A list of new nearby dwarf galaxy candidates

Astronomy and Astrophysics Supplement Series, Jul 2018

To increase completeness of the distance limited sample of nearby galaxies from the the Kraan-Korteweg & Tammann (1979) catalogue we undertook a search for small companions of larger known galaxies which have corrected radial velocities within 500 km/s. Based primarely on the POSS-II and ESO/SERC films we found 260 nearby dwarf galaxy candidates with angular diameters  arcmin. More than 50% of the objects were revealed for the first time. As we suppose, a significant part of them (about 30%) may really belong to the Local Volume sample.

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A list of new nearby dwarf galaxy candidates

Astron. Astrophys. Suppl. Ser. A list of new nearby dwarf galaxy candidates? V.E. Karachentseva 1 2 I.D. Karachentsev 0 2 0 Special Astrophysical Observatory, Russian Academy of Sciences, N.Arkhyz , Stavropolsky Kraj, 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 To increase completeness of the distance lim- V0 < 500 km/s due to large peculiar velocities). Then ited sample of nearby galaxies from the Kraan-Korteweg Huchtmeier & Richter (1988, 1989a) and Karachentsev & Tammann (1979) catalogue we undertook a search for (1994) increased the sample in the number to N = 215 small companions of larger known galaxies which have by including new objects detected in the HI 21 cm line by corrected radial velocities within 500 km/s. Based pri- di erent authors. Independent e orts to extend the KKTmarely on the POSS-II and ESO/SERC lms we found sample were undertaken by Schmidt & Boller (1992). 260 nearby dwarf galaxy candidates with angular diame- However, their catalogue of 289 nearby galaxies contains ters a > 0:5 arcmin. More than 50% of the objects were many questionable cases when the radial velocity refers to revealed for the rst time. As we suppose, a signi cant a globular cluster or a star projected on a distant galaxy. part of them (about 30%) may really belong to the Local Moreover, the catalogue of Schmidt & Boller (1992) inVolume sample. cludes about 40 galaxies without radial velocity, assuming their membership in the nearby groups, but later HI ob- 1. Introduction In a study of the general properties and evolution of galaxies it is necessary to have quite a complete and representative sample biased minimally by observational selection. A catalogue of all nearby galaxies with distances D < Dmax would be an ideal realization of such \fair" sample. To be representative such a sample must contain at least (100 − 300) objects, which can be achieved at Dmax = (5 − 10) Mpc. The knowledge of distances for all nearby galaxies permits both local dynamics and the shape of the gravitational potential eld to be evaluated. Unfortunately, the distances remain one of the most de cient observing parameters for the galaxies even in the vicinity of the Local Group. Being short of distance data, Kraan-Korteweg & Tammann (1979) compiled a catalogue of nearby galaxies with radial velocities V0 (corrected for solar motion) not exceeding Vmax = 500 km/s. Their sample (=KKT) contains 179 galaxies (after excluding probable Virgo cluster members which satisfy the condition ? Tables 1 and 2 also available in electronic form at CDS via anonymous ftp to cdsarc.u-strasbg.fr ( or via ftp://cdsweb.u-strasbg.fr/Abstract.html The galaxy number distribution of the KKT- sample with radial velocities as well as angular diameters show that its expected completeness is (40 − 60)% only. For instance, a median value of the linear diameter of the Local Group galaxies equals 0.9 kpc (Karachentsev 1996) . Being at D = 3:5 Mpc (the distance of the nearest groups M 81 and Cent.A), a \median" dwarf system has an angular diameter a = 0:9 arcmin. In fact a lot of the faintest objects in the KKT- sample are represented by the galaxies from the catalogues: DDO (van den Bergh 1966) , UGC (Nilson 1973) , and ESO/Uppsala (Lauberts 1982) with a limiting angular diameter a = 1 arcmin. So, we expect that about (100 − 250) smaller galaxies really situated at D = 4 − 6 Mpc have not yet been covered by radial velocity surveys (and even have not yet been discovered). The situation is complicated by the presence of the \Zone of avoidance" because of the light extinction in the Milky Way plane. As experience of the last two decades have shown, the increase in the number of the known galaxies with V0 < 500 km/s grows slowly (with a rate of 2 − 3 galaxies per year) and a discovery of very nearby galaxies happens in fact by chance. That is a reason for us to make special searches for the nearby dwarf galaxy candidates. We consider this work a part of a more extended program of studying the Local Volume galaxies (Karachentsev 1994) . 