As published in:
``Building Galaxies: from the Primordial Universe to the Present''
eds.: F. Hammer, T.X. Thuan, V. Cayatte, B, Guiderdoni, & J. Tran Thanh Van, 2000, p. 33-36.

Rolf A. Jansen ^1,2

¹ Kapteyn Astronomical Institute, Groningen, The Netherlands.
² Harvard-Smithsonian Center for Astrophysics, Cambridge, USA.

Abstract

I report on the results of an optical spectrophotometric and photometric study of 196 nearby galaxies, that form a representative sample of the local field population. Integrated (drift scanned) and major axis (pointed) spectra were obtained, as well as UBR surface photometry to aid in the interpretation of the spectroscopic data. Follow up observations include high resolution spectra ([8], internal kinematics) and HK NIR surface photometry ([10], dominant stellar population). The main goal of this program is to provide a comparison sample for high redshift studies and to study the variation in star formation rates (SFR), star formation history (SFH), excitation, metallicity, and internal kinematics over a large range in galaxy luminosity and morphological type. In particular, the work of Kennicutt [9] is extended to lower luminosity systems.

Introduction With the advent of 8 m class telescopes a flood of spectroscopic, photometric, and morphological data has become available on faint distant galaxies. For the first time, information can be obtained on normal galaxies or even -- perhaps -- their progenitors. No longer is study restricted to the very brightest objects: instead, galaxies are routinely sampled well down the galaxy luminosity function. The Nearby Field Galaxy Survey was motivated by the observation, that one of the limiting factors in the interpretation of these distant spectroscopic data has become the difficulty of obtaining good nearby comparison samples. Distant galaxies subtend small angles on the sky and their spectra are -- unavoidably -- integrated spectra. Most of the nearby galaxy spectra, on the other hand, are nuclear spectra only. A direct comparison of distant and nearby galaxy spectra, therefore, is difficult. In a pioneering effort Kennicutt [9] obtained integrated spectra for galaxies accross the Hubble sequence, providing a valuable local reference. But the range in luminosity sampled per type is small. Also, no uniform photometry is available to aid in the interpretation of the spectra. With the Nearby Field Galaxy Survey we aim to remedy this situation. The purpose of our study is to obtain integrated and nuclear spectrophotometry, as well as UBR surface photometry, for a sample of galaxies in the nearby field that includes all morphological types and which spans as large a range in luminosity as possible. With ``field'' I imply a selection that includes galaxies in clusters, groups and in low density environments, as opposed to a selection favoring any single one of these. The data will be used to study the emission and absorption line strengths, star formation rate and star formation history, morphologies, structural parameters, colors and magnitudes. We thus aim to extend the work of Kennicutt to lower luminosity galaxies accross the Hubble sequence and to study the variation in star formation rates and history, and metallicity over a large range in absolute magnitude and type. The data may also serve as templates for theoretical and modelling work, once the relative occurance rates per type and magnitude have been calibrated using a larger complete sample (e.g. [1]).

The Sample Galaxies were selected from the first CfA redshift catalog ([4], CfA I), which contains 2400 galaxies to a limiting blue photographic magnitude of 14.5 . The CfA I has several virtues: (1) it is nearly complete within its selection limits, (2) it contains galaxies with a large range in absolute magnitude, and (3) all galaxies have been morphologocally classified. As we wanted to avoid a bias towards a high density cosmic environment, we excluded galaxies in the Virgo Cluster. Given the length of the slit of the FAST spectrograph [2], we also wanted to minimize the number of galaxies larger than 3'. We did not want to impose a diameter limit, though, as that would increase the bias against low surface brightness galaxies inherent to any magnitude limited catalog. We therefore opted to select more luminous galaxies from farther away, arguing that, on average, more luminous galaxies are physically larger. The final selection from the remaining 1007 galaxies was such as to preserve the full range in luminosity present in the CfA I, and to preserve the changing morphological mix as a function of luminosity, and to give a sample with a luminosity distribution that approximates the local galaxy luminosity function over most of the range in absolute magnitude. The selected sample contains 196 galaxies, spans over 8 magnitudes in luminosity -- reaching 5 magnitudes below the characteristic local galaxy luminosity -- and contains all morphological types. The distribution in color is broad, even though color was not a selection criterion. Several types of nuclear activity are present in this sample. Although the distribution in effective surface brightness is peaked, we do find several low surface brightness galaxies in this sample. With very few caveats, this sample can be considered a fair representation of the local galaxy population.

Observations Observations were made over a period of 4 years at the F.L. Whipple Observatories 1.2 m and 1.5 m telescopes. The former telescope is dedicated to CCD and NIR array photometry; the latter, with the FAST spectrograph to spectroscopy. Per galaxy we obtained a spectrophotometrically calibrated integrated spectrum, and a nuclear one for reference, as well as moderately deep surface photometry in U, B and R. The integrated spectra were obtained by drift scanning the spectrograph slit back and forth accross the face of a galaxy while exposing. The spectrograph slit was rotated to approximate the position angle of the galaxies' major axis.

