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A Statistical Model of Galaxies

Mattia Vaccari

Department of Astronomy, University of Padova
Vicolo dell'Osservatorio 2, I-35122, Padova, Italy

CISAS ``G. Colombo'', University of Padova
Via Venezia 15, I-35131, Padova, Italy

Erik Høg

Astronomical Observatory, University of Copenhagen
Juliane Maries Vej 30, Dk-2100, Copenhagen Ø, Denmark

Date: GAIA-CUO-104 30 January 2002


Average optical properties of galaxies as available in recent literature are summarized. The differential and integral galaxy number density, the effective radius and the surface brightnes profile of ``typical'' galaxies are given as function of the galaxy morphological type and total $ I$ magnitude, down to a limiting magnitude of $ I=24$.

These typical values are believed to be useful when only galaxy statistical properties, i.e. properties averaged over large galaxy samples, are of interest, e.g. when planning future large-scale surveys and data reduction undertakings.

A simple model giving the number and average photometric properties of galaxies as function of a limited set of parameters is useful for different purposes, e.g. in the planning of future galaxy observations, where it is required to estimate the number of galaxies that could be detected, the angular radius to which the surface brightness profile could be followed and the connected scientific yield.

The observations that are needed in order to build such a model are the number density, angular size and surface brightness distribution of the galaxies observed on the sky. Recent literature, and more specifically the Medium Deep Survey (MDS, [Ratnatunga et al.(1999)]) and Hubble Deep Field North (HDF-N, [Williams et al.(1996)]) databases obtained with the HST WFPC2 camera, offer a substantial amount of data in optical $ UBVRI$-like bands, extending to large sky regions and faint magnitudes. The model's results, however, are here expressed in the $ I$ band, since mostly $ I$-band data were used. A magnitude limit of $ I=24$ mag was adopted as the best trade-off between the model's depth and reliability.

It turns out that, to a first approximation, the galaxy statistical properties that are here of interest can be conveniently expressed as function of two parameters only, namely the galaxy total magnitude and morphological type. It must be emphasized how, under our assumptions, these two parameters completely characterize the photometric properties of a galaxy. The predictions of our model are thus much different in nature from the results of conventional galaxy surface photometry analysis. While in the latter a set of parameters is fitted to galaxy images in order to obtain information about galactic structural properties, in our model the results of this analysis are combined to derive analytical expressions predicting the photometric properties of typical galaxies. While this model obviously cannot do justice to the strong individuality displayed by many galaxies, it is believed to yield sufficiently reliable results when only statistical properties, i.e. properties averaged over large samples, are of interest.

The results of this study have in fact been developed and used for the planning of future galaxy observations, namely to investigate the possibilities for multi-colour photometry of galaxies with the GAIA satellite ([Perryman et al.(2001)]). In this framework, they have proved a valuable tool e.g. in discussing the difficulties related to galaxy detection ([Høg et al.(1999)]) and observation ([ESA(2000)]). The model herein presented, as well as the tools developed for the simulation of galaxy observations and the performance expected from GAIA in this respect, are more thoroughly described in [Vaccari(2000)] and [Vaccari(2001)].

next up previous
Next: 1 Morphological Classification
Mattia Vaccari 2002-01-31