arXiv subjects

Piero Madau

Publications and source records attributed to Piero Madau.

212 records · Page 12Linked to original sources

arXiv1998

Probing The High Redshift Universe with the NGST

Through a combination of deep wide-field imaging and multi-object spectroscopy, the Next Generation Space Telescope will be able to chart with unprecedented accuracy the evolution of cosmic structures after the `dark ages' (z<5), when galaxies are thought to assemble and form the bulk of their stars. In this talk I will discuss how NGST observations might determine the history of the stellar birthrate in the universe and reveal the character of cosmological ionizing sources at high redshifts.

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arXiv1998

Star Formation at High Redshift: A Population of Early Dwarfs?

The history of the transition from a neutral intergalactic medium (IGM) to one that is almost fully ionized can reveal the character of cosmological ionizing sources and set important constraints on the stellar birthrate at high redshifts. The hydrogen component in a highly inhomogeneous universe is completely reionized when the number of photons emitted above 1 ryd in one recombination time equals the mean number of hydrogen atoms. If stellar sources are responsible for photoionizing the IGM at z=5, the rate of star formation at this epoch must be comparable or greater than the one inferred from optical observations of galaxies at z=3, and the mean metallicity per baryon in the universe greater than 1/500 solar. In hierarchical clustering scenarios, high-z dwarfs (i.e. an early generation of stars in dark matter halos with circular velocities of order 50 km/s) are expected to be one of the main source of UV photons and heavy elements at early epochs. They would be very numerous, >0.2 arcsec^{-2}, and faint, I_AB> 29.5 mag: their detection may have to wait for the Next Generation Space Telescope.

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arXiv1998

Constraints from the Hubble Deep Field on High Redshift Quasar Models

High resolution, deep imaging surveys are instrumental in setting constraints on semi-analytical structure formation models in Cold Dark Matter (CDM) cosmologies. We show here that the lack of unresolved B-band ``dropouts'' with V > 25 mag in the Hubble Deep Field (HDF) appears to be inconsistent with the expected number of quasars if massive black holes form with a constant universal efficiency in all CDM halos. To reconcile the models with the data, a mechanism is needed that suppresses the formation of quasars in halos with circular velocities v(circ) < 50-75 km/s. This feedback naturally arises due to the photoionization heating of the gas by the UV background. We consider several alternative effects that would help reduce the quasar number counts, and find that these can not alone account for the observed lack of detections. If reddening by dust can be neglected at early epochs, consistency with the optical data also requires that the luminous extent of dwarf galaxies at high redshifts be larger than a few percent of the virial radii of their dark matter halos, in order not to overpredict the number of point-like B-band dropouts. Future deep observations in the J and H bands with NICMOS might reveal several z > 5 objects per field or provide even stronger constraints on the models than existing B, V, and I data.

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arXiv1998

What Keeps the Universe Ionized at z=5?

The history of the transition from a neutral intergalactic medium to one that is almost fully ionized can reveal the character of cosmological ionizing sources. In this talk I will discuss the implications for rival reionization scenarios of the rapid decline observed in the space density of quasars and star-forming galaxies at redshifts z>3. The hydrogen component in a highly inhomogeneous universe is completely reionized when the number of ionizing photons emitted in one recombination time equals the mean number of hydrogen atoms. At z=5, the local character of the UV metagalactic flux allows one to define a critical emission rate of hydrogen-ionizing photons per unit comoving volume. Models based on photoionization by bright QSOs and/or young galaxies with star formation rates in excess of 0.3-1 M_sun/yr appear to fail to provide the required number of hydrogen-ionizing photons at these redshifts by large factors. If stellar sources are responsible for keeping the universe ionized at z=5, the rate of star formation per unit comoving volume at this epoch must be comparable or greater than observed at z=3.

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arXiv1998

Galaxy Evolution and the Cosmic Rate of Supernovae

Ongoing searches for supernovae (SNe) at cosmological distances have recently started to provide a link between SN Ia statistics and galaxy evolution. In this talk I will use recent estimates of the global history of star formation to compute the theoretical Type Ia and Type II SN rates as a function of cosmic time. I will compare the predicted values with the rates observed in the range 0<z<0.4, and show how accurate measurements of the frequency of SN events at intermediate redshifts are valuable probes of the evolution of the stellar birthrate in the universe and the nature of Type Ia progenitors. The Next Generation Space Telescope should detect of order 20 Type II SNe per field per year in the interval 1<z<4.

