A list of nearby dwarf galaxies towards the Local Void in Hercules-Aquila

Astronomy and Astrophysics Supplement Series, Jul 2018

Based on film copies of the POSS-II we inspected a wide area of □° in the direction of the nearest cosmic void: {RA = 18h 38m, , 1500 km s-1}. As a result we present a list of 78 nearby dwarf galaxy candidates which have angular diameters and a mean surface brightness , mag/□″. Of them 22 are in the direction of the Local Void region. To measure their redshifts, a HI survey of these objects is undertaken on the 100 m Effelsberg telescope.

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A list of nearby dwarf galaxies towards the Local Void in Hercules-Aquila

Astron. Astrophys. Suppl. Ser. A list of nearby dwarf galaxies towards the Local Void in Hercules-Aquila V.E. Karachentseva 1 2 I.D. Karachentsev 0 2 G.M. Richter 3 0 Special Astrophysical Observatory, Russian Academy of Sciences , N. Arkhyz, KChR, 357147 , Russia 1 Astronomical Observatory of Kiev University , Observatorna 3, 254053, Kiev , Ukraine 2 Visiting astronomer, Astrophysikalisches Institut Potsdam , An der Sternwarte 16, D-14482 Potsdam , Germany 3 Astrophysicalisches Institut Potsdam , an der Sternwarte 16, D-14482 Potsdam , Germany Based on lm copies of the POSS-II we in- Our main goal was to search for new nearby dwarf spected a wide area of 6000ut in the direction of the galaxies in this Local Void. Because the Local Void begins nearest cosmic void: fRA = 18h 38m, D = +18 , V0 < actually just beyond the Local Group edge, we obtain here 1500 km s−1g. As a result we present a list of 78 nearby unprecedently a low threshold for detection of very faint dwarf galaxy candidates which have angular diameters dwarf galaxies in a void, which is an order of magnitude > 0:5 and a mean surface brightness < 26 mag/ut00. Of lower than for detection in other voids. 0 them 22 are in the direction of the Local Void region. To measure their redshifts, a HI survey of these objects is undertaken on the 100 m E elsberg telescope. 2. Field of the search galaxies; general | galaxies; dwarf - To outline the position of the Local Void on the sky, we reproduce in Fig. 1 the distribution of galaxies with known corrected radial velocities V0 < 1500 km s−1 in a region RA = [10.0, 23.5]h, D = [−20, +60] . The 19 nearest 1. Introduction galaxies with V0 < 750 km s−1 are shown as larger open circles. The radial velocities were taken from the PGC As it is well known now, empty cosmic volumes (=voids) catalogue (Paturel et al. 1992) and updated with data are the most prominent features of the large scale struc- from recent publications. The curves correspond to the ture of the Universe. Besides huge voids of about 100 Mpc Milky Way avoidance zone within galactic latitudes +10 size which are revealed in deep galaxy redshift surveys and −10 . As it can be seen from Fig. 1, inside a circle (Joeveer et al. 1978; Kirshner et al. 1981; de Lapparent of 60 diameter around a point of fRA = 18h38m, D = et al. 1986) , in the very nearby Universe one can nd also +18 g there is no galaxy having a radial velocity V0 < \minivoids" with a diameter of 3 − 5 Mpc (Karachentsev 1500 km s−1. Searches for new nearby galaxies in the most 1994) . Are the voids completely empty of galaxies? The obscured part of this area undertaken by Nakanishi et al. question may be answered more easily by studying nearby (1997) among the IRAS sources, as well for HI-rich dwarf voids. galaxies (Ho man et al. 1992) , added nothing within the Compiling the Catalogue and the Atlas of nearby drawn circle. galaxies which covers the Local Supercluster volume, Tully The distribution of more distant galaxies in the same (1988) noted a wide sky region towards RA 19h, sky region is shown in Fig. 2. The rst galaxies inside the and D +20 which is absolutely avoided by galaxies Local Void zone appear just beyond V0 = 1500 km s−1. from known catalogues. Extinction at low galactic lat- Adopting for the Hubble parameter H = 75 km s−1/Mpc, itudes cannot be the main cause of this phenomenon. we nd that the empty volume is > 3 103 Mpc3. Unfortunately, Tully did not indicate even approximately It should be emphasized that there is still no systemthe Local Void boundary. This led to a confusion in which atic search for nearby dwarf galaxies in the whole Local di erent authors ascribed the same name to di erent vol- Void area. Therefore, the possibility for such a large volumes. Below we use our own de nition of the center and ume to be completely devoided of galaxies remains unextent of the Local Void. proved. In the next section we describe the results of a -20 o )( 