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The Westerbork HI Survey of Spiral and Irregular Galaxies I. HI Imaging of Late-Type Dwarf Galaxies

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The Westerbork HI Survey of Spiral and Irregular Galaxies I. HI Imaging of Late-Type Dwarf Galaxies Astronomy & Astrophysics manuscript no. (will be inserted by hand later) The Westerbork HI Survey of Spiral and Irregular Galaxies I. HI Imaging of Late-type Dwarf Galaxies R.A. Swaters1,2,3, T.S. van Albada1, J.M. van der Hulst1, and R. Sancisi4,1 1 Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands 2 Dept. of Physics and Astronomy Johns Hopkins University, 3400 N. Charles Str., Baltimore, MD 21218, U.S.A. 3 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, U.S.A. 4 Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy Received date; accepted date Abstract. Neutral hydrogen observations with the Westerbork Synthesis Radio Telescope are presented for a sample of 73 late-type dwarf galaxies. These observations are part of the WHISP project (Westerbork H i Survey of Spiral and Irregular Galaxies). Here we present H i maps, velocity fields, global profiles and radial surface density profiles of H i, as well as H i masses, H i radii and line widths. For the late-type galaxies in our sample, we find that the ratio of H i extent to optical diameter, defined as 6.4 disk scale lengths, is on average 1.8±0.8, similar to that seen in spiral galaxies. Most of the dwarf galaxies in this sample are rich in H i with a typical MH i/LB of 1.5. The relative H i content MH i/LR increases towards fainter absolute magnitudes and towards fainter surface brightnesses. Dwarf galaxies with lower average H i column densities also have lower average optical surface brightnesses. We find that lopsidedness is as common among dwarf galaxies as it is in spiral galaxies. About half of the dwarf galaxies in our sample have asymmetric global profiles, a third has a lopsided H i distribution, and about half shows signs of kinematic lopsidedness. Key words. Surveys – Galaxies: dwarf – Galaxies: structure 1. Introduction available. In addition to these studies, there have also been studies of the H i distribution in cluster galaxies. Over the past decades there have been numerous surveys Warmels (1988a) and Cayatte et al. (1990) studied the i arXiv:astro-ph/0204525v1 30 Apr 2002 of the H properties of galaxies using single dish tele- Virgo Cluster, Verheijen & Sancisi (2001, hereafter VS) scopes. These have been useful for determining the global the Ursa Major cluster. H i properties of galaxies and for finding relations between H i content, morphological type and other global proper- To date, little effort has been put into making a large, ties. They have been particularly useful for obtaining accu- homogeneous H i imaging survey of the local galaxy pop- rate redshifts and line widths for many galaxies. However, ulation. Clearly, such a survey is important for a sys- due to the large beam size, imaging galaxies in H i was tematic study of the H i component, including its kine- restricted to the largest galaxies. matical properties and their relation with optical proper- With the advent of synthesis radio telescopes H i imag- ties. For this reason, the Westerbork H i Survey of Spiral ing became routinely possible. Their high spatial resolu- and Irregular Galaxies (WHISP) with the Westerbork tion inspired many studies of the detailed distribution and Synthesis Radio Telescope (WSRT) was started. The kinematics of H i in galaxies. However, most of these stud- WHISP survey aims at mapping about 500 spiral and ies focused on one or a few galaxies, and only a few stud- irregular galaxies in H i. There are two main aspects to ies aimed at obtaining a large sample of field galaxies. the WHISP project. First, a systematic study of the H i Bosma (1978, 1981a,b) and Wevers (1984) both investi- component in itself, covering topics such as the distribu- gated a sample of about 20 galaxies in H i, but these stud- tion and kinematics of H i in and around galaxies, the ies focused on large and H i bright galaxies. Broeils & van frequency of lopsidedness and tidal features in H i, and Woerden (1994) and Rhee & van Albada (1996) investi- the variation of these H i properties with the luminous gated larger samples of about 50 galaxies and over a wider properties and morphological types. Second, the provision range of galaxy properties, but both studies used short ob- of data for other studies, not directly related to the H i servation with the Westerbork Synthesis Radio Telescope component. For example, the kinematic data are of great (WSRT) so that essentially only one-dimensional data are importance for studies of rotation curves and dark matter 2 R.A. Swaters