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C. K. Xu

Publications and source records attributed to C. K. Xu.

At least 73 records · Page 4Linked to original sources

The HerMES SPIRE submillimeter local luminosity function

Local luminosity functions are fundamental benchmarks for high-redshift galaxy formation and evolution studies as well as for models describing these processes. Determining the local luminosity function in the submillimeter range can help to better constrain in particular the bolometric luminosity density in the local Universe, and Herschel offers the first opportunity to do so in an unbiased way by imaging large sky areas at several submillimeter wavelengths. We present the first Herschel measurement of the submillimeter 0<z<0.2 local luminosity function and infrared bolometric (8-1000 $μ$m) local luminosity density based on SPIRE data from the HerMES Herschel Key Program over 14.7 deg^2. Flux measurements in the three SPIRE channels at 250, 350 and 500 \mum are combined with Spitzer photometry and archival data. We fit the observed optical-to-submillimeter spectral energy distribution of SPIRE sources and use the 1/V_{max} estimator to provide the first constraints on the monochromatic 250, 350 and 500 \mum as well as on the infrared bolometric (8-1000 \mum) local luminosity function based on Herschel data. We compare our results with modeling predictions and find a slightly more abundant local submillimeter population than predicted by a number of models. Our measurement of the infrared bolometric (8-1000 \mum) local luminosity function suggests a flat slope at low luminosity, and the inferred local luminosity density, 1.31_-0.21^+0.24 x 10^8 Lsun Mpc^-3, is consistent with the range of values reported in recent literature.

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HerMES: The Submillimeter Spectral Energy Distributions of Herschel/SPIRE-Detected Galaxies

We present colours of sources detected with the Herschel/SPIRE instrument in deep extragalactic surveys of the Lockman Hole, Spitzer-FLS, and GOODS-N fields in three photometric bands at 250, 350 and 500 micrometers. We compare these with expectations from the literature and discuss associated uncertainties and biases in the SPIRE data. We identify a 500 micrometer flux limited selection of sources from the HerMES point source catalogue that appears free from neighbouring/blended sources in all three SPIRE bands. We compare the colours with redshift tracks of various contemporary models. Based on these spectral templates we show that regions corresponding to specific population types and redshifts can be identified better in colour-flux space. The redshift tracks as well as the colour-flux plots imply a majority of detected objects with redshifts at 1<z<3.5, somewhat depending on the group of model SEDs used. We also find that a population of S_250/S_350<0.8 at fluxes above 50 mJy as observed by SPIRE is not well represented by contemporary models and could consist of a mix of cold and lensed galaxies.

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HerMES: Far-infrared properties of known AGN in the HerMES fields

Nuclear and starburst activity are known to often occur concomitantly. Herschel-SPIRE provides sampling of the FIR SEDs of type 1 and type 2 AGN, allowing for the separation between the hot dust (torus) and cold dust (starburst) emission. We study large samples of spectroscopically confirmed type 1 and type 2 AGN lying within the Herschel Multi-tiered Extragalactic Survey (HerMES) fields observed during the science demonstration phase, aiming to understand their FIR colour distributions and constrain their starburst contributions. We find that one third of the spectroscopically confirmed AGN in the HerMES fields have 5-sigma detections at 250um, in agreement with previous (sub)mm AGN studies. Their combined Spitzer-MIPS and Herschel-SPIRE colours - specifically S(250)/S(70) vs. S(70)/S(24) - quite clearly separate them from the non-AGN, star-forming galaxy population, as their 24-um flux is dominated by the hot torus emission. However, their SPIRE colours alone do not differ from those of non-AGN galaxies. SED fitting shows that all those AGN need a starburst component to fully account for their FIR emission. For objects at z > 2, we find a correlation between the infrared luminosity attributed to the starburst component, L(SB), and the AGN accretion luminosity, L(acc), with L(SB) propto L(acc)^0.35. Type 2 AGN detected at 250um show on average higher L(SB) than type 1 objects but their number is still too low to establish whether this trend indicates stronger star-formation activity.

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HerMES: The SPIRE confusion limit

We report on the sensitivity of SPIRE photometers on the Herschel Space Observatory. Specifically, we measure the confusion noise from observations taken during the Science Demonstration Phase of the Herschel Multi-tiered Extragalactic Survey. Confusion noise is defined to be the spatial variation of the sky intensity in the limit of infinite integration time, and is found to be consistent among the different fields in our survey at the level of 5.8, 6.3 and 6.8 mJy/beam at 250, 350 and 500 microns, respectively. These results, together with the measured instrument noise, may be used to estimate the integration time required for confusion-limited maps, and provide a noise estimate for maps obtained by SPIRE.

