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Register a new userThe Cosmic Chemical Evolution as seen by the Brightest Events in the Universe
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Sandra Savaglio
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Gamma-ray bursts (GRBs) are the brightest events in the universe. They have been used in the last five years to study the cosmic chemical evolution, from the local universe to the first stars. The sample size is still relatively small when compared to field galaxy surveys. However, GRBs show a universe that is surprising. At z > 2, the cold interstellar medium in galaxies is chemically evolved, with a mean metallicity of about 1/10 solar. At lower redshift (z < 1), metallicities of the ionized gas are relatively low, on average 1/6 solar. Not only is there no evidence of redshift evolution in the interval 0 < z < 6.3, but also the dispersion in the ~ 30 objects is large. This suggests that the metallicity of host galaxies is not the physical quantity triggering GRB events. From the investigation of other galaxy parameters, it emerges that active star-formation might be a stronger requirement to produce a GRB. Several recent striking results strongly support the idea that GRB studies open a new view on our understanding of galaxy formation and evolution, back to the very primordial universe at z ~ 8.
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We study the dust properties of galaxies in the redshift range 0.1<z<2.8 observed by the Herschel Space Observatory in the field of the Great Observatories Origins Deep Survey-North as part of PEP and HerMES key programmes. Infrared (IR) luminosity (L_IR) and dust temperature (T_dust) of galaxies are derived from the spectral energy distribution (SED) fit of the far-infrared (FIR) flux densities obtained with PACS and SPIRE instruments onboard Herschel. As a reference sample, we also obtain IR luminosities and dust temperatures of local galaxies at z<0.1 using AKARI and IRAS data in the field of the Sloan Digital Sky Survey. We compare the L_IR-T_dust relation between the two samples and find that: the median T_dust of Herschel-selected galaxies at z>0.5 with L_IR>5x10^{10} L_odot, appears to be 2-5 K colder than that of AKARI-selected local galaxies with similar luminosities; and the dispersion in T_dust for high-z galaxies increases with L_IR due to the existence of cold galaxies that are not seen among local galaxies. We show that this large dispersion of the L_IR-T_dust relation can bridge the gap between local star-forming galaxies and high-z submillimeter galaxies (SMGs). We also find that three SMGs with very low T_dust (<20 K) covered in this study have close neighbouring sources with similar 24-mum brightness, which could lead to an overestimation of FIR/(sub)millimeter fluxes of the SMGs.
A gamma-ray burst (GRB) is a strong and fast gamma-ray emission from the explosion of stellar systems (massive stars or coalescing binary compact stellar remnants), happening at any possible redshift, and detected by space missions. Although GRBs are the most energetic events after the Big Bang, systematic search (started after the first localization in 1997) led to only 374 spectroscopic redshift measurements. For less than half, the host galaxy is detected and studied in some detail. Despite the small number of known hosts, their impact on our understanding of galaxy formation and evolution is immense. These galaxies offer the opportunity to explore regions which are observationally hostile, due to the presence of gas and dust, or the large distances reached. The typical long-duration GRB host galaxy at low redshift is small, star-forming and metal poor, whereas, at intermediate redshift, many hosts are massive, dusty and chemically evolved. Going even farther in the past of the Universe, at z > 5, long-GRB hosts have never been identified, even with the deepest NIR space observations, meaning that these galaxies are very small (stellar mass < 10^7 M_sun). We considered the possibility that some high-z GRBs occurred in primordial globular clusters, systems that evolved drastically since the beginning, but would have back then the characteristics necessary to host a GRB. At that time, the fraction of stellar mass contained in proto globular clusters might have been orders of magnitude higher than today. Plus, these objects contained in the past many massive fast rotating binary systems, which are also regarded as a favorable situation for GRBs. The common factor for all long GRBs at any redshift is the stellar progenitor: it is a very massive rare/short-lived star, present in young regions, whose redshift evolution is closely related to the star-formation history of the Universe.
Star formation in massive galaxies is quenched at some point during hierarchical mass assembly. To understand where and when the quenching processes takes place, we study the evolution of the total star formation rate per unit total halo mass (Sigma(SFR/M)) in three different mass scales: low mass halos (field galaxies), groups, and clusters, up to a redshift ~1.6. We use deep far-infrared PACS data at 100 and 160 um to accurately estimate the total star formation rate of the Luminous Infrared Galaxy population of 9 clusters with mass ~10^{15} M_{odot}, and 9 groups/poor clusters with mass ~ 5 x 10^{13} M_{odot}. Estimates of the field Sigma(SFR/M) are derived from the literature, by dividing the star formation rate density by the mean comoving matter density of the universe. The field Sigma(SFR/M) increases with redshift up to z~1 and it is constant thereafter. The evolution of the Sigma(SFR/M)-z relation in galaxy systems is much faster than in the field. Up to redshift z~0.2, the field has a higher Sigma(SFR/M) than galaxy groups and galaxy clusters. At higher redshifts, galaxy groups and the field have similar Sigma(SFR/M), while massive clusters have significantly lower Sigma(SFR/M) than both groups and the field. There is a hint of a reversal of the SFR activity vs. environment at z~1.6, where the group Sigma(SFR/M) lies above the field Sigma(SFR/M)-z relation. We discuss possible interpretations of our results in terms of the processes of downsizing, and star-formation quenching.
We have determined the O/H and N/O of a sample of 122751 SFGs from the DR7 of the SDSS. For all these galaxies we have also determined their morphology and their SFH using the code STARLIGHT. The comparison of the chemical abundance with the SFH allows us to describe the chemical evolution in the nearby universe (z < 0.25) in a manner which is consistent with the formation of their stellar populations and morphologies. A 45% of the SFGs in our sample show an excess of abundance in nitrogen relative to their metallicity. We also find this excess to be accompanied by a deficiency of oxygen, which suggests that this could be the result of effective starburst winds. However, we find no difference in the mode of star formation of the nitrogen rich and nitrogen poor SFGs. Our analysis suggests they all form their stars through a succession of bursts of star formation extended over a few Gyr period. What produces the chemical differences between these galaxies seems therefore to be the intensity of the bursts: the galaxies with an excess of nitrogen are those that are presently experiencing more intense bursts, or have experienced more intense bursts in their past. We also find evidence relating the chemical evolution process to the formation of the galaxies: the galaxies with an excess of nitrogen are more massive, have more massive bulges and earlier morphologies than those showing no excess. As a possible explanation we propose that the lost of metals consistent with starburst winds took place during the formation of the galaxies, when their potential wells were still building up, and consequently were weaker than today, making starburst winds more efficient and independent of the final mass of the galaxies. In good agreement with this interpretation, we also find evidence consistent with downsizing, according to which the more massive SFGs formed before the less massive ones.
Modern cosmology has sharpened questions posed for millennia about the origin of our cosmic habitat. The age-old questions have been transformed into two pressing issues primed for attack in the coming decade: How did the Universe begin? and What physical laws govern the Universe at the highest energies? The clearest window onto these questions is the pattern of polarization in the Cosmic Microwave Background (CMB), which is uniquely sensitive to primordial gravity waves. A detection of the special pattern produced by gravity waves would be not only an unprecedented discovery, but also a direct probe of physics at the earliest observable instants of our Universe. Experiments which map CMB polarization over the coming decade will lead us on our first steps towards answering these age-old questions.
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