2. Approach to the search The availability of high-quality Sky Surveys, POSS-II and ESO/SERC, made on new photographic emulsions stimulated strongly the present search. The high angular resolution and the sensitivity to low contrast details permit the detection and classi cation with con dence of galaxies of small angular dimensions, especially low surface brightness objects. Using our previous experience of searching for low surface brightness objects on the POSS-I prints (Karachentseva 1968, 1972, 1973) , we undertook analogous searches based on new observational material. Due to the di erent morphology of dwarf galaxies there are no distinct criteria to distinguish them from a normal distant galaxy of the same angular diameter (or from a part of a reflecting nebula). Typical cases of such visual misclassi cation were described by Karachentseva et al. (1985) . Indeed, a search for the nearby dwarf galaxy candidates seems to be a rather delicate subject. To complete the KKT- sample with new very nearby galaxies we used the following strategy: 1. Dwarf companions of the known nearby galaxies were assumed to be the main supplier of new objects for the Local Volume (=LV). Following this idea we searched on the POSS-II or ESO/SERC two-colour lms for dwarf galaxies down to the limiting angular diameter 0.5 arcmin around each of 215 LV galaxies with V0 < 500 km/s from the list by Karachentsev (1994) , which is an up-dated version of the KKT-sample. The diameter of the inspected area around each of the LV galaxies was chosen equal to 50 times its diameter. So, for a normal galaxy 20 kpc in diameter the survey area corresponds to a 1 Mpc diameter circle and exceeds the crossing length for the satellite during a cosmological time of 1/H. Because of the present incompleteness of the POSS-II survey we used in 15% of the cases the POSS-I prints too. 2. In addition to the inspected neighbourhood of individual galaxies we carried out a total visual examination of wide regions of the well-known nearby galaxy groups: the M 31, M 81, M 101, IC 342/Ma ei, Sculptor, Centaurus and the Canes Venatici cloud, indicated in the Nearby Galaxy Catalog of Tully (1988) . In case of CnV the survey area was limited to R:A: = [11:5h, 14:5h], D: = [+20 , +60 ]. 3. Using the POSS-II and ESO/SERC lms we revised the images of dwarf galaxies from the catalogues of Karachentseva & Sharina (1988) , and Arp & Madore (1989). The objects with signs of possible resolution into stars have been included, as a rule, in the candidate list. 4. To ll in the empty cone evoked by excluding all galaxies in the Virgo cluster direction, we inspected the galaxy images of the angular diameter a > 0:5 arcmin within the boundaries R:A: = [11:5h, 13:5h], D: = [0 , +20 ]. As an example, we considered the dwarf system GR 8 (Reaves 1983) situated in front of the Virgo cluster. 5. Into our list of the LV candidates there were included also galaxies with recent radial velocity determinations, mainly from the HI surveys (Huchtmeier & Richter 1989b; Shombert et al. 1992; Kraan-Korteweg et al. 1994; Garcia et al. 1994; Gallagher et al. 1995; Huchtmeier et al. 1995; Mattews et al. 1995; Huchtmeier & van Driel 1996; Mattews & Gallagher 1996) when their corrected velocities did not exceed 500 km/s. Note that due to the influence of Galactic hydrogen, the low signal-to-noise ratio or other reasons the small value of galaxy radial velocity would often seem doubtful. That is why our decision to include a galaxy in the list took into account the visual impression about its structure. We used also the available