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HerMES: SPIRE galaxy number counts at 250, 350 and 500 microns

Emission at far-infrared wavelengths makes up a significant fraction of the total light detected from galaxies over the age of Universe. Herschel provides an opportunity for studying galaxies at the peak wavelength of their emission. Our aim is to provide a benchmark for models of galaxy population evolution and to test pre-existing models of galaxies. With the Herschel Multi-tiered Extra-galactic survey, HerMES, we have observed a number of fields of different areas and sensitivity using the SPIRE instrument on Herschel. We have determined the number counts of galaxies down to ~20 mJy. Our constraints from directly counting galaxies are consistent with, though more precise than, estimates from the BLAST fluctuation analysis. We have found a steep rise in the Euclidean normalised counts at <100 mJy. We have directly resolved 15% of the infrared extra-galactic background at the wavelength near where it peaks.

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Powerful H$_2$ Line-cooling in Stephan's Quintet : I - Mapping the Significant Cooling Pathways in Group-wide Shocks

We present results from the mid-infrared spectral mapping of Stephan's Quintet using the Spitzer Space Telescope. A 1000 km/s collision has produced a group-wide shock and for the first time the large-scale distribution of warm molecular hydrogen emission is revealed, as well as its close association with known shock structures. In the main shock region alone we find 5.0 $\times10^{8}$ M$_{\odot}$ of warm H$_2$ spread over $\sim$ 480 kpc$^2$ and additionally report the discovery of a second major shock-excited H$_2$ feature. This brings the total H$_2$ line luminosity of the group in excess of 10$^42$ erg/s. In the main shock, the H$_2$ line luminosity exceeds, by a factor of three, the X-ray luminosity from the hot shocked gas, confirming that the H$_2$-cooling pathway dominates over the X-ray. [Si II]34.82$μ$m emission, detected at a luminosity of 1/10th of that of the H$_2$, appears to trace the group-wide shock closely and in addition, we detect weak [FeII]25.99$μ$m emission from the most X-ray luminous part of the shock. Comparison with shock models reveals that this emission is consistent with regions of fast shocks (100 < $V_{s}$ < 300 km/s) experiencing depletion of iron and silicon onto dust grains. Star formation in the shock (as traced via ionic lines, PAH and dust emission) appears in the intruder galaxy, but most strikingly at either end of the radio shock. The shock ridge itself shows little star formation, consistent with a model in which the tremendous H$_{2}$ power is driven by turbulent energy transfer from motions in a post-shocked layer. The significance of the molecular hydrogen lines over other measured sources of cooling in fast galaxy-scale shocks may have crucial implications for the cooling of gas in the assembly of the first galaxies.

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Collisional debris as laboratories to study star formation

In this paper we address the question whether star formation is driven by local processes or the large scale environment. To do so, we investigate star formation in collisional debris where the gravitational potential well and velocity gradients are shallower and compare our results with previous work on star formation in non-interacting spiral and dwarf galaxies. We have performed multiwavelength spectroscopic and imaging observations (from the far-ultraviolet to the mid-infrared) of 6 interacting systems, identifying a total of 60 star-forming regions in their collision debris. Our analysis indicates that in these regions a) the emission of the dust is at the expected level for their luminosity and metallicity, b) the usual tracers of star formation rate display the typical trend and scatter found in classical star forming regions, and c) the extinction and metallicity are not the main parameters governing the scatter in the properties of intergalactic star forming regions; age effects and variations in the number of stellar populations, seem to play an important role. Our work suggests that local properties such as column density and dust content, rather than the large scale environment seem to drive star formation. This means that intergalactic star forming regions can be used as a reliable tool to study star formation.

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2MASS/SDSS Close Major-Merger Galaxy Pairs: Luminosity Functions and Merger Mass Dependence

We select a close "major-merger candidate" galaxy pair sample in order to calculate the K_{s} luminosity function (LF) and pair fraction representative of the merger/interaction component of galaxy evolution in the local universe. The pair sample (projected separation 5 h$^{-1}$ kpc $\leq$ r $\leq$ 20 h$^{-1}$ kpc, $K_{s}$-band magnitude difference $Δ$$K_{s}$ $\leq$ 1 mag) is selected by combining the Two Micron All Sky Survey (2MASS) with the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5). The resulting data set contains 340 galaxies covering 5800 sq. degrees. A stellar mass function is also translated from the LF. A differential pair fraction displays nearly constant fraction of galaxy pairs as a function of galaxy mass from 10$^{9}$ to 10$^{11.5}$ M$_{\sun}$ . The differential pair fraction is less subject to absolute magnitude bias due to survey limitations than the standard total pair fraction. These results suggest that major-merger candidate pairs in the 0$<$z$<$0.1 universe are developed from $\sim$1.6% of the galaxy population without dependance on galaxy mass for pair components below 10$^{11}$ M$_{\sun}$. The derived LF combined with merger model time scales give local merger rates per unit volume which decrease with masses greater than 10$^{11}$ M$_{\sun}$ .

