4 Data analysis

In Fig. 1 we plot the Gunn r and Johnson B total apparent magnitude histograms of the UCM Survey galaxies. They were arranged in 0.5 magnitude bins. Both distributions cover a range of about seven magnitudes and present a rather symmetric shape around 16.5$^{\rm m}$ in the B bandpass and 16.0$^{\rm m}$ in the r filter. The average of the Johnson B distribution is 16.1 $\pm$ 1.1. In the Gunn r filter the average is 15.5 $\pm$ 1.0. These values are plotted at the top of the diagram. Both histograms show a sharp bright magnitude cutoff (around 14.5-15.0 in the B-band and 13.75-14.25 in the r band) due to detection problems (the objective-prism spectra of very bright objects are saturated, not allowing the detection of the emission lines); there is also a faint magnitude limit around 19 magnitudes in the blue filter and 18 in the red one.

Figure 1: Johnson B and Gunn r histograms of the UCM Survey. The top error bar shows the average and the standard deviation of the data (see text). The average colour results 0.71 magnitudes. The Gunn rdata have been extracted from Vitores et al. ([1996a])

We plot the absolute total magnitudes versus the effective radii of the UCM galaxies in Fig. 2. Galaxies were labelled depending of their spectroscopic type (see Gallego et al. [1996] for details):

SBN --Starburst Nuclei-- Originally defined by Balzano ([1983]), they show high extinction values, with very low [NII]/H$\alpha$ ratios and faint [OIII]$\lambda$5007 emission. Their H$\alpha$ luminosities are always higher than 10⁸ $L_{\hbox{$\odot$ }}$.

DANS --Dwarf Amorphous Nuclear Starburst-- Introduced by Salzer et al. ([1989]), they show very similar spectroscopic properties to SBN objects, but with H$\alpha$ luminosities lower than $5\ 10^{7}$ $L_{\hbox{$\odot$ }}$.

HIIH --HII Hotspot-- The HII Hotspot class shows similar H$\alpha$ luminosities to those measured in SBN galaxies but with large [OIII]$\lambda$5007/H$\beta$ ratios, that is, higher ionization.

DHIIH --Dwarf HII Hotspot-- This is an HIIH subclass with identical spectroscopic properties but H$\alpha$ luminosities lower than 5$\$10⁷ $L_{\hbox{$\odot$ }}$.

BCD --Blue Compact Dwarf-- The lowest luminosity and highest ionization objects have been classified as Blue Compact Dwarf galaxies, showing in all cases H$\alpha$ luminosities lower than 5$\$10⁷ $L_{\hbox{$\odot$ }}$. They also show large [OIII]$\lambda$5007/H$\beta$ and H$\alpha$/[NII]$\lambda$6584 line ratios and intense [OII]$\lambda$3727 emission.

All these spectroscopic classes are usually collapsed in two main categories: starburst disk-like (SB hereafter) and HII-like galaxies (see Guzmán et al. [1997]; Gallego [1998]). The SB-like class includes SBN and DANS spectroscopic types, whereas the HII-like includes HIIH, DHIIH and BCD type galaxies.

The UCM Survey does not contain objects brighter than an absolute magnitude of -22.9 or fainter than -16.3. Despite the considerable scatter, we observe a correlation between M_B, r_1/2 and the spectroscopic type in Fig. 2. BCD galaxies appear as small and faint objects in the bottom left corner of the plot. SBN galaxies are more concentrated in the largest effective radius and luminosity zone of the diagram. This should be the place for normal grand-design spirals. The existence of a bright starburst in the nucleus of SBN objects turns them into objects redder than those with the starburst located out of the nucleus (see below the discussion of Figs. 4 and 6). Only UCM 1612+1308 shows the typical small size of nucleated compact dwarfs. Most of the DANS and HIIH galaxies are also located in the small effective radii zone, below 5 kpc.

As reference, we have plotted the constant surface brightness lines corresponding to -14, -16 and -18 mag$\$kpc^-2 in Fig. 2.

Figure 2: Absolute magnitude corrected from Galactic extinction versus effective radius, measured in kpc as the circular aperture that contains half the light of the galaxy. As reference, constant surface brightness lines corresponding to -14, -16 and -18 mag$\$kpc-2 are plotted

In Fig. 3 we plot the histograms of $(B-r)_{\rm ef}$ colours of UCM galaxies corrected from Galactic extinction according to their morphological (Vitores et al. [1996a]) and spectroscopic classification (Gallego et al. [1996]). The averaged colours of each Hubble type are listed in Table 4, jointly with the mean colours calculated by Fukugita et al. ([1995]). The vertical ticks in these diagrams show Fukugita et al. ([1995]) colours and averaged colours for each spectroscopic type.

Overall, early-type spirals show a bluer colour than those of Fukugita et al. ([1995]), probably related to the presence of the star-forming process. On the other hand, irregulars and BCDs do show redder B-r colours than Fukugita's sample; this could be a selection effect, given that very blue objects would not show up at the original objective-prism plates as they were taken in the H$\alpha$ region.

