Emanuele Ripamonti

A Metal-rich Molecular Cloud Surrounds GRB 050904 at Redshift 6.3

GRB 050904 is the gamma-ray burst with the highest measured redshift. We performed time resolved X-ray spectroscopy of the late GRB and early afterglow emission. We find robust evidence for a decrease with time of the soft X-ray absorbing column. We model the evolution of the column density due to the flash ionization of the GRB and early afterglow photons. This allows us to constrain the metallicity and geometry of the absorbing cloud. We conclude that the progenitor of GRB 050904 was a massive star embedded in a dense metal enriched molecular cloud with Z > 0.03Z⊙. This is the first local measurement of metallicity in the clo se environment of a GRB and one of the highest redshift metallicity measurements. We also find that the dust associated with the cloud cannot be similar to that of our Galaxy but must be either sizably depleted or dominated by silicate grains. We discuss the implications of these results for GRB progenitors and high redshift star formation. Subject headings: gamma-rays: bursts — stars: formation — ISM: abundances — ISM: dust, extinction

Signatures of Planets and Protoplanets in the Galactic Center: A Clue to Understanding the G2 Cloud?

Several hundred young stars lie in the innermost parsec of our Galaxy. The super-massive black hole (SMBH) might capture planets orbiting these stars, and bring them onto nearly radial orbits. The same fate might occur to planetary embryos (PEs), i.e. protoplanets born from gravitational instabilities in protoplanetary disks. In this paper, we investigate the emission properties of rogue planets and PEs in the Galactic center. In particular, we study the effects of photoevaporation, caused by the ultraviolet background. Rogue planets can hardly be detected by current or forthcoming facilities, unless they are tidally disrupted and accrete onto the SMBH. In contrast, photoevaporation of PEs (especially if the PE is being tidally stripped) might lead to a recombination rate as high as ~10^45 s^-1, corresponding to a Brackett-gamma luminosity ~10^31 erg s^-1, very similar to the observed luminosity of the dusty object G2. We critically discuss the possibility that G2 is a rogue PE, and the major uncertainties of this model.

WIMP annihilation effects on primordial star formation

We study the effects of WIMP dark matter (DM) annihilations on the thermal and chemical evolution of the gaseous clouds where the first generation of sta rs in the Universe is formed. We follow the collapse of the gas inside a typical halo virializing at very high redshift, from well before virialization until a stage where the heating from DM annihilations exceeds the gas cooling rate. The DM energy input is estimated by inserting the energy released by DM annihilations (as predicted by an adiabatic contraction of the original DM profile) in a spherically symmetric radiative transfer scheme. In addition to the heating effec ts of the energy absorbed, we include its feedback upon the chemical properties of the gas, which is critical to determine the cooling rate in the halo, and hence the fragmentation scale and Jeans mass of the first stars. We find that DM annihilation does alter the free electron and especially the H 2 fraction when the gas density is n >10 4 #/cm 3 , for our fiducial parameter values. However, even if the chan ge in the H2 abundance and the cooling efficiency of the gas is large (some times exceeding a factor 100), the effects on the temperature of the collapsing gas are far s maller (a reduction by a factor < 1.5), since the gas cooling rate depends very strongly on temperature: then, the fragmentation mass scale is reduced only slightly, hinting towards no dramatic change in the initial mass function of the first stars.

Are ring galaxies the ancestors of giant low surface brightness galaxies?: Are ring galaxies the ancestors of GLSBs?

We simulate the collisional formation of a ring galaxy and we integrate its evolution up to 1.5 Gyr after the interaction. About 100-200 Myr after the collision, the simulated galaxy is very similar to observed ring galaxies (e.g. Cartwheel). After this stage, the ring keeps expanding and fades. Approximately 0.5-1 Gyr after the interaction, the disc becomes very large (similar to 100kpc) and flat. Such extended discs have been observed only in giant low surface brightness galaxies (GLSBs). We compare various properties of our simulated galaxies (surface brightness profile, morphology, H I spectrum and rotation curve) with the observations of four well-known GLSBs (UGC 6614, Malin 1, Malin 2 and NGC 75 89). The simulations match quite well the observations, suggesting that ring galaxies could be the progenitors of GLSBs. This result is crucial for the cold dark matter (CDM) model, as it was very difficult, so far, to explain the formation of GLSBs within the CDM scenario.

