Feminist Collaboration in the Art Academy

Women\u27s activity in the visual arts both in and outside of the art institutions of Europe and the United States reveals a history of collaboration in artistic production and political activism This paper analyzes the effects of feminist collaboration upon the disciplines of art, the pedagogy of art, and the administration of art institutions. In Part I, the authors review the impact of feminist collaboration in art history, aesthetics, art criticism, and art production. Part II provides examples of collaborative experiences of women in higher education art institutions and in some art communities in the United States, Scandinavia, and Italy. Three conclusions emerged from the review: (a) Collaboration facilitated women\u27s entry into the visual arts; (b) collaborative dialogue has changed the academic structures of art criticism and art history, but collaboration has had a minimal effect in the areas of aesthetics and art production; and (c) collaboration has not resulted in a significant change in the administration or pedagogy of art institutions

Chemistry and Radiative Feedback of Early Galaxies: Seeding the First Supermassive Black Holes

The abundance of molecular hydrogen (H2), the primary coolant in primordial gas, is critical for the thermodynamic evolution and star–formation histories in early protogalaxies. Suppression of H2–cooling in early protogalaxies can occur via photodissociation of H2 (by ultraviolet Lyman–Werner [LW] photons) or by photodetachment of H−, a precursor in H2 formation (by infrared [IR] photons). It is widely believed that the formation of the first massive black hole “seeds,” with masses 104−6 M⊙, in primordial halos may be enabled if H2–cooling is suppressed. We study the radiative feedback processes that suppress H2–cooling in primordial proto- galaxies. Previous studies have typically adopted idealized spectra, with a blackbody or a power–law shape, in modeling the chemistry of metal–free protogalaxies, and utilized a single parameter, the critical UV flux, or Jcrit, to determine whether H2–cooling is prevented. This can be misleading, as independent of the spectral shape, there is a a critical curve in the (kLW,kH−) plane, where kLW and kH− are the H2–dissociation rates by LW and IR photons, which determines whether a protogalaxy can cool below ∼ 1000 Kelvin. In Chapter 1, we use a one–zone model to follow the chemical and thermal evolution of gravitationally collapsing protogalactic gas, to compute this critical curve, and provide an accurate analytical fit for it. We improve on previous works by considering a variety of more realistic Pop III or Pop II-type spectra from population synthesis models and perform fully frequency–dependent calculations of the H2–photodissociation rates for each spectrum. We compute the ratio kLW/kH− for each spectrum, as well as the minimum stellar mass M∗, for various IMFs and metallicities, required to prevent cooling in a neighboring halo a distance d away. We provide critical M∗/d2 values for suppression of H2–cooling, with analytic fits, which can be used in future studies. Determining the photodissociation rate of H2 by an incident LW flux is crucial, but prohibitively expensive to calculate on the fly in simulations. The rate is sensitive to the H2 rovibrational distribution, which in turn depends on the gas density, temperature, and incident LW radiation field. In Chapter 2, we use the publicly available cloudy package to model primordial gas clouds and compare exact photodissociation rate calculations to commonly–used fitting formulae. We find the fit from Wolcott-Green et al. (2011) is most accurate for moderate densities n ∼ 103cm−3 and temperatures, T ∼ 103K, and we provide a new fit, which captures the increase in the rate at higher densities and temperatures, owing to the increased excited rovibrational populations in this regime. Our new fit has typical errors of a few percent percent up to n ≤ 107 cm−3, T ≤ 8000K, and H2 column density NH2 ≤ 1017 cm−2, and can be easily utilized in simulations. We also show that pumping of the excited rovibrational states of H2 by a strong LW flux further modifies the level populations when the gas density is low, and noticeably decreases self-shielding for J21 > 103 and n < 102cm−3. This may lower the “critical flux” at which primordial gas remains H2–poor in some protogalaxies, enabling massive black hole seed formation. In Chapter 3, we study the thermal evolution of UV–irradiated atomic cooling halos using high–resolution three–dimensional hydrodynamic simulations. We consider the effect of H− photodetachment by Lyα cooling radiation in the optically–thick cores of three such halos, a process which has not been included in previous simulations. H− is a precursor of molecular hydrogen, and therefore, its destruction can diminish the H2 abundance and cooling. We find that the critical UV flux for suppressing H2–cooling is decreased by up to a factor of a few when H− photodetachment by Lyα is included. In a more conservative estimate of the trapped Lyα energy density, we find the critical flux is decreased by ∼ 15 − 50 per cent. Our results suggest that Lyα radiation may have an important effect on the thermal evolution of UV–irradiated halos, and therefore on the potential for massive black hole formation

