arXiv subjects

Joseph Silk

Publications and source records attributed to Joseph Silk.

At least 469 records · Page 26Linked to original sources

arXiv1997

How small were the first cosmological objects?

The minimum mass that a virialized gas cloud must have in order to be able to cool in a Hubble time is computed, using a detailed treatment of the chemistry of molecular hydrogen. With a simple model for halo profiles, we reduce the problem to that of numerically integrating a system of chemical equations. The results agree well with numerically expensive 3D simulations, and our approach has the advantage of rapidly being able to explore large regions of parameter space. The minimum baryonic mass M_b is found to be strongly redshift dependent, dropping from 10^6 solar masses at z=15 to 5000 solar masses at z=100 as molecular cooling becomes effective. For z>>100, M_b rises again, as CMB photons inhibit H_2-formation through the H^- channel. Finally, for z>>200, the H_2^+ channel for H_2-formation becomes effective, driving M_b down towards 10^3 solar masses. With a standard CDM power spectrum with sigma_8=0.7, this implies that a fraction 10^{-3} of all baryons may have formed luminous objects by z=30, which could be sufficient to reheat the universe.

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arXiv1997

Constraints on a Primordial Magnetic Field

We derive an upper limit of $B_0<3.4\times 10^{-9}(Ω_0h_{50}^2)^{1/2}$ Gauss on the present strength of any primordial homogeneous magnetic field. The microwave background anisotropy created by cosmological magnetic fields is calculated in the most general flat and open anisotropic cosmologies containing expansion-rate and 3-curvature anisotropies. Our limit is derived from a statistical analysis of the 4-year Cosmic Background Explorer data for anisotropy patterns characteristic of homogeneous anisotropy averaged over all possible sky orientations with respect to the COBE receiver. The limits we obtain are considerably stronger than those imposed by primordial nucleosynthesis and ensure that other magnetic field effects on the microwave background structure are unobservably small.

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arXiv1996

Feedback, Disk Self-regulation and Galaxy Formation

Self-regulation of star formation in disks is controlled by two dimensionless parameters: the Toomre parameter for gravitational instability and the porosity of the interstellar medium to supernova remnant-heated gas. An interplay between these leads to expressions for the gas fraction and star formation efficiency in disks, and to a possible explanation of the Tully-Fisher relation. I further develop feedback arguments that arise from the impact of massive star formation and death on protogalaxies in order to account for the characteristic luminosity of a galaxy and for early winds from forming spheroids.

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arXiv1996

Peaks in the Cosmic Microwave Background: flat versus open models

We present properties of the peaks (maxima) of the CMB anisotropies expected in flat and open CDM models. We obtain analytical expressions of several topological descriptors: mean number of maxima and the probability distribution of the gaussian curvature and the eccentricity of the peaks. These quantities are calculated as functions of the radiation power spectrum, assuming a gaussian distribution of temperature anisotropies. We present results for angular resolutions ranging from 5' to 20' (antenna FWHM), scales that are relevant for the MAP and COBRAS/SAMBA space missions and the ground-based interferometer experiments. Our analysis also includes the effects of noise. We find that the number of peaks can discriminate between standard CDM models, and that the gaussian curvature distribution provides a useful test for these various models, whereas the eccentricity distribution can not distinguish between them.

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arXiv1996

Colors and K-Band Counts of Extremely Faint Field Galaxies

We combine deep K-band (Keck) with V- and I-band (NTT) observations of two high-Galactic latitude fields, surveying a total of ~2 sq. arcmin. The K-band galaxy counts continue to rise above K=22, reaching surface densities of few x 10^5 per sq. degree. The slope for the counts is (d log(N) per mag per sq. degree) = 0.23 +/- 0.02 between K=18-23, consistent with other deep K surveys. The numbers of galaxies in each mag bin is about two times greater than the galaxy counts of Djorgovski et al. (1995). The optical and near infrared magnitudes of all objects detected in the V+I+K image are discussed in the context of grids of isochrone synthesis galaxy evolutionary models (Bruzual & Charlot 1993, 1995). The colors of most of the observed galaxies are consistent with a population drawn from a broad redshift distribution. A few galaxies at K=19-20 are red in both colors (V-I>3; I-K>2), consistent with being early-type galaxies having undergone a burst of star formation at z>5 and viewed at z~1. At K>20, we find ~8 ``red outlier'' galaxies with I-K>4 and V-I<2.5, whose colors are difficult to mimic by a single evolving or non-evolving stellar population at any redshift. They are likely either low-metallicity, dusty dwarf galaxies, or old galaxies at high redshift (z>1.2). Their surface density is several per square arcminute, which is so high that they are probably common objects of low luminosity $L<L_*$.

