Primordial Galaxies Research Articles

A Cosmic Milestone: Constraints from Metal-poor Halo Stars on the Cosmological Reionization Epoch

Theoretical studies and current observations of the high-redshift intergalactic medium (IGM) indicate that at least two cosmic transitions occur by the time the universe reaches gas metallicities of about $10^{-3}$ of solar values. These are the cosmological reionization of the IGM, and the transition from a primordial to present-day mode of star formation. We quantify this relation through new calculations of the ionizing radiation produced in association with the elements carbon, oxygen and silicon observed in Galactic metal-poor halo stars, which are likely second-generation objects formed in the wake of primordial supernovae. We demonstrate that sufficient ionizing photons per baryon are created by enrichment levels of [Fe/H] of about -3 in the environment of metal-poor halo stars to provide the optical depth in the cosmic microwave background of about 0.1 detected by $WMAP$. We show, on a star by star basis, that a genuine cosmic milestone in IGM ionization and star formation mode occurred at metallicities of $10^{-4}$ to $10^{-3}$ solar in these halo stars. This provides an important link in the chain of evidence for metal-free first stars having dominated the process of reionization by redshift 6. We conclude that many of the Fe-poor halo stars formed close to the end of or soon after cosmological reionization, making them the ideal probe of the physical conditions under which the transition from first- to second-generation star formation happened in primordial galaxies.

The Radiative Transport of Dust in Primordial Galaxies and Second‐Generation Star Formation

We investigate the radiative transport of dust in primordial galaxies in the presence of the UV radiation field from the first metal-free stars. We find that dust created in the first supernova (SN) explosions can be driven through the interior of the SN remnant to accumulate in the SN shells, where second-generation stars may form from compressed cooling gas. This scenario requires metal-free stars to form continuously over timescales of up to 10 Myr, consistent with recent estimates. Silicate and graphite grains, as well as iron-bearing magnetites, are transported to the shells for reasonable parameter assumptions, but their relative yields from primordial SNe is an important factor in the resulting abundance ratios. We compare the results of segregated grain transport with the current nucleosynthetic data on extremely metal-poor Galactic halo stars. Fossil signatures of this process may already have been detected in those iron-poor stars with enhanced carbon and silicate elements such as magnesium, silicon and oxygen. We discuss the implications of our results for the transition from first- to second-generation star formation in primordial galaxies, and the role played by the radiative transport of dust in this process.

On the Unusual Depletions toward Sk 155, or What Are the Small Magellanic Cloud Dust Grains Made Of?

The dust in the Small Magellanic Cloud (SMC), an ideal analog of primordial galaxies at high redshifts, differs markedly from that in the Milky Way by exhibiting a steeply rising far-ultraviolet extinction curve, an absence of the 2175 Angstrom extinction feature, and a local minimum at ~12 micron in its infrared emission spectrum, suggesting the lack of ultrasmall carbonaceous grains (i.e. polycyclic aromatic hydrocarbon molecules) which are ubiquitously seen in the Milky Way. While current models for the SMC dust all rely heavily on silicates, recent observations of the SMC sightline toward Sk 155 indicated that Si and Mg are essentially undepleted and the depletions of Fe range from mild to severe, suggesting that metallic grains and/or iron oxides, instead of silicates, may dominate the SMC dust. However, in this Letter we apply the Kramers-Kronig relation to demonstrate that neither metallic grains nor iron oxides are capable of accounting for the observed extinction; silicates remain as an important contributor to the extinction, consistent with current models for the SMC dust.