Feedback from the IR Background in the Early Universe

It is commonly believed that the earliest stages of star-formation in the Universe were self-regulated by global radiation backgrounds - either by the ultraviolet Lyman-Werner (LW) photons emitted by the first stars (directly photodissociating H_2), or by the X-rays produced by accretion onto the black hole (BH) remnants of these stars (heating the gas but catalyzing H_2 formation). Recent studies have suggested that a significant fraction of the first stars may have had low masses (a few M_sun). Such stars do not leave BH remnants and they have softer spectra, with copious infrared (IR) radiation at photon energies around 1eV. Similar to LW and X-ray photons, these photons have a mean-free path comparable to the Hubble distance, building up an early IR background. Here we show that if soft-spectrum stars, with masses of a few M_sun, contributed more than 1% of the UV background (or their mass fraction exceeded 90%), then their IR radiation dominated radiative feedback in the early Universe. The feedback is different from the UV feedback from high-mass stars, and occurs through the photo-detachment of H^- ions, necessary for efficient H_2 formation. Nevertheless, we find that the baryon fraction which must be incorporated into low-mass stars in order to suppress H_2-cooling is only a factor of few higher than for high-mass stars.Comment: Accepted for publication in MNRAS (Letters). 5 pages with 2 figure

The First Stars: Mass Growth Under Protostellar Feedback

We perform three-dimensional cosmological simulations to examine the growth of metal-free, Population III (Pop III) stars under radiative feedback. We begin our simulation at z=100 and trace the evolution of gas and dark matter until the formation of the first minihalo. We then follow the collapse of the gas within the minihalo up to densities of n = 10^12 cm^-3, at which point we replace the high-density particles with a sink particle to represent the growing protostar. We model the effect of Lyman-Werner (LW) radiation emitted by the protostar, and employ a ray-tracing scheme to follow the growth of the surrounding H II region over the next 5000 yr. We find that a disk assembles around the first protostar, and that radiative feedback will not prevent further fragmentation of the disk to form multiple Pop III stars. Ionization of neutral hydrogen and photodissociation of H_2 by LW radiation leads to heating of the dense gas to several thousand Kelvin, and this warm region expands outward at the gas sound speed. Once the extent of this warm region becomes equivalent to the size of the disk, the disk mass declines while the accretion rate onto the protostars is reduced by an order of magnitude. This occurs when the largest sink has grown to ~ 20 M_sol while the second sink has grown to 7 M_sol, and we estimate the main sink will approach an asymptotic value of ~ 30 M_sol by the time it reaches the main sequence. Our simulation thus indicates that the most likely outcome is a massive Pop III binary. However, we simulate only one minihalo, and the statistical variation between minihaloes may be substantial. If Pop III stars were typically unable to grow to more than a few tens of solar masses, this would have important consequences for the occurence of pair-instability supernovae in the early Universe as well as the Pop III chemical signature in the oldest stars observable today.Comment: 21 pages, 11 figures, to appear in MNRA