version of the ZCAT (Huchra 1995) which contains 386 objects with V0 < 500 km=s. Of them 215 are in the Karachentsev (1994) list, and 97 objects are members of the Virgo cluster. The remaining 74 ones formed a set of strange cases. Note some typical examples: { the radial velocity refers to a bright star projected on a distant galaxy (for example, 0232 + 3725), { the object is a globular cluster, not a galaxy (NGC 2420), { the velocity belongs to a HII region in M33 or another large galaxy (0130 + 3041), { the velocity belongs to the Galactic hydrogen in the empty area (0106 + 2137), { the object is a planetary nebula (NGC 5844) or a cometary nebula (2022 + 3541), { the small value of the galaxy radial velocity is a result of misprint (2318 − 4200). 3. The list of candidates The results of our search for new dwarf galaxy candidates are presented in Table 1 where Cols. 1, 2 are the equatorial coordinates (epoch 1950.0); Cols. 3, 4 are the major and minor angular diameters in arcmin measured on blue lms; Col. 5 is the morphological type in usual designations (\d" as a \dwarf" was omitted); Col. 6 is the rough estimate of surface brightness (=SB): H | high (equal or brighter than the SB of a normal spiral galaxy), L | low, VL | very low, EL | extremely low (usually invisible on POSS-I but visible on POSS-II); Fig. 1 illustrates the appearence on the digital POSS-I of four objects with the di erent SB estimates; Col. 7 is the galaxy name or its number in the catalogues/lists : AM | Arp & Madore (1987) , FG | Feitzinger & Galinski (1985) , K | Karachentseva (1968, 1972, 1973), BK | B¨orngen & Karachentseva (1985) , DDO | van den Bergh (1966), UGC, UGCA (Nilson 1973, 1974) , F | Schombert et al. (1992), VCC | Binggeli et al. (1985) , MCG | VorontsovVelyaminov et al. (1962 {1968); Col. 8 contains the data on heliocentric radial velocities and galaxy shape and indicates some notes at the end of the Table 1. In preparing this article two HI surveys of nearby galaxies (Huchtmeier et al. 1997; Cote et al. 1997) revealed 15 our objects to have radial velocities greater than 1000 km/s. A list of them is presented in Table 2 where columns are the same as in Table 1. The distribution of 260 galaxies from Table 1 is presented in Fig. 2 in equatorial coordinates. Because our survey does not cover the sky continuously, we do not discuss here the details of this distribution. Note only that the presented distribution delineates clearly the concentration toward the Local Supercluster plane and also the wide empty region coinciding with the Local Void (Tully 1988) around the direction f19:0h, +15 g. 4. Discussion As it is seen from Table 1, among 260 galaxy candidates 132 are absent in the previous catalogues and lists. In other words, 51% of our list objects are new. All but one are visible on two di erent colour lms and we suppose they are real objects. From them 33 galaxies have radial velocity data including 7 recent HI measurements made by Huchtmeier et al. (1997) . Obviously, there is no direct way to estimate the resulting e ciency of our searches. We did not meet in the literature a similar attempt just concerning the search for very nearby dwarf galaxies. Binggeli et al. (1990) were searching for dwarf galaxies on deep IIIa-J Palomar Schmidt telescope plates in several sky belts in the range of D.= [+28 , +57 ], R:A: = [9h, 16h]. Among 179 objects considered by them to be dwarf galaxies there are only 2 objects in common with our list. Schombert et al. (1992) published a catalogue of LSB galaxies found on the POSS-II survey in the declination zone D. = [0 , +25 ]. Their catalogue contains 369 LSB galaxies with angular diameters greater than 0.5 arcmin. Basing on the results of the HI survey of the catalogue galaxies, Schombert et al. (1992) concluded that most of the detected objects are spirals with V > 2000 km/s. Only 4 dwarf galaxies have V0 < 500 km/s. Impey et al. (1996) carried out a search for LSB galaxies in the equatorial zone covering an area of 786 sq.degrees. Using a combination of both automated and visual examination of glass copies of the UKST IIIa-J survey plates they detected 516 low surface brightness objects with a > 0:5 