et al.: HI Imaging of Late-type Dwarf Galaxies properties, the frequency of warps and the understanding of spiral structure. For statistically meaningful results, and for studies of variations with morphological type and lu- minosity, many hundreds of galaxies need to be observed. The WHISP project will provide the H i maps and veloc- ity fields necessary for this work. These data will be useful for other studies of individual galaxies as well. In addition to the H i data, for all the galaxies in the WHISP sample R-band CCD photometry has also been obtained (Swaters & Balcells 2000, hereafter Paper II). These will be used to obtain photometric parameters such as integrated magni- tudes, surface brightnesses scale lengths. This paper focuses on the WHISP H i data for late- type dwarf galaxies. These galaxies in particular have been underrepresented in previous H i synthesis surveys. Nonetheless, as has already been found, these galaxies have interesting properties. Single dish observations have Fig. 1. Histogram of Hubble types in the primary WHISP i shown that they are generally rich in H , often with sample. The shaded area gives the distribution over MH i/LB ratios larger than unity (e.g., Roberts & Haynes Hubble types of the sample presented in this paper. 1994). The largest H i disks, relative to the optical extent, are found in late-type dwarf galaxies, such as NGC 2915 (Meurer et al. 1996), DDO 154 (Carignan & Purton 1998) larger than 100 mJy, where the H i flux density is the ratio and NGC 4449 (Hunter et al. 1998). Rotation curve stud- of the integrated H i flux and profile width at the 50% level ies show that these galaxies may have even larger dark as listed in the RC3. This resulted in our primary WHISP matter fractions than ordinary spirals (e.g., Carignan & sample of 409 galaxies. The distribution over morphologi- Beaulieu 1989; Broeils 1992b; Swaters 1999; Cˆot´eet al. cal types of this sample is shown in Fig. 1. Almost half of 2000). the galaxies in the sample selected as described above are In this paper we present H i observations for a sam- late-type dwarf galaxies. ple of 73 late-type dwarf galaxies and we compare their From the primary sample we selected late-type dwarf H i properties with their luminous properties. Sect. 2 de- galaxies, i.e., all galaxies with Hubble types later than scribes the sample selection and the observations. Sect. 3 Sd, supplemented with galaxies of earlier Hubble type, describes the reduction of the data and the derivation of but with absolute magnitudes fainter than MB = −17. the H i maps and velocity fields, the radial and global Furthermore, we only selected galaxies with H i flux den- profiles, and global properties such as the H i mass, line sities larger than 200 mJy and with Galactic latitudes widths and H i diameters. In section 4 the H i properties of |b| > 10◦. This resulted in a sample of 109 galaxies. To these galaxies are compared to the optical properties, and this sample we added galaxies that met all selection cri- to the H i and optical properties of bright spiral galaxies. teria of the primary WHISP sample, except the diameter Sect. 5 discusses the occurrence of asymmetries in late- criterion. This additional sample consists of 4 galaxies, type dwarf galaxies. In Sect. 6 the conclusions are pre- and these were added to the sample presented in this pa- sented. per because of the possible uncertainties in using the op- To facilitate the reading of this paper, all long tables tical diameter D25 as an indicator of the H i diameter (see have been put into Appendix A. An atlas of the H i ob- Sect. 4.1). The combined sample constructed in this way servations is presented in Appendix B. consists of 113 galaxies. The distribution over morpholog- ical types of the sample is shown in Fig. 1 as the shaded 2. The sample area. Because the selection is largely based on morpholog- ical type, a few galaxies have been included in the sample The galaxies for the WHISP survey have so far been se- that have been classified as irregulars, but that proved to lected from the Uppsala General Catalogue of Galaxies be large interacting galaxies rather than dwarf galaxies. (UGC, Nilson 1973), taking observability with the WSRT Their data are presented here as well. into account. The typical resolution of the WSRT is Out of the sample thus selected, 73 have been observed 14′′ × 14′′/ sin δ. To make sure that the galaxies will be (these are listed in Table A.1). For most of the 40 other sufficiently resolved by the WSRT beam, we first selected galaxies H i data have been obtained by others, although galaxies with blue diameters larger than 1.5′ and decli- some of these data are as yet unpublished. These 40 galax- nations north of 20◦.
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