1551M . THE ASTRONOMICAL JOURNAL VOLUME 111, NUMBER 4 APRIL 1996 111 NGC 2915. II. A DARK SPIRAL GALAXY WITH A BLUE COMPACT DWARF CORE Gerhardt R. Meurer Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218-2695 1996AJ Electronic mail: [email protected] Claude Carignan Département de Physique and Observatoire du Mont Mégantic, Université de Montréal, C.P. 6128, Suce. “A”, Montréal, Québec, Canada H3C3J7 Electronic mail: [email protected] Sylvie F. Beaulieu and Kenneth C. Freeman Mount Stromlo and Siding Spring Observatories, The Australian National University, Private Bag, P.O. Weston Creek, ACT 2611, Australia Electronic mail: [email protected], [email protected] Received 1995 December 4; revised 1996 January 16 ABSTRACT This paper presents Australia Telescope Compact Array HI synthesis observations of the weak blue compact dwarf (BCD) galaxy NGC 2915. It is shown that NGC 2915 has the HI properties of a late-type spiral galaxy (Sd-Sm), including a double horn global profile, and HI spiral arms. The HI extends out to over five times the Holmberg radius, and 22 times the exponential scale length in the B band. The optical counterpart corresponds to a central HI bar. The HI distribution and kinematics are discussed in detail. A rotation curve is derived and fitted with a mass model consisting of a stellar disk, a neutral gas disk, and a dark matter (DM) halo. The DM halo dominates at nearly all radii. The total mass to blue light ratio, <ÆtILb=16 within the last measured point. Thus NGC 2915 is one of the darkest disk galaxies known. The complex HI dynamics of the central region results in a high uncertainty of many of the fitted parameters. Nevertheless it is clear that the core of the DM halo is unusually dense (po^O.l Pc~3) and compact 9 (Rc^l kpc). The neutral gas component, with mass = 1.27X 10 is probably more massive than the stellar disk. Split and broad Hi lines (velocity dispersion ^35 kms-1) are seen in the central region. Pressure support is probably significant, and it is not clear whether the core is in equilibrium. Beyond the optical disk the average HI line of sight velocity dispersion is 8 km s_1, which is normal for disk galaxies. NGC 2915 does not obey the Tully-Fisher [A&A, 54, 661 (1977)] relation, being underluminous for its Vrot=88 km s“1 by a factor of nine. It also does not obey the star-formation threshold model of Kennicutt [ApJ, 344, 685 (1989)], when only the neutral gas is considered. A simple HI surface density threshold of 21 -2 2h i,crit ^ 10 cm adequately describes the location of current star formation. Although the HI properties of NGC 2915 are extreme relative to normal galaxies they appear less extreme in comparison to other BCDs, which have similar radial profiles of HI density and velocity dispersion, and HI extending well beyond the optical disk. © 1996 American Astronomical Society. 1. INTRODUCTION 1995, 1996; Hunter et al. 1994) but their primary aims were other than to search for DM. The quest for understanding dark matter (DM) halos of Here we present Hi observations of NGC 2915 whose galaxies using HI as a probe is predominantly weighted to- optical properties were presented by Meurer et al. (1994; Pa- wards spiral and dwarf irregular galaxies. The H l-rich blue per I). There it was shown to be a weak BCD, that is its compact dwarf (BCD; Thuan & Martin 1981) class has been integrated star-formation rate is only 0.05 yr_1- Most of largely ignored, and is under represented in Hi synthesis this star formation is near the very center of the galaxy with studies. This may be because their small optical angular some enhanced star formation along the major axis to the SE sizes, often smaller than typical synthesized beam sizes, sug- of the center. NGC 2915 resolves into stars; the brightest of gests that they will not be well resolved at Á.21 cm. Also, these yields the distance D=5.3±1.6 Mpc. At this distance only 8% of the BCDs in Thuan and Martin’s sample have 1" corresponds to 26 pc, and 1' corresponds to 1.54 kpc. double horn profiles indicative of extended rotating disks, Previous single dish HI observations indicate that it has a suggesting that they are not ideal candidates for DM studies. very extended HI disk (Becker et al. 1988). The HI obser- Some important Hi synthesis observations of BCDs, and vations presented here were made primarily to determine similar galaxies, have been made (e.g., Taylor et al. 1993, NGC 2915’s Hi rotation curve, and from it, constrain the 1551 Astron. J. Ill (4), April 1996 0004-6256/96/111 (4)/l 551/15/$6.00 © 1996 Am. Astron. Soc. 1551 © American Astronomical Society • Provided by the NASA Astrophysics Data System 1551M . 