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A deconvolution technique to correct deep images of galaxies from instrumental scattered light
摘要： Deep imaging of the di?use light that is emitted by stellar ?ne structures and outer halos around galaxies is often now used to probe their past mass assembly. Because the extended halos survive longer than the relatively fragile tidal features, they trace more ancient mergers. We use images that reach surface brightness limits as low as 28.5?29 mag arcsec?2 (g-band) to obtain light and color pro?les up to 5–10 e?ective radii of a sample of nearby early-type galaxies. These were acquired with MegaCam as part of the CFHT MATLAS large programme. These pro?les may be compared to those produced using simulations of galaxy formation and evolution, once corrected for instrumental e?ects. Indeed they can be heavily contaminated by the scattered light caused by internal re?ections within the instrument. In particular, the nucleus of galaxies generates arti?cial ?ux in the outer halo, which has to be precisely subtracted. We present a deconvolution technique to remove the arti?cial halos that makes use of very large kernels. The technique, which is based on PyOperators, is more time e?cient than the model-convolution methods that are also used for that purpose. This is especially the case for galaxies with complex structures that are hard to model. Having a good knowledge of the point spread function (PSF), including its outer wings, is critical for the method. A database of MegaCam PSF models corresponding to di?erent seeing conditions and bands was generated directly from the deep images. We show that the di?erence in the PSFs in di?erent bands causes arti?cial changes in the color pro?les, in particular a reddening of the outskirts of galaxies having a bright nucleus. The method is validated with a set of simulated images and applied to three representative test cases: NGC 3599, NGC 3489, and NGC 4274, which exhibits a prominent ghost halo for two of them. This method successfully removes this.
关键词： galaxies: photometry,techniques: photometric,galaxies: elliptical and lenticular, cD,galaxies: evolution,galaxies: stellar content,techniques: image processing
更新于2025-09-04 15:30:14
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Towards a census of high-redshift dusty galaxies with Herschel
摘要： Context. Over the last decade a large number of dusty star-forming galaxies has been discovered up to redshift z = 2 ? 3 and recent studies have attempted to push the highly confused Herschel SPIRE surveys beyond that distance. To search for z ≥ 4 galaxies they often consider the sources with ?uxes rising from 250 μm to 500 μm (so-called “500 μm-risers”). Herschel surveys offer a unique opportunity to ef?ciently select a large number of these rare objects, and thus gain insight into the prodigious star-forming activity that takes place in the very distant Universe. Aims. We aim to implement a novel method to obtain a statistical sample of 500 μm-risers and fully evaluate our selection inspecting different models of galaxy evolution. Methods. We consider one of the largest and deepest Herschel surveys, the Herschel Virgo Cluster Survey. We develop a novel selection algorithm which links the source extraction and spectral energy distribution ?tting. To fully quantify selection biases we make end-to-end simulations including clustering and lensing. Results. We select 133 500 μm-risers over 55 deg2, imposing the criteria: S 500 > S 350 > S 250, S 250 > 13.2 mJy and S 500 > 30 mJy. Differential number counts are in fairly good agreement with models, displaying a better match than other existing samples. The estimated fraction of strongly lensed sources is 24+6 Conclusions. We present the faintest sample of 500 μm-risers down to S 250 = 13.2 mJy. We show that noise and strong lensing have an important impact on measured counts and redshift distribution of selected sources. We estimate the ?ux-corrected star formation rate density at 4 < z < 5 with the 500 μm-risers and ?nd it to be close to the total value measured in far-infrared. This indicates that colour selection is not a limiting effect to search for the most massive, dusty z > 4 sources.
关键词： galaxies: photometry,galaxies: evolution,galaxies: statistics,galaxies: high-redshift,infrared: galaxies,galaxies: star formation
更新于2025-09-04 15:30:14