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Clustering of galaxies at 3.6 microns in the Spitzer Wide-area Infrared Extragalactic legacy survey

We investigate the clustering of galaxies selected in the 3.6 micron band of the Spitzer Wide-area Infrared Extragalactic (SWIRE) legacy survey. The angular two-point correlation function is calculated for eleven samples with flux limits of S_3.6 > 4-400 mujy, over an 8 square degree field. The angular clustering strength is measured at >5-sigma significance at all flux limits, with amplitudes of A=(0.49-29)\times10^{-3} at one degree, for a power-law model, Aθ^{-0.8}. We estimate the redshift distributions of the samples using phenomological models, simulations and photometric redshifts, and so derive the spatial correlation lengths. We compare our results with the GalICS (Galaxies In Cosmological Simulations) models of galaxy evolution and with parameterized models of clustering evolution. The GalICS simulations are consistent with our angular correlation functions, but fail to match the spatial clustering inferred from the phenomological models or the photometric redshifts. We find that the uncertainties in the redshift distributions of our samples dominate the statistical errors in our estimates of the spatial clustering. At low redshifts (median z<0.5) the comoving correlation length is approximately constant, r_0=6.1\pm0.5h^{-1} Mpc, and then decreases with increasing redshift to a value of 2.9\pm0.3h^{-1} Mpc for the faintest sample, for which the median redshift is z=1. We suggest that this trend can be attributed to a decrease in the average galaxy and halo mass in the fainter flux-limited samples, corresponding to changes in the relative numbers of early- and late-type galaxies. However, we cannot rule out strong evolution of the correlation length over 0.5<z<1.

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Clustering Properties of restframe UV selected galaxies I: the correlation length derived from GALEX data in the local Universe

We present the first measurements of the angular correlation function of galaxies selected in the far (1530 A) and near (2310 A) Ultraviolet from the GALEX survey fields overlapping SDSS DR5 in low galactic extinction regions. The area used covers 120 sqdeg (GALEX - MIS) down to magnitude AB = 22, yielding a total of 100,000 galaxies. The mean correlation length is ~ 3.7 \pm 0.6 Mpc and no significant trend is seen for this value as a function of the limiting apparent magnitude or between the GALEX bands. This estimate is close to that found from samples of blue galaxies in the local universe selected in the visible, and similar to that derived at z ~ 3 for LBGs with similar rest frame selection criteria. This result supports models that predict anti-biasing of star forming galaxies at low redshift, and brings an additional clue to the downsizing of star formation at z<1.

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UV to IR SEDs of UV selected galaxies in the ELAIS fields: evolution of dust attenuation and star formation activity from z=0.7 to z=0.2

We study the ultraviolet to far-infrared (hereafter UV-to-IR) SEDs of a sample of intermediate redshift (0.2 < z < 0.7) UV-selected galaxies from the ELAIS-N1 and ELAIS-N2 fields by fitting a multi-wavelength dataset to a library of GRASIL templates. Star formation related properties of the galaxies are derived from the library of models by using the Bayesian statistics. We find a decreasing presence of galaxies with low attenuation and low total luminosity as redshift decreases, which does not hold for high total luminosity galaxies. In addition the dust attenuation of low mass galaxies increases as redshift decreases, and this trend seems to disappear for galaxies with M* > 10^11 M_sun. This result is consistent with a mass dependent evolution of the dust to gas ratio, which could be driven by a mass dependent efficiency of star formation in star forming galaxies. The specific star formation rates (SSFR) decrease with increasing stellar mass at all redshifts, and for a given stellar mass the SSFR decreases with decreasing redshift. The differences in the slope of the M*--SSFR relation found between this work and others at similar redshift could be explained by the adopted selection criteria of the samples which, for a UV selected sample, favours blue, star forming galaxies.

