include both collapsed minihalos, which are generally self-shielding, and lower density structures, which are not. The minihalos are dense and sufficiently optically thick to trap intergalactic ionization fronts, blocking their path and robbing them of ionizing photons until the minihalo gas is expelled as an evaporative wind. The lower density structures do not trap these fronts, but they can slow them down by increasing the overall recombination rate in the intergalactic medium (IGM). In this paper we study the effects of both types of inhomogeneities, including nonlinear clustering effects, and we find that both IGM clumping and collapsed minihalos have significant yet qualitatively different impacts on reionization. While the number density of minihalos on average increases strongly with time, the density of minihalos inside H II regions around ionizing sources is largely constant. Thus the impact of minihalos is essentially to decrease the number of ionizing photons available to the IGM at all epochs, which is equivalent to a reduction in the luminosity of each source. On the other hand, the effect of IGM clumping increases strongly with time, slowing down reionization and extending it. Thus while the impact of minihalos is largely degenerate with the unknown source efficiency, IGM clumping can help significantly in reconciling the recent observations of cosmic microwave background polarization with quasar absorption spectra at z ~ 6, which together point to an early but extended reionization epoch.