A&A 405, 951-957 (2003)
DOI: 10.1051/0004-6361:20030706

New optical velocities of nearby dwarf LSB galaxies

D. I. Makarov1,2 - I. D. Karachentsev1 - A. N. Burenkov1,2


1 - Special Astrophysical Observatory, Russian Academy of Sciences, N. Arkhyz, KChR, 369167, Russia
2 - Isaac Newton Institute of Chile, SAO Branch, Russia

Received 11 February 2003 / Accepted 16 April 2003

Abstract
We present radial velocities for nearby dwarf galaxy candidates found by Karachentseva & Karachentsev (1998) on the POSS-II films. Out of 118 observed objects, 88 have been detected in the H$_{\alpha}$ line. Their median radial velocity is 1750 km s-1. A quarter of the galaxies belong to the Local Volume, having corrected radial velocities $V_{\rm LG} < 500$ km s-1. Some of them are members of the nearby groups around Maffei/IC 342, M 81, and NGC 6946.

Key words: galaxies: dwarf

1 Introduction

Kraan-Korteweg & Tammann (1979) defined the Local Volume (LV) as our extragalactic neighbourhood out to $\sim$7 Mpc, which includes galaxies with radial velocities $V_{\rm LG} < 500$ km s-1 corrected to the Local Group centroid. Apparently, the LV census becomes increasingly incomplete for smaller galaxies, especially those of low surface brightness (LSB). Over the last two decades many observational teams undertook efforts to update the LV sample. As a result, the original Kraan-Korteweg & Tammann list of 179 galaxies in the LV is now doubled. A significant number of nearby dwarf galaxies have been revealed in 1998-2001 by Karachentseva & Karachentsev after visual inspection of the POSS-II films. On the northern sky they found about 400 nearby galaxy candidates with angular diameters $a\ga0\hbox{$.\mkern-4mu^\prime$ }5$, being mainly LSB. All these objects were observed then in the HI line with the 100-m Effelsberg radio telescope (Huchtmeier et al. 1997, 2000a,b, 2003). About 50% of the galaxies were detected. Due to their median radial velocity, 1200 km s-1, and the median HI line width, 60 km s-1, most of the objects seem to be nearby dwarf galaxies, like DDO irregular galaxies. More than 50 of them are situated within the Local Volume.

As known, some nearby galaxies with heliocentric radial velocities $V_{\rm h}$ inside [-300, +300] km s-1 remain still undetected or marginally detected in the HI line due to the Galactic emission. Gas rich galaxies can be undetected in HI when their radial velocities lie outside the interval [-470, +3970] km s-1 used in the Effelsberg HI survey. Some target galaxies may be confused with their close bright neighbours, if their separations do not exceed the half power beam width of the Effelsberg telescope ( $9\hbox{$.\mkern-4mu^\prime$ }3$ at 21 cm). For these reasons, measurements of optical velocities for nearby galaxy candidates are desirable.

2 Observations

The northern sky objects from the KK-lists (Karachentseva & Karachentsev 1998; Karachentseva et al. 1999, 2001) were observed with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences with the prime focus CCD spectrograph UAGS in 2000-2002. A $\rm 1k\times1k$pixels CCD with a pixel size of 24 microns together with a 1302 grooves/mm grating provide a dispersion of 1.2 Å/pixel and a spectral resolution of 4 Å. The slit length and width were 150 $\hbox{$^{\prime\prime}$ }$ and 2 $\hbox{$^{\prime\prime}$ }$, respectively. A typical exposure time was 600-900 s in the H$_{\alpha}$ region. The image processing was carried out with the LONG context in the ESO-MIDAS reduction package. We obtained about 230 spectra for 118 objects.

Table 1: List of northern objects from the KK-lists observed with the 6-m telescope.

3 Results


  \begin{figure} \par\mbox{\includegraphics[width=8.7cm,clip]{ms3591f1a.eps}\hspace*{1mm} \includegraphics[width=8.7cm,clip]{ms3591f1b.eps} }\end{figure} Figure 1: CCD images of two LSB objects observed with the 6-m telescope: a) dwarf irregular galaxy KKH 18 imaged in the V band, b) KKH 91, a new planetary nebula, imaged in H-alpha filter by S. Kajsin.
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  \begin{figure} \par\includegraphics[width=8.8cm,clip]{ms3591f2.eps} \end{figure} Figure 2: Distribution of the observed galaxies according to heliocentric radial velocities in km s-1.
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The basic parameters of the observed objects are given in Table 1. Its columns contain:

(1) galaxy number in the lists of Karachentseva & Karachentsev (1998) (KK); Karachentseva et al. (1999) (KKR); and Karachentsev et al. (2001) (KKH);

(2) equatorial coordinates at the 1950.0 epoch;

(3) major and minor angular diameters of the galaxy in arcmin, measured on blue plates;

(4) morphological type in the usual designations: Ir - irregular, Im - irregular magellanic, Sm - spiral magellanic; Sph - dwarf spheroidal, PN? - probable planetary nebula;