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Clustering Properties of restframe UV selected galaxies II: Migration of Star Formation sites with cosmic time from GALEX and CFHTLS

We analyze the clustering properties of ultraviolet selected galaxies by using GALEX-SDSS data at z<0.6 and CFHTLS deep u' imaging at z=1. These datasets provide a unique basis at z< 1 which can be directly compared with high redshift samples built with similar selection criteria. We discuss the dependence of the correlation function parameters (r0, delta) on the ultraviolet luminosity as well as the linear bias evolution. We find that the bias parameter shows a gradual decline from high (b > 2) to low redshift (b ~ 0.79^{+0.1}_{-0.08}). When accounting for the fraction of the star formation activity enclosed in the different samples, our results suggest that the bulk of star formation migrated from high mass dark matter halos at z>2 (10^12 < M_min < 10^13 M_sun, located in high density regions), to less massive halos at low redshift (M_min < 10^12 M_sun, located in low density regions). This result extends the ``downsizing'' picture (shift of the star formation activity from high stellar mass systems at high z to low stellar mass at low z) to the dark matter distribution.

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The ultraviolet properties of Luminous Infrared Galaxies at z ~ 0.7: is there any evolution in their dust attenuation?

The TIR and FUV luminosity functions of galaxies and the related luminosity densities rho(TIR) and rho(FUV) are known to evolve from z=0 to z~1 but with a different rate: the galaxy populations appear brighter in the past at both wavelengths but the evolution in TIR is larger than in FUV. It leads to an increase of the ratio of TIR to FUV luminosity densities rho(TIR)/rho(FUV) which can be interpreted as a global increase of the dust attenuation from z=0 to z~1. Our aim is to understand the origin of this increase. We focus on infrared galaxies with L(TIR>10^{11} Lsun at z~0.7 observed by SPITZER/MIPS and we measure their UV emission at 2310 A from GALEX. These Luminous InfraRed Galaxies (LIRGs) represent the bulk of the TIR luminosity density at intermediate redshift. Some evolution of L(TIR)/L(FUV) and therefore of dust attenuation is found for these galaxies: LIRGs at z=0.7 span a larger range of L(TIR)/L(FUV) ratios than at z=0 and their mean dust attenuation at FUV is found ~0.5 mag lower than for their local counterparts. The decrease of dust attenuation is found lower than that reported in other studies for bright galaxies selected in UV rest-frame at z=1 and 2. From a semi-quantitative analysis we find that the slight decrease of dust attenuation for LIRGs at z=0.7 remains consistent with the increase of rho(TIR)/rho(FUV) with redshift.

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The Local Universe as Seen in Far-Infrared and in Far-Ultraviolet: A Global Point of View on the Local Recent Star Formation

We select far-infrared (FIR-60 microns) and far-ultraviolet (FUV-1530 A) samples of nearby galaxies in order to discuss the biases encountered by monochromatic surveys (FIR or FUV). Very different volumes are sampled by each selection and much care is taken to apply volume corrections to all the analyses. The distributions of the bolometric luminosity of young stars are compared for both samples: they are found to be consistent with each other for galaxies of intermediate luminosities but some differences are found for high (>5 10^{10} L_sun) luminosities. The shallowness of the IRAS survey prevents us from securing comparison at low luminosities (<2 10^9 L_sun). The ratio of the total infrared (TIR) luminosity to the FUV luminosity is found to increase with the bolometric luminosity in a similar way for both samples up to 5 10^{10} L_sun. Brighter galaxies are found to have a different behavior according to their selection: the L_TIR/L_FUV ratio of the FUV-selected galaxies brighter than 5 10^{10} L_sun reaches a plateau whereas L_TIR/L_FUV continues to increase with the luminosity of bright galaxies selected in FIR. The volume-averaged specific star formation rate (SFR per unit galaxy stellar mass, SSFR) is found to decrease toward massive galaxies within each selection. The SSFR is found to be larger than that measured for optical and NIR-selected sample over the whole mass range for the FIR selection, and for masses larger than 10^{10} M_sun for the FUV selection. Luminous and massive galaxies selected in FIR appear as active as galaxies with similar characteristics detected at z ~ 0.7.

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Morphological Studies of the SWIRE Galaxy Population in the UGC 10214 HST/ACS field

We present results of a morphological analysis of a small subset of the Spitzer Wide-area InfraRed Extragalactic survey (SWIRE) galaxy population. The analysis is based on public ACS data taken inside the SWIRE N1 field, which are the deepest optical high-resolution imaging available within the SWIRE fields as of today. Our reference sample includes 156 galaxies detected by both ACS and SWIRE. Among the various galaxy morphologies, we disentangle two main classes, spheroids (or bulge-dominated galaxies) and disk-dominated ones, for which we compute the number counts as a function of flux. We then limit our sample to objects with IRAC fluxes brighter than 10 microJy, estimated ~90% completeness limit of the SWIRE catalogues, and compare the observed counts to model predictions. We find that the observed counts of the spheroidal population agree with the expectations of a hierarchical model while a monolithic scenario predicts steeper counts. Both scenaria, however, under-predict the number of late-type galaxies. These observations show that the large majority (close to 80 per cent) of the 3.6 and 4.5 micron galaxy population, even at these moderately faint fluxes, is dominated by spiral and irregular galaxies or mergers.

