arXiv subjects

Asantha Cooray

Publications and source records attributed to Asantha Cooray.

At least 145 records · Page 8Linked to original sources

arXiv2014

Dusty Star-Forming Galaxies at High Redshift

Far-infrared and submillimeter wavelength surveys have now established the important role of dusty, star-forming galaxies (DSFGs) in the assembly of stellar mass and the evolution of massive galaxies in the Universe. The brightest of these galaxies have infrared luminosities in excess of 10$^{13}$ L$_{\odot}$ with implied star-formation rates of thousands of solar masses per year. They represent the most intense starbursts in the Universe, yet many are completely optically obscured. Their easy detection at submm wavelengths is due to dust heated by ultraviolet radiation of newly forming stars. When summed up, all of the dusty, star-forming galaxies in the Universe produce an infrared radiation field that has an equal energy density as the direct starlight emission from all galaxies visible at ultraviolet and optical wavelengths. The bulk of this infrared extragalactic background light emanates from galaxies as diverse as gas-rich disks to mergers of intense starbursting galaxies. Major advances in far-infrared instrumentation in recent years, both space-based and ground-based, has led to the detection of nearly a million DSFGs, yet our understanding of the underlying astrophysics that govern the start and end of the dusty starburst phase is still in nascent stage. This review is aimed at summarizing the current status of DSFG studies, focusing especially on the detailed characterization of the best-understood subset (submillimeter galaxies, who were summarized in the last review of this field over a decade ago, Blain et al., 2002), but also the selection and characterization of more recently discovered DSFG populations. We review DSFG population statistics, their physical properties including dust, gas and stellar contents, their environments, and current theoretical models related to the formation and evolution of these galaxies.

astro-ph.CO↗

arXiv2013

Herschel SPIRE-FTS Observations of Excited CO and [CI] in the Antennae (NGC 4038/39): Warm and Cold Molecular Gas

We present Herschel SPIRE-FTS observations of the Antennae (NGC 4038/39), a well studied, nearby ($22$ Mpc) ongoing merger between two gas rich spiral galaxies. We detect 5 CO transitions ($J=4-3$ to $J=8-7$), both [CI] transitions and the [NII]$205μm$ transition across the entire system, which we supplement with ground based observations of the CO $J=1-0$, $J=2-1$ and $J=3-2$ transitions, and Herschel PACS observations of [CII] and [OI]$63μm$. Using the CO and [CI] transitions, we perform both a LTE analysis of [CI], and a non-LTE radiative transfer analysis of CO and [CI] using the radiative transfer code RADEX along with a Bayesian likelihood analysis. We find that there are two components to the molecular gas: a cold ($T_{kin}\sim 10-30$ K) and a warm ($T_{kin} \gtrsim 100$ K) component. By comparing the warm gas mass to previously observed values, we determine a CO abundance in the warm gas of $x_{CO} \sim 5\times 10^{-5}$. If the CO abundance is the same in the warm and cold gas phases, this abundance corresponds to a CO $J=1-0$ luminosity-to-mass conversion factor of $α_{CO} \sim 7 \ M_{\odot}{pc^{-2} \ (K \ km \ s^{-1})^{-1}}$ in the cold component, similar to the value for normal spiral galaxies. We estimate the cooling from H$_2$, [CII], CO and [OI]$63μm$ to be $\sim 0.01 L_{\odot}/M_{\odot}$. We compare PDR models to the ratio of the flux of various CO transitions, along with the ratio of the CO flux to the far-infrared flux in NGC 4038, NGC 4039 and the overlap region. We find that the densities recovered from our non-LTE analysis are consistent with a background far-ultraviolet field of strength $G_0\sim 1000$. Finally, we find that a combination of turbulent heating, due to the ongoing merger, and supernova and stellar winds are sufficient to heat the molecular gas.

