next up previous
Next: Comparison with the ULDA Up: Normal Galaxies INES Guide Previous: Selection of galaxies for

Presentation of the guide data

Given the large number of normal galaxies in the sample, we decided to present for each object the most relevant information on a single page of this Guide. An individual page contains, therefore, the following information:

  1. A header with the name of the galaxy as given in INES, its J2000 celestial coordinates ($\alpha$ and $\delta$), and its Galactic coordinates (l and b).

  2. A list of alternative names for the galaxy culled from LEDA.

  3. General information about the object, collected from the LEDA data base: morphological T-type, morphological type, redshift, and logarithmic size parameters d$_{25}$ and r$_{25}$.

  4. Information about the two IUE spectra (SW and LW) presented on the page: image number, date when each spectrum was acquired, exposure of each spectrum in seconds, and position angle of the aperture's major axis.

  5. Image numbers of at most ten additional IUE spectra of the object. These are selected by decreasing order of exposure time.

  6. A plot of the combined UV spectrum of the galaxy, from 1150Å to 3350Å. Note that the LW spectrum was, in some instances, scaled to match the long wavelength end of the SW spectrum, as explained below.

  7. Two images of the galaxy, extracted from the Digitized Sky Survey (DSS), with the overlaid outline of the IUE aperture relevant for the specific spectrum displayed here. The left image is always that of the SW aperture position and the right image is that of the LW.

The selection of spectra to be combined into the representative UV spectrum of the galaxy was done according to the following rules:

  1. Two spectra, one SW and another LW, with the longest exposures, were selected from INES.

  2. The positions of the two IUE entrance apertures, for the SW and LW spectra, were checked in the images with the aperture overlays. If the locations were very similar, the two spectra were retained and combined into the final representative spectrum. If not, INES was searched for a suitable pair of deep spectra conforming to the similar-location criterion.

  3. In a number of instances, only SW or only LW spectra were available. In these cases, the left or the right image of the galaxy, with the suitable aperture overlaid, is missing.

The two selected SW and LW spectra were then combined into a single representative UV spectrum of the object. Sometimes, the mean spectral energy density (SED) levels of the SW and LW spectra of the same object were very different from each other. A simple combination, e.g. by concatenation of the SW and LW spectra would then result in a step-like SED. This is clearly visible in some of the spectra displayed in the ULDA Access Guide No. 3 (Longo & Capaccioli 1992), e.g. A1223+4846.

To prevent such an occurence, we decided to bring the LW spectrum to the level of the SW so as to ensure a smooth linkage between the spectra. We averaged the flux density in a 50Å segment at the long wavelength end of the SW spectrum, and in a 50Å segment at the short wavelength end of the LW spectrum. The normalization constant was determined from these flux density averages. The difference between the two averages was added to the LW spectrum to bring it into smooth continuation of the SW spectrum, and is indicated on the spectral plot. This matching was not performed in cases when it would have driven any part of the LW spectrum to negative flux levels. Such cases can be noticed by the gap between the SW and LW spectra displayed on the relevant pages (e.g. IC1613). A marginal case, where the shift was from a spectrum at zero LW flux level, was for AOO ANON1244-53. In few a cases (e.g. NGC 1147) the LW spectrum seems contaminated by additional light between 1900 and 2200Å. In such cases, no flux-matching procedure was applied.

For some extended galaxies, much larger than the IUE entrance apertures, spectra were obtained at a number of physically different regions in the galaxy. When two spectra, one SW and the other LW, could be identified in the same location for such a region, one page was dedicated to the separate presentation of this information. Multiple pages are shown for the following galaxies: NGC 3034, NGC 3690, NGC 4449, NGC 4861, NGC 5236.

For some spectra, the aperture coordinates reported in the image header are incorrect and point to a region outside the target, while the spectra show a significant signal. For these spectra, a new aperture position was computed (Solano, E., private communication) based on the guide star position used by the Guest Observer. Table 2 lists the image number and the revised aperture position.

We emphasize here that the morphological type of an object included in this compilation is the one listed in LEDA. Perusing the contents of this INES Guide, we found that some objects with which we are familiar are obviously mis-classified in LEDA. Such an example is Mrk 49, an E galaxy in LEDA but which is really a compact, starbursting dwarf. It is possible that there are more such mis-classifications, but we have not attempted to sort them out. Along with the morphological type listed in Table 1, the numerical T-type associated with a galaxy should also change. We believe this change not to be significant when considering the statistical distribution of the normal galaxy population displayed here (cf., Figure 3).

next up previous
Next: Comparison with the ULDA Up: Normal Galaxies INES Guide Previous: Selection of galaxies for