Early Stages Of Galaxy Formation Research Articles

Self-regulated growth of supermassive black holes by a dual jet-heating active galactic nucleus feedback mechanism: methods, tests and implications for cosmological simulations

We develop a new sub-grid model for the growth of supermassive Black Holes (BHs) and their associated Active Galactic Nuclei (AGN) feedback in hydrodynamical cosmological simulations. Assuming that BHs are created in the early stages of galaxy formation, they grow by mergers and accretion of gas at a Eddington-limited Bondi accretion rate. However this growth is regulated by AGN feedback which we model using two different modes: a quasar-heating mode when accretion rates onto the BHs are comparable to the Eddington rate, and a radio-jet mode at lower accretion rates. In other words, our feedback model deposits energy as a succession of thermal bursts and jet outflows depending on the properties of the gas surrounding the BHs. We assess the plausibility of such a model by comparing our results to observational measurements of the coevolution of BHs and their host galaxy properties, and check their robustness with respect to numerical resolution. We show that AGN feedback must be a crucial physical ingredient for the formation of massive galaxies as it appears to be the only physical mechanism able to efficiently prevent the accumulation of and/or expel cold gas out of halos/galaxies and significantly suppress star formation. Our model predicts that the relationship between BHs and their host galaxy mass evolves as a function of redshift, because of the vigorous accretion of cold material in the early Universe that drives Eddington-limited accretion onto BHs. Quasar activity is also enhanced at high redshift. However, as structures grow in mass and lose their cold material through star formation and efficient BH feedback ejection, the AGN activity in the low-redshift Universe becomes more and more dominated by the radio mode, which powers jets through the hot circum-galactic medium.

The Disk-Halo Connection and Where Has All The Gas Gone?

The wealth of data in the past decades, and especially in the past 15 years has transformed our picture of the gas around the Milky Way and other spiral galaxies. There is good evidence for extraplanar gas that is a few kpc in height and is seen in all gaseous phases: neutral; warm atomic; and hot, X-ray emitting gas. This medium is seen not only around the Milky Way, but other spiral galaxies and it is related to the star formation rate, so it is likely produced by the activity in the disk through a galactic fountain. More extended examples of halo gas are seen, such as the HVC around the Milky Way and around M 31. This gas is typically 10–20 kpc from the galaxy and is not seen beyond 50 kpc. This gas is most likely being accreted. A hot dilute halo (106 K) is present with a similar size, although its size is poorly determined.An ongoing controversy surrounds the relative amounts of outflow from the disk and accretion onto galaxies such as the Milky Way. There is good evidence for accretion of cold material onto the Milky Way and other galaxies, but it is not clear if there is enough to modify the gas content and star formation properties in the disk. The reservoir of accretion material is as yet unidentified. Some of these findings may be related to the issue that galaxies are baryon-poor: their baryon to dark matter ratio is well below the cosmological value. The absence of baryons may be due to extremely violent outflow events in the early stages of galaxy formation. We may be able to understand this stage of galaxy evolution by applying our deepening understanding of our local disk-halo environment.

The First Magnetic Fields

We review current ideas on the origin of galactic and extragalactic magnetic fields. We begin by summarizing observations of magnetic fields at cosmological redshifts and on cosmological scales. These observations translate into constraints on the strength and scale magnetic fields must have during the early stages of galaxy formation in order to seed the galactic dynamo. We examine mechanisms for the generation of magnetic fields that operate prior during inflation and during subsequent phase transitions such as electroweak symmetry breaking and the quark-hadron phase transition. The implications of strong primordial magnetic fields for the reionization epoch as well as the first generation of stars is discussed in detail. The exotic, early-Universe mechanisms are contrasted with astrophysical processes that generate fields after recombination. For example, a Biermann-type battery can operate in a proto-galaxy during the early stages of structure formation. Moreover, magnetic fields in either an early generation of stars or active galactic nuclei can be dispersed into the intergalactic medium.