astro-ph.GA↗

arXiv2013

Disentangling a group of lensed submm galaxies at z~2.9

MS$\,$0451.6$-$0305 is a rich galaxy cluster whose strong lensing is particularly prominent at submm wavelengths. We combine new SCUBA-2 data with imaging from Herschel SPIRE and PACS and HST in order to try to understand the nature of the sources being lensed. In the region of the "giant submm arc," we uncover seven multiply imaged galaxies (up from the previously known three), of which six are found to be at a redshift of $z\sim2.9$, and possibly constitute an interacting system. Using a novel forward-modelling approach, we are able to simultaneously deblend and fit SEDs to the individual galaxies that contribute to the giant submm arc, constraining their dust temperatures, far infrared luminosities and star formation rates. The submm arc first identified by SCUBA can now be seen to be composed of at least five distinct sources, four of these within the galaxy group at $z\sim2.9$. The total unlensed luminosity for this galaxy group is $(3.1\pm0.3) \times 10^{12}\,\mathrm{L}_\odot$, which gives an unlensed star formation rate of $(450\pm50)$ M$_\odot$ yr$^{-1}$. From the properties of this system, we see no evidence of evolution towards lower temperatures in the dust temperature versus far-infrared luminosity relation for high redshift galaxies.

astro-ph.CO↗

arXiv2013

Far-infrared Fine-Structure Line Diagnostics of Ultraluminous Infrared Galaxies

We present Herschel observations of six fine-structure lines in 25 Ultraluminous Infrared Galaxies at z<0.27. The lines, [O III]52, [N III]57, [O I]63, [N II]122, [O I]145, and [C II]158, are mostly single gaussians with widths <600 km s-1 and luminosities of 10^7 - 10^9 Solar. There are deficits in the [O I]63/L_IR, [N II]/L_IR, [O I]145/L_IR, and [C II]/L_IR ratios compared to lower luminosity systems. The majority of the line deficits are consistent with dustier H II regions, but part of the [C II] deficit may arise from an additional mechanism, plausibly charged dust grains. This is consistent with some of the [C II] originating from PDRs or the ISM. We derive relations between far-IR line luminosities and both IR luminosity and star formation rate. We find that [N II] and both [O I] lines are good tracers of IR luminosity and star formation rate. In contrast, [C II] is a poor tracer of IR luminosity and star formation rate, and does not improve as a tracer of either quantity if the [C II] deficit is accounted for. The continuum luminosity densities also correlate with IR luminosity and star formation rate. We derive ranges for the gas density and ultraviolet radiation intensity of 10^1 < n < 10^2.5 and 10^2.2 < G_0 < 10^3.6, respectively. These ranges depend on optical type, the importance of star formation, and merger stage. We do not find relationships between far-IR line properties and several other parameters; AGN activity, merger stage, mid-IR excitation, and SMBH mass. We conclude that these far-IR lines arise from gas heated by starlight, and that they are not strongly influenced by AGN activity.

astro-ph.CO↗

arXiv2013

The extragalactic background light from the measurements of the attenuation of high-energy gamma-ray spectrum

The attenuation of high-energy gamma-ray spectrum due to the electron-positron pair production against the extragalactic background light (EBL) provides an indirect method to measure the EBL of the universe. We use the measurements of the absorption features of the gamma-rays from blazars as seen by Fermi Gamma-ray Space Telescope to explore the EBL flux density and constrain the EBL spectrum, star formation rate density (SFRD) and photon escape fraction from galaxies out to z=6. Our results are basically consistent with the existing determinations of the quantities. We find a larger photon escape fraction at high redshifts, especially at z=3, compared to the result from the recent Ly-alpha measurements. Our SFRD result is consistent with the data from both gamma-ray burst and UV observations in 1-sigma level. However, the average SFRD we obtain at z>~3 matches the gamma-ray data better than the UV data. Thus our SFRD result at z>~6 favors that it is sufficiently high enough to reionize the universe.

astro-ph.CO↗

arXiv2013

New Approaches to Probing Minkowski Functionals

We generalize the concept of the ordinary skew-spectrum to probe the effect of non-Gaussianity on the morphology of Cosmic Microwave Background (CMB) maps in several domains: in real-space (where they are commonly known as cumulant-correlators), and in harmonic and needlet bases. The essential aim is to retain more information than normally contained in these statistics, in order to assist in determining the source of any measured non-Gaussianity, in the same spirit as Munshi & Heavens (2010) skew-spectra were used to identify foreground contaminants to the CMB bispectrum in Planck data. Using a perturbative series to construct the Minkowski Functionals (MFs), we provide a pseudo-Cl based approach in both harmonic and needlet representations to estimate these spectra in the presence of a mask and inhomogeneous noise. Assuming homogeneous noise we present approx- imate expressions for error covariance for the purpose of joint estimation of these spectra. We present specific results for four different models of primordial non-Gaussianity local, equilateral, orthogonal and enfolded models, as well as non-Gaussianity caused by unsubtracted point sources. Closed form results of next-order corrections to MFs too are obtained in terms of a quadruplet of kurt-spectra. We also use the method of modal decomposition of the bispectrum and trispectrum to reconstruct the MFs as an alternative method of reconstruction of morphological properties of CMB maps. Finally, we introduce the odd-parity skew-spectra to probe the odd-parity bispectrum and its impact on the morphology of the CMB sky. Although developed for the CMB, the generic results obtained here can be useful in other areas of cosmology.

