Saumyadip Samui

Herschel-ATLAS: Dust Temperature and Redshift Distribution of SPIRE and PACS Detected Sources Using Submillimetre Colours

We present colour-colour diagrams of detected sources in the Herschel-ATLAS science demonstration field from 100 to 500 mu m using both PACS and SPIRE. We fit isothermal modified black bodies to the spectral energy distribution (SED) to extract the dust temperature of sources with counterparts in Galaxy And Mass Assembly (GAMA) or SDSS surveys with either a spectroscopic or a photometric redshift. For a subsample of 330 sources detected in at least three FIR bands with a significance greater than 3 sigma, we find an average dust temperature of (28 +/- 8) K. For sources with no known redshift, we populate the colour-colour diagram with a large number of SEDs generated with a broad range of dust temperatures and emissivity parameters, and compare to colours of observed sources to establish the redshift distribution of this sample. For another subsample of 1686 sources with fluxes above 35 mJy at 350 mu m and detected at 250 and 500 mu m with a significance greater than 3s, we find an average redshift of 2.2 +/- 0.6.

Herschel -ATLAS: extragalactic number counts from 250 to 500 microns

Aims. The Herschel-ATLAS survey (H-ATLAS) will be the largest area survey to be undertaken by the Herschel Space Observatory. It will cover 550 sq. deg. of extragalactic sky at wavelengths of 100, 160, 250, 350 and 500 μm when completed, reaching flux limits (5σ) from 32 to 145 mJy. We here present galaxy number counts obtained for SPIRE observations of the first ∼14 sq. deg. observed at 250, 350 and 500 μm. Methods. Number counts are a fundamental tool in constraining models of galaxy evolution. We use source catalogs extracted from the H-ATLAS maps as the basis for such an analysis. Correction factors for completeness and flux boosting are derived by applying our extraction method to model catalogs and then applied to the raw observational counts. Results. We find a steep rise in the number counts at flux levels of 100−200 mJy in all three SPIRE bands, consistent with results from BLAST. The counts are compared to a range of galaxy evolution models. None of the current models is an ideal fit to the data but all ascribe the steep rise to a population of luminous, rapidly evolving dusty galaxies at moderate to high redshift.

Herschel-ATLAS: The dust energy balance in the edge-on spiral galaxy UGC 4754

We use Herschel PACS and SPIRE observations of the edge-on spiral galaxy UGC4754, taken as part of the H-ATLAS SDP observations, to investigate the dust energy balance in this galaxy. We build detailed SKIRT radiative models based on SDSS and UKIDSS maps and use these models to predict the far-infrared emission. We find that our radiative transfer model underestimates the observed FIR emission by a factor of two to three. Similar discrepancies have been found for other edge-on spiral galaxies based on IRAS, ISO, and SCUBA data. Thanks to the good sampling of the SED at FIR wavelengths, we can rule out an underestimation of the FIR emissivity as the cause for this discrepancy. Instead we support highly obscured star formation that contributes little to the optical extinction as a more probable explanation.

Herschel-ATLAS: Evolution of the 250 µm luminosity function out to z = 0.5

We have determined the luminosity function of 250 mu m-selected galaxies detected in the similar to 14 deg(2) science demonstration region of the Herschel-ATLAS project out to a redshift of z = 0.5. Our findings very clearly show that the luminosity function evolves steadily out to this redshift. By selecting a sub-group of sources within a fixed luminosity interval where incompleteness effects are minimal, we have measured a smooth increase in the comoving 250 mu m luminosity density out to z = 0.2 where it is 3.6(-0.9)(+1.4) times higher than the local value.

Phonon confinement in oxide-coated silicon nanowires

Raman spectroscopy of micron-long crystalline Si nanowires covered with a thick SiO2 layer showed a downshift and asymmetric broadening of the Raman first order TO phonon peak when compared with the bulk (q=0) mode. The Raman shift and broadening were attributed to phonon confinement in the nanowires. A phenomenological phonon confinement model, incorporating the effects of nanowire diameter distribution, is presented. This model is shown to accurately describe the broadening of the Raman peak and is consistent with the microstructure of Si nanowires. In addition to the work a distribution of the phonon wave vector was directly taken into consideration replacing the diameter distribution to fit the Raman TO peak. The effects of the nano-Si:SiO2 boundary on the Raman spectra are discussed in terms of softening of the phonon confinement.

Herschel ATLAS: The cosmic star formation history of quasar host galaxies

We present a derivation of the star formation rate per comoving volume of quasar host galaxies, derived from stacking analyses of far-infrared to mm-wave photometry of quasars with redshifts 0 IAB > -32 We use the science demonstration observations of the first ~16 deg2 from the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) in which there are 240 quasars from the Sloan Digital Sky Survey (SDSS) and a further 171 from the 2dF-SDSS LRG and QSO (2SLAQ) survey. We supplement this data with a compilation of data from IRAS, ISO, Spitzer, SCUBA and MAMBO. H-ATLAS alone statistically detects the quasars in its survey area at >5σ at 250,350 and 500 μm. From the compilation as a whole we find striking evidence of downsizing in quasar host galaxy formation: low-luminosity quasars with absolute magnitudes in the range -22 > IAB > -24 have a comoving star formation rate (derived from 100 μm rest-frame luminosities) peaking between redshifts of 1 and 2, while high-luminosity quasars with IAB IAB > -24 quasars evolves as (1 + z)2.3±0.7 at z IAB > -28. We tentatively interpret this as a combination of a declining major merger rate with time and gas consumption reducing fuel for both black hole accretion and star formation.

