A simple model to interpret the ultraviolet, optical and infrared SEDs of galaxies

We present a simple, largely empirical but physically motivated model, which is designed to interpret consistently multi-wavelength observations from large samples of galaxies in terms of physical parameters, such as star formation rate, stellar mass and dust content. Our model is both simple and versatile enough to allow the derivation of statistical constraints on the star formation histories and dust contents of large samples of galaxies using a wide range of ultraviolet, optical and infrared observations. We illustrate this by deriving median-likelihood estimates of a set of physical parameters describing the stellar and dust contents of local star-forming galaxies from the Spitzer Infrared Nearby Galaxy Sample (SINGS) and from a newly-matched sample of SDSS galaxies observed with GALEX, 2MASS, and IRAS. The model reproduces well the observed spectral energy distributions of these galaxies across the entire wavelength range from the far-ultraviolet to the far-infrared. We find important correlations between the physical parameters of galaxies which are useful to investigate the star formation activity and dust properties of galaxies. Our model can be straightforwardly applied to interpret observed ultraviolet-to-infrared spectral energy distributions (SEDs) from any galaxy sample.Comment: 4 pages, 3 figures, to appear in the Conference Proceedings of IAU Symposium No. 262: Stellar Populations - Planning for the Next Decade, G. Bruzual & S. Charlot ed

A Fossil Record of Galaxy Encounters

The cosmic infrared background (CIRB) is a record of a large fraction of the emission of light by stars and galaxies over time. The bulk of this emission has been resolved by the Infrared Space Observatory camera. The dominant contributors are bright starburst galaxies with redshift z ~ 0.8; that is, in the same redshift range as the active galactic nuclei responsible for the bulk of the x-ray background. At the longest wavelengths, sources of redshift z>2 tend to dominate the CIRB. It appears that the majority of present-day stars have been formed in dusty starbursts triggered by galaxy-galaxy interactions and the build-up of large-scale structures.Comment: 5 pages, 3 figures, published in 11 April 2003 issue of Science (review

Tracing Recent Star Formation of Red Early-type Galaxies out to zz ∼\sim 1

We study the mid-infrared (IR) excess emission of early-type galaxies (ETGs) on the red-sequence at $z <$ 1 using a spectroscopic sample of galaxies in the fields of Great Observatories Origins Deep Survey (GOODS). In the mass-limited sample of 1025 galaxies with $M_{star}$ $>$ 10$^{10.5}$ $M_{\odot}$ and $0.4<z<1.05$, we identify 696 $Spitzer$ 24 $\mu$m detected (above the 5$\sigma$) galaxies and find them to have a wide range of NUV-$r$ and $r$-[12 $\mu$m] colors despite their red optical $u-r$ colors. Even in the sample of very massive ETGs on the red sequence with $M_{star}$ $>$ 10$^{11.2}$ $M_{\odot}$, more than 18% show excess emission over the photospheric emission in the mid-IR. The combination with the results of red ETGs in the local universe suggests that the recent star formation is not rare among quiescent, red ETGs at least out to $z \sim 1$ if the mid-IR excess emission results from intermediate-age stars or/and from low-level ongoing star formation. Our color$-$color diagram including near-UV and mid-IR emissions are efficient not only for identifying ETGs with recent star formation, but also for distinguishing quiescent galaxies from dusty star-forming galaxies.Comment: 25 pages, 9 figures, accepted for publication in Ap

MACHOs, White Dwarfs, and the Age of the Universe

(Abridged Abstract) A favored interpretation of recent microlensing measurements towards the Large Magellanic Cloud implies that a large fraction (i.e. 10--50%) of the mass of the galactic halo is composed of white dwarfs. We compare model white dwarf luminosity functions to the data from the observational surveys in order to determine a lower bound on the age of any substantial white dwarf halo population (and hence possibly on the age of the Universe). We compare various theoretical white dwarf luminosity functions, in which we vary hese three parameters, with the abovementioned survey results. From this comparison, we conclude that if white dwarfs do indeed constitute more than 10% of the local halo mass density, then the Universe must be at least 10 Gyr old for our most extreme allowed values of the parameters. When we use cooling curves that account for chemical fractionation and more likely values of the IMF and the bolometric correction, we find tighter limits: a white dwarf MACHO fraction of 10% (30%) requires a minimum age of 14 Gyr (15.5 Gyr). Our analysis also indicates that the halo white dwarfs almost certainly have helium-dominated atmospheres.Comment: Final version accepted for publication, straight TeX formate, 6 figs, 22 page

Electronic data interchange : an assessment of the factors leading to EDI adoption

Electronic Data Interchange (EDI) is a revolutionary way to conduct business. The electronic exchange of information between business partners can have a significant impact on the performance and profitability of businesses. However, despite the numerous potential advantages EDI can provide, only fraction of the market is using EDI. The current study attempts to find the explanation for this phenomenon. Thus, which factors are affecting the decision to adopt EDI? This study is based on Iacovou et al. (1995) research where seven cases of EDI adoption were studied. Iacovou et al. (1995) identified three major variables affecting the adoption of EDI: perceived benefits, external pressure and organisational readiness. The current study modified Iacovou et al.'s (1995) model by introducing the awareness variable. This study also changed the organisational readiness variable to financial strength and technology used. The current study was conducted using questionnaires that were sent to 1000 businesses in different industries in Quebec. A sample size of 149 companies was used to test the revised model. The results suggest that there is some relationship between EDI adoption and awareness, financial strength and technology used. Furthermore, this study suggests also a strong relationship between EDI adoption and pressure from important partne