arcmin. For 264 and 160 objects the optical and HI velocities, respectively, have been measured. Our analysis of their catalogue data showed the following: (a) Only two corrected radial velocities do not exceed the limit of 500 km/s. But in both cases the measurements refer to stars projected on the galaxies. (b) There are no common objects in the list of Impey et al. (1996) and our Table 1. (c) Many Impey et al. (1996) LSB objects of irregular shape on their nding charts, looking like nearby dwarf candidates (for example, 0123 − 0029, 0129 − 0024, 0217 + 0031, 0224 + 0238, 0224 + 0233, 0227 + 0040 etc.), are unrecognized absolutely on both (blue and red) lms of POSS-II. In carrying out the search, we found independently all three dwarf spheroidal companions of the M 31: And I, And II, And III which had been discovered by van den Bergh (1972 ). However, we did not nd any new spheroidal nor did we nd irregular candidate member of this group. In the zone of strong obscuration in the vicinity of spiral galaxies IC 342 and Ma ei 2 we found independently the galaxies Cas 1, Cas 2, MB1 discovered recently by Huchtmeier et al. (1995) , Kraan-Korteweg et al. (1994) and McCall & Buta (1995) . Moreover we believe that our list contains probably additional members of this nearby galaxy complex. It should be emphasized that Table 1 does not contain other nearby galaxies which have been presented in their basic list (Karachentsev 1994) . This is the reason why LGS 3, GR 8, K 52, and some other well-known dwarf galaxies do not appear in Table 1. To clarify the nature of galaxies from our list one needs to measure their radial velocities. The pilot HI survey of the sample of 26 objects from this list made by Huchtmeier et al. (1997) shows that about 70% of them are detected in the 21 cm line, and the fraction of very nearby dwarfs with V0 < 500 km/s consists of 1/4. 5. Concluding remarks Due to the incompletness of the available set of the POSSII lms we did not aim at the global task of search for candidates in the sample of galaxies with D < 7 Mpc on the whole sky. The region inspected by us covers about 20% of the whole sky area. However in our list we expect to see the main part of new members of the Local Volume because our search covers the area occupied by all nearby groups. We suppose that a systematic study of these galaxies in the HI line as well as their multicolour imaging up to a deep limit for photometric distance determinations are tasks of current concern for large radio and optical telescopes. As regards the theories of galaxy formation and evolution, the search for nearby dwarf galaxies outside galaxy groups is very important too. Especially it is reasonable to reveal dwarf systems in the volume of the Local Void, occupying an area of several steradians on the sky (Tully 1988) . A survey of the Local Void region on the POSSII and ESO/SERC is planned as the second part of this project. Acknowledgements. We are grateful to the director and sta of the Astrophysical Institute Potsdam for hospitality and opportunity to work with the POSS-II Survey. We thank Dr. G.M. Richter for helpful discussion. We are indebted to G.G. Korotkova for the assistance in preparing the manuscript. This project is supported by INTAS-RFBR grant No. 95-INRU-1390. 0.6 1.0 0.7 0.8 2.0 2.0 2.2 1.1 1.2 0.9 0.6 1.6 0.7 0.8 1.2 0.3 0.7 0.5 0.8 0.8 0.7 0.3 0.6 0.5 0.4 0.4 0.6 0.7 0.6 0.6 Type 5 Ir Ir Ir Ir Ir Ir? Ir? Sph? Ir Ir Sph? Ir Im Im Im Arp H.C. , Madore B.F. , 1987 , A Catalogue of southern peculiar galaxies and associations . Cambridge University Press, I, II Binggeli B. , Sandage A. , Tammann G.A. , 1985 , AJ 90 , 1681 Binggeli B. , Tarengui M. , Sandage A. , 1990 , A &A 228 , 42 B¨orngen F. , Karachentseva V.E. , 1985 , Astron. 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The system discovered by R.Cannon; near NGC 7531 with V = 1598 km/s; perhaps a case similar to the LSBD \0110+008" near NGC 428 (Smoker et al. 1996 ).

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V. E. Karachentseva, I. D. Karachentsev. A list of new nearby dwarf galaxy candidates, Astronomy and Astrophysics Supplement Series, 409-419, DOI: 10.1051/aas:1998109