High-resolution CO and radio imaging of z similar to 2 ULIRGs: extended CO structures and implications for the universal star formation law

We present high spatial resolution (0.4 arcsec, Graphic kpc) Plateau de Bure Interferometer interferometric data on three ultraluminous infrared galaxies (ULIRGs) at Graphic: two submillimetre galaxies (SMGs) and one submillimetre faint star-forming radio galaxy. The three galaxies have been robustly detected in CO rotational transitions, either 12CO (J= 4Graphic3) or 12CO (J= 3Graphic2), allowing their sizes and gas masses to be accurately constrained. These are the highest spatial resolution observations observed to date (by a factor of Graphic2) for intermediate-excitation CO emission in Graphic ULIRGs. The galaxies appear extended over several resolution elements, having a mean radius of 3.7 kpc. High-resolution (0.3 arcsec) combined Multi-Element Radio-Linked Interferometer Network-Very Large Array observations of their radio continua allow an analysis of the star formation behaviour of these galaxies, on comparable spatial scales to those of the CO observations. This ‘matched beam’ approach sheds light on the spatial distribution of both molecular gas and star formation, and we can therefore calculate accurate star formation rates and gas surface densities: this allows us to place the three systems in the context of a Kennicutt–Schmidt (KS)-style star formation law. We find a difference in size between the CO and radio emission regions, and as such we suggest that using the spatial extent of the CO emission region to estimate the surface density of star formation may lead to error. This size difference also causes the star formation efficiencies within systems to vary by up to a factor of 5. We also find, with our new accurate sizes, that SMGs lie significantly above the KS relation, indicating that stars are formed more efficiently in these extreme systems than in other high-z star-forming galaxies

X-ray hiccups from Sagittarius A* observed by XMM-Newton: The second brightest flare and three moderate flares caught in half a day

Context. Our Galaxy hosts at its dynamical center Sgr A*, the closest supermassive black hole. Surprisingly, its luminosity is several orders of magnitude lower than the Eddington luminosity. However, the recent observations of occasional rapid X-ray flares from Sgr A* provide constraints on the accretion and radiation mechanisms at work close to its event horizon. Aims. Our aim is to investigate the flaring activity of Sgr A* and to constrain the physical properties of the X-ray flares. Methods. In Spring 2007, we observed Sgr A* with XMM-Newton with a total exposure of ~230 ks. We have performed timing and spectral analysis of the new X-ray flares detected during this campaign. To study the range of flare spectral properties, in a consistent manner, we have also reprocessed, using the same analysis procedure and the latest calibration, archived XMM-Newton data of previously reported rapid flares. The dust scattering was taken into account during the spectral fitting. We also used Chandra archived observations of the quiescent state of Sgr A* for comparison. Results. On April 4, 2007, we observed for the first time within a time interval of roughly half a day, an enhanced incidence rate of X-ray flaring, with a bright flare followed by three flares of more moderate amplitude. The former event represents the second brightest X-ray flare from Sgr A* on record with a peak amplitude of about 100 above the quiescent luminosity. This new bright flare exhibits similar light-curve shape (nearly symmetrical), duration (~3 ks) and spectral characteristics to the very bright flare observed in October 3, 2002 by XMM-Newton. The measured spectral parameters of the new bright flare, assuming an absorbed power law model taken into account dust scattering effect, are $N_{\rm H}$ = $12.3^{+2.1}_{-1.8}\times 10^{22}$ cm-2 and $\Gamma$ = 2.3 $\pm$ 0.3 calculated at the 90% confidence level. The spectral parameter fits of the sum of the three following moderate flares, while lower ($N_{\rm H}$ = $8.8^{+4.4}_{-3.2} \times 10^{22}$ cm-2 and $\Gamma = 1.7^{+0.7}_{-0.6}$), are compatible within the error bars with those of the bright flares. The column density found, for a power-law model taking into account the dust scattering, during the flares is at least two times higher than the value expected from the (dust) visual extinction toward Sgr A* ($A_{\rm V}$ ~ 25 mag), i.e., 4.5 $\times$ 1022 cm-2. However, our fitting of the Sgr A* quiescent spectra obtained with Chandra, for a power-law model taking into account the dust scattering, shows that an excess of column density is already present during the non-flaring phase. Conclusions. The two brightest X-ray flares observed so far from Sgr A* exhibited similar soft spectra

Evolution of dust temperature of galaxies through cosmic time as seen by Herschel

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 the PACS Extragalactic Probe (PEP) and Herschel Multi-tiered Extragalactic Survey (HerMES) key programmes. Infrared (IR) luminosity (LIR) and dust temperature (Tdust) of galaxies are derived from the spectral energy distribution fit of the far-IR (FIR) flux densities obtained with the 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 LIR–Tdust relation between the two samples and find that the median Tdust of Herschel-selected galaxies at z≳ 0.5 with LIR≳ 5 × 1010 L⊙ appears to be 2–5 K colder than that of AKARI-selected local galaxies with similar luminosities, and the dispersion in Tdust for high-z galaxies increases with LIR due to the existence of cold galaxies that are not seen among local galaxies. We show that this large dispersion of the LIR−Tdust relation can bridge the gap between local star-forming galaxies and high-z submillimetre galaxies (SMGs). We also find that three SMGs with very low Tdust (≲20 K) covered in this study have close neighbouring sources with similar 24-μm brightness, which could lead to an overestimation of FIR/(sub)millimetre fluxes of the SMGs