Photometric properties We [5] reproduce the well-known trend of absolute magnitude with type for the morphological types later than Sa. There are not many very bright very late type galaxies. We find a significant number of low luminosity early type systems. In a plot of the effective B-R color as a function of type, the bulk of the galaxies follow a strong trend: galaxies become progressively bluer towards later types. Lower luminosity systems tend to be bluer than luminous galaxies. The ellipticals and S0's follow the kown color-magnitude relation. To a lesser degree, many of the late type spirals and irregulars also appear to follow a trend, offset from that followed by the ellipticals. The early and intermediate type spirals, however, show a much larger scatter. In a color-color diagram the galaxies lie on a tight relation, despite the large range in galaxy properties. The slope of this relation seems to be steeper for the early type galaxies than for the galaxies with types later than Sa. The interstellar extinction vector points almost parallel to the part of the relation followed by the later type systems. In general, the more luminous galaxies are physically bigger as measured by their effective (half-light) radii. There is, however, a spread with lower surface brightness systems having larger sizes at a given luminosity than high surface brightness systems. This, in effect, also produces a small type separation, as early type galaxies tend to have a higher surface brightness at their effective radius. We use results like these to check the validity of the morphological classifications for the low luminosity systems.

Spectrophotometric results Common knowledge dictates that the optical spectrum of an elliptical galaxy is dominated by old stars and shows no line emission; the spectrum of an intermediate type spiral -- say Sb -- has a roughly flat spectrum and a modest amount of nebular Balmer, Oxygen, Nitrogen and Sulphur emission lines; and that of late type galaxies should have a blue continuum and lots of nebular emission. Going from high luminosity galaxies to low luminosity galaxies in the NFGS sample, however, a trend becomes apparent [6]: galaxies become bluer going towards lower luminosities and they show stronger emisison lines, relative to their continuum (and more frequently so). This trend is seen accross the Hubble sequence, but is most pronounced for the early and intermediate type spirals. This observed shift in spectroscopic properties going towards lower luminosities cannot be explained by uncertainties in the morphological typing. The total range in spectroscopic properties that can be found at a given type and given luminosity can be large however. These spectroscopic results should be taken into account when modelling galaxy populations and their evolution over time, especially if a large portion of the galaxy luminosity function is to be represented.

[O II] 3727Å as a tracer of the current SFR When H is redshifted out of the optical wavelength window, [O II] 3727Å is the strongest line remaining for analysis to redshifts of ~ 1. Since a correlation seems to exist [9] (but note [11]!) between equivalent widths (EW) of [O II] and H, [O II] has been widely used to estimate current high mass star formation rates. We [6, 7] find that, while galaxies with small emission line EWs can be of any luminosity, there is a strong preference for the systems with strong [O II] lines to be of lower luminosity. Moreover, high luminosity galaxies tend to follow the relation found by [9], while most of the low luminosity galaxies tend to show significantly stronger [O II] emission for the same H EW (Figure 1).

Figure 1: [O II] as a function of H EW.

The driving `force' for this effect appears to be metallicity. Lower luminosity systems tend to have lower metallicities, resulting in a less effective cooling of the interstellar medium and a higher average temperature. Despite the lower abundance, the [O II] line will be boosted in strength with respect to H. Metallicity contributes in a second way as well: lower metallicity systems are less likely to be very dusty than high metallicity systems. These findings add to the work by other authors (e.g. [3]; CFRS) and show that the excess strength of [O II] with respect to H is not restricted to high redshifts.

Acknowledgements The NFGS team includes R.A. Jansen, M. Franx, D.G. Fabricant, N. Caldwell, S.J. Kannappan and M. Pahre. Should any part of this paper fail to reflect their contributions correctly, I apologize. I gratefully acknowledge grants from the European Union (TMR), the Kapteyn Institute and the Leiden Kerkhoven-Bosscha Fund that made my participation in this conference possible.

References [1] Carter, B., & Fabricant, D. G., (in prep.) [2] Fabricant, D., Cheimets, P., Caldwell, N., & Geary, J., 1998, Publ. Astr. Soc. Pac. 110, 79 [3] Hammer, F., Flores, H., Lilly, S. J., Crampton, D., Le F\`evre, O., Rola, C., Mallen-Ornelas, G., Schade, D., & Tresse, L., 1997, Astrophys. J. 481, 49 [4] Huchra, J. P., Davis, M., Latham, D., & Tonry, J., 1983, Astrophys. J. Suppl. Ser. 52, 89 (CfA I) [5] Jansen, R. A., Franx, M., Fabricant, D. G., & Caldwell, N., 2000a, Astrophys. J. Suppl. Ser. 126, 271 [6] Jansen, R. A., Fabricant, D. G., Franx, M., & Caldwell, N., 2000b, Astrophys. J. Suppl. Ser. 126, 331 [7] Jansen, R. A., 2000, Ph.D. thesis, University of Groningen [8] Kannappan, S. J., Fabricant, D. G., & Franx, M., (in prep.) [9] Kennicutt, R. C. Jr., 1992, Astrophys. J. 388, 310 [10] Pahre, M., Jansen, R. A., Kannappan, S. J., Fabricant, D. G., & Willner, S., (in prep.) [11] Schaerer, D., 1999 (these proceedings)