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Mattia Vaccari

Publications and source records attributed to Mattia Vaccari.

84 records · Page 5Linked to original sources
arXiv2009

On the nature of the first galaxies selected at 350 microns

[abridged] We present constraints on the nature of the first galaxies selected at 350 microns. The sample includes galaxies discovered in the deepest blank-field survey at 350 microns (in the Bootes Deep Field) and also later serendipitous detections in the Lockman Hole. Spectral energy distribution templates are fit to identified counterparts, and the sample is found to comprise IR-luminous galaxies at 1<z<3 predominantly powered by star formation. The first spectrum of a 350-micron-selected galaxy provides an additional confirmation, showing prominent dust grain features typically associated with star-forming galaxies. Compared to submillimeter galaxies selected at 850 and 1100 microns, galaxies selected at 350 microns have a similar range of far-infrared color temperatures. However, no 350-micron-selected sources are reliably detected at 850 or 1100 microns. Galaxies in our sample with redshifts 1<z<2 show a tight correlation between the far- and mid-infrared flux densities, but galaxies at higher redshifts show a large dispersion in their mid- to far-infrared colors. The 350-micron flux densities (15<S(350)<40 mJy) place these objects near the Herschel/SPIRE 350-micron confusion threshold, with the lower limit on the star formation rate density suggesting the bulk of the 350-micron contribution will come from less luminous infrared sources and normal galaxies.

astro-ph.CO↗
arXiv2009

A New Approach to Multi-wavelength Associations of Astronomical Sources

One of the biggest problems faced by current and next-generation astronomical surveys is trying to produce large numbers of accurate cross identifications across a range of wavelength regimes with varying data quality and positional uncertainty. Until recently simple spatial "nearest neighbour" associations have been sufficient for most applications. However as advances in instrumentation allow more sensitive images to be made the rapid increase in the source density has meant that source confusion across multiple wavelengths is a serious problem. The field of far-IR and sub-mm astronomy has been particularly hampered by such problems. The poor angular resolution of current sub-mm and far-IR instruments is such that in a lot of cases there are multiple plausible counterparts for each source at other wavelengths. Here we present a new automated method of producing associations between sources at different wavelengths using a combination of spatial and SED information set in the Bayesian framework presented by Budavari & Szalay (2008). Testing of the technique is performed on both simulated catalogues of sources from GaLICS and real data from multi-wavelength observations of the SXDF. It is found that a single figure of merit, the Bayes factor, can be effectively used to describe the confidence in the match. Further applications of this technique to future Herschel datasets are discussed.

astro-ph.CO↗
arXiv2008

The extragalactic optical-infrared background radiations, their time evolution and the cosmic photon-photon opacity

The background radiations in the optical and the infrared constitute a relevant cause of energy loss in the propagation of high energy particles through space. In particular, TeV observations with Cherenkov telescopes of extragalactic sources are influenced by the opacity effects due to the interaction of the very high-energy source photons with the background light. With the aim of assessing with the best possible detail these opacity terms, we have modelled the extragalactic optical and IR backgrounds using available information on cosmic sources in the universe from far-UV to sub-mm wavelengths over a wide range of cosmic epochs. We have exploited the relevant cosmological survey data - including number counts, redshift distributions, luminosity functions - from ground-based observatories in the optical, near-IR, and sub-mm, as well as multi-wavelength information coming from space telescopes, HST, ISO and Spitzer. Additional constraints have been used from direct measurements or upper limits on the extragalactic backgrounds by dedicated missions (COBE). All data were fitted and interpolated with a multi-wavelength backward evolutionary model, allowing us to estimate the background photon density and its redshift evolution. From the redshift-dependent background spectrum, the photon-photon opacities for sources of high-energy emission at any redshifts were then computed. The same results can also be used to compute the optical depths for any kind of processes in the intergalactic space involving interactions with background photons (like scattering of cosmic-ray particles). We have applied our photon-photon opacity estimates to the analysis of spectral data at TeV energies on a few BLAZARs of particular interest. [abridged]

