The UCM Survey: Main Results

• Summary
• Optical imaging
• nIR Observations
• Spectroscopic follow-up
• Density of Star Formation Rate
• Star Formation Rate and [OII] emission

UCM Survey

Summary

The UCM Survey has found (Lists I and II) 264 candidates in 17 fields (471.4 square degrees). Only 191 (72%) have been confirmed as emission-line objects. The overall density (emission-line galaxies per square degree) is 0.4, i.e. around four times that of the Markarian Survey.

It is well established that surveys based on different techniques select samples of ELGs which are not identical. With the results presented in the first and second lists it becomes clear that only an objective-prism survey which selects galaxies by the presence of emission lines, both in the blue and Hα, and blue or UV-excess color will yield a less-biased sample of ELGs. The prototype of this kind of survey is the Second Byurakan Spectral Sky Survey (SBS) (Markarian, Stepanian & Erastova 1987). Some comparisons between the samples detected with different surveys were performed in the discovery papers of UCM List I (Zamorano et al. 1994 ApJS 95,387 and List II (Zamorano et al. 1996 ApJS 105,343).

The fraction of CGCGs found with emission by the UCM Survey is 15% for field galaxies with known redshift z<0.04. We are not claiming that this is the actual fraction of galaxies with emission lines in their spectra. The comparison between UCM and IRAS samples can be found in Gallego 1992, Rego et al. 1993.

It should be noted that our survey is also limited by the total flux and the equivalent width of the Hα+[NII] blend (aprox. 20 A), the red magnitude (continuum flux around Hα) and surface brightness. This is why we are losing an important fraction of bright CGCGs whose spectra appear saturated on the PO plates. Some other CGCGs have EW

Using the Zwicky catalog as reference, from the sample of the UCM Survey a smaller ratio has been found in the Coma cluster (7%) than in the field (15%) for the galaxies with emission. This means that the fraction of emission-line galaxies is around half in this cluster environment than that of the field. The density of acro name="UCM"> galaxies in plate A228, covering the Coma cluster, is aprox. 1.5 s per sq. deg. while this number reduces to aprox. 0.4 for the remaining of the survey (field galaxies). This result is not surprising due to the high density of galaxies of any kind in the cluster. However the relative number of UCM and CGCGs with z<0.04 in both regions is 0.4 (5 CGCGs for each UCM) for the field and 0.2 for the Coma cluster. We have obtained again a factor of two between the fraction of emission-line galaxies in the field and a cluster environment.

Follow-up Optical Imaging

Survey

Follow-up optical imaging (Gunn r) observations of the UCM galaxies included in the first two lists have been performed with the 2.2m and 3.5m telescopes of the German-Spanish observatory of Calar Alto (Almería, Spain). The results have been published in Alvaro G. Vitores PhD dissertation 1995 , Vi ores et al 1996a and Vi ores et al 1996b.

Morphologies, magnitudes, spatial surface brightness profiles, effective radii, concentration indexes etc. have been obtained and the main results are summarized here:

• Heterogeneous morphological population, mainly late-type galaxies (66% Sb or later)
• Emission coming in 80% of galaxies from nuclear regions. The intense star-formation region dominates the optical output of the galaxy
• Galaxies smaller and less luminous than normal spirals.
• Median luminosity ~1 mag fainter than galaxies found in blue surveys. Mr = -20.6 D₂₄ = 19.4 kpc (median values for Ho=50 km/s/Mpc) .

Follow-up imaging in Johnson B and H-alpha has been also obtained. See Pérez González et al 2000 A&A 141,409 and Pérez-González et al 2001 A&A 365,370.

Follow-up Near Infrared Imaging

Follow-up near-infrared (J and K band) imaging observations of the UCM survey using a number of telescopes have been obtained. These include the 3.8m UKIRT on Mauna Kea (Hawaii, USA), the 1m telescope on Lick Observatory (California, USA) and the 2.2m telescope on Calar Alto Observatory (Almería, Spain). Preliminary results from these campaigns appear in Almudena Alonso Herrero's PhD dissertation (1996) and Alonso-Herrero et al. (1996).