Supermassive black hole ancestors

We study a model in which supermassive black holes (SMBHs) can grow by the combined action of gas accretion on heavy seeds and mergers of both heavy (m_s^h=10^5 Msol) and light (m_s^l = 10^2 Msol) seeds. The former result from the direct collapse of gas in T_s^h >1.5x10^4K, H_2-free halos; the latter are the endproduct of a standard H_2-based star formation process. The H_2-free condition is attained by exposing halos to a strong (J_21 > 10^3) Lyman-Werner UV background produced by both accreting BHs and stars, thus establishing a self-regulated growth regime. We find that this condition is met already at z close to 18 in the highly biased regions in which quasars are born. The key parameter allowing the formation of SMBHs by z=6-7 is the fraction of halos that can form heavy seeds: the minimum requirement is that f_heavy>0.001; SMBH as large as 2x10^10 Msol can be obtained when f_heavy approaches unity. Independently of f_heavy, the model produces a high-z stellar bulge-black hole mass relation which is steeper than the local one, implying that SMBHs formed before their bulge was in place. The formation of heavy seeds, allowed by the Lyman-Werner radiative feedback in the quasar-forming environment, is crucial to achieve a fast growth of the SMBH by merger events in the early phases of its evolution, i.e. z>7. The UV photon production is largely dominated by stars in galaxies, i.e. black hole accretion radiation is sub-dominant. Interestingly, we find that the final mass of light BHs and of the SMBH in the quasar is roughly equal by z=6; by the same time only 19% of the initial baryon content has been converted into stars. The SMBH growth is dominated at all epochs z > 7.2 by mergers (exceeding accretion by a factor 2-50); at later times accretion becomes by far the most important growth channel. We finally discuss possible shortcomings of the model.Comment: 12 pages, 9 figures, 1 table, MNRAS in pres

Photodissociation of H2 in Protogalaxies: Modeling Self-Shielding in 3D Simulations

The ability of primordial gas to cool in proto-galactic haloes exposed to Lyman-Werner (LW) radiation is critically dependent on the self-shielding of H_2. We perform radiative transfer calculations of LW line photons, post-processing outputs from three-dimensional adaptive mesh refinement (AMR) simulations of haloes with T_vir > 10^4 K at redshifts around z=10. We calculate the optically thick photodissociation rate numerically, including the effects of density, temperature, and velocity gradients in the gas, as well as line overlap and shielding of H_2 by HI, over a large number of sight-lines. In low-density regions (n<10^4 cm^-3) the dissociation rates exceed those obtained using most previous approximations by more than an order of magnitude; the correction is smaller at higher densities. We trace the origin of the deviations primarily to inaccuracies of (i) the most common fitting formula (Draine & Bertoldi 1996) for the suppression of the dissociation rate and (ii) estimates for the effective shielding column density from local properties of the gas. The combined effects of gas temperature and velocity gradients are comparatively less important, typically altering the spherically averaged rate only by a factor of less than two. We present a simple modification to the DB96 fitting formula for the optically thick rate which improves agreement with our numerical results to within approx. 15 per cent, and can be adopted in future simulations. We find that estimates for the effective shielding column can be improved by using the local Sobolev length. Our correction to the H_2 self-shielding reduces the critical LW flux to suppress H_2-cooling in T_vir>10^4 K haloes by an order of magnitude; this increases the number of such haloes in which supermassive (approx. M=10^5 M_sun) black holes may have formed.Comment: 17 pages, 11 figures. Submitted to MNRA

Suppression of HD-cooling in protogalactic gas clouds by Lyman-Werner radiation

It has been shown that HD molecules can form efficiently in metal-free gas collapsing into massive protogalactic halos at high redshift. The resulting radiative cooling by HD can lower the gas temperature to that of the cosmic microwave background, T_CMB=2.7(1+z)K, significantly below the temperature of a few 100 K achievable via H_2-cooling alone, and thus reduce the masses of the first generation of stars. Here we consider the suppression of HD-cooling by UV irradiation in the Lyman-Werner (LW) bands. We include photo-dissociation of both H_2 and HD, and explicitly compute the self-shielding and shielding of both molecules by neutral hydrogen as well as the shielding of HD by H_2. We use a simplified dynamical collapse model, and follow the chemical and thermal evolution of the gas, in the presence of a UV background. We find that a LW flux of J_crit = 1e-22 erg/cm^2/sr/s/Hz is able to suppress HD cooling and thus prevent collapsing primordial gas from reaching temperatures below 100 K. The main reason for the lack of HD cooling for J>J_crit is the partial photo-dissociation of H_2, which prevents the gas from reaching sufficiently low temperatures (T<150K) for HD to become the dominant coolant; direct HD photo-dissociation is unimportant except for a narrow range of fluxes and column densities. Since the prevention of HD-cooling requires only partial H_2 photo-dissociation, the critical flux J_crit is modest, and is below the UV background required to reionize the universe at redshift z=10-20. We conclude that HD-cooling can reduce the masses of typical stars only in rare halos forming well before the epoch of reionization.Comment: 14 pages with 9 figures, submitted to MNRA