Confinement in silicon nanowires: Optical properties

The blueshift of the optical absorption edge along with the intense red photoluminescence (PL) peak has been observed from micron-long crystalline silicon nanowires prepared by pulsed-laser vaporization of heated Si (mixed with metal catalyst) targets. Previous studies on the confinement in silicon nanostructures resulted in a dispute regarding the application of theoretical models to explain their optical properties. Based on the microstructure a phenomenological confinement model, incorporating the nanowire diameter distribution is used, which is found to describe the optical properties including the shape of absorption spectra, the band gap, and the PL peak position of the Si nanowires very well.

Probing the star formation history using the redshift evolution of luminosity functions

We present a self-consistent, semi-analyticalCDM model of star formation and reioniza- tion. For the cosmological parameters favored by the WMAP data, our models consistently reproduce the electron scattering optical depth to reioniz ation, redshift of reionization and the observed luminosity functions (LF) and hence the star formation rate (SFR) density at 3 ≤ z ≤ 6 for a reasonable range of model parameters. While simple photoionization feed- back produces the correct shape of LF at z = 6, for z = 3 we need additional feedback that suppresses star formation activities in halos with 10 10 . (M/M⊙) . 10 11 . Models with prolonged continuous star formation activities are prefer red over those with short bursts as they are consistent with the existence of a Balmer break in considerable fraction of observed galaxies even at z ∼ 6. The halo number density evolution from the standardCDM structure formation model that fits LF up to z = 6 is consistent with the upper limits on z ≃ 7 LF and source counts at 8 ≤ z ≤ 12 obtained from the Hubble Ultra Deep Field (HUDF) observations without requiring any dramatic change in the nature of star formation. However, to reproduce the observed LF at 6 ≤ z ≤ 10, obtained from the near-IR observations around strong lensing clusters, we need a strong evolution in the initial mass func tion, reddening correction and the mode of star formation at z & 8. We show that low mass molecular cooled halos, which may be important for reionizing the universe, are not detectabl e in the present deep field obser- vations even if a considerable fraction of its baryonic mass goes through a star burst phase. However, their presence and contribution to reionization can be inferred indirectly from the redshift evolution of the luminosity function in the redshi ft range 6 ≤ z ≤ 12. In our model calculations, the contribution of low mass halos to global SFR density prior to reionization reveals itself in the form of second peak at z ≥ 6. However this peak will not be visible in the observed SFR density as a function of z as most of these galaxies have luminosity below the detection threshold of various ongoing deep field survey s. Accurately measuring the LF at high redshifts can be used to understand the nature of star formation in the dark ages and probe the history of reionization.

Cosmic ray driven outflows from high-redshift galaxies

We study winds in high redshift galaxies driven by a relativistic cosmic ray (proton) component in addition to the hot thermal gas component. Cosmic rays (CRs) are likely to be efficiently generated in supernova (SNe) shocks inside galaxies. We obtain solutions of such CR driven free winds in a gravitational potential of the Navarro-Frenk-White (NFW) form, relevant to galaxies. Cosmic rays naturally provide the extra energy and/or momentum input to the system, needed for a transonic wind solution in a gas with adiabatic index = 5=3. We show that cosmic rays can effectively drive winds even when the thermal energy of the gas is lost due to radiative cooling. These wind solutions predict an asymptotic wind speed closely related to the circular velocity of the galaxy. Furthermore, the mass outflow rate per unit star formation rate ( w) is predicted to be 0:2 0:5 for massive galaxies, with masses M 10 11 10 12 M . We show w to be inversely proportional to the square of the circular velocity. Magnetic fields at the G levels are also required in these galaxies to have a significant mass loss. A large w for small mass galaxies implies that cosmic ray driven outflows could provide a strong negative feedback to the star formation in dwarf galaxies. Further, our results will also have important implications to the metal enrichment of the intergalactic medium. These conclusions are applicable to the class of free wind models where the source region is confined to be within the sonic point.

Understanding the redshift evolution of the luminosity functions of Lyman α emitters

We present a semi-analytical model of star formation which explains simultaneously the observed ultraviolet (UV) luminosity function (LF) of high-redshift Lyman break galaxies (LBGs) and LFs of Lyman α emitters. We consider both models that use the Press–Schechter (PS) and Sheth–Tormen (ST) halo mass functions to calculate the abundances of dark matter haloes. The Lyman α LFs at z 4 are well reproduced with only 10 per cent of the LBGs emitting Lyman α lines with rest equivalent width greater than the limiting equivalent width of the narrow band surveys. However, the observed LF at z> 5 can be reproduced only when we assume that nearly all LBGs are Lyman α emitters. Thus, it appears that 4 5 could be less dusty, more clumpy and having more complex velocity field. All of these will enable easier escape of the Lyman α photons. At z> 5, the observed Lyman α LF are well reproduced with the evolution in the halo mass function along with very minor evolution in the physical properties of high-redshift galaxies. In particular, up to z = 6.5, we do not see the effect of evolving intergalactic medium opacity on the Lyman α escape from these galaxies.