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arXiv1996

Late reionization by supernova-driven winds

A model is presented in which supernova-driven winds from early galaxies reionize the intergalactic medium by z=5. This scenario can explain the observed absence of a Gunn-Peterson trough in the spectra of high-redshift quasars providing that the bulk of these early galaxies are quite small, no more massive than about 10^8 solar masses. It also predicts that most of the IGM was enriched to at least 10% of current metal content by z=5 and perhaps as early as z=15. The existence of such early mini-galaxies violates no spectral constraints and is consistent with a pure CDM model with b<2. Since the final radius of a typical ionized bubble is only around 100 kpc, the induced modification of the galaxy autocorrelation function is negligible, as is the induced angular smoothing of the CMBR. Some of the gas swept up by shells may be observable as pressure-supported Lyman-alpha forest clouds.

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arXiv1996

On the inevitability of reionization: implications for cosmic microwave background fluctuations

Early photoionization of the intergalactic medium is discussed in a nearly model-independent way, in order to investigate whether early structures corresponding to rare Gaussian peaks in a CDM model can photoionize the intergalactic medium sufficiently early to appreciably smooth out the microwave background fluctuations. We conclude that this is indeed possible for a broad range of CDM normalizations and is almost inevitable for unbiased CDM, provided that the bulk of these early structures are quite small, no more massive than about 10^8 solar masses. Typical parameter values predict that reionization occurs around z=50, thereby suppressing fluctuations on degree scales while leaving the larger angular scales probed by COBE reasonably unaffected. However, for non-standard CDM, incorporating mixed dark matter, vacuum density or a tilted primordial power spectrum, early reionization plays no significant role.

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arXiv1996

Did the universe recombine? New spectral constraints on reheating

One still cannot conclusively assert that the universe underwent a neutral phase, despite the new COBE FIRAS limit y<0.000025 on Compton y-distortions of the cosmic microwave background. Although scenarios where the very early (z=1000) ionization is thermal, caused by IGM temperatures exceeding 10000K, are clearly ruled out, there is a significant loophole for cosmologies with typical CDM parameters if the dominant ionization mechanism is photoionization. If the ionizing radiation has a typical quasar spectrum, then the y-constraint implies roughly h^{3/2}Ω_b Omega_0^{-1/4}<0.1 for fully ionized models. This means that BDM models with Omega_0 around 0.15 and reionization at about z=1000 are strongly constrained even in this very conservative case, and can survive the y test only if most of the baryons form BDM around the reionization epoch.

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arXiv1996

Reionization in an open cdm universe: implications for cosmic microwave background fluctuations

We generalize previous work on early photoionization to CDM models with Omega<1. Such models have received recent interest because the excess power in the large-scale galaxy distribution is phenomenologically fit if the ``shape parameter" Gamma=h Omega_0 is about 0.25. It has been argued that such models may require early reionization to suppress small-scale anisotropies in order to be consistent with experimental data. We find that if the cosmological constant lambda=0, the extent of this suppression is quite insensitive to Omega_0. Given a sigma_8-normalization today, the loss of small-scale power associated with a lower Omega_0 is partially canceled by higher optical depth from longer lookback times and by structures forming at higher redshifts before the universe becomes curvature-dominated. The maximum angular scale on which fluctuations are suppressed decreases when Omega_0 is lowered, but this effect is also rather weak and unlikely to be measurable in the near future. For flat models, on the other hand, where lambda_0 = 1-Omega_0, the negative effects of lowering Omega_0 dominate, and early reionization is not likely to play a significant role if Omega_0<<1. The same applies to CDM models where the effective Gamma is lowered by increasing the number of relativistic particle species.

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arXiv1996

Passive Evolution: Are the Faint Blue Galaxy Counts Produced by a Population of Eternally Young Galaxies?

A constant age population of blue galaxies, postulated in the model of Gronwall & Koo (1995), seems to provide an attractive explanation of the excess of very blue galaxies in the deep galaxy counts. Such a population may be generated by a set of galaxies with cycling star formation rates, or at the other extreme, be maintained by the continual formation of new galaxies which fade after they reach the age specified in the Gronwall and Koo model. For both of these hypotheses, we have calculated the luminosity functions including the respective selection criteria, the redshift distributions, and the number counts in the B_J and K bands. We find a substantial excess in the number of galaxies at low redshift (0 < z < 0.05) over that observed in the CFH redshift survey (Lilly et al. 1995) and at the faint end of the Las Campanas luminosity function (Lin et al. 1996). Passive or mild evolution fails to account for the deep galaxy counts because of the implications for low redshift determinations of the I-selected redshift distribution and the r-selected luminosity function in samples where the faded counterparts of the star-forming galaxies would be detectable.

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arXiv1996

Angular Sizes of Faint Field Disk Galaxies: Intrinsic Luminosity Evolution

In order to explain the small scale-lengths detected in the recent deep field observations performed from large ground-based telescopes and from the Hubble Space Telescope, we investigate the predictions at high redshifts for disk galaxies that formed by infall. Changes with redshift in the observed properties of field galaxies are directly related to the evolution of the disks and of the stellar populations. We see that changes in the rest frame luminosity of a galaxy induce smaller values of half-light radii than are predicted assuming no evolution. Comparisons are presented with two observed samples from Mutz et al. (1994) and Smail et al. (1995).

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arXiv1996

How Anisotropic is our Universe?

