408 S.A. Cellone: Dwarf <strong>galaxies</strong> <strong>in</strong> <strong>the</strong> <strong>NGC</strong> <strong>5044</strong> <strong>Group</strong>Fig. 1. (Cont<strong>in</strong>ued)
S.A. Cellone: Dwarf <strong>galaxies</strong> <strong>in</strong> <strong>the</strong> <strong>NGC</strong> <strong>5044</strong> <strong>Group</strong> 409orS e = S 0 + 5 log(reα) [ Γ(2− 2.5 logN )N]. (5)It can be shown that reα depends only on N, and <strong>the</strong>n S e doesnot depend on α. These calculated parameters are also shown <strong>in</strong>Table 4. The magnitudes and colours <strong>in</strong> Table 3, <strong>in</strong>stead, wereobta<strong>in</strong>ed from <strong>the</strong> observed <strong>pr<strong>of</strong>iles</strong> with no model assumption,to allow for those <strong>galaxies</strong> whose <strong>pr<strong>of</strong>iles</strong> couldn’t be fitted with<strong>the</strong> model.4. See<strong>in</strong>g effectsThe effects <strong>of</strong> see<strong>in</strong>g on <strong>the</strong> photometric parameters<strong>of</strong> elliptical <strong>galaxies</strong> have been studied us<strong>in</strong>g differenttechniques (e. g.: Franx et al. 1989; Peletier et al. 1990;Saglia et al. 1993b). These studies have shown that <strong>the</strong> effects <strong>of</strong>see<strong>in</strong>g on surface <strong>brightness</strong> and ellipticity <strong>pr<strong>of</strong>iles</strong> may extendup to several PSF radii.The observations presented <strong>in</strong> this paper were obta<strong>in</strong>ed underra<strong>the</strong>r mediocre see<strong>in</strong>g conditions and, <strong>in</strong> addition, <strong>the</strong> telescopehad small track<strong>in</strong>g errors. This resulted <strong>in</strong> a non-circularPSF, with Gaussian fits to its m<strong>in</strong>or and major axis giv<strong>in</strong>gFWHM Y ≃ 2.0 ′′ and FWHM X ≃ 2.8 ′′ , respectively, withsmall variations between different frames. However, <strong>the</strong> actualshape <strong>of</strong> <strong>the</strong> PSF was not Gaussian, hav<strong>in</strong>g notably larger w<strong>in</strong>gs.The adopted approach, <strong>the</strong>n, was to evaluate <strong>the</strong> effects <strong>of</strong> see<strong>in</strong>gon this particular set <strong>of</strong> observations us<strong>in</strong>g an empirical PSFobta<strong>in</strong>ed from <strong>the</strong> same data us<strong>in</strong>g IRAF–DAOPHOT rout<strong>in</strong>es.A set <strong>of</strong> artificial <strong>galaxies</strong> follow<strong>in</strong>g a perfect Sérsic law andspann<strong>in</strong>g a broad range <strong>in</strong> <strong>the</strong> relevant parameters was generated,and <strong>the</strong>n convolved with <strong>the</strong> empirical PSF. The surface<strong>brightness</strong> <strong>pr<strong>of</strong>iles</strong> <strong>of</strong> <strong>the</strong>se convolved artificial <strong>galaxies</strong> were<strong>the</strong>n obta<strong>in</strong>ed and fitted with a Sérsic law, <strong>in</strong> <strong>the</strong> same way aswas previously done with <strong>the</strong> observed <strong>galaxies</strong>. The effects<strong>of</strong> see<strong>in</strong>g were evaluated by compar<strong>in</strong>g <strong>the</strong> “observed” and <strong>the</strong>“orig<strong>in</strong>al” parameters for <strong>the</strong> convolved artificial <strong>galaxies</strong>.A total <strong>of</strong> 55 artificial <strong>galaxies</strong> were generated with fixedcentral surface <strong>brightness</strong>, and spann<strong>in</strong>g <strong>the</strong> follow<strong>in</strong>g ranges<strong>in</strong> scale length and shape parameter: 0.001 ′′ ≤ α ≤ 10 ′′ , and0.25 ≤ N ≤ 1.50, respectively. Most <strong>of</strong> <strong>the</strong>m were round, butseveral non-circular (ɛ =0.5) and a few “nucleated” artificial<strong>galaxies</strong> (i. e., with a po<strong>in</strong>t source 10% <strong>the</strong> lum<strong>in</strong>osity <strong>of</strong> <strong>the</strong>galaxy added at its centre, before <strong>the</strong> convolution) were alsogenerated, <strong>in</strong> order to check for any differences compared tocircular, non-nucleated <strong>galaxies</strong>. No noise or constant sky levelwere added. Although a complete study <strong>of</strong> see<strong>in</strong>g and <strong>in</strong>strumentaleffects would require <strong>the</strong> <strong>in</strong>clusion <strong>of</strong> <strong>the</strong>se factors, alongwith <strong>the</strong> generation <strong>of</strong> a larger sample <strong>of</strong> artificial <strong>galaxies</strong> convolvedwith different PSFs, <strong>the</strong> present approach is sufficient toevaluate <strong>the</strong> global effects <strong>of</strong> see<strong>in</strong>g on <strong>the</strong> measured photometricparameters <strong>of</strong> this particular sample <strong>of</strong> <strong>galaxies</strong>.Fig. 2 shows <strong>the</strong> differences between each measured parameterand <strong>the</strong> orig<strong>in</strong>al one [∆S 0 , ∆ log(α), and ∆ log(N), respectively]aga<strong>in</strong>st <strong>the</strong> measured effective radius r e . It is evident thatfor sufficiently large <strong>galaxies</strong> <strong>the</strong> orig<strong>in</strong>al parameters are recovabcFig. 2a–c. Measured (after convolution with PSF) m<strong>in</strong>us orig<strong>in</strong>al parametersaga<strong>in</strong>st measured effective radius for 55 artificial <strong>galaxies</strong>:a central surface <strong>brightness</strong>, b logarithm <strong>of</strong> scale length, and c logarithm<strong>of</strong> shape parameter. Squares: round, non-nucleated <strong>galaxies</strong>;triangles: ɛ =0.5, non-nucleated <strong>galaxies</strong>; crosses: round, nucleated<strong>galaxies</strong>. Dashed l<strong>in</strong>es are least-squares fits for log(r e) ≤ 0.5 ′′ .ered with small errors, but for <strong>galaxies</strong> with r e 5 ′′ <strong>the</strong> parametersare systematically underestimated because <strong>of</strong> see<strong>in</strong>g andsampl<strong>in</strong>g effects. (Note that, at least <strong>in</strong> pr<strong>in</strong>ciple, it seems possibleto recover <strong>the</strong> orig<strong>in</strong>al parameters for <strong>the</strong> smaller <strong>galaxies</strong>;however, <strong>the</strong> scatter is large, specially for N, and try<strong>in</strong>g to apply
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