Extremely metal-poor stars in classical dwarf spheroidal galaxies: Fornax, Sculptor, and Sextans

We present the results of a dedicated search for extremely metal-poor stars in the Fornax, Sculptor and Sextans dSphs. Five stars were selected from two earlier VLT/Giraffe and HET/HRS surveys and subsequently followed up at high spectroscopic resolution with VLT/UVES. All of them turned out to have [Fe/H] <= -3 and three stars are below [Fe/H] -3.5. This constitutes the first evidence that the classical dSphs Fornax and Sextans join Sculptor in containing extremely metal-poor stars and suggests that all of the classical dSphs contain extremely metal-poor stars. One giant in Sculptor at [Fe/H]=-3.96 +- 0.10 is the most metal-poor star ever observed in an external galaxy. We carried out a detailed analysis of the chemical abundances of the alpha, iron peak, and the heavy elements, and we performed a comparison with the Milky Way halo and the ultra faint dwarf stellar populations. Carbon, barium and strontium show distinct features characterized by the early stages of galaxy formation and can constrain the origin of their nucleosynthesis.

Measuring the GC luminosity function up to z ~ 0.2

AbstractGlobular clusters (GCs) are stellar systems (~106 M⊙) with very regular symmetry, single age, and single metallicity. Spectroscopic studies have revealed very old ages, suggesting that GCs were formed in the earliest stages of galaxy formation and assembly. The aim of this work is to find out how far we can measure the GC luminosity function, specific frequency, and radial distribution, applying the surface-brightness-fluctuations (SBF) technique to deep ACS images. To this end, we apply the effects caused by higher redshift to HST/ACS images (in two optical bands, F606W and F814W) of M87, an elliptical galaxy with a very well-studied GC system. The effects involved are: (i) evolution, (ii) inverse k correction, (iii) binning of the image to smaller angular size, (iv) cosmological dimming of surface brightness, and (v) noise addition to account for different exposure times. After processing the images we detect the brightest GCs through direct photometry (e.g., with SExtractor), whereas the unresolved clusters are measured through SBFs. The above treatment is repeated for z=0.05, 0.1, 0.14, and 0.18, and the results are compared to the measurements at z=0 to estimate biases and incompleteness.

HALO DENSITY REDUCTION BY BARYONIC SETTLING?

We test the proposal by El-Zant et al that the dark matter density of halos could be reduced through dynamical friction acting on heavy baryonic clumps in the early stages of galaxy formation. Using N-body simulations, we confirm that the inner halo density cusp is flattened to 0.2 of the halo break radius by the settling of a single clump of mass \ga 0.5% of the halo mass. We also find that an ensemble of 50 clumps each having masses \ga 0.2% can flatten the cusp to almost the halo break radius on a time scale of \sim9 Gyr, for an NFW halo of concentration 15. We summarize some of the difficulties that need to be overcome if this mechanism is to resolve the apparent conflict between the observed inner densities of galaxy halos and the predictions of LCDM.

Galactic archeology with extremely metal-poor stars

AbstractExtremely metal-poor (EMP) stars are thought to be formed in the low-mass protogalaxies as building blocks of the Milky Way and can be probes to investigate the early stage of galaxy formation and star formation in the early universe. We study the formation history of EMP stars in the Milky Way halo using a new model of chemical evolution based on the hierarchical theory of the galaxy formation. We construct the merging history of the Milky Way halo based on the extended Press-Schechter formalism, and follow the star formation and chemical evolution along the merger tree. The abundance trends and number of low-mass stars predicted in our model are compared with those of observed EMP stars. Additionally, in order to clarify the origin of hyper metal poor stars, we investigate the change of the surface metal abundances of stars by accretion of interstellar matter. We also investigate the pre-enrichment of intergalactic matter by the first supernovae.