astro-ph↗
arXiv2008

Photometric redshifts in the SWIRE Survey

We present the SWIRE Photometric Redshift Catalogue, 1025119 redshifts of unprecedented reliability and accuracy. Our method is based on fixed galaxy and QSO templates applied to data at 0.36-4.5 mu, and on a set of 4 infrared emission templates fitted to infrared excess data at 3.6-170 mu. The code involves two passes through the data, to try to optimize recognition of AGN dust tori. A few carefully justified priors are used and are the key to supression of outliers. Extinction, A_V, is allowed as a free parameter. We use a set of 5982 spectroscopic redshifts, taken from the literature and from our own spectroscopic surveys, to analyze the performance of our method as a function of the number of photometric bands used in the solution and the reduced chi^2. For 7 photometric bands the rms value of (z_{phot}-z_{spec})/(1+z_{spec}) is 3.5%, and the percentage of catastrophic outliers is ~1%. We discuss the redshift distributions at 3.6 and 24 mu. In individual fields, structure in the redshift distribution corresponds to clusters which can be seen in the spectroscopic redshift distribution. 10% of sources in the SWIRE photometric redshift catalogue have z >2, and 4% have z>3, so this catalogue is a huge resource for high redshift galaxies. A key parameter for understanding the evolutionary status of infrared galaxies is L_{ir}/L_{opt}, which can be interpreted as the specific star-formation rate for starbursts. For dust tori around Type 1 AGN, L_{tor}/L_{opt} is a measure of the torus covering factor and we deduce a mean covering factor of 40%.

astro-ph↗
arXiv2007

The SCUBA HAlf Degree Extragalactic Survey (SHADES) -- VI. 350 micron mapping of submillimetre galaxies

A follow-up survey using the Submillimetre High-Angular Resolution Camera (SHARC-II) at 350 microns has been carried out to map the regions around several 850 micron-selected sources from the Submillimetre HAlf Degree Extragalactic Survey (SHADES). These observations probe the infrared luminosities and hence star-formation rates in the largest existing, most robust sample of submillimetre galaxies (SMGs). We measure 350 micron flux densities for 24 850 micron sources, seven of which are detected at >2.5-sigma within a 10 arcsec search radius of the 850 micron positions. When results from the literature are included the total number of 350 micron flux density constraints of SHADES SMGs is 31, with 15 detections. We fit a modified blackbody to the far-infrared (FIR) photometry of each SMG, and confirm that typical SMGs are dust-rich (Mdust~9x10^8 Msun), luminous (Lfir~2x10^12 Lsun), star-forming galaxies with intrinsic dust temperatures of ~35 K and star-formation rates of ~400 Msun/yr. We have measured the temperature distribution of SMGs and find that the underlying distribution is slightly broader than implied by the error bars, and that most SMGs are at 28 K with a few hotter. We also place new constraints on the 350 micron source counts, N350(>25mJy)~200-500 deg^-2.

astro-ph↗
arXiv2007

Galaxy Counts at 24 Microns in the SWIRE Fields

This paper presents galaxy source counts at 24 microns in the six Spitzer Wide-field InfraRed Extragalactic (SWIRE) fields. The source counts are compared to counts in other fields, and to model predictions that have been updated since the launch of Spitzer. This analysis confirms a very steep rise in the Euclidean-normalized differential number counts between 2 mJy and 0.3 mJy. Variations in the counts between fields show the effects of sample variance in the flux range 0.5-10 mJy, up to 100% larger than Poisson errors. Nonetheless, a "shoulder" in the normalized counts persists at around 3 mJy. The peak of the normalized counts at 0.3 mJy is higher and narrower than most models predict. In the ELAIS N1 field, the 24 micron data are combined with Spitzer-IRAC data and five-band optical imaging, and these bandmerged data are fit with photometric redshift templates. Above 1 mJy the counts are dominated by galaxies at z less than 0.3. By 300 microJy, about 25% are between z ~ 0.3-0.8, and a significant fraction are at z ~ 1.3-2. At low redshifts the counts are dominated by spirals, and starbursts rise in number density to outnumber the spirals' contribution to the counts below 1 mJy.

astro-ph↗
arXiv2007

The SCUBA HAlf Degree Extragalactic Survey (SHADES) - IV: Radio-mm-FIR photometric redshifts