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arXiv1997

The Evolution of Luminous Matter in the Universe

I review a technique for interpreting faint galaxy data which traces the evolution with cosmic time of the galaxy luminosity density, as determined from several deep spectroscopic samples and the HDF. The method relies on the rest frame UV and near-IR continua of galaxies as indicators, for a given IMF and dust content, of their instantaneous SFR and total stellar mass, and offers the prospect of addressing in a coherent framework an important set of subjects: cosmic star formation history, dust in primeval galaxies, shape of the IMF, stellar mass-to-light ratios of present-day galaxies, extragalactic background light, Type II supernovae and heavy element enrichment history of the universe. The global spectrophotometric properties of field galaxies are well fit by a simple stellar evolution model, defined by a time-dependent SFR per unit comoving volume, a universal IMF which is relatively rich in massive stars, and a modest amount of dust reddening. The model is able to account for the entire background light recorded in the galaxy counts down to the very faint magnitude levels probed by the HDF, and produces visible mass-to-light ratios at the present epoch which are consistent with the values observed in nearby galaxies of various morphological types. The bulk (>60%) of the stars present today formed relatively recently (z<1.5), consistently with the expectations from a broad class of hierarchical clustering cosmologies, and in good agreement with the low level of metal enrichment observed at high redshifts in the damped Lyman-alpha systems. Throughout this review I emphasize how the poorly constrained amount of starlight that was absorbed by dust and reradiated in the far-IR at early epochs represents one of the biggest uncertainties in our understanding of the evolution of luminous matter in the universe.

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arXiv1997

The Star Formation History of Field Galaxies

We develop a method for interpreting faint galaxy data which focuses on the integrated light radiated from the galaxy population as a whole. The emission history of the universe at ultraviolet, optical, and near-infrared wavelengths is modeled from the present epoch to z~4 by tracing the evolution with cosmic time of the galaxy luminosity density, as determined from several deep spectroscopic samples and the Hubble Deep Field (HDF) imaging survey. The global spectrophotometric properties of field galaxies can be well fit by a simple stellar evolution model, defined by a time-dependent star formation rate (SFR) per unit comoving volume and a universal IMF extending from 0.1 to 125 M_sun. In the best-fit models, the global SFR rises sharply, by about an order of magnitude, from a redshift of zero to a peak value at z~1.5, to fall again at higher redshifts. The models are able to account for the entire background light recorded in the galaxy counts down to the very faint magnitude levels probed by the HDF. Since only 20% of the current stellar content of galaxies is produced at z>2, a rather low cosmic metallicity is expected at these early times, in good agreement with the observed enrichment history of the damped Lyman-αsystems. A ``monolithic collapse'' model, where half of the present-day stars formed at z>2.5 and were shrouded by dust, can be made consistent with the global history of light, but overpredicts the metal mass density at high redshifts as sampled by QSO absorbers.

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arXiv1997

The Hubble Deep Field and the Early Evolution of Galaxies

I review some recent progress made in our understanding of galaxy evolution and the cosmic history of star formation. The Hubble Deep Field (HDF) imaging survey has achieved the sensitivity to capture the bulk of the extragalactic background light from discrete sources. No evidence is found in the optical number-magnitude relation down to AB=29 mag for a large amount of star formation at high redshifts. The emission history of the universe at ultraviolet, optical, and near-infrared wavelengths can be modeled from the present epoch to z~4 by tracing the evolution with cosmic time of the galaxy luminosity density, as determined from several deep spectroscopic samples and the HDF. The global spectrophotometric properties of field galaxies are well fitted by a simple stellar evolution model, defined by a time-dependent star formation rate (SFR) per unit comoving volume and a universal initial mass function which is relatively rich in massive stars. The SFR density is found to rise sharply, by about an order of magnitude, from a redshift of zero to a peak value at z~1.5, to fall again by a factor of 2 (4) out of a redshift of 3 (4). Since only 10% of the current stellar content of galaxies is produced at z>2.5, a rather low cosmic metallicity is predicted at these early times, in good agreement with the observed enrichment history of the damped Lyman-αsystems. The biggest uncertainty is represented by the poorly constrained amount of starlight that was absorbed by dust and reradiated in the IR at early epochs.