Large-scale cosmic microwave background anisotropies in homogeneous, globally anisotropic cosmologies are investigated. We perform a statistical analysis in which the four-year data from the Cosmic Background Explorer satellite is searched for the specific anisotropy patterns predicted by these models and thereby set definitive upper limits on the amount of shear, $(σ/H)_0$ and vorticity, $(ω/H)_0$, which are orders of magnitude stronger than previous constraints. We comment on how these results might impact our understanding of primordial global anisotropy.

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arXiv1996

The Physics of Microwave Background Anisotropies

Cosmic microwave background anisotropies provide a vast amount of cosmological information. Their full physical content and detailed structure can be understood in a simple and intuitive fashion through a systematic investigation of the individual mechanisms for anisotropy formation.

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arXiv1996

CMB radiation power spectrum in CDM open universes up to 2nd order perturbations

A second--order perturbation theory approach is developed to calculate temperature anisotropies in the cosmic microwave background. Results are given for open universes and fluctuations corresponding to CDM models with either Harrison-Zeldovich (HZ) or Lyth-Stewart-Ratra-Peebles (LSRP) primordial energy--density fluctuation power spectrum. Our perturbation theory approach provides a distinctive multipole contribution as compared to the primary one, the amplitude of the effect being very dependent on normalization. For low--$Ω$ models, the contribution of the secondary multipoles to the radiation power spectrum is negligible both for standard recombination and reionized scenarios, with the 2--year COBE--DMR normalization. For a flat universe this contribution is $\approx 0.1-10\%$ depending on the reionization history of the universe and on the normalization of the power spectrum.

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arXiv1996

On the Effects of Bursts of Massive Star Formation During the Evolution of Elliptical Galaxies

We consider the hypothesis that the formation of elliptical galaxies includes a phase in which star formation is mostly restricted to massive stars, with the bias towards high mass stars increasing with elliptical galaxy mass. The mass fraction of stars in this top-heavy mode of star formation is constrained by requiring the resulting stellar remnants to account for the observed increase in the mass-to-light ratio of ellipticals with increasing galaxy mass. We then consider the implications of this population of massive stars for the intracluster medium and the extragalactic background at various wavelengths. The mass and abundance ratios of metals produced by our proposed population of massive stars are consistent with the observations of the mass and abundance ratios of metals in the hot gas of galaxy clusters for most of the standard range of IMF slopes and SN II yields. The predicted energy density produced by this stellar population approaches current limits on the extragalactic background at both optical wavelengths, into which the ultraviolet radiation of the massive stars is likely to be redshifted, and far-infared wavelengths, at which starlight reprocessed by dust associated with the starburst will be observed. In either case, the background is predicted to be significantly clustered since massive ellipticals are clustered.

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arXiv1995

Baryonic Dark Halos: A Model with MACHOS and Cold Gas Globules

The dark matter in the halos of galaxies may well be baryonic, and much of the mass within them could be in the form of clusters of substellar objects within which are embedded cold gas globules. Such halos might play an active role in galaxy formation and evolution.

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arXiv1995

Baryonic Dark Halos: Machos and Cold Gas?

We consider the possibility that the dark matter in the halos of galaxies may be in the form of clusters of \macho s within which are embedded cold, dense gas clouds. Microlensing experiments have found evidence that the Galactic halo contains up to half of its mass in the form of low-mass \macho s. A number of observational and dynamical arguments point to the existence of hitherto unobserved cold gas around galaxies. We show that the cold gas can be stabilized by \macho\ clusters. Within the framework of a simple two-component model, we derive constraints on the \macho\ clusters and on the halo cold gas content. Typical cluster masses are $\sim 10 \msun$, typical \macho\ masses are $\sim 0.01 \msun$, and the gas content could be up to of order 50 \%. Various predictions are given for testing the hypothesis that such objects could constitute most of the mass in the dark halos of galaxies. If halos are indeed baryonic and contain significant amounts of cold gas, they are likely to play a much more active role in galaxy formation and evolution than is commonly supposed.

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arXiv1995

Gravitational Microlensing by Clustered MACHOs

It has been proposed that the MACHOs in our galaxy could be clumped in globular cluster--like associations or RAMBOs (robust associations of massive baryonic objects) (Moore \& Silk 1995). Here we investigate the effect such clustering has on the microlensing of stars in the Large Magellanic Cloud. We find that the lensing in a one square degree field could be dominated by just a few clusters. As a result the lensing properties vary widely depending on the position and velocity of those clusters which happen to lie between us and the LMC. Moreover we find a large variance in time scale distributions that suggests that the small number statistics could easily be dominated by events in the tails of the unclustered distribution ({\it e.g.} by long periods). We compare our results with the MACHO collaboration's data and find that a ``standard'' halo made entirely of MACHOs is not strongly disfavored if the clusters have masses of $10^6 M_{\odot}$. For less massive clusters such a halo is not as likely. For $10^4 M_{\odot}$ clusters the microlensing statistics are essentially unchanged from the unclustered case. It may be possible to detect very massive clusters from the distribution of events in time-scale and space. We provide some example time- scale distributions.

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