20 c e D 60 40 0 -20 o )( 20 c e D Vo<1500 New objects 60 40 0 -20 o )( 20 c e D 22 20 18 16 14 22 20 18 16 14 special survey for nearby dwarf galaxy candidates on almost the whole area of the Local Void. 3. The list of dwarf galaxy candidates To detect new dwarf galaxies we used J and R lm copies of the Second Palomar Sky Survey, POSS-II. The high angular resolution of new photographic emulsions and their sensitivity to low contrast details permit the detection and classi cation with con dence of galaxies of small angular dimensions, especially of low surface brightness objects. Following the method described earlier (Karachentseva & Karachentsev 1998 = Survey I) , we searched for dwarf galaxies with angular diameters more than about 0.5 arcmin in a wide region limited to RA = [14.0, 23.5]h, D = [−3; +63] . Because in the previous Survey-I RA (h) we searched for very nearby dwarf galaxies with V0 < 500 km s−1, i.e. about 3 times lower than in the present survey, we relaxed slightly our criterion, not requiring signs of possible resolution of into stars. We found 78 dwarf galaxy candidates, which are presented in Table 1. Here Col. 1 gives the object number indicating the POSS-II eld number; Col. 2 contain the equatorial coordinates (epoch 1950.0); Col. 3 gives the major and minor angular diameters in arcmin measured on the blue and red lms, respectively; Col. 4 gives the morphological type in the usual designations (\d" as a \dwarf" was omitted); Col. 5 gives a rough estimate of surface brightness: H-high (equal or brighter than for a normal spiral galaxy), L | low, VL | very low, EL | extremely low, i.e. invisible on POSS-I (this means a mean SB brighter than about 26 mag/ut00); Col. 6 gives the crossidenti cation with other lists (D or F | Schombert et al. 1992, BCh, 83 | Balkowski & Chamaraux 1983) ; Col. 7 gives the galaxy morphology description, or the heliocentric radial velocity and magnitude estimated from NED. The distribution of the 78 galaxies from Table 1 is presented in Fig. 3 by diamonds. Obviously, the detected new objects are distributed more or less regularly over the whole studied area. This picture gives no indication of the Local Void. However, only redshift measurements for the new objects can show, if they ll the Local Void volume or not. A HI survey of the objects from Table 1 is presently in progress at the 100-m E elsberg radio telescope. (Huchtmeier 1998) . 4. Discussion As can be seen from Table 1, among 78 galaxy candidates, 59 are not in previous lists. For the 8 galaxies with known radial velocities the median is +1690 km s−1, which shows that they are actually dwarfs. (3) (6) VL VL L EL L L L L L L L L L L L L L L L L L L L L L L L VL F520-3 CGMW 3-4255 Roman et al., 96 (7) bright nucl.+ blue halo blue smoothed, blue no nuclea blue comp.? of N6384 having Vh = 1665 (1) 144.5 144.3 blue Considering the distribution of nearby galaxies in approximately the same sky region, Nakanishi et al. (1997) determined the position of the Local Void to be about RA = 21.0h, D = +12 , which is 32 away from the above mentioned location. According to them the Local Void center corresponds to V h = 2500 km s−1. Apparently, we have here two di erent voids which are located near each other. To prevent confusions, we propose to call the nearest empty volume the \Hercules-Aquila Void", and the other one the \Pegasus-Delphinus Void". Ho man et al. (1992) have studied the Hercules-Aquila Void center in the HI line for low-mass gas clouds and HIrich dwarf galaxies. None were found within 2200 km s−1. For the mean Local Volume distance, 10 Mpc, a detection threshold in the HI line about 0.5 Jy km s−1 corresponds to a hydrogen mass limit of 107 solar masses. 5. Concluding remarks Using the present data on redshifts for nearby galaxies we determined a more accurate center position and a Acknowledgements. We have made extensive use of the Lyon-Meudon Extragalactic Database (LEDA, supplied by the LEDA team at the CRAL-Observatoire de Lyon), the NASA/IPAC Extragalactic Database (NED, which is operated by the Jet Propulsion Laboratory, Caltech, under contract with the National Aeronautics and Space Administration), and the Digitized Sky Survey (DSS-1) produced at the Space Telescope Science Institute under U.S. Goverment grant NAG W-2166. Our project is supported by INTAS{RFBR grant 95{IN{RU{ 1390 and DFG grant KS 9112. 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V. E. Karachentseva, I. D. Karachentsev, G. M. Richter. A list of nearby dwarf galaxies towards the Local Void in Hercules-Aquila, Astronomy and Astrophysics Supplement Series, 221-226, DOI: 10.1051/aas:1999173