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arXiv1996

Cosmic Star Formation History

I review some recent progress made in our understanding of galaxy evolution and the cosmic history of star formation. Like bookends, the results obtained from deep ground-based spectroscopy and from the Hubble Deep Field imaging survey put brackets around the intermediate redshift interval, $1<z<2$, where starbirth probably peaked at a rate 10 times higher than today. The steady decline observed since $z\sim 1$ is largely associated with late-type galaxies. At $z\gtrsim 2.5$, the Lyman-break selected objects may represent the precursors of present-day spheroids, but appear, on average, quite underluminous relative to the expectations of the standard early-and-rapidly forming picture for spheroidal systems. The observed ultraviolet light density accounts for the bulk of the metals seen today in ``normal'' massive galaxies.

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arXiv1996

The instrinsic UV/soft x-ray spectrum of quasars

The detection of a HeII absorption trough in the spectra of three high-redshift quasars provides new constraints on the spectral shape of the UV extragalactic background. The relative strengths of the observed flux decrements at the rest-frame wavelengths of 1216 and 304 A require a relatively soft radiation field at 4 ryd compared to 1 ryd. If the ionizing metagalactic flux is dominated by the integrated light from QSOs, and the HeII reionization of the universe was completed well before z~3, then the UV/soft X-ray spectrum of individual quasars at these epochs must, once the cosmological ``filtering'' through material along the line of sight is taken into account, satisfy similar constraints on the average. We model the propagation of AGN-like ionizing radiation through the intergalactic medium using CUBA, a numerical code developed in our prior work. We show that, in order to explain the reported HeII absorption, any thermal component responsible for the ``soft X-ray excess'' observed in the spectra of AGNs at z<0.3 must, at z~3, have a typical temperature >80 eV, and a luminosity not exceeding 20% of that of the ``UV bump''.

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arXiv1996

21-cm Tomography of the Intergalactic Medium at High Redshift

We investigate the 21-cm signature that may arise from the intergalactic medium (IGM) prior to the epoch of full reionization (z>5). In scenarios in which the IGM is reionized by discrete sources of photoionizing radiation, the neutral gas which has not yet been engulfed by an H II region may easily be preheated to temperatures well above that of the cosmic background radiation (CBR), rendering the IGM invisible in absorption against the CBR. We identify three possible preheating mechanisms: (1) photoelectric heating by soft X-rays from QSOs, (2) photoelectric heating by soft X-rays from early galactic halos, and (3) resonant scattering of the continuum UV radiation from an early generation of stars. In the presence of a sufficiently strong ambient flux of Lyman-alpha photons, the hyperfine transition in the warmed H I will be excited. A beam differencing experiment would detect a patchwork of emission, both in frequency and in angle across the sky. This patchwork could serve as a valuable tool for understanding the epoch, nature, and sources of the reionization of the universe, and their implications for cosmology. We demonstrate that isolated QSOs will produce detectable signals at meter wavelengths within their ``spheres of influence'' over which they warm the IGM. As a result of the redshifted 21-cm radiation emitted by warm H I bubbles, the spectrum of the radio extragalactic background will display frequency structure with velocity widths up to 10,000 km/s. Broad beam observations would reveal corresponding angular fluctuations in the sky intensity with (delta T/T)=1E-3 on degree-scales. Radio measurements near 235 and 150 MHz, as will be possible in the near future using the Giant Metrewave Radio Telescope, may provide the first detection of a neutral IGM at 5<z<10.

