Protogalaxies Research Articles

Hyperluminous infrared galaxies

39 galaxies are now known, from follow-up of faint IRAS sources and from submm observations of high redshift AGN, with far infrared luminosities > 10^{13} Lo. 13 of these, which have been found in 60 or 850 mu surveys, form an important unbiased sub-sample. 12 have been found by comparison of 60 mu surveys with quasar or radio-galaxy catalogues, or from infrared surveys with colour selection biased towards AGN, while a further 14 have been found through submm observations of known high redshift AGN. In this paper I argue, on the basis of detailed modelling of the spectral energy distributions of hyperluminous galaxies with accurate radiative transfer models, and from evidence of high gas-mass in several cases, that the bulk of the emission from these galaxies at rest-frame wavelengths >= 50 mu is due to star formation. Even after correction for the effects of lensing, hyperluminous galaxies with emission peaking at rest-frame wavelengths >= 50 mu are therefore undergoing star-formation at rates > 1000 Mo/yr and are strong candidates for being primeval galaxies, in the process of a major episode of star formation.

Submillimeter Observations of IC 10: The Dust Properties and Neutral Carbon Content of a Low‐Metallicity Starburst

We present submillimeter observations of the Local Group, metal-poor, irregular dwarf galaxy IC 10, directly relevant to the interaction between interstellar medium and star formation activity in primeval galaxies. Using the JCMT, we have observed the fine structure neutral carbon transition at 492 GHz and the rotational J=3-2 transition of 12CO and 13CO in the most massive giant molecular cloud complex in this galaxy, IC10-SE. We find that, although the I([CII])/I(CO) ratio for this object is a factor of 4 larger than the typical Milky Way value, its [C~I] to CO intensity ratio is I([CI])/I(CO)~18 is similar (only about 50% larger) to that of the Milky Way. Modelling of the [CII]/CO and [CI]/CO intensity ratios with metallicity indicates that, if C+ and C are chiefly produced by UV photodissociation, both ratios should increase sharply with decreasing metallicity (and consequently diminished UV shielding; Bolatto, Jackson, and Ingalls 1999). These data then suggest a different origin for an important fraction of C in these clouds, unrelated to photodissociation. We have also mapped the 850 um continuum in this region using SCUBA. Employing these data in conjunction with KAO and IRAM measurements we find an extremely low emissivity exponent, b~0.5. We conclude that this low exponent is most likely due to the destruction of small dust grains, brought about by the increased penetration of UV radiation in the low metallicity ISM. If a low b in the submillimeter is a general property of metal-poor systems then the interpretation of millimeter and submillimeter surveys of high-z galaxies should be revised.

Magnetization of the Intergalactic Medium by Primeval Galaxies

We review what has recently been learned from starburst-driven superwinds, galaxy-scale outflows, dwarf galaxy surveys, and evidence for early-universe merging scenarios. We apply this knowledge to simplified model populations of dwarf galaxies in the primeval galaxy era to demonstrate how a substantial fraction of the intergalactic medium can be permeated with magnetic fields by dwarf galaxy outflow winds in the volcanic early Mechanisms of both magnetic field regeneration and diffusion are discussed as they apply to galaxies at both primeval and later epochs of the universe. We propose that, in the past, magnetic field strengths in outflow superbubbles and halos were stronger than those now prevailing in the disks of current-epoch galaxies such as the Milky Way. We also show how subsequent acausal diffusion of magnetic fields (which we have not modeled) can propagate the fields over great distances in the universe within a Hubble time. We conclude that dwarf galaxies can effectively seed the intergalactic medium with magnetic fields for a wide range of outflow and galaxy density parameters. If the first galaxies form at or before z~10 in a bottom-up hierarchical merging scenario, a substantial fraction of the intergalactic medium will become permeated with a magnetic field at the present epoch. This intergalactic field seeding is largely accomplished by z~6.

Galaxy Evolution from Deep Field Galaxy Counts

We present deep galaxy number counts and colours of K—band selected galaxy surveys. We argue that primeval galaxies are present within the survey data, but have remained unidentified. There are few objects with the colours of an L* elliptical galaxy at a redshift of z ≈ 1, in contradiction to standard luminosity evolution models. We present K—band photometry of the objects in a spectroscopic redshift survey selected at 21 < B < 22.5. The absolute K magnitudes of the galaxies are consistent with the no-evolution or pure luminosity evolution models. The excess faint blue galaxies seen in the B—band number counts at intermediate magnitudes are a result of a low normalization, and do not dominate the population until B ≈ 25. Extreme merging or excess dwarf models are not needed at z < 1.