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Complete Multiwavelength Characterization of Faint Chandra X-ray Sources Seen in the Spitzer Wide-Area IR Extragalactic (SWIRE) Survey

We exploit deep combined observations with Spitzer and Chandra of the SWIRE survey in the ELAIS-N1 region, to investigate the nature of the faint X-ray and IR sources in common, to identify AGN/starburst diagnostics, and to study the sources of the X-ray and IR cosmic backgrounds. In the 17'x17' area of the Chandra ACIS-I image there are 3400 SWIRE near-IR sources with 4 sigma detections in at least 2 IRAC bands and 988 sources detected at 24micron with MIPS brighter than 0.1 mJy. Of these, 102 IRAC and 59 MIPS sources have Chandra counterparts, out of a total of 122 X-ray sources present in the area with S(0.5-8 kev)>10^(-15) erg/cm^2/s. We have constructed SEDs for each source using data from the 4 IRAC wavebands, Chandra fluxes, and optical follow-up data in the wavebands U, g', r', i', Z, and H. We fit a number of spectral templates to the SEDs at optical and infrared wavelengths to determine photometric redshifts and spectral categories, and also make use of diagnostics based on the X-ray luminosities, hardness ratios, X-ray to infrared spectral slopes and optical morphologies. Although we have spectroscopic redshifts for only a minority of the Chandra sources, the available SEDs constrain the redshifts for most of the sample sources, which turn out to be typically at 0.5<z<2. We find that 39% of the Chandra sources are dominated by type-1 AGN emission, 23% display optical/IR spectra typical of type-2 AGNs, while the remaining 38% fraction show starburst-like or even normal galaxy spectra. Since we prove that all these galaxies are dominated by AGN emission in X-rays this brings the fraction of type-1 AGNs to be 80% of the type-2: even assuming that all the Chandra sources undetected by Spitzer are type-2 AGNs, the type-1 fraction would exceed 1/3 of the total population (abridged).

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Dust attenuation in the nearby Universe: comparison between galaxies selected in the ultraviolet or in the infrared

We compare the dust attenuation properties of two samples of galaxies purely selected in the near-ultraviolet (NUV) band (1750-2750 A, lambda_m = 2310 A) and in the far-infrared (FIR) at 60micron. These samples are built using the GALEX and IRAS sky surveys over ~600 square degrees. The NUV selected sample contains 95 galaxies detected down to NUV=16mag (AB system). 83 galaxies in this sample are spirals or irregulars and only two of them are not detected at 60micron. The FIR selected sample is built from the IRAS PSCz catalog complete down to 0.6Jy. Among the 163 sources, we select 118 star forming galaxies well measured by IRAS, all but 1 are detected in NUV and 14 galaxies are not detected in the far-ultraviolet (FUV) band (1350-1750 A, lambda_m = 1530 A). The dust to ultraviolet (NUV and FUV) flux ratio is calibrated to estimate the dust attenuation at both wavelengths. The median value of the attenuation in NUV is found to be ~1 mag for the NUV selected sample versus ~2 mag for the FIR selected one. Within both samples, the dust attenuation is found to correlate with the luminosity of the galaxies. Almost all the NUV selected galaxies and 2/3 of the FIR selected sample exhibit a lower dust attenuation than that expected from the tight relation found previously for starburst galaxies between the dust attenuation and the slope of the ultraviolet continuum. The situation is inverse for one third of the FIR selected galaxies: their extinction is higher than that deduced from their FUV-NUV color and the relation valid for starbursts.

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Sloan Digital Sky Survey Quasars in the SWIRE ELAIS N1 Field: Properties and Spectral Energy Distributions

We present a mid-infrared analysis of 35 quasars with spectroscopic redshifts selected from the Spitzer Wide-area InfraRed Extragalactic Survey (SWIRE). We discuss their optical and mid-infrared (MIR) colors, and show that these quasars occupy well defined regions in MIR color-color space. We examine the issue of type-I AGN candidate selection in detail and propose new selection methods based on mid-IR colors. The available multi-band data allows us to construct two new, well-sampled quasar templates, covering wavelengths from the ultraviolet to the MIR.

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