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arXiv1996

High Redshift Galaxies in the Hubble Deep Field. Color Selection and Star Formation History to z=4

The Lyman decrement associated with the cumulative effect of HI in QSO absorption systems along the line of sight provides a distinctive feature for identifying galaxies at z>2.5. The Hubble Deep Field (HDF) observations offer the opportunity to exploit the ubiquitous effect of intergalactic absorption and obtain useful statistical constraints on the redshift distribution of galaxies considerably fainter than current spectroscopic limits. We model the HI cosmic opacity as a function of redshift, including scattering in resonant lines of the Lyman series and Lyman-continuum absorption, and use stellar population synthesis models with a wide variety of ages, metallicities, dust contents, and redshifts, to derive color selection criteria that provide a robust separation between high redshift and low redshift galaxies. From the HDF images we construct a sample of star-forming galaxies at 2<z<4.5. While none of the more than 60 objects in the HDF having known Keck/LRIS spectroscopic redshifts in the range 0<z<1.4 is found to contaminate our high-redshift sample, our color criteria are able to efficiently select the 2.6<z<3.2 galaxies identified by Steidel et al. (1996b). The ultraviolet (and blue) dropout technique opens up the possibility of investigating cosmic star and element formation in the early universe.

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arXiv1995

Cosmic Metal Production and the Contribution of QSO Absorption Systems to the Ionizing Background

The recent discovery by Cowie \etal and Tytler \etal of metals in the \Lya clouds shows that the intergalactic medium (IGM) at high redshift is contaminated by the products of stars, and suggests that ionizing photons from massive star formation may be a significant contributor to the UV background radiation at early epochs. We assess the validity of the stellar photoionization hypothesis. Based on recent computations of metal yields and O-star Lyman continuum (LyC) fluxes, we find that 0.2\% of the rest-mass energy of the metals produced is radiated as LyC. By modeling the transfer of ionizing radiation through the IGM and the rate of chemical enrichment, we demonstrate that the background intensity of photons at 1 Ryd that accompanies the production of metals in the \Lya forest clouds may be significant, approaching $0.5\times 10^{-21}\uvunits$ at $z\approx 3$ if the LyC escape fraction is $\gta 0.25$. Together with quasars, massive stars could then, in principle, provide the hydrogen and helium LyC photons required to ionize the universe at high redshifts. We propose that observations of the \HeII Gunn-Peterson effect and of the metal ionization states of the \Lya forest and Lyman-limit absorbers should show the signature of a stellar spectrum. We also note that the stellar photoionization model fails if a large fraction of the UV radiation emitted from stars cannot escape into the IGM, as suggested by the recent {\it Hopkins Ultraviolet Telescope} observations by Leitherer \etal of low-redshift starburst galaxies, or if most of the metals observed at $z\approx 3$ were produced at much earlier epochs.

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arXiv1995

Radiative Transfer in a Clumpy Universe: II. The Utraviolet Extragalactic Background

We present a detailed calculation of the propagation of AGN-like ionizing radiation through the intergalactic space. We model the ionization state of absorbing clouds, and show that the universe will be more opaque above $4\,$Ryd than previously estimated. Singly ionized helium in \Lya forest clouds and Lyman-limit systems is found to be very efficient in reprocessing soft X-ray, helium-ionizing photons into ultraviolet, hydrogen-ionizing ones. We demonstrate that a significant fraction of the absorbed primary photons (emitted, e.g., by quasar sources) will be reradiated by the photoionized gas through \Lya line emission, two-photon continuum, and recombination continuum radiation. In the light of new data and recent studies, we also reassess the contribution of the QSOs observed in optical surveys to the UV extragalactic background, and find that the stochastic reprocessing of quasar Lyman continuum radiation by hydrogen and helium along the line of sight will significantly affect the amplitude, spectral shape, and fluctuations properties of the metagalactic flux. In a scenario in which QSOs are the primary source of ionizing photons in the universe, the integrated \HI \Lya emission at $z=0$ from photoionized \Lya clouds and Lyman-limit systems is found to be at a level of less than 5\% of current observational limits on the far-UV extragalactic radiation flux. We show that $J_{912}$ increases from $\approx 10^{-23} \uvunits$ at the present epoch to $\approx 5\times 10^{-22}\uvunits$ at $z=2.5$. The attenuated direct quasar emission plus recombination radiation from intergalactic gas appears to provide enough hydrogen-ionizing photons to satisfy the proximity effect at large redshift. The \HeII/\HI ratio in the diffuse intergalactic medium and the \Lya clouds increases from $\approx25$ at

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