Searches for primeval galaxies

A primeval galaxy represents the earliest stages of a galaxy's life and as such provides clues to the early history of the Universe and the evolution of stars and galaxies. Over the last 20 years astronomers have been engaged in the quest to detect the faint signals from these objects, believed to lie at a distance comparable with the size of the Universe. A wide variety of observational techniques have been employed in this search, with astronomers eagerly awaiting each new generation of astronomical telescope or detector in the hope of finally solving the mystery to the origin of galaxies- or at least placing new and interesting constraints. Until recently, primeval galaxies have eluded detection in these searches, however experiments over the last couple of years which use either 10 m-class optical telescopes or state-of-the-art submillimetre array detectors, may hold the clue to the origin of structure as they have finally uncovered what appears to be a widespread population of young galaxies.

Metal Enrichment of Ly Alpha Clouds and the Intergalactic Medium

We have considered the metal enrichment of the intergalactic medium (IGM) and Lyα clouds at high redshifts. The CIV absorption lines associated with the Lyα forest have been observed (e.g. Songaila &amp; Cowie 1996) and the metallicity is estimated as ~0.01Z⊙. Here, based on a galactic wind model, we have examined how the IGM is metal-enriched by the outflows driven by supernova explosions in primeval galaxies and minihalos.

Evolutionary Population Synthesis Models of Primeval Galaxies: A Critical Appraisal

A recognized problem when searching for primeval galaxies at cosmological distances is a definition of a firm selection criterion (alternative to a direct but extremely time-demanding measure of z) to single out high-redshift candidates from the plethora of other more or less peculiar objects (i.e. AGN, starburst galaxies etc.) at lower distances.The case of the HST “Deep Field” (HDF) observations (Williams et al. 1996) is especially relevant in this regard as galaxies up to z = 3.2 have been detected in the field (Steidel et al. 1996).

Polarization of the L[CLC]y[/CLC]α Line from an Anisotropically Expanding H [CSC]i[/CSC] Shell in Primeval Galaxies

We compute the polarization of the Lyα photons emerging from an anisotropically expanding and optically thick medium, which is expected to operate in many Lyα-emitting primeval objects, including damped Lyα absorbers and Lyman break galaxies. In the case of a highly optically thick medium, the escape of resonance-line photons is achieved by a large number of local resonant scatterings followed by a small number of damping-wing scatterings. We show that some polarization can develop because wing scatterings are coupled with strong spatial diffusion that depends on the scattering geometry and kinematics. The case of a slab with a finite scattering optical depth and an expansion velocity of ~100 km s-1 is investigated. When the typical scattering optical depth τ104, it is found that the emergent Lyα photons are linearly polarized with a degree of polarization up to 0.1 in the direction perpendicular to the slab plane. We subsequently investigate the polarization of the Lyα photons emerging from a hemispherical shell obscured partially by an opaque component, and we obtain ~5% of the polarization. We propose that the polarimetry of Lyα emission may provide important clues to the geometrical and kinematical structures of the H I shells in primeval galaxies and nearby starburst galaxies.

A Galaxy at [ITAL]z[/ITAL] = 5.34

We report the discovery of Lyman alpha emission from a galaxy at z=5.34, the first object at z>5 with a spectroscopically confirmed redshift. The faint continuum emission (m(8000A)=27 AB mag), relatively small rest-frame equivalent width of the emission line (rest equivalent width of 95A), and limits on the NV/Lya ratio suggest that this is a star-forming galaxy and not an AGN. The star-formation rates implied by the UV continuum emission and the Lyman alpha emission are (in the absence of dust extinction) fairly modest (about 6 Msun/yr for q0=0.5, H0=50). The continuum luminosity is similar to that of sub-L*(1500) star-forming galaxies at z~3, and the width of the Lyman alpha line yields an upper limit to the mass of < 2.6 x 10^{10} Msun. The strong emission line detected in this low-luminosity galaxy provides hope for the discovery of higher luminosity primeval galaxies at redshifts z>5.

The seeds of rich galaxy clusters in the Universe

The discovery1 of a population of young galaxies at a redshift when the Universe was about a tenth of its current age has shed new light on the question of when and how galaxies formed. Within the context of popular models2, this is the population of primeval galaxies that built themselves up to the size of present-day galaxies through the process of repeated mergers called hierarchical clustering. But the recent detection3 of a large concentration of these primeval galaxies appears to be incompatible with hierarchical clustering models, which generally predict that clusters of this size are fully formed later in time. Here we use a combination of theoretical techniques — semi-analytic modelling and n-body simulations — to show that such large concentrations should be quite common in a universe dominated by cold dark matter, and that they are the progenitors of the rich galaxy clusters seen today. We predict the clustering properties of primeval galaxies which should, when compared with data that will be collected in the near future, test our current understanding of galaxy formation within the framework of a universe dominated by cold dark matter.