M82, Starbursts, Star Clusters, and the Formation of Globular Clusters

We observed the nearby starburst galaxy M82 in CO in the higher frequency (2-1) transition to achieve an angular resolution below 1'' or 17 pc at the target. We resolved the molecular gas into a large number of compact clouds, with masses ranging from ~2 × 103 to 2 × 106 M☉. The mass spectrum scales as N(M) ∝ M-1.5±0.1, similar to the mass spectra of young massive star clusters, suggesting that individual molecular clouds are transformed in the starburst into individual star clusters. The larger clouds are surrounded by supernovae and H II regions, suggesting that star formation proceeds from the outside of the clouds and progresses inward, consistent with triggering by a sudden increase in external pressure. The clouds with internal star formation have velocity gradients and inverse P Cygni spectral line profiles indicating inward motions of 35 km s-1, consistent with shock-driven compression. Diffuse free-free radio emission and X-ray emission around the clouds provide evidence for superheated ionized gas sufficient to drive the compression. Clouds with spectral lines indicating expansion show little internal star formation, suggesting that the dynamics precedes and is responsible for the star formation rather than the inverse. M82 is known to be in interaction with the neighboring galaxy M81. The overall picture is consistent with the formation of massive star clusters from individual giant molecular clouds crushed by a sudden galactic-scale increase in external pressure generated by the changing dynamics that result from a near-collision with a neighboring galaxy. Present-day globular clusters may have formed in a similar fashion in primordial galaxies.

FALCON: multi-object AO

FALCON is a wide-field, multi-object integral field spectrograph equipped with adaptive optics. It is dedicated to the study of the formation process of primordial galaxies. The AO system uses natural guide stars, and the high sky coverage required for these studies is obtained using tomographic techniques for the wavefront analysis. The structure of the OA system is very new, and particularly suited for a future implementation on extremely large telescopes. To cite this article: E. Gendron et al., C. R. Physique 6 (2005).

The Nature of Lyα Blobs: Supernova-dominated Primordial Galaxies

We consider a forming galaxy undergoing multitudinous supernova (SN) explosions, as a possible model of \lya blobs (LABs). For this purpose, an ultra--high resolution hydrodynamic simulation is performed using $1024^3$ grid points, where SN remnants are resolved with sufficient accuracy. It is found that multiple SN explosions produce kpc--size expanding hot bubbles, which drive cool, dense shells by strong shock. The colliding high--density cooling shells radiate intensive \lya emission, resulting in a high \lya luminosity of $\sim10^{43}$ erg s$^{-1}$, comparable to the observed level in LABs. Also, recently discovered bubbly features in some LABs are quite similar to the structure predicted in the present simulation. Furthermore, the result demonstrates that LABs are representative of evolving primordial galaxies; they could hold direct information on the early chemical enrichment of galaxies, contrary to present--day galaxies which have undergone intense recycling of interstellar matter, thus erasing most of the early chemical history. It turns out that the metal mixing proceeds in a very inhomogeneous fashion, so that there appears a large spread of metallicity, that is,$[{\rm Fe/H}] \approx 0 {\rm ~to} -5$ or $[{\rm O/H}] \approx 1 {\rm ~to} -4$. Hence, the early galactic chemical evolution may have proceeded in a different manner from that hitherto considered in one--zone models.

The Radio Continuum of the Metal‐deficient Blue Compact Dwarf Galaxy SBS 0335−052

We present new Very Large Array observations at five frequencies, from 1.4 to 22 GHz, of the extremely low metallicity blue compact dwarf SBS 0335-052. The radio spectrum shows considerable absorption at 1.49 GHz and a composite thermal plus nonthermal slope. After fitting the data with a variety of models, we find the best-fitting geometry to be one with free-free absorption homogeneously intermixed with the emission of both thermal and nonthermal components. The best-fitting model gives an emission measure EM ~ 8 × 107 pc cm-6 and a diameter of the radio-emitting region D ≈ 17 pc. The inferred density is ne ~ 2000 cm-3. The thermal emission comes from an ensemble of ~9000 O7 stars, with a massive star formation rate (≥5 M☉) of 0.13-0.15 yr-1 and a supernova (SN) rate of 0.006 yr-1. We find evidence for ionized gas emission from stellar winds, since the observed Brα line flux significantly exceeds that inferred from the thermal radio emission. The nonthermal fraction at 5 GHz is ~0.7, corresponding to a nonthermal luminosity of ~2 × 1020 W Hz-1. We derive an equipartition magnetic field of ~0.6-1 mG and a pressure of ~3 × 10-8 to 1 × 10-7 dynes cm-2. Because of the young age and compact size of the starburst, it is difficult to interpret the nonthermal radio emission as resulting from diffusion of SN-accelerated electrons over 107-108 yr timescales. Rather, we attribute the nonthermal radio emission to an ensemble of compact SN remnants expanding in a dense interstellar medium. If the radio properties of SBS 0335-052 are representative of star formation in extremely low metallicity environments, derivations of the star formation rate from the radio continuum in high-redshift primordial galaxies need to be reconsidered. Moreover, photometric redshifts inferred from "standard" spectral energy distributions could be incorrect.