Introduction: recent developments in the study of gamma-ray bursts

Gamma-ray bursts (GRBs) are immensely powerful explosions, originating at cosmological distances, whose outbursts persist for durations ranging from milliseconds to tens of seconds or more. In these brief moments, the explosions radiate more energy than the Sun will release in its entire 10Gyr lifetime. Current theories attribute these phenomena to the final collapse of a massive star, or the coalescence of a binary system induced by gravity wave emission. New results from Swift and related programmes offer fresh understanding of the physics of GRBs, and of the local environments and host galaxies of burst progenitors. Bursts found at very high red shifts are new tools for exploring the intergalactic medium, the first stars and the earliest stages of galaxy formation. This Royal Society Discussion Meeting has brought together leading figures in the field, together with young researchers and students, to discuss and review the latest results from NASA's Swift Gamma-ray Burst Observatory and elsewhere, and to examine their impact on current understanding of the observed phenomena.

Chemodynamics of Lyman alpha emitters, Lyman break galaxies and elliptical galaxies

We report an ultra-high-resolution simulation that follows evolution from the earliest stages of galaxy formation through the period of dynamical relaxation. The bubble structures of gas revealed in our simulation ( 8 years) resemble closely the high-redshift Lyman α emitters (LAEs). After 10 9 years these bodies are dominated by stellar continuum radiation and look like the Lyman break galaxies (LBGs) known as the high-redshift star-forming galaxies at which point the abundance of elements appears to be solar. After 1.3×10 10 years, these galaxies resemble present-day ellipticals. The comparisons of simulation results with the observations of elliptical galaxies allow us to conclude that LAEs and LBGs are infants of elliptical galaxies or bulge systems in the nearby universe.

Angular Momentum Transfer in Dark Matter Halos: Erasing the Cusp

We propose that angular momentum transfer from the baryons to the Dark Matter (DM) during the early stages of galaxy formation can flatten the halo inner density profile and modify the halo dynamics. We compute the phase-space distribution function of DM halos, that corresponds to the density and anisotropy profiles obtained from N-body simulations in the concordance cosmology. We then describe an injection of angular momentum into the halo by modifying the distribution function, and show that the system evolves into a new equilibrium configuration; the latter features a constant central density and a tangentially-dominated anisotropy profile in the inner regions, while the structure is nearly unchanged beyond 10% of the virial radius. Then we propose a toy model to account for such a halo evolution, based on the angular momentum exchange due to dynamical friction; at the epoch of galaxy formation this is efficiently exerted by the DM onto the gas clouds spiralling down the potential well. The comparison between the angular momentum profile gained by the halo through dynamical friction and that provided by the perturbed distribution function reveals a surprising similarity, hinting at the reliability of the process.

Early star formation, nucleosynthesis and chemical evolution in proto-galactic clouds

We present numerical simulations to describe the nucleosynthesis and evolution of pre-galactic clouds in a model which is motivated by cold dark-matter simulations of hierarchical galaxy formation. We adopt a SN-induced star-formation mechanism within a model that follows the evolution of chemical enrichment and energy input to the clouds by Type II and Type Ia supernovae. We utilize metallicity-dependent yields for all elements at all times and include effects of finite stellar lifetimes. We derive the metallicity distribution functions for stars in the clouds, their age-metallicity relation and relative elemental abundances for a number of alpha- and Fe-group elements. The stability of these clouds against destruction is discussed, and results are compared for different initial mass functions. We find that the dispersion of the metallicity distribution function observed in the outer halo is naturally reproduced by contributions from many clouds with different initial conditions. The scatter in metallicity as a function of age for these stars is very large, implying that no age-metallicity relation exists in the early stages of galaxy formation. Clouds with initial masses ≳ presently observed globular clusters are found to survive the first 0.1 Gyr from the onset of star formation, suggesting that such systems may have contributed to the formation of the first stars and could have been self-enriched. More massive clouds are only stable when one assumes an initial mass function that is not biased towards massive stars, indicating that even if the first stars were formed according to a top-heavy mass function, subsequent star formation was likely to have proceeded with a present-day mass function or happened in an episodic manner. The predicted relative abundances of some alpha- and Fe-group elements show good agreement with the observed values down to metallicities below [Fe/H] ∼−4 when the iron yields are reduced relative to stellar models. The observed scatter is also reproduced for most elements including the observed bifurcation in [α/Fe] for stars with low [Fe/H]. However, the predicted dispersion may be too large for some elements (particularly alpha elements) unless a limited range of progenitor masses contributing to the abundances of these elements is assumed. The contributions to the abundances from supernovae with different progenitor masses and metallicity are discussed. The results suggest that the low-mass end of SNeII was probably absent at the very lowest metallicities and that the upper mass limit for the first stars that contributed to nucleosynthesis may be ≲40 M⊙.