1552 MEURER ETAL: NGC 2915. IL 1552 111 Table 1. H i observing log. frame (CVEL), and the data from the three runs were com- bined (DBCON). The data were imaged and “CLEANed” config. UT date baseline on source (MX; Schwab 1984; see also Clark 1980; Högbom 1974) to name range11 (hrs) about the noise level per resultant channel to make both uni- 1996AJ 0.375 3 May, 1992 31 - 459 11.34 form and natural weighted data cubes (hereafter UN and NA, respectively). For the NA cube, two spectral channels at a 0.75 C 22 Jan, 1993 46 - 750 10.52 time were averaged at the imaging stage. The UN data were 1.5 D 12 Mar, 1993 107 - 1439 10.80 imaged at full spectral resolution. The beam sizes at the 50% level are W5o=45" (circular) for the NA data cube and W =27"X23" (PA=0°) for the UN cube. The final cubes aIn meters, excluding baselines with 50 were made by reconvolving the clean component images antenna 6. with two-dimensional Gaussian beams having the above sizes. Planes of the resultant NA data cube are shown in Fig. 1. structure of its DM halo. In addition we wish to examine From each cube, maps of total intensity, mean velocity, how the HI properties of this BCD differ from other types of and line broadening were constructed from the zeroth, first, galaxies, and whether they give any indication of what regu- and second moments of the data cubes (w.r.t. velocity; MO- lates star formation in BCDs. MNT). The moment maps and data cubes were then cor- The observations are presented in Sec. 2. Section 3 pre- rected for the fall-off in primary beam response with distance sents an overview of the H I properties. The dynamics of the from the pointing center (PBCOR). Table 2 summarizes ISM are analyzed in Sec. 4. The rotation curve is measured, some of the properties of the NA and UN data sets including pressure support corrections applied, and mass models fitted. the beam size, pixel size, and noise level in the resultant data Section 5 presents a discussion of the results including a cubes. comparison with other galaxies, a discussion of the Tully- In order to check the moment analysis results, single Fisher (1977) relation and the star-formation threshold law. Gaussian fits to the UN data cube were made (XGAUS). As Our conclusions are summarized in Sec. 6. In the Appendix a further check, and to examine the line profiles in detail the we discuss a possible interaction partner of NGC 2915. UN data cube was block averaged into 30"X30" pixels, and all the resulting profiles were examined and fit with a mul- 2. OBSERVATIONS AND DATA REDUCTION tiple Gaussian fitting algorithm using the IRAF program SPLOT. NGC 2915 was observed with three configurations of the Australia Telescope Compact Array using all six antennas. However, baselines with antenna 6 (which gives the longest 3. OVERVIEW OF H I PROPERTIES baselines—up to 6 km) were discarded since they produced insignificant correlation amplitudes. The dual polarization AT A cursory examination of the HI properties of NGC 2915 receiver was employed with the correlator set to 256 chan- shows that they are very different from its optical properties. nels per polarization, and with each channel separated by Figure 2 (Plate 52) shows three Hi surface density maps, 15.7 KHz (3.31 km/s). The observing logs for the three con- plotted as greyscales. Panel (a) shows the NA moment 0 map figurations are given in Table I. The integration time was 20 (45" beam); panel (b), the UN moment 0 map (27"X23" s per visibility measurement. Some data in each run were lost beam), and panel (c), the XGAUS peak amplitude map. to equipment failure and weather. NGC 2915 clearly has a spiral morphology, and this is most The data were reduced using standard software in the readily apparent in the NA map. There appears to be two AIPS package (ATNF version). The relevant tasks used are arms which can be traced back to the ends of the central bar noted here in parentheses. Bad or suspect data were edited structure. Figure 3 shows a contour plot of the NA moment 0 out (TYFLAG, SPFLAG). Temporal gain and phase drifts map, while the UN moment 0 contours are shown in Fig. 4 were calibrated (CALIB) using the secondary calibrator, (Plate 53) overlaid on an I band image obtained with the 0906-682 which was observed at 50 min intervals for 3-6 Anglo-Australian Telescope. This clearly shows that the H I, min.