Rate Coefficient for H + + H 2 ( X [FORMULA][F][SUP]1[/SUP][SUP]+[/SUP][INF]g[/INF][/F][/FORMULA], = 0, J = 0) H(1 s ) + [FORMULA][F][RM]H[/RM][SUP]+[/SUP][INF]2[/INF][/F][/FORMULA] Charge Transfer and Some Cosmological Implications

Krstic has carried out the first quantum mechanical calculations near threshold for the charge transfer (CT) process H^+ + H_2(X ^1Sigma_g^+, nu=0, J=0) --> H(1s) + H_2^+. These results are relevant for models of primordial galaxy and first star formation that require reliable atomic and molecular data for obtaining the early universe hydrogen chemistry. Using the results of Krstic, we calculate the relevant CT rate coefficient for temperatures between 100 and 30,000 K. We also present a simple fit which can be readily implemented into early universe chemical models. Additionally, we explore how the range of previously published data for this reaction translates into uncertainties in the predicted gas temperature and H_2 relative abundance in a collapsing primordial gas cloud. Our new data significantly reduce these cosmological uncertainties that are due to the uncertainties in the previously published CT rate coefficients.

X-ray spectroscopy of Gamma-Ray Bursts

In this review we briefly summarize the recent developments in the research on Gamma-Ray Bursts (GRB), and discuss in more detail the recent results derived from X-ray spectroscopy, in particular the detection of X-ray narrow features and their implication on our understanding on the origin of GRB. Finally, we outline the importance of high-resolution spectroscopy in X-rays, which can provide new clues on the nature of progenitors, and a powerful probe of the early universe and primordial galaxy formation.

Feedback from the First Supernovae in Protogalaxies: The Fate of the Generated Metals

We investigate the chemo-dynamical effects of multiple supernova explosions in the central region of primordial galaxies using three-dimensional hydrodynamical simulations of the inhomogenous interstellar medium down to parsec-scales. We find that the final protogalactic structure and metal distribution depend strongly on the number of SNe. Specifically, 1) 1000 SNe after an instantaneous burst of star formation are sufficient to almost completely blow away the gas in these systems, whereas 2) 100 SN explosions trigger the collapse of the protogalactic cloud, leading to the formation of a cold, dense clumpy disk (n > 300 cm^-3) with metallicity, Z = 4 10^-4 Z_sun. These results imply that the metallicity of the ``second generation'' of stars could be Z ~ 10^-4 Z_sun, and that the environment to form metal-free stars in protogalaxies may be lost relatively quickly (< 10^7 yr) after the first burst of Z=0 star formation. The recently discovered ultra-metal-poor star (Christlieb et al. 2002) may represent surviving members of such second-generation star formation.