astro-ph.CO↗

arXiv2013

The roles of star formation and AGN activity of IRS sources in the HerMES fields

In this work we explore the impact of the presence of an active galactic nucleus (AGN) on the mid- and far-infrared (IR) properties of galaxies as well as the effects of simultaneous AGN and starburst activity in these same galaxies. To do this we apply a multi-component, multi-band spectral synthesis technique to a sample of 250 micron selected galaxies of the Herschel Multi-tiered Extragalactic Survey, with IRS spectra available for all galaxies. Our results confirm that the inclusion of the IRS spectra plays a crucial role in the spectral analysis of galaxies with an AGN component improving the selection of the best-fit hot dust model (torus). We find a correlation between the obscured star formation rate (SFR) derived from the IR luminosity of the starburst component, SFR_IR and SFR_PAH, derived from the luminosity of the PAH features, L_PAH, with SFR_FIR taking higher values than SFR_PAH. The correlation is different for AGN- and starburst-dominated objects. The ratio of L_PAH to that of the starburst component, L_PAH/L_SB, is almost constant for AGN-dominated objects but decreases with increasing L_SB for starburst-dominated objects. SFR_FIR increases with the accretion luminosity, L_acc, with the increase less prominent for the very brightest, unobscured AGN-dominated sources. We find no correlation between the masses of the hot and cold dust components. We interpret this as a non-constant fraction of gas driven by the gravitational effects to the AGN while the starburst is ongoing. We also find no evidence of the AGN affecting the temperature of the cold dust component, though this conclusion is mostly based on objects with a non-dominant AGN component. We conclude that our findings do not provide evidence that the presence of AGN affects the star formation process in the host galaxy, but rather that the two phenomena occur simultaneously over a wide range of luminosities.

astro-ph.CO↗

arXiv2013

Constraints on Spatial Variations in the Fine-Structure constant from Planck

We use the Cosmic Microwave Background (CMB) anisotropy data from Planck to constrain the spatial fluctuations of the fine-structure constant α. Through Thompson scattering of CMB photons, spatial anisotropies of αlead to higher-order correlations in the CMB anisotropies. We use a quadratic estimator based on the four-point correlation function of the CMB temperature anisotropy to extract the angular power spectrum of the spatial variation of the fine-structure constant projected along the line of sight at the last scattering surface. At tens of degree angular scales and above, we constrain the rms fluctuations of the fine structure constant to be δα/α_0= (1.34 +/- 5.82) x 10^-2 at the 95% confidence level with respect to the standard value α_0. We find no evidence for a spatially varying αat a redshift of 10^3.

astro-ph.CO↗

arXiv2013

The rapid assembly of an elliptical galaxy of 400 billion solar masses at a redshift of 2.3

Stellar archeology shows that massive elliptical galaxies today formed rapidly about ten billion years ago with star formation rates above several hundreds solar masses per year (M_sun/yr). Their progenitors are likely the sub-millimeter-bright galaxies (SMGs) at redshifts (z) greater than 2. While SMGs' mean molecular gas mass of 5x10^10 M_sun can explain the formation of typical elliptical galaxies, it is inadequate to form ellipticals that already have stellar masses above 2x10^11 M_sun at z ~ 2. Here we report multi-wavelength high-resolution observations of a rare merger of two massive SMGs at z = 2.3. The system is currently forming stars at a tremendous rate of 2,000 M_sun/yr. With a star formation efficiency an order-of-magnitude greater than that of normal galaxies, it will quench the star formation by exhausting the gas reservoir in only ~200 million years. At a projected separation of 19 kiloparsecs, the two massive starbursts are about to merge and form a passive elliptical galaxy with a stellar mass of ~4x10^11 M_sun. Our observations show that gas-rich major galaxy mergers, concurrent with intense star formation, can form the most massive elliptical galaxies by z ~ 1.5.

astro-ph.CO↗

arXiv2013

Herschel-ATLAS/GAMA: What determines the far-infrared properties of radio-galaxies?