The evolution of galaxies from primeval irregulars to present-day ellipticals

Galaxy formation is believed to proceed in a 'bottom up' manner, starting with the formation of small clumps of gas and stars that then merge hierarchically into giant systems. The baryonic gas loses thermal energy by radiative cooling and falls towards the centres of the new galaxies, while supernovae blow gas out. Any realistic model therefore requires a proper treatment of these processes, but hitherto this has been far from satisfactory. Here we report a simulation that follows evolution from the earliest stages of galaxy formation through the period of dynamical relaxation, at which point the resulting galaxy is in its final form. The bubble structures of gas revealed in our simulation (for times of less than 3 x 10(8) years) resemble closely high-redshift Lyman-alpha emitters. After 10(9) years, these bodies are dominated by stellar continuum radiation and then resemble the Lyman break galaxies, which are high-redshift star-forming galaxies. At this point, the abundance of elements heavier than helium ('metallicity') appears to be solar. After 1.3 x 10(10) years, these galaxies resemble present-day ellipticals.

Mission Concept for the Single Aperture Far-Infrared (SAFIR) Observatory

The Single Aperture Far-InfraRed (SAFIR) Observatory’s science goals are driven by the fact that the earliest stages of almost all phenomena in the universe are shrouded in absorption by and emission from cool dust and gas that emits strongly in the far-infrared (40μ–200μ) and submillimeter (200μ–1 mm). In the very early universe, the warm gas of newly collapsing, unenriched galaxies will be revealed by molecular hydrogen emission lines at these long wavelengths. High redshift quasars are found to have substantial reservoirs of cool gas and dust, indicative of substantial metal enrichment early in the history of the universe. As a result, even early stages of galaxy formation will show powerful far-infrared emission. The combination of strong dust emission and large redshift (1 < z < 7) of these galaxies means that they can only be studied in the far-infrared and submillimeter. For nearby galaxies, many of the most active galaxies in the universe appear to be those whose gaseous disks are interacting in violent collisions. The details of these galaxies, including the effect of the central black holes that probably exist in most of them, are obscured to shorter wavelength optical and ultraviolet observatories by the large amounts of dust in their interstellar media. Within our own galaxy, the earliest stages of star formation, when gas and dust clouds are collapsing and the beginnings of a central star are taking shape, can only be observed in the far-infrared and submillimeter. The cold dust that ultimately forms the planetary systems, as well as the cool “debris” dust clouds that indicate the likelihood of planetary sized bodies around more developed stars, can only be observed at wavelengths longward of 20μ.

Collision-induced galaxy formation: semi-analytical model and multiwavelength predictions

A semi-analytical model is proposed that couples the Press‐Schechter formalism for the number of galaxies with a prescription for galaxy‐galaxy interactions that enables us to follow the evolution of galaxy morphologies along the Hubble sequence. Within this framework, we calculate the chemo-spectrophotometric evolution of galaxies to obtain spectral energy distributions. We find that such an approach is very successful in reproducing the statistical properties of galaxies as well as their time evolution. We are able to make predictions as a function of galaxy type. For clarity, we restrict ourselves to two categories of galaxies: early and late types that are identified with ellipticals and discs. In our model, irregulars are simply an early stage of galaxy formation. In particular, we obtain good matches for the galaxy counts and redshift distributions of sources from ultraviolet to submillimetre wavelengths. We also reproduce the observed cosmic star formation history and the diffuse background radiation, and make predictions as to the epoch and wavelength at which the dust-shrouded star formation of spheroids begins to dominate over the star formation that occurs more quiescently in discs. A new prediction of our model is a rise in the far-infrared luminosity density with increasing redshift, peaking at about z ∼ 3, and with a ratio to the local luminosity density ρ L,ν(z = zpeak)/ρ L,ν(z = 0) about 10 times higher than that in the blue (B band) which peaks near z ∼ 2.