On the possibility of observing the double emission line feature of H2 and HD from primordial molecular cloud cores

We study the prospects for observing H2 and HD emission during the assembly of primordial molecular cloud cores. The primordial molecular cloud cores, which resemble those at the present epoch, can emerge around 1 +z∼ 20 according to recent numerical simulations. A core typically contracts to form the first generation of stars and the contracting core emits H2 and HD line radiation. These lines show a double-peak feature. The higher peak is the H2 line of the J= 2–0 (v= 0) rotational transition, and the lower peak is the HD line of the J= 4–3 (v= 0) rotational transition. The ratio of the peaks is about 20, this value characterizing the emission from primordial galaxies. The expected emission flux at the redshift of 1 +z∼ 20 (e.g. ΩM= 0.3 and ΩΛ= 0.7), in the J= 2–0 (v= 0) line of H2 occurs at a rate ∼2 × 10−7 Jy, and in the J= 4–3 (v= 0) line of HD at a rate ∼8 × 10−9 Jy. The former has a frequency of 5.331 79 × 1011 Hz and the latter is at 5.333 88 × 1011 Hz, respectively. Because the frequency resolution of the Atacama Large Millimetre Array (ALMA) is about 40 kHz, the double peak is resolvable. While an individual object is not observable even by ALMA, the expected assembly of primordial star clusters on subgalactic scales can result in fluxes at the 2000–50 μJy level. These are marginally observable. The first peak of H2 is produced when the core gas cools due to HD cooling, while the second peak of HD occurs because the medium maintains thermal balance by H2 cooling which must be enhanced by three-body reactions to form H2 itself.

Towards observations of Population III galaxies

We are undertaking various studies of primordial galaxies with the following main aims: (i)synthesize the properties of rest-frame UV-optical spectra of Population III galaxies and ‘transition objects’ between Pop III and normal starbursts;(ii)devise optimal search strategies for the detection of Pop III and related objects with ground-based and space observations;(iii)simulate photometric and spectroscopic observations of Pop III galaxies to quantify the feasibility of such studies especially for future ground-based instruments; and(iv)study the physics of Pop III and ‘transition’ galaxies and their implications on cosmology and the physics of galaxies

Radiation-Hydrodynamical Simulations of Primordial Galaxy Formation in the UV Background

We study the effects of the ultraviolet background radiation (UVB) on the formation of primordial galaxies, solving self-consistently radiative transfer of photons, non-equilibrium H2 chemistry, and one-dimensional hydrodynamics. The cut-off scale of collapse is shown to drop significantly compared to previous optically thin predictions, due to both self-shielding of the gas and H2 cooling. For plausible radiation intensities and spectra, the collapsing gas can cool efficiently to temperatures well below 104K before rotationally supported. Our results imply that star formation can take place in low-mass objects collapsing in the UVB.

Galaxy Populations and Evolution in Clusters. III. The Origin of Low-Mass Galaxies in Clusters: Constraints from Stellar Populations

Low-mass galaxies in nearby clusters are the most numerous galaxy type in the universe, yet their origin and properties remain largely unknown. To study basic questions concerning these galaxies we present the results of a survey designed to constrain the characteristics and properties of the stellar populations in a magnitude complete sample of low-mass cluster galaxies (LMCGs) in the center of the Perseus cluster. Using deep, high-quality WIYN UBR images, we demonstrate that the 53 LMCGs in our sample have a significant scatter about the color-magnitude relationship at M_B > -15. By comparing single-burst stellar population models to our photometry, we argue that stellar populations in LMCGs all have ages > 1 Gyrs, with redder LMCGs having stellar metallicities [Fe/H] > -0.5. By assuming that the colors of LMCGs reflect metallicity, and have co-evolved with the giant ellipticals, we find a wide range of values, from solar to [Fe/H] ~ -3. We argue from this that LMCGs have multiple origins, and fundamentally differ from Local Group dwarf spheroidals/ellipticals. The inferred lower metallicities of the bluer LMCGs implies that these are possibly primordial galaxies formed through self enrichment and stellar feedback provided by winds from supernova. We also investigate several other formation scenarios for these LMCGS, including: self enrichment induced by the confinement of metals in halos by the intracluster medium, in situ formation out of intracluster gas, systems with extreme dark halos, and as remnants of higher mass systems. We conclude that roughly half of all low-mass cluster galaxies have stellar populations and kinematic properties, discussed in previous papers in this series, consistent with being the remnants of stripped galaxies accreted into clusters a few Gyrs ago.