We perform a stacking analysis of H-ATLAS data in order to obtain isothermal dust temperatures and rest-frame luminosities at 250um (L250), for 1599 radio sources over the H-ATLAS P1 GAMA area. The radio sample is generated using a combination of NVSS data and K-band UKIDSS-LAS data, over 0.01 1.5 L_{K}^{*}) may have systematically lower FIR luminosities (~25%) than their colour-matched non radio-detected counterparts. Compact radio sources (<30kpc) are associated with higher L250s and dust temperatures than their more extended (>30kpc) counterparts. The higher dust temperature suggests that this may be attributed to enhanced SFRs, but whether this is directly or indirectly due to radio activity (e.g. jet induced or merger-driven SF) is as yet unknown.

astro-ph.CO↗

arXiv2013

Herschel/SPIRE Sub-millimeter Spectra of Local Active Galaxies

We present the sub-millimeter spectra from 450 GHz to 1550 GHz of eleven nearby active galaxies observed with the SPIRE Fourier Transform Spectrometer (SPIRE/FTS) onboard Herschel. We detect CO transitions from J_up = 4 to 12, as well as the two [CI] fine structure lines at 492 and 809 GHz and the [NII] 461 GHz line. We used radiative transfer models to analyze the observed CO spectral line energy distributions (SLEDs). The FTS CO data were complemented with ground-based observations of the low-J CO lines. We found that the warm molecular gas traced by the mid-J CO transitions has similar physical conditions (n_H2 ~ 10^3.2 - 10^3.9 cm^-3 and T_kin ~ 300 - 800 K) in most of our galaxies. Furthermore, we found that this warm gas is likely producing the mid-IR rotational H2 emission. We could not determine the specific heating mechanism of the warm gas, however it is possibly related to the star-formation activity in these galaxies. Our modeling of the [CI] emission suggests that it is produced in cold (T_kin < 30 K) and dense (n_H2 > 10^3 cm^-3) molecular gas. Transitions of other molecules are often detected in our SPIRE/FTS spectra. The HF J=1-0 transition at 1232 GHz is detected in absorption in UGC05101 and in emission in NGC7130. In the latter, near-infrared pumping, chemical pumping, or collisional excitation with electrons are plausible excitation mechanisms likely related to the AGN of this galaxy. In some galaxies few H2O emission lines are present. Additionally, three OH+ lines at 909, 971, and 1033 GHz are identified in NGC7130.

astro-ph.GA↗

arXiv2012

Intensity Mapping of Lyman-alpha Emission During the Epoch of Reionization

We calculate the absolute intensity and anisotropies of the Lyman-alpha radiation field present during the epoch of reionization. We consider emission from both galaxies and the intergalactic medium (IGM) and take into account the main contributions to the production of Lyman-alpha photons: recombinations, collisions, continuum emission from galaxies and scattering of Lyman-n photons in the IGM. We find that the emission from individual galaxies dominates over the IGM with a total Lyman-alpha intensity (times frequency) of about (1.43-3.57)x10^{-8} erg s^{-1} cm^{-2} sr^{-1} at a redshift of 7. This intensity level is low so it is unlikely that the Lyman-αbackground during reionization can be established by an experiment aiming at an absolute background light measurement. Instead we consider Lyman-αintensity mapping with the aim of measuring the anisotropy power spectrum which has rms fluctuations at the level of 1 x 10^{-16} [erg s^[-1} cm^{-2} sr^{-1}]^2 at a few Mpc scales. These anisotropies could be measured with a spectrometer at near-IR wavelengths from 0.9 to 1.4 μm with fields in the order of 0.5 to 1 sq. degrees. We recommend that existing ground-based programs using narrow band filters also pursue intensity fluctuations to study statistics on the spatial distribution of faint Lyman-αemitters. We also discuss the cross-correlation signal with 21 cm experiments that probe HI in the IGM during reionization. A dedicated sub-orbital or space-based Lyman-αintensity mapping experiment could provide a viable complimentary approach to probe reionization, when compared to 21 cm experiments, and is likely within experimental reach.

