In order to delineate the morphology of galaxies as observed in their rest-frame mid-ultraviolet, we have carried out a systematic imaging survey with HST/WFPC2 of 37 nearby galaxies the mid-UV filter F300W (centered at 2930Å), as well as in the I-band (filter F814W centered at 8230Å). These mid-UV images will be useful to more reliably classify the numerous faint galaxy images seen with HST in deep I-band surveys at high redshifts (z 1-2). Eleven galaxies located in the HST Continuous Viewing Zone were also imaged in the F255W filter (centered at 2550Å).
Our sample is carefully selected for size and surface brightness, and included galaxies of small enough radius and high enough predicted mid-UV SB to be observable with HST/WFPC2 in one orbit. Together with 17 galaxies imaged in F300W available in the HST Archive, our survey covers a wide range of Hubble types and inclinations. Complementary data at other wavelengths are available for our galaxy sample: most objects have ground-based UBVR images, some have IJHK images, and 15 have far-UV images from the Astro/UIT missions. Mid-UV images (2000-3200Å) have been the missing ingredient thus far.
In summary, our mid-UV imaging data base shows that in galaxies where star-formation is sufficiently pronounced, it can dominate the morphology from the mid-UV though the optical, resulting in very little change in morphology from the UV to the red. However, when stellar populations older than about 1 Gyr produce most of the optical light, we see changes in morphology between the optical - where these stars tend to produce relatively regular structures due to the effects of orbital mixing within the galaxies - and the mid-UV - where younger stars whose locations still reflect the distribution of their birthplaces. An additional complication is introduced by the presence of dust obscuration. Dust lanes or clouds that are nearly transparent in the visible can be opaque in the mid-UV, thereby changing the apparent morphology.
As a function of intrinsic galaxy type, our first qualitative results from this Cycle 9 project can be summarized as following:
(1) Early-type galaxies (T = - 6 to 0) show a significant change in SB in going from the mid-UV to the red, reflecting their relative paucity of a young stellar population. Some early types appear rather dim in the mid-UV due to significant central dust lanes, while others show mostly a small blue nuclear feature in the mid-UV. Three ellipticals become nearly point sources in the mid-UV (e.g., dominated by LINER's, Seyferts). This is in part due to our sample selection, which required the objects to have high (predicted) mid-UV SB. But it leads us to wonder to what extent the apparently strong cosmological evolution of weak AGN in early-type galaxies is due to the morphological K-correction plus SB-dimming, which causes the surrounding UV-faint early-type host to no longer be visible at higher redshifts. In summary, high SB early-type galaxies in the optical show a variety of morphologies in the mid-UV that can lead to a different morphological classification, although not necessarily always as later-type. The often rather peculiar mid-UV morphology of early-type galaxies is generally quite different than that of the real late-type galaxies as seen in the mid-UV, as discussed below.
(2) Mid-type spiral galaxies (T = 1-5) and star-forming galaxies can appear as later morphological types in the mid-UV, as Astro / UIT has shown primarily in the far-UV. About half of the mid-type spirals in the optical appear as later morphological types in the mid-UV, but not all mid-type spiral galaxies do look dramatically different in the mid-UV. One mid-type spiral, NGC 6782 (Section 3.15), shows a spectacular resonance ring full of hot stars in the F255W and F300W filters. The mid-UV images show a considerable range in the scale and SB of individual star-forming regions. A comparison of F300W to F814W images yields good sensitivity to dust features. Almost without exception, the mid-type spirals in our sample have their small nuclear bulges bisected by a dust-lane, which is often connected to the inner spiral arm structure.
(3) The late-type, irregular, peculiar, and merging galaxies (T=6-11) in our sample show diverse properties in the mid-UV. The majority of these galaxies show a F300W morphology that is similar to that in F814W, but with important differences due to recognizable dust-features absorbing the F300W light, and hot stars, star-clusters, or star-formation "ridges" that are better visible in the mid-UV. Less than a third of galaxies classified as late-type in the optical show significantly different structures in the mid-UV to result in a different classification.
In conclusion, our HST mid-UV survey of nearby galaxies suggests that it is more likely to misclassify true early-mid type galaxies in the rest-frame mid-UV as later types, while known late-type galaxies are less likely to be misclassified (as earlier types) in the mid-UV. This is because truly late-type galaxies are dominated by young and hot stars in all filters from the mid-UV to the red, and so have to first order the same morphology and a very small morphological K-correction in general. However, early-type galaxies (ellipticals and early-type spirals) can, although do not have to look significantly different when one goes from the rest-frame mid-UV to the optical-red part of the spectrum.
Classification of faint galaxies in the rest-frame mid-UV will thus likely result in some fraction of early-mid type galaxies being misclassified as later-types, probably a larger fraction than vice versa. But it is unlikely that this morphological K-correction can explain all of the faint blue galaxy excess as misclassified earlier-type galaxies. And the morphological K-correction cannot explain the slight excess of early-mid type galaxies at faint magnitudes (B 24 mag) with respect to passively evolving models, as found by Odewahn et al. (1996) and Cohen et al. (2002), since the main misclassification error goes in the opposite direction. Instead, our mid-UV survey seems to support the conclusion of Cohen et al. (2002) that the number of faint galaxies is larger than the non-evolving predictions for all galaxy types, but more significantly so for the later types. Cohen et al. (2002) give a possible explanation of this finding in terms of hierarchical formation models in a Lambda-dominated Universe.
To address these issues further quantitatively, we will in a sequel paper use the Fourier decomposition method of Odewahn et al. (1995, 2002a), where we will quantify the morphological K-correction based on each galaxy's mid-UV photometric parameters as a function of rest-frame wavelength and observed type. Our goal is to use the current HST mid-UV sample to quantify the morphological K-correction, and apply it to a large complete sample of faint HST galaxies with know photometric redshifts to get reliable and consistent rest-frame classifications. These will then be used to compute the redshift distribution as a function of morphological type, which will help delineate the formation and evolution of galaxies along the Hubble sequence (see Driver et al. 1998).
The US authors acknowledge support from NASA grants GO-8645.* and AR-8765.*, and RSdJ acknowledges support from NASA through Hubble Fellowship grant HF-01106.01-A, awarded by STScI, which is operated by AURA for NASA under contract NAS 5-26555. RAW, CAC and SCO acknowledge support from NASA ADP grant NAG-6740. CAC and VAT acknowledge support from a NASA Space Grant Graduate Fellowship at ASU. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology (Caltech) under contract with NASA, and has used NASA's Astrophysics Data System Bibliographic Services. The Second Palomar Observatory Sky Survey (POSS-II) was made by Caltech with funds from the NSF, NASA, the National Geographic Society, the Sloan Foundation, the Samuel Oschin Foundation, and the Eastman Kodak Corporation. The Oschin Schmidt Telescope is operated by Caltech and Palomar Observatory.
We thank Tony Roman and the STScI staff for their excellent help in getting these HST observations scheduled. We thank the staff of the Vatican Advanced Technology Telescope, in particular Richard Boyle, Chris Corbally, and Matt Nelson, for their excellent help in obtaining a large fraction of the ground-based images presented in this paper.
We thank Seth Cohen for advice in the data reduction, Dr. Arthur Code for a careful reading of the manuscript and for fruitful discussions, and Prof. Peter Strittmatter and the University of Arizona for their hospitality during a working visit. We thank the referee, Dr. Rob Kennicutt, for a careful and helpful review of the manuscript, and for suggesting a new way to publish a paper like the current one.