The transition from Population III to normal galaxies: Lyα and He II emission and the ionising properties of high redshift starburst galaxies

Using new sets of stellar evolution models at very low metallicities (, 10-5) and previously published grids we examine spectral properties of the ionising continua, the Lyman-break, and the Lyα and He ii λ1640 recombination lines in starbursts. The metallicity dependence of these properties, especially the transition from primordial galaxies (Population III) to currently observed metallicities, is examined for various IMFs and star formation histories. For the average properties of starbursts, approximated by a model with constant star formation, the main findings are: For young bursts the maximum Lyα equivalent width is shown to increase strongly with decreasing metallicity from Lyα –350 Å at to 400–850 Å or higher at Z between 10-5 and 0 (Pop III) for the same Salpeter IMF. However, for well known reasons, the Lyα emission predicted likely represents an upper limit. Non-negligible He ii λ1640 emission due to stellar photoionisation appears to be limited to very small metallicities ( ) and Population III objects. The predictions, available on the Web through the CDS and at http://webast.ast.obs-mip.fr/sfr/, should be useful for a variety of studies regarding high redshift galaxies, cosmological reionisation, and others.

A Bridge from Optical to Infrared Galaxies: Explaining Local Properties and Predicting Galaxy Counts and the Cosmic Background Radiation

We give an explanation for the origin of various properties observed in local infrared galaxies and make predictions for galaxy counts and cosmic background radiation (CBR) using a new model extended from that for optical/near-infrared galaxies. Important new characteristics of this study are that (1) mass scale dependence of dust extinction is introduced based on the size-luminosity relation of optical galaxies and that (2) the large-grain dust temperature Tdust is calculated based on a physical consideration for energy balance rather than by using the empirical relation between Tdust and total infrared luminosity LIR found in local galaxies, which has been employed in most previous works. Consequently, the local properties of infrared galaxies, i.e., optical/infrared luminosity ratios, LIR-Tdust correlation, and infrared luminosity function are outputs predicted by the model, while these have been inputs in a number of previous models. Our model indeed reproduces these local properties reasonably well. Then we make predictions for faint infrared counts (in 15, 60, 90, 170, 450, and 850 μm) and CBR using this model. We found results considerably different from those of most previous works based on the empirical LIR-Tdust relation; especially, it is shown that the dust temperature of starbursting primordial elliptical galaxies is expected to be very high (40-80 K), as often seen in starburst galaxies or ultraluminous infrared galaxies in the local and high-z universe. This indicates that intense starbursts of forming elliptical galaxies should have occurred at z ~ 2-3, in contrast to the previous results that significant starbursts beyond z ~ 1 tend to overproduce the far-infrared (FIR) CBR detected by COBE/FIRAS. On the other hand, our model predicts that the mid-infrared (MIR) flux from warm/nonequilibrium dust is relatively weak in such galaxies making FIR CBR, and this effect reconciles the prima facie conflict between the upper limit on MIR CBR from TeV gamma-ray observations and the COBE detections of FIR CBR. The intergalactic optical depth of TeV gamma rays based on our model is also presented.

Low Frequency Science with the Square Kilometre Array

Over the past several years an international community of scientists and engineers has emerged with a common goal to solve the technical challenge required to construct a giant radio telescope with a collecting area of one square kilometre. The Square Kilometre Array (SKA) will have a hundred times more collecting area than our most powerful existing radio telescopes, providing sensitivity of a few tens of nanoJy in the centimetre/decimetre wavelength continuum. With a spatial resolution better than the Hubble Space Telescope, a field of view larger than the full moon, and the ability to simultaneously image a wide range of red shift, the SKA will be the worlds premier spectroscopic imaging telescope at any waveband.At long wavelengths the SKA will be able to detect emission from atomic hydrogen gas at extreme redshifts, allowing study of the “Dark Ages” of the Universe, before, and during, the transition phase when the initial stars formed and reionization occurred. The combination of sensitivity, wide field of view and high angular resolution, will allow high resolution imaging of the interstellar media and magnetic field of a vast number of galaxies to high redshift. Measurements of atomic hydrogen emission and continuum emission will trace the star formation history of the Universe from primordial galaxies to the present.