astro-ph.CO↗

arXiv2012

Probing the Pre-Reionization Epoch with Molecular Hydrogen Intensity Mapping

Molecular hydrogen is now understood to be the main coolant of the primordial gas clouds leading to the formation of the very first stars and galaxies. The line emissions associated with molecular hydrogen should then be a good tracer of the matter distribution at the onset of reionization of the universe. Here we propose intensity mapping of H2 line emission in rest-frame mid-infrared wavelengths to map out the spatial distribution of gas at redshifts z > 10. We calculate the expected mean intensity and clustering power spectrum for several H2 lines. We find that the 0-0S(3) rotational line at a rest wavelength of 9.66 microns is the brightest line over the redshift range of 10 to 30 with an intensity of about 5 to 10 Jy/sr at z~15. To reduce astrophysical and instrumental systematics, we propose the cross-correlation between multiple lines of the H2 rotational and vibrational line emission spectrum. Our estimates of the intensity can be used as a guidance in planning instruments for future mid-IR spectroscopy missions such as SPICA.

astro-ph.CO↗

arXiv2012

H-ATLAS: The cosmic abundance of dust from the far-infrared background power spectrum

We present a measurement of the angular power spectrum of the cosmic far-infrared background (CFIRB) anisotropies in one of the extragalactic fields of the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) at 250, 350 and 500 μm bands. Consistent with recent measurements of the CFIRB power spectrum in Herschel-SPIRE maps, we confirm the existence of a clear one-halo term of galaxy clustering on arcminute angular scales with large-scale two-halo term of clustering at 30 arcminutes to angular scales of a few degrees. The power spectrum at the largest angular scales, especially at 250 μm, is contaminated by the Galactic cirrus. The angular power spectrum is modeled using a conditional luminosity function approach to describe the spatial distribution of unresolved galaxies that make up the bulk of the CFIRB. Integrating over the dusty galaxy population responsible for the background anisotropies, we find that the cosmic abundance of dust, relative to the critical density, to be between Ω_dust=10^{-6} and 8 x 10^{-6} in the redshift range z ~ 0-3. This dust abundance is consistent with estimates of the dust content in the Universe using quasar reddening and magnification measurements in the SDSS.

astro-ph.CO↗

arXiv2012

HerMES: Candidate Gravitationally Lensed Galaxies and Lensing Statistics at Submillimeter Wavelengths

We present a list of 13 candidate gravitationally lensed submillimeter galaxies (SMGs) from 95 square degrees of the Herschel Multi-tiered Extragalactic Survey, a surface density of 0.14\pm0.04deg^{-2}. The selected sources have 500um flux densities (S_500) greater than 100mJy. Gravitational lensing is confirmed by follow-up observations in 9 of the 13 systems (70%), and the lensing status of the four remaining sources is undetermined. We also present a supplementary sample of 29 (0.31\pm0.06deg^{-2}) gravitationally lensed SMG candidates with S_500=80--100mJy, which are expected to contain a higher fraction of interlopers than the primary candidates. The number counts of the candidate lensed galaxies are consistent with a simple statistical model of the lensing rate, which uses a foreground matter distribution, the intrinsic SMG number counts, and an assumed SMG redshift distribution. The model predicts that 32--74% of our S_500>100mJy candidates are strongly gravitationally lensed (mu>2), with the brightest sources being the most robust; this is consistent with the observational data. Our statistical model also predicts that, on average, lensed galaxies with S_500=100mJy are magnified by factors of ~9, with apparently brighter galaxies having progressively higher average magnification, due to the shape of the intrinsic number counts. 65% of the sources are expected to have intrinsic 500micron flux densities less than 30mJy. Thus, samples of strongly gravitationally lensed SMGs, such as those presented here, probe below the nominal Herschel detection limit at 500 micron. They are good targets for the detailed study of the physical conditions in distant dusty, star-forming galaxies, due to the lensing magnification, which can lead to spatial resolutions of ~0.01" in the source plane.

astro-ph.CO↗

arXiv2012

A measurement of the intrahalo light fraction with near-infrared background anisotropies