“Baade's Red Sheet” Resolved into Stars with [ITAL]Hubble Space Telescope[/ITAL] in the Blue Compact Dwarf Galaxy VII Zw 403

Hubble Space Telescope Wide Field Planetary Camera 2 observations of the nearby blue compact dwarf galaxy VII Zw 403 (=UGC 6456) resolve single stars down to MI ≈ -2.5, deep enough to identify red giants. This population has a more uniform spatial distribution than the young main-sequence stars and supergiants, forming the structure known as "Baade's red sheet." We conclude that VII Zw 403 is not a primeval galaxy.

Evolution of X-Ray Clusters of Galaxies with Active Protogalaxies

We consider heating due to proto galaxies in the formation process of clusters of galaxies, since much metal is observed in the intracluster gas which must be ejected from protogalaxies vir strong galactic winds and the metal abundance in the intracluster gas correlates with the fraction of early type galaxies in clusters (Arnaud 1993). We also consider radiative cooling. From the difference between the observed LX - TX relation of X-ray cluster (Hatsukade 1989), LX ∝ T2.7-3.3X, and the prediction from the self-similar model (Kaiser 1986), LX ∝ T2X, it is pointed out that physical processes which are not taken into account in the self-similar model, e.g., effect of radiative cooling, and/or effect of proto galaxy heating, play a important role in the formation process (Evrard and Henry 1991, Kaiser 1991).

Detection of candidate Lukhcy-emitting galaxies at z=5.7

We report the detection of seven candidate Lyα-emitting galaxies which have been found during the preliminary phase of the ‘Calar Alto Deep Imaging Survey’ (CADIS). They are selected from 147 emission-line galaxies detected in a field of 60 arcmin2 with a Fabry–Perot interferometer, following the exclusion of 140 sources identified as foreground galaxies by a combination of broad- and narrow-band filters. Although no conclusive spectroscopic confirmation is yet available, we expect from an analysis of possible residual contamination by foreground objects that two or more of the candidates will turn out to be genuine Lyα-emitting primeval galaxies at a redshift of z=5.7. For a flat universe (q0=0.5) this would be in perfect agreement with refined predictions of the abundance of primeval galaxies and various recent results on the space density of young galaxies at z≤4.5.

The Calar Alto Deep Imaging Survey: First Results

The Calar Alto Deep Imaging Survey (CADIS) is a very deep emission line survey using a Fabry-Pérot (FP), combined with deep broad- and medium-band photometry (for an overview see Hippelein et al. 1996). This survey is specifically designed to detect primeval galaxies, but it will in addition produce a large data base for investigations of faint galaxies at intermediate redshifts (0.2 &lt; z &lt; 1.2). We present some first results from the initial data recorded with the CADIS strategy.

The optical identification of a primeval galaxy atz 4.4

We have obtained with the SUSI CCD camera on the ESO 3.5m NTT deep images in the BVRI bands of the field centered on the QSO BRI 1202-0725 ($z_{\it em}=4.694$). In the final combined frames the stellar images have FWHM of 1,1,0.6 and 0.65 arcsec respectively. The R and I images show clearly a galaxy $2.2''$ from the QSO, corresponding to $13h^{-1}_{50}$ kpc at $z\sim 4.5$. Possible identification with the metal absorption systems seen in the line of sight to the QSO, including the highest redshift damped system known to date at $z=4.383$, are discussed. We conclude that its colours can be reconciled only with the spectrum of a primeval galaxy at z >~ 4.4, making it the most distant galaxy detected so far. From its magnitudes and models of young galaxy evolution we deduce that it is forming stars at a rate $\sim 30 M_{\odot}$ yr$^{-1}$ and has an estimated age of the order of $10^8$ yr or less, implying that the bulk of the stellar population formed at $z < 6$.

The Quest for Protogalaxies

The current state and the future prospects of searches for protogalaxies (PGs) are reviewed. Many high-redshift objects are now known, mostly associated in some way with AGN, and at least some of them may be young galaxies. Quasars at z &gt; 4 and high-z quasar pairs may mark galaxy formation sites. Deep field surveys for Lyα luminous galaxies powered by star formation alone have failed so far to reveal a large population of such objects, and the observed limits are in conflict with simple model predictions by three orders of magnitude. Some extinction by dust can account for this. However, strong limits from COBE place severe constraints on models of completely obscured PGs. New searches in the near IR are now beginning to probe the relevant line flux and number density regime, and first interesting PG candidates are being discovered. Searches from mid-IR to mm wavelengths would complement these efforts.

Did the Bulge Form All at Once?