We present the redshift distribution of the SHADES galaxy population based on the rest-frame radio-mm-FIR colours of 120 robustly detected 850um sources in the Lockman Hole East (LH) and Subaru XMM-Newton Deep Field (SXDF). The redshift distribution derived from the full SED information is shown to be narrower than that determined from the radio-submm spectral index, as more photometric bands contribute to a higher redshift accuracy. The redshift distribution of sources derived from at least two photometric bands peaks at z ~ 2.4 and has a near-Gaussian distribution, with 50 per cent (interquartile range) of sources at z=1.8-3.1. We find a statistically-significant difference between the measured redshift distributions in the two fields; the SXDF peaking at a slightly lower redshift (median z ~ 2.2) than the LH (median z ~ 2.7), which we attribute to the noise-properties of the radio observations. We demonstrate however that there could also be field-to-field variations that are consistent with the measured differences in the redshift distributions, and hence, that the incomplete area observed by SHADES with SCUBA, despite being the largest sub-mm survey to date, may still be too small to fully characterize the bright sub-mm galaxy population. Finally we present a brief comparison with the predicted, or assumed, redshift distributions of sub-mm galaxy formation and evolution models, and we derive the contribution of these SHADES sources and the general sub-mm galaxy population to the star formation-rate density at different epochs.

astro-ph↗
arXiv2006

Understanding Infrared Galaxy Populations: the SWIRE Legacy Survey

We discuss spectral energy distributions, photometric redshifts, redshift distributions, luminosity functions, source-counts and the far infrared to optical luminosity ratio for sources in the SWIRE Legacy Survey. The spectral energy distributions of selected SWIRE sources are modelled in terms of a simple set of galaxy and quasar templates in the optical and near infrared, and with a set of dust emission templates (cirrus, M82 starburst, Arp 220 starburst, and AGN dust torus) in the mid infrared. The optical data, together with the IRAC 3.6 and 4.5 mu data, have been used to determine photometric redshifts. For galaxies with known spectroscopic redshifts there is a notable improvement in the photometric redshift when the IRAC data are used, with a reduction in the rms scatter from 10% in (1+z) to 5%. While further spectroscopic data are needed to confirm this result, the prospect of determining good photometric redshifts for the 2 million extragalactic objects in SWIRE is excellent. The distribution of the different infrared sed types in the L{ir}/L{opt} versus L{ir} plane, where L{ir} and L{opt} are the infrared and optical bolometric luminosities, is discussed. Source-counts at 24, 70 and 160 mu are discussed, and luminosity functions at 3.6 and 24 mu are presented.

astro-ph↗
arXiv2002

The LARI Method for ISO-CAM/PHOT Data Reduction and Analysis

The techniques and software tools developed for the reduction and analysis of ISO-CAM/PHOT data with the LARI method are presented. The method, designed for the detection of faint sources in ISO raster observations, is based on the assumption of the existence of two different time scales in the detectors' transient behaviour, accounting either for fast or slow detectors' response. The specifically developed IDL software includes: a reduction pipeline performing basic operations such as deglitching and background determination; the fitting procedures proper, modelling the time history of individual pixels and detecting any flux excess with respect to the local background ascribable to potential sources; mapping, source extraction and flux estimation procedures; simulation procedures allowing one to estimate the errors arising from different instrumental and reduction effects. Moreover, an easy-to-use graphical user interface allows one to quickly browse the data and carry out the substantial amount of interactive analysis required when the automatic fit fails and to check the reliability of detected sources. This method provides source lists of great reliability and completeness and an outstanding photometric accuracy, particularly at low redundancy levels, where the reliability of ISO-CAM/PHOT source lists, even at moderately bright flux levels, has been a long-standing issue. In this work a description of the techniques and software tools that have been developed is given, alongside with some highlights from the results obtained thanks to their application to different fields.

astro-ph↗
arXiv2001

GAIA Galaxy Survey: a multi-colour galaxy survey with GAIA

The performance expected from a galaxy survey to be carried out with GAIA, the GAIA Galaxy Survey, is outlined. From a statistical model of galaxy number density, size and surface brightness distribution, and from detailed numerical simulations based on real images, it is conservatively estimated that GAIA would be able to detect and observe about 3 million galaxies brighter than $V \simeq 17$ and to provide multi-colour and multi-epoch broad-band photometry of these with an end-of-mission angular resolution of $\simeq$ 0.35 \as and a photometric accuracy of $simeq$ 0.2 {mag/arcsec$^2$} at $μ_V = 20$ {mag/arcsec$^2$}. The substantial scientific case for performing such a survey and the additional efforts required in terms of mission preparation, operations and telemetry are also discussed.

astro-ph↗