Multiwavelength Observations of the Low-Metallicity Blue Compact Dwarf Galaxy SBS 0335−052

New infrared and millimeter observations from Keck, Palomar, ISO, and OVRO and archival data from the NRAO^(**) VLA and IRAS are presented for the low-metallicity blue compact dwarf galaxy SBS 0335-052. Mid-infrared imaging shows this young star-forming system is compact (0farcs31; 80 pc) at 12.5 μm. The large Brγ equivalent width (235 A) measured from integral field spectroscopy is indicative of a ~5 Myr starburst. The central source appears to be optically thin in emission, containing both a warm (~80 K) and a hot (~210 K) dust component, and the overall interstellar radiation field is quite intense, about 10,000 times the intensity in the solar neighborhood. CO emission is not detected, though the galaxy shows an extremely high global H I gas-to-dust mass ratio, high even for blue compact dwarfs. Finally, the galaxy's ratio of mid-infrared-to-optical and mid-to-near-infrared luminosities are quite high, whereas its far-infrared-to-radio and far-infrared-to-optical flux ratios are surprisingly similar to what is seen in normal star-forming galaxies. The relatively high bolometric infrared-to-radio ratio is more easily understood in the context of such a young system with negligible nonthermal radio continuum emission. These new lines of evidence may outline features common to primordial galaxies found at high redshift.

Feedback from Galaxy Formation: Production and Photodissociation of Primordial H2

We use one-dimensional radiative transfer simulations to study the evolution of H2 gas-phase (H- catalyzed) formation and photodissociation regions in the primordial universe. We find a new positive feedback mechanism capable of producing shells of H2 in the intergalactic medium (IGM), which are optically thick in some Lyman-Werner bands. While these shells exist, this feedback effect is important in reducing the H2 dissociating background flux and the size of photodissociation spheres around each luminous object. The maximum background opacity of the IGM in the H2 Lyman-Werner bands is ? ? 1-2 for a relic molecular fraction x = 2 ? 10-6, about 6 times greater than that found by Haiman, Abel, & Rees. Therefore, the relic molecular hydrogen can decrease the photodissociation rate by about an order of magnitude. The problem is relevant to the formation of small primordial galaxies with masses MDM 108 M? that rely on molecular hydrogen cooling to collapse. Alternatively, the universe may have remained dark for several hundred million years after the birth of the first stars, until galaxies with virial temperature Tvir 104 K formed.

H2 Formation in Low-Metallicity Galaxies

Abstract A possible formation mechanism of hydrogen molecules on a galactic scale is examined. We are interested especially in the role of hydrogen molecules for the formation and evolution of primordial galaxies. Thus, the formation process of hydrogen molecules in a very low-metallicity galaxy (I Zw 18; the most typical metal-deficient galaxy) is studied. Adopting a recent observational result of the absorption lines of hydrogen molecules in I Zw 18, we obtain the upper limit for the ionization degree in the case where hydrogen molecules can form via the $\mathrm{H}^{-}$-process, although they are generally believed to form on the surface of dust grains. Furthermore, we present a critical ionization degree, above which the $\mathrm{H}^{-}$-process can be dominant over the formation process on the surface of grains. Interestingly, this critical ionization degree is comparable to the upper limit of the ionization degree for I Zw 18. For determining the formation process of hydrogen molecules, future observational facilities can be useful. Thus, we examine the detectability in some wavelengths for metal-deficient galaxies. According to our estimate, the near-infrared line emission of hydrogen molecules is observable at the level of $10\,\mu\mathrm{Jy}$, the free–free radio emission is at the level of mJy, and the far-infrared emission from the dust on which hydrogen molecules form can also be detected at the 10-mJy level with its temperature of 16 K. The near-infrared line and the far-infrared continuum are feasible for ASTRO-F observations.