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Proceedings Title IAU Symposium Proceedings IAU Symposium No. 244, 2007 A.C. Editor, B.D. Editor & C.E. Editor, eds.

c 2007 International Astronomical Union DOI: 00.0000/X000000000000000X

Prob e of dark galaxies via disturb ed/ lopsided isolated galaxies
Igor D. Karachentsev1 , Valentina E. Karachentseva2 , Walter K. Huchtmeier3 , Dmitry I. Makarov1 , and Serafim S. Kaisin1
1

Sp ecial Astrophysical Observatory, Russian Academy of Sciences, N.Arkhyz, KChR, 369167, Russia email:ikar@sao.ru,dim@sao.ru, skai@sao.ru 2 Astronomical Observatory of Kiev University, Observatorna 3, 04053 Kiev, Ukraine, email:vkarach@observ.univ.kiev.ua 3 Max-Planck-Institut fur Radioastronomy, Auf dem Hugel 69, D-53121 Bonn, Germany email:p083huc@mpifr-b onn.mpg.de lopsided/interacting ob jects among 1500 isolated galaxies yields only galaxies which may b e explained as a result of their interaction with present results of sp ectral and photometric observations of these galaxm telescop e that lead to significant restriction on cosmic abundance of

arXiv:0708.1046v1 [astro-ph] 8 Aug 2007

Abstract. Searching for eight strongly disturb ed massive dark ob jects. We ies p erformed with the 6dark galaxies.

Keywords. dark matter, interaction galaxies, p eculiar galaxies

1. Intro duction
The standard Lambda-CDM cosmology assumes that besides dark halos with luminous galaxies in their cores, completely dark clumps (sub-halos) should also exist with masses 108 - 1011 M (van den Bosch et al. 2003, Tully 2005, Yang et al. 2005). The total number of dark sub-halos may exceed in the number of the usual galaxies by a factor tens. Within the Local Group, Klypin et al. (1999) predict about 300 satellites with masses greater than 3 108 M that is significantly higher than the observed one. A possible explanation for this discrepancy is the physical processes inhibiting star formation especially in low mass clumps, thus implying the existence of a large number of dark satellites. Here, we use an approach proposed by van den Bergh (1969) and Trentham et al. (2001), which relies on searching for signs of non-motivated distortion visible on images of spatially isolated galaxies. As known, galaxies in close encounters show a significant signature of interaction in the form of a distortion of their structure, the presence of tails and bridges, or a common diffuse envelope. All these features have been quantitatively explained based on numerous N-body simulations ever since Toomre & Toomre (1972). Signs of interaction are seen in more than 50% of those binary galaxies where the separation is comparable to the sum of the diameters. In systems with greater separation, as in triplets, the relative number of peculiar galaxies is about 30%. In magnitude-limited galaxy catalogs, the fraction of interacting ob jects is equal to 8% (Karachentsev,"Binary galaxies", 1987). When considering more and more scattered systems of galaxies and single galaxies of the "general field", one can expect a fraction of the interacting ob jects among them to 1


I 2gor D. Karachentsev1, Valentina E. Karachentseva2, Walter K. Huchtmeier3 , Dmitry I. Makarov1, and Serafim S. Kaisin1 be nearby zero. This would occur if there were no other ob jects except the luminous galaxies. However, if completely dark galaxies with masses of 108 - 1011 M exist, the phenomena of interaction will occur in the case of extremely isolated galaxies too. Hence, an asymptotic relative number of peculiar shapes among the most spatially isolated galaxies may be a sensitive tool for estimating the cosmic abundance of massive dark galaxies.