Gil de Paz et al. analyzed near-infrared and optical images together with optical spectroscopy for a representative sample of 67 UCM star forming galaxies. Here we compared the data with outputs of evolutionary synthesis models to derive properties of the current star formation activity: burst strength (mass ratio of the newly formed stars to the stellar mass of the galaxy) and age, star formation rates, as well as the stellar mass of the galaxies ( Gil de Paz et al 1998 and Gil de Paz et al 2000).

Since 1998 we have been using the 90" (2.3 m) Bok Telescope on Kitt Peak of the University of Arizona (USA) to obtain near-infrared images of the remaining 65% of the UCM sample. The JK-band imaging of the complete UCM sample is necessary to constrain the amount of underlying old stars and their spatial distribution relative to young stars.

Multi-frequency (B, r, Halpha, J, K) observations with good spatial resolution, in which the near-infrared plays a pivotal role, together with detailed modeling will be used to disentangle the age, burst strength, metallicity and obscuration effects. In particular, the near-infrared data will be used to determine the burst strength and mass distribution of star-forming galaxies in the Local Universe. This investigation is an important part of the PhD dissertation os Pablo G. Pérez-González (2003) and is described in Pérez-González et al. 2003 MNRAS 338,508 and Pérez-González et al. 2003 MNRAS 338,525

Spectroscopic follow-up

Follow-up spectroscopic observations of the UCM galaxies included in the first two lists have been performed with 2.2m and 3.5m telescopes of Calar Alto observatory (Almeria, Spain) and with the 2.5m INT of the observatorio del Roque de los Muchachos (La Palma, Canary Islands, Spain).

The results have been published in J. Gallego PhD dissertation 1995, Ga lego et al 1996 and Gallego et al 1997.

Spectral classification, fluxes and equivalent widths of the emission lines, redshifts, Hα luminosities etc has been obtained for the whole sample.

Main results are as follows:

• 59% with low ionization or high extinction properties. These galaxies can not be found with blue objective-prism surveys.
• Lack of sensitivity to low-luminosity objects.
• Only 9 Blue Compact Dwarf Galaxies (~5%) with Mr~-18 and EW(Hα) ~300Å

Higher spectral resolution spectra have been taken in order to derive dynamical masses and a thesis is under way. Spectra taken in the blue region of the spectra allowed us to mesure the [OII 3727Å line (see bellow)

Density of the Star Formation rate in the Local Universe

Analyzing a complete sample of emission-line galaxies from the UCM survey we have computed the Hα luminosity function for the star-forming galaxies in the surveyed volume of the Universe. A Schechter function provides a good fit to the Hα luminosity function.

(H₀ = 50 km/s/Mpc and a Scalo Initial Mass Function and have been used).

The luminosity density for Hα has been obtained integrating over the full range of luminosities. Finally we convert to the SFR density of the local universe (valid for star-forming galaxies with EW(Ha+[NII]) > 10 A and z <0.045) using the Hα emission as SFR estimator.

This is the first observational determination of this quantity, which will provide a direct test of current galaxy formation and evolution models (Gallego et al 1995).

Star Formation Rate and [OII] emission

Recently we have analyzed the relationship between the [OII]3727 and the Hα emission lines for the UCM sample to derive a good calibration between the SFR and the [OII] emission line (Aragón-Salamanca et al. 2001).

Although the [OII] line is routinely used as a SFR estimator for galaxies at redshifts z < 1, its dependence with metallicity, excitation and galaxy type (i.e., HII type versus disk-type) is not well known.

Our study has demostrated that the use of the absolute K-band magnitude (which traces the stellar mass of the galaxy) rather than the optical B-band magnitudes in the [OII] vs. Hα relation alleviates the dependence with the galaxy type. This is because HII-type and disk-type galaxies are well segregated in terms of their M_K magnitudes (that is, mass).