Poster paper presented at the 52nd Dutch Astronomers Conference (NAC), May 14–16 1997, Dalfsen


R.A. Jansen, M. Franx, D.G. Fabricant, N. Caldwell

Objective Measurement of Morphology

As with the advent of the Hubble Space Telescope and 8 m class ground-based telescopes galaxies are sampled at fainter magnitudes and higher redshifts the need arises for a means of objective measurement of galaxy morphology for the purpose of galaxy evolution studies.
Following Abraham et al.(1994, 1996a,b) we use a quantitative classification system based on measurements of the rotational asymmetry and central concentration of galaxian light, to measure the morphologies of 158 galaxies in the Nearby Field Galaxy Survey (NFGS) in the U, B and Rc filters (fig.1a-c).
The NFGS galaxy sample is well suited for the calibration of this system in the local universe. The range in luminosity and type is very large, while the range in redshift is small (z=0.01+0.05-0.01; hence the wavelength of measurement closely approximates the rest-frame wavelength of the emitted light).
The use of multi-filter data allows us to study directly the dependence of morphology on rest-frame wavelength (figs.2,3), luminosity and galaxy colour (fig.1).

The Importance of Rest-Frame Wavelength

We find a strong dependence of morphology on wavelength, Hubble type and galaxy colour: galaxies are more asymmetric and somewhat less concentrated at shorter wavelengths and/or later types, and get bluer as they get less concentrated and more asymmetric.
The values of the asymmetry and central concentration (as measured thru a particular filter) critically depend on galaxy type. The values of central concentration — and to a lesser extent of asymmetry — are also sensitive to the cutoff isophote within which the measurement takes place.
The amplitude of the shift in the peak of the distribution of the asymmetries going from Rc to U sits in an interesting range, as it is of the same order of magnitude as that observed between measurements in the Medium Deep Survey and Hubble Deep Field (Abraham et al. 1996a,b) (see fig.4).

Concentration and Asymmetry

We fit for each galaxy in B an elliptical aperture E(1) with major axis position angle φ, semi-major and semi-minor axes a and b and ellipticity ε = (1 − b/a) to the pixels at a specified isophote μlim.
Concentration index C is then defined by:

                     Σij in E(α) ( Iij )
                 C = ------------------  ,
                     Σij in E(1) ( Iij )

where α is an empirically chosen ``shrinking factor'' (in units of semi-major axis a for fixed φ and ε. We adopted Abraham et al.'s optimal choice value α = 0.3.
Rotational asymmetry index A is defined by rotating a galaxy image about its center and self-subtracting this image from the original one, using aperture E(1) to define the galaxy pixels:

    1   Σij in E(1) | Iij - IRij |
A = - * -----------------------   - k  ,
    2      Σij in E(1) ( Iij )

where Iij is the intensity in pixel (i,j), and IRij is the corresponding intensity after the object's image has been rotated by 180° about its center. The k term is a small correction accounting for signal introduced into A by noise in the sky background.
Aperture E(1) is also used in U and Rc, to define the galaxy pixels.

The Nearby Field Galaxy Survey

The Nearby Field Galaxy Survey (NFGS) is an attempt to remedy the situation noted by (e.g.) Koo et al. (1993): one of the major problems with the interpretation of various measurements on distant galaxies is the absence of a good nearby comparison sample.
We obtained over the last three years U, B and Rc filter photometry and both nuclear and integrated (luminosity weighted) spectrophotometry (3600–7200 Angstrom) for a total of close to 200 nearby field galaxies of all types and spanning a wide range in luminosity.
This unique data set will provide us with an accurate description of the distributions of magnitude, structural parameters, colour and spectral types in the local universe.
The observed emission line strengths will allow us to measure the star formation rates; absorption line diagnostics will be used to study the star formation history of these galaxies, as demonstrated by Caldwell et al. (1993) for galaxies in the Coma Cluster.
Magnitudes and structural parameters (effective radius, surface brightness, concentration and asymmetry) will allow us to model detection rates at increasing redshift. These data will be used as an aid in understanding the spectra of galaxies at higher redshift, and in measuring the changes in star formation rates over time.

The Sample

Because many galaxy properties correlate with magnitude, our sample must span a broad range in luminosity. Imposing a diameter limit would bias against low surface brightness galaxies. As a result, we want to avoid a purely magnitude or diameter limited sample. Instead we impose a luminosity dependent lower limit on the recessional velocity.
We selected galaxies from the first CfA redshift catalogue, which contains galaxies to a limiting photographic magnitude of mZ=14.5 (Huchra et al. 1983), and split up the sample into absolute magnitude bins of width 1 magnitude. For each of these bins we sorted the list by type and randomly took out galaxies in such proportions as to end up with a total of 198 galaxies with a luminosity distribution approximating the observed local galaxy luminosity function.
The final sample contains galaxies with the desired range of absolute magnitudes (−14 < MB < −22), extending five magnitudes fainter than the chararacteristic luminosity L* ~ −19.2 (de Lapparent et al. 1989).

Figure captions:

  • Fig.1: Distribution of the 158 measured NFGS galaxies in the asymmetry versus central concentration plane (for definitions see text) for a) U, b) B, and c) Rc filters
    Point shapes are coded according to Hubble type, point sizes according to blue luminosity and point colour according to (B−R) colour.
  • Fig.2: Average values of asymmetry and central concentration as a function of wavelength for two cutoff isophotes. Point shapes are coded according to Hubble type as in Fig.1 .
  • Fig.3: Fractional number distributions of asymmetry as a function of filter of measurement for two cutoff isophotes. For reference, the U filter distributions have been overplotted in the panels for B and Rc.
  • Fig.4: Histograms of the asymmetry index A for galaxies in the HDF (solid line) and MDS (dotted line), reproduced from Abraham et al. (1996a). The amplitude of the shift between the peaks of the two distributions is 0.24.


Abraham, R.G, Valdes, F., Yee, H.K.C., and van den Bergh, S., 1994, ApJ 432, 75
Abraham, R.G., Tanvir, N.R., Santiago, B.X., Ellis, R.S., Glazebrook, K., and van den Bergh, S., 1996a, MNRAS 279, L47
Abraham, R.G., van den Bergh, S., Glazebrook, K., Ellis, R.S., Santiago, B.X., Surma, P., and Griffiths, R.E., 1996b, ApJS 107, 1
van den Bergh, S., Abraham, R.G., Ellis, R.S., Tanvir, N.R., Santiago, B.X., Glazebrook, K., 1996, AJ 112
Koo, D.C., Gronwall, C. & Bruzual, G. A. 1993, ApJ L 415, L21
Caldwell, N., Rose, J., Sharples, R., Ellis, R. & Bower, R. 1993, AJ 106, 473
Huchra, J., Davis, M., Latham, D. & Tonry, J. 1983, Ap J Sup 52, 89
de Lapparent, V., Geller, M. J. & Huchra, John P. 1989, ApJ 343, 1

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