Direct estimate of cirrus noise in Herschel Hi-GAL images

摘要

In Herschel images of the Galactic plane and manystar forming regions, amajor factor limiting our ability toextract faint compact sources is cirrus confusion noise, operationallydefined as the ``statistical error to be expected in photometricmeasurements dueto confusion in a background of fluctuating surface brightness''. Thehistogram of the flux densities of extracted sources shows adistinctive faint-end cutoff below which the catalog suffers fromincompleteness and the flux densities become unreliable. This empiricalcutoff should be closely related to the estimated cirrus noise and weshow that this is the case. We compute the cirrus noise directly, bothon Herschel images from which the bright sourceshave been removed and on simulated images of cirrus with statisticallysimilar fluctuations. We connect these direct estimates with those frompower spectrum analysis, which has been used extensively to predict thecirrus noise and provides insight into how it depends on variousstatistical properties and photometric operational parameters. Wereport multi-wavelength power spectra of diffuse Galactic dust emissionfrom Hi-GAL observations at70 to500within Galactic plane fields atand .We find that the exponent of the power spectrum is about-3.At,the amplitude of the power spectrum increases roughly as the square ofthe median brightness of the map and so the expected cirrus noisescales linearly with the median brightness. Fora givenregion, the wavelength dependence of the amplitude can be described bythe square of the spectral energy distribution (SED) of the dustemission. Generally, theconfusion noise will be a worseproblem at longer wavelengths, because of the combination of lowerangular resolution and the rising power spectrum of cirrus toward lowerspatial frequencies, but the photometric signal to noise will alsodepend on the relative SED of the source compared tothecirrus.Key words: ISM: structure - ISM: general -stars: formation - stars: protostars - submillimeter: ISM - infrared:ISM