Galaxies Detected by the Dwingeloo Obscured Galaxies Survey

A. J. Rivers , P. A. Henning , R.C. Kraan-Korteweg ,, PASA, 16 (1), in press.
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Subsections

Results

Approximately 60% of the DOGS survey was completed using the Dwingeloo 25-m telescope. In this portion of the survey, 36 galaxies were confirmed, 23 of which were previously unknown (no NED counterpart). The number of galaxies detected is consistent with calculations based on an assumed HI mass function (Zwaan et al. 1997) and the survey sensitivity which predict between 50 and 100 detections within the survey range. Recent Westerbork observations completed the survey and galaxies discovered will soon be incorporated into the Dwingeloo sample (cf. figure 3 for location of detected galaxies).

Figure 3: Spatial distribution of DOGS sources (indicated by $\star $) combined with Lyon-Meudon Extragalactic Database (LEDA) galaxies out to V

$\mathrm{_{LSR}} \leq$ 4000 km s-1 ($\bullet $). The 60% of the survey covered by the 25-m and analyzed in this paper is shown by the dashed line.
\begin{figure} \begin{center} \centerline{\psfig{file=DOGSspatial.eps,height=5cm,width=12.5cm}}\end{center}\end{figure}

Five of the 36 sources were originally identified by the shallow survey including Dwingeloo 1 and Maffei 2, both members of the nearby Maffei / IC 342 group of galaxies (Kraan-Korteweg et al. 1994). During the deep survey another known group member, MB 1 (McCall & Buta 1995), was identified and two additional members still await confirmation observations.

The most significant nearby, previously unknown galaxy identified by DOGS was Dwingeloo 1. Given the 80% coverage of the survey region by the shallow survey (Henning et al. 1998), chances are low that a massive nearby spiral was missed, since nearby galaxies appear in many adjacent pointings, all of which would have to be missed for the galaxy to escape detection. Thus, it is fairly unlikely that there is another previously unidentified massive galaxy whose gravitational influence significantly impacts Local Group peculiar motion or internal dynamics in the area covered by the survey.

The Supergalactic Plane Crossing Region and the Local Void

Although the Dwingeloo survey is limited in survey depth (V

$\mathrm{_{LSR}} \leq$ 4000 km s-1), two primary large scale structures fall partially within the survey range: the Local Void and the Local Supercluster. Eleven galaxies were discovered in the survey coverage of the Local Supercluster crossing region ($\ell\sim$140$^\circ$; |b| $\leq$ 5$^{\circ}\!\!$.25); 6 of these sources are noted in NED. Known structures appear continuous and well defined across the Galactic plane with a narrow bridge of galaxies visible at $\ell\sim$142$^\circ$ and V

LSR $\sim$ 1400 km s-1 .

Near the Local Void ($\ell\sim$33$^\circ$, b$\sim$-15$^\circ$), Marzke et al. (1996) and Roman et al. (1998) found evidence for a nearby cluster at $\sim$1500 km s-1. Two previously unknown galaxies were detected in this region, adding support for the hypothesized overdensity. These galaxies, Dw030.6-2.4 ($\ell$=30.60, b=-2.48, V

LSR=1480 km s-1) and Dw030.1-4.3 ($\ell$=30.09, b=-4.35, V

LSR=1528 km s-1), were independently identified by the Parkes ZOA survey (Henning et al., this volume).

NGC 6946 Group

Three dwarf galaxies were detected in the region of NGC 6946 ($\ell$=95.72; b=11.67) suggesting the possibility of a new nearby group. Of these, Dw095.0+1.0 was originally recorded as a compact High Velocity Cloud (HVC) (Wakker 1990), but present data suggest it is in fact a nearby dwarf galaxy. With a velocity of VGSR=368 km s-1, it has the highest redshift of any HVC in the Wakker catalog and the 50% velocity width of $\Delta $V50=100 km s-1 seen in the Dwingeloo spectrum is significantly broader than the 20-30 km s-1 velocity dispersion generally observed in HVCs. Naively assuming this galaxy is at the same distance as NGC 6946 ($\sim$6 Mpc, Sharina et al. 1997) yields M

$\mathrm{_{HI}}\simeq$ 4 x 108 M$_\odot$.