Although the spectroscopic histograms show a great dispersion we observe that SBN galaxies are redder than other types. The bluest objects appear to be BCDs and DHIIHs. These two facts could be explained in two different ways: SBNs could be affected by larger dust reddening or the starburst could be more relevant in BCD and DHIIH galaxies, making them bluer. In fact, Gallego et al. ([1997]) showed that the mean B-V colour excess for SBN galaxies is 0.2$^{\rm m}$ higher than for HII-like galaxies.

Both kind of data are mixed in Fig. 4. SBN galaxies dominate the spiral zone (from T=1 -Sa- to T=6 -Sc-), adding a great colour dispersion to our sample. There are also 7 very blue objects, all of them late-type spirals (Sc+) or irregulars. some of these objects are low metallicity galaxies, for example UCM 2304+1640 ( $(B-r)_{\rm ef}$=-0.18, metallicity ${Z_{\hbox{$\odot$ }}}/{7}$) or UCM 0049+0017 ( $(B-r)_{\rm ef}$=-0.33, metallicity ${Z_{\hbox{$\odot$ }}}/{20}$).

 

Table 4: Mean colours according to Hubble type

Hubble type	$\overline{(B-r)}_{\rm UCM}$	$\overline{(B-r)}_{\rm F95}$	$N_{\rm gal}$
(1)	(2)	(3)	(4)
Sa	0.74	0.97 (Sab)	40
Sb	0.75	0.73 (Sbc)	44
Sc+	0.72	0.65 (Scd)	45
Irr	0.42	0.24 (Irr)	8
BCD	0.34	0.24 (Irr)	4

Table 4: (1) Hubble type. (2) Mean total B-r colours of the UCM sample. (3) Mean total B-r colours tabulated in Fukugita et al. ([1995]). (4) Number of galaxies used in the calculated mean colours.

Figure 3: Histograms of the $(B-r)_{\rm ef}$ colours of the UCM galaxies corrected from Galactic extinction according to their morphological and spectroscopic classification as established in Vitores et al. ([1996a]) and Gallego et al. ([1996]), respectively. The vertical marks in the left diagram are the typical colours of each morphological type as tabulated in Fukugita et al. ([1995]); mean colours are listed in Table 4. In the right diagram we have marked the averaged colour of each spectroscopic type. The values are: 0.83 for SBN type, 0.73 for DANS, 0.62 for HIIH, 0.34 for DHIIH & BCD

Figure 4: Relation between spectroscopic and morphological types and $(B-r)_{\rm ef}$ colour. We have selected the main spectroscopic types of our sample: SBN, DANS, HIIH and BCD & DHIIH, as classified in Gallego et al. ([1996]) and morphological types from S0 to Irr; the galaxies classified as Sc+ by Vitores et al. ([1996a]) are included in T=6 -corresponding to a Sc galaxy

The B-r histogram for the whole sample is plotted in Fig. 5. The averaged effective colour of the UCM sample is $0.73\pm0.41$. The distribution is rather flat, being dominated by galaxies with a colour corresponding to a typical spiral.

Figure 5: B-r histogram of the UCM Survey Lists I and II. The averaged colours of Fukugita et al. ([1995]) have been marked at the top

In Fig. 6 we plot the B absolute magnitude M_B versus the effective colour $(B-r)_{\rm ef}$. Labels correspond to the spectroscopic type of each object. An extinction vector of 0.4 magnitudes in the B band has been drawn. SBN galaxies are located in the most luminous and reddest part of the plot, jointly with Sy2 galaxies. In the other hand, BCDs appear to be the bluest and faintest objects in our sample. UCM objects are compared with a normal sample of galaxies from the literature in Fig. 7; we have selected common galaxies in the Nearby Universe from the NGC, IC and Mrk catalogs extracted from the NED database. The BCD data have been extracted from Doublier et al. ([1997]). Both sets of reference data are drawn lightened.

In the top panel we have compared our colours with those of spirals. As expected, most of the UCM sample is located in the region where normal spiral galaxies are found in this colour-magnitude diagram; some of our galaxies have similar colours to those of early-type galaxies though this could be due to internal reddening. The BCD galaxies in our sample seem to be about 0.7$^{\rm m}$ brighter and 0.2$^{\rm m}$ bluer than the Doublier et al. ([1997]) sample.

Figure 6: Absolute B magnitude MB corrected from Galactic extinction versus $(B-r)_{\rm ef}$ (effective colour). Bottom marks are B-r colours from Fukugita et al. ([1995]). An extinction vector corresponding to 0.4 magnitudes in the Johnson B band is given and also the averaged error bars of both sets of data

Figure 7: Colour-magnitude diagram of the UCM survey galaxies compared with other galaxies in the Nearby Universe (drawn lightened as diamonds -spirals- and five-points stars -BCD's-). In the top panel we have used isophote 25 colours while in the second panel we have represented total colours (extracted from the $2\cdot r_{\rm Kron}$aperture). All absolute magnitudes are integrated total magnitudes. Typical error is that shown in Fig. 6