(5) mean surface brightness in a scale: H - high (22-23 $^{\rm m}/\sq\hbox{$^{\prime\prime}$ }$), L - low ($\sim$ $24^{\rm m}/\sq\hbox{$^{\prime\prime}$ }$), VL - very low ($\sim$ $25^{\rm m}/\sq\hbox{$^{\prime\prime}$ }$), and EL - extremely low ($\sim$ $26^{\rm m}/\sq\hbox{$^{\prime\prime}$ }$);

(6) heliocentric optical velocity and its internal error in km s-1, measured from our spectra;

(7) heliocentric HI velocity and its error in km s-1, measured by Huchtmeier et al. (2000a,b, 2003) and Karachentsev et al. (2001); blank lines in (6) and (7) mean that the objects were observed but not detected in H$_{\alpha}$ or HI, respectively;

(8) range of internal motions, W, in km s-1 measured along the slit position. It corresponds usually to a difference between maximal and minimal measured velocity in cases significant internal motions;

(9) asterisk addressing the table footnotes.

As an illustration, we present in Fig. 1 large-scale CCD images of two objects obtained with the 6-m telescope: a semi-resolved irregular galaxy, KKH 18, having $V_{\rm LG} = 372$ km s-1, and KKH 91, which turns out to be a planetary nebula.

  \begin{figure} \par\includegraphics[width=8.5cm,clip]{ms3591f3a.eps}\hspace*{3mm} \includegraphics[width=8.5cm,clip]{ms3591f3b.eps} \end{figure} Figure 3: Reproduction of spectra of two dwarf irregular LSB galaxies in the Maffei/IC 342 group, Cam A and KKH 6, with heliocentric velocities -49 km s-1 and +23 km s-1, respectively. Upper panel: rough spectra; middle panel: after sky subtraction; bottom panel: extracted 1D spectra.
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  \begin{figure} \par\mbox{\includegraphics[width=5.8cm,clip]{ms3591f4a.eps}\hspac... ...}\hspace*{1mm} \includegraphics[width=5.8cm,clip]{ms3591f4c.eps} }\end{figure} Figure 4: Red DSS images of three LV galaxy candidates: KKH 37, KKH 60, and KKR 60 with corrected radial velocities, $V_{\rm LG}$, respectively, +214, +104, and +296 km s-1.
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In total we obtained spectra for 118 nearby galaxy candidates, and 88 of them were detected in the H$_{\alpha}$ line. Figure 2 shows the object distribution according to their heliocentric radial velocity. The median velocity for the detected objects is 1750 km s-1.

When six local Galactic objects: KKH 26, KKH 29, KKH 33, KKR 4, KKH 91, and KKR 62 with heliocentric velocities in a range of  [-39, +55] km s-1 are excluded, the median velocity of the sample increases to 1920 km s-1. Therefore, the majority of the observed galaxies are true nearby dwarf galaxies situated inside the Local supercluster.

From 34 galaxies with optical as well radio velocities we obtained the mean-square velocity difference $\sigma(V_{\rm opt} - V_{\rm HI}) = 19$ km s-1, and the mean velocity difference $\langle V_{\rm opt} - V_{\rm HI}\rangle = -3\pm3$ km s-1, showing a good agreement between H$_{\alpha}$ and HI velocity measurements.

The derived optical spectra allow us to clarify the nature of some objects that have been marginally detected before (see Table footnotes). For instance, due to their velocities, KKH 6, KK 35, and  $\rm KK~41 = Cam~A$ are confirmed as members of the Maffei/IC 342 group. Reproductions of spectra of Cam A and KKH 6 are presented in Fig. 3, where a faint H-alpha emission is seen in both the objects.

Further, the blue dwarf compact galaxy $\rm KK~78 = UGC~5272b$ is recognized by us as a companion to another dwarf galaxy, UGC 5272, with a radial velocity difference of only $(19\pm4)$ km s-1. Together with KKH 54, UGC 5209, and UGC 5186, they probably form a new nearby group of dwarf galaxies, as discussed by Tully et al. (2002).

A more accurate radial velocity of DDO 82, the member of the M 81 group, also should be noted. Its new velocity differs from the old velocity in NED by 124 km s-1, decreasing the group velocity dispersion.

New radial velocities of three dwarf members of the NGC 6946 group, KKR 55, KKR 56, and KKR 59, make the velocity dispersion in the group lower (67 km s-1 instead of 95 km s-1), which decreases the virial mass-to-luminosity ratio of the group from 56 (Karachentsev et al. 2000) to 28  $M_{\odot}/L_{\odot}$.

The three objects KKH 37, KKH 60, and KKR 60 are unclear. Their DSS images are shown in Fig. 4. These objects have radial velocities with respect to the Local Group centroid in the range of  [+100, +300] km s-1, and are probable new Local Volume members. However, their possible nearby location should be confirmed with large-scale images.

Acknowledgements
Support for this work was provided by RFBR grant 01-02-16001 and DFG-RFBR grant 02-02-04012. The Digitized Sky Surveys were produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The images of these surveys are based on photographic data obtained using the Oschin Schmidt Telescope on the Palomar Mountain and the UK Schmidt Telescope. The plates were processed into the present compressed digital form with permission of these institutions. This work made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, Caltech, under contract with the National Aeronautics and Space Administration.

References


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