Unresolved near-infrared background anisotropies are expected to have contributions from the earliest galaxies during reionization and faint, dwarf galaxies at intermediate redshifts. Previous measurements were unable to conclusively pinpoint the dominant origin because they did not sample spatial scales that were sufficiently large to distinguish between these two possibilities. Here we report a measurement of the anisotropy power spectrum from sub-arcminute to one degree angular scales and find the clustering amplitude to be larger than the model predictions involving the two existing explanations. As the shot-noise level of the power spectrum is consistent with that expected from faint galaxies, a new source population on the sky is not necessary to explain the observations. A physical mechanism that increases the clustering amplitude, however, is needed. Motivated by recent results related to the extended stellar light profile in dark matter halos, we consider the possibility that the fluctuations originate from diffuse intrahalo stars of all galaxies. We find that the measured power spectrum can be explained by an intrahalo light fraction of 0.07 to 0.2 % relative to the total luminosity in dark matter halos of masses log(M/M_Sun) ~ 9 to 12 at redshifts of ~ 1 to 4.

astro-ph.CO↗

arXiv2012

A Conditional Luminosity Function Model of the Cosmic Far-Infrared Background Anisotropy Power Spectrum

The cosmic far-infrared background (CFIRB) is expected to be generated by faint, dusty star-forming galaxies during the peak epoch of galaxy formation. The anisotropy power spectrum of the CFIRB captures the spatial distribution of these galaxies in dark matter halos and the spatial distribution of dark matter halos in the large-scale structure. Existing halo models of CFIRB anisotropy power spectrum are either incomplete or lead to halo model parameters that are inconsistent with the galaxy distribution selected at other wavelengths. Here we present a conditional luminosity function approach to describe the far-IR bright galaxies. We model the 250 um luminosity function and its evolution with redshift and model-fit the CFIRB power spectrum at 250 um measured by the Herschel Space Observatory. We introduce a redshift dependent duty-cycle parameter so that we are able to estimate the typical duration of the dusty star formation process in the dark matter halos as a function of redshifts. We find the duty cycle of galaxies contributing to the far-IR background is 0.3 to 0.5 with a dusty star-formation phase lasting for \sim0.3-1.6 Gyrs. This result confirms the general expectation that the far-IR background is dominated by star-forming galaxies in an extended phases, not bright starbursts that are driven by galaxy mergers and last \sim10-100 Myrs. The halo occupation number for satellite galaxies has a power-law slope that is close to unity over 0 10^{10} L_Sun is 2\times10^{11}M_Sun and 3\times 10^{10}M_Sun at z=1 and 2, respectively. Integrating over the galaxy population with L_250>10^{9} L_Sun, we find that the cosmic density of dust residing in the dusty, star-forming galaxies responsible for the background anisotropies Ω_{dust}\sim3\times10^{-6} to 2\times10^{-5}.

astro-ph.CO↗

arXiv2012

The dust energy balance in the edge-on spiral galaxy NGC 4565

We combine new dust continuum observations of the edge-on spiral galaxy NGC 4565 in all Herschel/SPIRE (250, 350, 500 micron) wavebands, obtained as part of the Herschel Reference Survey, and a large set of ancillary data (Spitzer, SDSS, GALEX) to analyze its dust energy balance. We fit a radiative transfer model for the stars and dust to the optical maps with the fitting algorithm FitSKIRT. To account for the observed UV and mid-infrared emission, this initial model was supplemented with both obscured and unobscured star-forming regions. Even though these star-forming complexes provide an additional heating source for the dust, the far-infrared/submillimeter emission long wards of 100 micron is underestimated by a factor of 3-4. This inconsistency in the dust energy budget of NGC 4565 suggests that a sizable fraction (two-thirds) of the total dust reservoir (Mdust ~ 2.9e+8 Msun) consists of a clumpy distribution with no associated young stellar sources. The distribution of those dense dust clouds would be in such a way that they remain unresolved in current far-infrared/submillimeter observations and hardly comtribute to the attenuation at optical wavelengths. More than two-thirds of the dust heating in NGC 4565 is powered by the old stellar population, with localized embedded sources supplying the remaining dust heating in NGC 4565. The results from this detailed dust energy balance study in NGC 4565 is consistent with that of similar analyses of other edge-on spirals.

astro-ph.CO↗