Figure 4: Spectra from the Dwingeloo 25-m telescope of three galaxies found in the vicinity of NGC 6946. Pictured are, from left to right, IRAS 21189+4503, Dw092.8+0.5 and Dw095.0+1.0
\begin{figure} \psfig{file=outspec.epsi,height=5.5cm}\end{figure}

Also detected in the area were IRAS 21189+4503 (Nakanishi 1997) and Dw092.8+0.5. Assuming a 6 Mpc distance yields M$_{HI}\simeq$ 2 x 108 M$_\odot$ and 1 x 108 M$_\odot$ respectively. The estimated neutral hydrogen masses are typical of dwarf galaxies, consistent with the gaussian shape of the 21-cm profiles (figure 4). With the recent discovery of the LSB galaxy Cepheus 1, (Burton et al., in press) 7 galaxies with recessional velocites V

$\mathrm{_{LSR}} \leq$ 250 km s-1 have been identified within 15$^\circ$ of NGC 6946 (table 1). If these galaxies do in fact signify a new nearby group, this group would lie some 40$^\circ$ from the Supergalactic plane, considerably more than any other known group in the local universe. The potential exists for the discovery of additional galaxies in this group since the data recently collected by the Westerbork array for +1$^{\circ}\!\!$.05 $\leq$ b $\leq$ +5$^{\circ}\!\!$.25 has not yet been analyzed.


Table 1: Possible members of the NGC 6946 group. HI masses are calculated based on the assumption that all galaxies lie at the same distance as NGC 6946 (6 Mpc, Sharina et al. 1997). The HI mass of NGC 6946 is based on the mean of the 6 flux entries in LEDA which are corrected for the beam-filling effect. The uncertainty in the masses of the three Dwingeloo detected sources is primarily due to the uncertainty in the positions of the galaxies.
Galaxy Name $\ell$ b V

LSR

 log(M

HI)

 Reference
      km s-1 M$_\odot$  
HKK L150 96.03 12.35 132 7.1 Huchtmeier et al. 1997
UGC 11583 95.63 12.31 127 8.2 Huchtmeier et al. 1997
HKK L149 95.57 12.22 126 8.1 Huchtmeier et al. 1997
NGC 6946 95.72 11.67 48 10.1 LEDA
Cepheus 1 94.38 8.01 65 9.1 Burton et al. in press
Dw095.0+1.0 95.05 1.16 142 8.6: Henning et al. 1998
Dw092.8+0.5 92.82 0.53 230 7.8:  
IRAS 21189+4503 88.85 -3.20 228 8.3: Nakanishi et al. 1997

Results from Synthesis Follow-ups of DOGS Sources

Synthesis observations of Dwingeloo galaxies were conducted with the VLA and WSRT in 1997. Snapshot WSRT observations yielded positions of DOGS sources though sensitivities were not adequate for detailed mapping. VLA follow-ups yielded a few interesting results.

Figure 5: A synthesis map of DOGS detection Dw073.2-4.2 from the VLA. A galaxy group is revealed in a sample velocity slice (V

LSR=3219 km s-1, $\Delta $v $\simeq $ 10 km s-1).
\begin{figure} \centerline{\psfig{file=D42.eps,height=9cm}}\end{figure}

A galaxy group unresolved by the Dwingeloo

$0^{\circ}\!\!.6$ beam was resolved into 5 galaxies by follow-up VLA observations (figure 5). Since early synthesis follow-ups to Parkes ZOA detections also have revealed interacting galaxy groups (Staveley-Smith et al. 1998), it will be interesting to see if this is commonly seen in HI selected samples. The total HI mass measured for this group was 2 x 109 M$_\odot$h0-2 with a mean velocity of V

LSR=3222 km s-1 and a calculated group diameter of approximately 150 h0-1 kpc. No optical counterparts were identified on POSS E plates under significant extinction (A$_B \simeq$ 4.5 mag from DIRBE reddening maps, Schlegel et al. 1998). A previously unknown low mass companion to IRAS 05596+1451 was also discovered in a separate VLA synthesis map.

Next Section: Conclusions and Future Directions
Title/Abstract Page: Galaxies Detected by the
Previous Section: Telescope Parameters and Search
Contents Page: Volume 16, Number 1

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