It is reasonable to say that if Jan Oort were alive today, he would no doubt find recent developments in the study of the Galactic bulge to be fascinating. Oort considered the Galactic bulge in two contexts. First, he was interested in the use of the RR Lyrae stars as probes to determine the distance to the Galactic Center. No doubt, Oort would have been excited about the growing evidence of the bulge's triaxiality, as well as by the debate over the age of the bulge. His second interest was in the nature of activity at the center, an issue that I will not discuss in this review. The latter also remains an unsolved problem of the Milky Way, and (based on his work) one that might have been nearer to his heart than this one. Yet the question of when the bulge formed is ultimately a question about the formation history of the Galaxy. The oldest stars (those whose ages we are certain of) are found in Galactic globular clusters, the sum total of which are ≈ 5 × 107M⊙. The field population of the bulge is ≈ 2–3 × 1010M⊙, an order of magnitude more massive than the field population of the metal poor spheroid. So if the bulge formed all at once, and early, then the Milky Way had a luminous, even cataclysmic youth. But if the bulge formed later in the history of our galaxy, as a starburst or dynamical instability of the central disk, then the young Milky Way may have been inconspicuous and primeval galaxies may be hard to find indeed. If our bulge formed very early, its stellar population might have much in common with the giant ellipticals, while a late bulge might teach us much about processes that affect galaxy evolution.

Search for Primeval Galaxies with the “Calar Alto Deep Imaging Survey”

Understanding the formation of galaxies is certainly one of the major challenges in astrophysics. Observationally, the most important insight will be provided by the detection and study of primeval galaxies. With the large telescopes and the efficient detectors available today, it seems feasible to observe those early stages of galaxy evolution directly. Despite considerable efforts with different techniques (Cowie et al. 1990; De Propris et al. 1993; Pritchet &amp; Hartwick 1990; Thompson et al. 1995; Djorgovski et al. 1993) no primeval galaxy is found yet, in contradiction with predictions by model calculations (e.g. Baron &amp; White 1987). What could be the reason for this lack of sucess?

Number Density Predictions for Primeval Galaxies

In the last two decades several methods have been employed to search for the ancestors of present day galaxies undergoing their first starburst at high redshift, the so-called “primeval galaxies” (PGs). According to predictions of the Ly-α flux from the PG phase current surveys for Ly-α bright PGs should have detected 101 − 103 PGs. Yet, no really good candidate has been found. One possible explanation for this discrepancy would be the presence of dust which could strongly suppress Ly-α. In fact, models of early galaxy formation are able to produce gas to dust ratios of about 1/10 solar after some 108 years. This would suppress the emerging Ly-α flux below current detection limits. Thus, to detect a PG in its Ly-α bright phase, one has to hit exactly the redshift of their first star formation. On the other hand, the epoch of galaxy formation is likely to extend over much longer time. So when searching a finite volume of the universe (given by the depth Δz and the field ΔΩ), the protogalaxies are not simultaniously in the Ly-α bright PG phase and only a fraction of them exceed the detection limit. Our new attempt to predict the expected number of PGs takes this reduced propability into account. Further contraints of the Ly-α luminosity were derived from recent observations and theoretical work (for details see E.Thommes &amp; K.Meisenheimer in Galaxies in the Young Universe, Eds. H.Hippelein et al., Springer-Verlag, 1995, p.242). As expected, we find that the number density of Ly-α emitting PGs is drastically reduced in comparison to previous predictions. Thus previous surveys had no realistic chance to find them. Nevertheless, our calculations also show, that the detection of Ly-α bright PGs is in the scope of present day techniques. Specifically, we predict that the Calar-Alto-Deep-Imaging-Survey (see contribution of H. Hippelein et. al., these proc.) which will search for Ly-α in an area of ≈ 0.3□° down to a limiting line flux of 3 × 10−20W/m2 should detect 10 … 100 PGs in three redshift intervals Δz = 0.1. The uncertainty accounts for our ignorance of q0 and the unknown epoch of galaxy formation.

Searching the Continuum for Primeval Galaxies

A population of primeval galaxies (PG's) should be detectable by directly imaging with two intermediate-band filters tuned to either side of the Lyman break (DeRobertis, M. M., and McCall, M. L. 1995, A.J., 109, 1947). In the figure below, the solid and short-dashed curves show the flux (left scale) as a function of redshift from a PG 0.7 Gyr old with a total stellar mass of as seen through filters with rest-frame passbands 890 ± 30 Å (‘β’) and 1010 ± 30 Å (‘ρ’), respectively, moved to redshift 5. The upper curves depict the colour β – ρ (right scale); the dotted line is for the 0.7 Gyr population, and the dot-dashed line is for a 7.5 Gyr model. A source can be identified as a PG if it can be clearly detected in the ρ filter and if it has a colour greater than +0.75 mag. Confusion with any old stellar systems at lower redshifts can be eliminated by supplementing observations with Gunn r and i. The colour condition selects Lyman break objects between redshifts 4.7 and 5.4, a range over an order of magnitude greater than is achievable through an emission line survey. The discriminatory power of the technique is not affected by internal dust.