Connecting the evolution of thermally pulsing asymptotic giant branch stars to the chemistry in their circumstellar envelopes – I. Hydrogen cyanide

We investigate the formation of hydrogen cyanide (HCN) in the inner circumstellar envelopes of thermally pulsing asymptotic giant branch (TP-AGB) stars. A dynamic model for periodically shocked atmospheres, which includes an extended chemo-kinetic network, is for the first time coupled to detailed evolutionary tracks for the TP-AGB phase computed with the COLIBRI code. We carried out a calibration of the main shock parameters (the shock formation radius and the effective adiabatic index) using the circumstellar HCN abundances recently measured for a populous sample of pulsating TP-AGB stars. Our models recover the range of the observed HCN concentrations as a function of the mass-loss rates, and successfully reproduce the systematic increase of HCN moving along the M-S-C chemical sequence of TP-AGB stars, that traces the increase of the surface C/O ratio. The chemical calibration brings along two important implications: i) the first shock should emerge very close to the photosphere, and ii) shocks are expected to have a dominant isothermal character in the denser region close to the star (within ~ 3-4 R), implying that radiative processes should be quite efficient. Our analysis also suggests that the HCN concentrations in the inner circumstellar envelopes are critically affected by the H-H2 chemistry during the post-shock relaxation stages.

The population of ULXs in the spiral galaxy NGC 2276

We present results for X-ray point sources in the Sc galaxy NGC 2276, obtained by analyzing Chandra data. The galaxy is known to be very active in many wavelengths, possibly due to gravitational interaction with the central elliptical of the group, NGC 2300. However, previous XMM-Newton observations resulted in the detection of only one bright ULX and extended hot gas emission. We present here the X-ray population in NGC 2276 which comprises 17 sources. We found that 6 of them are new ULX sources in this spiral galaxy resolved for the first time by Chandra. We constructed the luminosity function that can be interpreted as mainly due of high mass X-ray binaries, and estimate the star formation rate (SFR) to be SFR ∼5–10 M⊙ yr–1 (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

NGC 2276: a remarkable galaxy with a large number of ultraluminous X-ray sources

The starbusting, nearby (D = 32:9 Mpc) spiral (Sc) galaxy NGC 2276 belongs to the sparse group dominated by the elliptical galaxy NGC 2300. NGC 2276 is a remarkable galaxy, as it displays a disturbed morphology at many wavelengths. This is possibly due to gravitational interaction with the central elliptical galaxy of the group. Previous ROSAT and XMM{Newton observations resulted in the detection of extended hot gas emission and of a single very bright ( 10 41 erg s 1 ) ultraluminous X-ray source (ULX) candidate. Here we report on a study of the X-ray sources of NGC 2276 based on Chandra data taken in 2004. Chandra was able to resolve 16 sources, 8 of which are ULXs, and to reveal that the previous ULX candidate is actually composed of a few distinct objects. We construct the luminosity function of NGC 2276, which can be interpreted as dominated by high mass X-ray binaries, and estimate the star formation rate (SFR) to be 5{15 M yr 1 , consistent with the values derived from optical and infrared observations. By means of numerical simulations, we show that both ram pressure and viscous transfer eects are necessary to produce the distorted morphology and the high SFR observed in NGC 2276, while tidal interaction have a marginal eect.

The host galaxies of double compact objects merging in the local Universe

We investigate the host galaxies of compact objects merging in the local Universe, by combining the results of binary population-synthesis simulations with the Illustris cosmological box. Double neutron stars (DNSs) merging in the local Universe tend to form in massive galaxies (with stellar mass

IMPACT OF DARK MATTER ON REIONIZATION AND HEATING

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Are ring galaxies the progenitors of giant low surface brightness galaxies

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