Implications of O and Mg abundances in metal-poor halo stars for stellar iron yields

Inhomogeneous chemical evolution models of galaxies that try to reproduce the scatter seen in element- to-iron ratios of metal-poor halo stars are heavily dependent on theoretical nucleosynthesis yields of core-collapse supernovae (SNe II). Thus, inhomogeneous chemical evolution models present themselves as a test for stellar nucleosynthesis calculations. Applying such a model to our Galaxy reveals a number of shortcomings of existing nucleosynthesis yields. One problem is the predicted scatter in (O/Fe) and (Mg/Fe) which is too large compared to that observed in metal-poor halo stars. This can be either due to the oxygen or magnesium yields or due to the iron yields (or both). However, oxygen and magnesium are -elements that are produced mainly during hydrostatic burning and thus are not aected by the theoretical uncertainties in the collapse and explosion of a massive star. Stellar iron yields, on the other hand, depend heavily on the choice of the mass-cut between ejecta and proto- neutron star and are therefore very uncertain. We present iron yield distributions as a function of progenitor mass that are consistent with the abundance distribution of metal-poor halo stars and are in agreement with observed 56 Ni yields of core-collapse supernovae with known progenitor masses. The iron yields of lower-mass SNe II (in the range 10 20 M) are well constrained by these observations. Present observations, however, do not allow us to determine a unique solution for higher-mass SNe. Nevertheless, the main dependence of the stellar iron yields as function of progenitor mass can be derived and may be used as a constraint for future core-collapse supernova/hypernova models. A prediction of hypernova models is the existence of ultra -element enhanced stars at metallicities (Fe/H) 2:5, which can be tested by future observations. The results are of importance for the earliest stages of galaxy formation when the ISM is dominated by local chemical inhomogeneities and the instantaneous mixing approximation is not valid.

Hubble Successor Gathers Support

NASA scientists have unfurled a blueprint for an ambitious new project: a $500 million successor to the Hubble Space Telescope that will peer in the infrared back to the early stages of galaxy formation. The NASA team has proposed an 8-meter reflector made of eight thin segments, which unfold like petals of a flower once in space. The proposal is being enthusiastically endorsed by astronomers, but it has many hurdles to clear before it flies.

Astronomy: Hubble Successor Gathers Support

NASA scientists have unfurled a blueprint for an ambitious new project: a $500 million successor to the Hubble Space Telescope that will peer in the infrared back to the early stages of galaxy formation. The NASA team has proposed an 8-meter reflector made of eight thin segments, which unfold like petals of a flower once in space. The proposal is being enthusiastically endorsed by astronomers, but it has many hurdles to clear before it flies.

HST images of nearby luminous quasars

Strong upper limits are placed on the visual-band brightnesses of galactic hosts for four luminous, radio-quiet quasars with redshifts between $0.16$ and~$0.24$ that were studied with the HST's Wide Field/Planetary Camera-2. Typical upper limits on the luminosities of galactic hosts are about $1.4$ mag fainter than $L^*$ for spirals and about $0.5$ mag fainter than $L^*$ for ellipticals. The galactic hosts of the quasars are more than a magnitude and a half fainter than the median integrated absolute magnitude of Seyfert galaxies. If the detection limits are determined using featureless simulated galaxies instead of observed galaxy images, then the detection limits for spirals are 0.5--1.0 mag less stringent. These results are consistent with the hypothesis that the quasar phenomenon corresponds to the early stages of galaxy formation, before extensive star formation occurs.

Plasma model of microwave background and primordial elements: an alternative to the big bang

A plasma model of the origin of the light elements and the microwave background is presented. In contrast to the conventional Big Bang hypothesis, the model assumes that helium, deuterium and the microwave background were all generated by massive stars in the early stages of galaxy formation. The microwave background is scattered and isotropized by multi-GeV electrons trapped in the jets emitted by active galactic nuclei. The model produces reasonable amounts of heavy elements, accurately predicts the gamma-ray background intensity and spectrum, and explains the statistics of quasars, compact and extended radio sources.