2. Searching for p eculiar isolated galaxies
To search for such strange cases of interaction where the second interacting companion is invisible, we used the "Catalog of Isolated Galaxies" (Karachentseva 1973 = KIG). This catalog contains 1050 galaxies without "significant" neighbors. According to our estimates, the catalog ob jects do not suffer essential perturbations from neighboring galaxies over some Gyrs. As it is a sample of northern galaxies with m < 15.7, the catalog includes only 4% of the CGCG galaxies from Zwicky et al. (19611968); hence KIG is a collection of a rather rare kind of galaxies. All KIG galaxies were inspected by us on the Digital Sky Survey In some cases, we also studied galaxy images in the 2MASS to check for the possibility of double-nuclei systems as recent merging remnants. The results of our inspections (Karachentsev et al. 2006 Karach06 yield five disturbed/lopsided galaxies. Then, we undertook a new search for isolated distorted galaxies in a sample limited by radial velocities VLG < 3200 km s-1 . Among 7500 such galaxies of the Local Supercluster, about 60% reside in groups of different populations (Makarov & Karachentsev, 2000). The remaining N 3000 galaxies are characterized by different degrees of isolation with respect to their neighbors. We selected 500 of the most isolated galaxies and inspected their images on DSS. Only 4 galaxies out of 500 show significant signs of interaction. One case, UGC 4722, turns out to be common with the KIG sample. Signs of interaction between galaxies are best developed when the ob jects have similar masses. If the number of completely dark sub-halos with typical masses of 108 - 1011M in any volume approximately corresponds to the number of luminous galaxies, then one may expect about 8% interacting galaxies among the isolated ones (interaction with an invisible ob ject). This rough estimate ignores, of course, properties of spatial distribution of dark sub-haloes with respect to the luminous galaxies. The observed relative number of KIG galaxies with clear features of interaction, 5/1050 = 0.5%, turns out to be one order of magnitude lower than expected. The observed frequency of disturbed shapes among very isolated galaxies in the Local Supercluster, 4/500 = 0.8%, is consistent with the previous estimate made for the KIG sample. In view of these results, it is rather unlikely that the number of massive dark sub-halos is similar to the number of the usual galaxies. Moreover, apart from interaction with a dark galaxy, the observed morphological irregularities of isolated galaxies may different origins, in particular, there could have been a merger, with the companion now merged and not visible anymore, or there could be large gas accretion from cosmic filaments, leading to perturbed morphologies via star bursts. As noted by Trentham, dark galaxies probably have low masses and negligible dynamical effects on massive galaxies like the ones in the KIG sample. They can have much more substantial effects on the nearby low-mass galaxies seen, say, in the Catalog of Neighboring Galaxies (Karachentsev et al. 2004). This sample contains 197 quite isolated galaxies with a "tidal index" T I < 0. (A


Probe of dark galaxies via disturbed/ lopsided isolated galaxies
Table 1. Isolated galaxies with disturb ed structure Galaxy UGC UGC UGC F6355101 11905 11871 2 V
LG

3

MB

log(LB ) log(MHI) ­ ­ 9.68 ­ 9.74 8.73 y: 9.41 9.89

+ + + + + + ly + +

ESO 539-7 NGC 244 Probab UGC 4722 ESO 545-5

Probable mergers: 11904 -21.1 10.60 7737 -20.6 10.40 8230 -21.0 10.56 12000: -20.7 10.44 Asymmetric SF: 3279 -18.6 9.60 1021 -17.0 8.96 disturb ed by a dark galax 1705 -16.9 8.92 2329 -18.9 9.72

negative T I means that the Keplerian cyclic period of the galaxy with respect main neighboring disturber exceeds the cosmic Hubble time.) About 90% of the low mass dwarfs. Pustilnik et al. (2005) found that an isolated nearby galaxy, DDO 68 with MB = and T I = -1.6, looks as a disturbed ob ject with a long curved tail. If DDO 68 is a such ob ject in the Local Volume, it yields a fraction of disturbed isolated galaxies, = 0.5%, the same as for the KIG catalog.

to its m are -14.3 single 1/197

3. Observations with the BTA 6-m telescop e
Spectral observations were performed by us for some faint galaxies around each of the eight peculiar ob jects. No physical companions to the considered peculiar galaxies were found, confirming their high isolation. The peculiar galaxies were imaged in B , V , R bands, and in the emission H line. Longslit spectra were obtained also across the target galaxies to study their kinematics. For the observations we used the multi-mode focal reducer SCORPIO installed at the BTA 6-m telescope. Figure 1 presents images of 8 the galaxies in different optical bands. The right hand images are given in larger scale for better seeing of the central regions. Preliminary analysis of the data allows us to separate eight the galaxies into three categories, as shown in Table 1: A) Galaxies of high luminosity with ripples, plumes and loops, caused apparently by a recent merging; two of them, UGC 5101 and UGC 11905, have been classified as mergers by Rothberg & Joseph (2004) based on K-band surface photometry. B) Galaxies of low luminosity with asymmetric star formation (ESO 539-7 and NGC 244); they have a quite regular undisturbed periphery and eccentric HII regions. C) Galaxies (UGC 4722 and ESO 545-5), whose disturbed peculiar shape may be really originated by a tight encounter with a massive dark ob ject. Note that both the galaxies have hydrogen mass-to-luminosity ratio, M (H I )/LB , higher than 1 in solar units. Results of our spectral and photometric observations for all eight the galaxies will be discussed in details in a separate paper. Finally, we suggest that the existence of dark galaxies can be a probable reason of signs of interactions observed in some isolated galaxies. However, a cosmic abundance of dark galaxies is likely less than 1/20 the population of usual luminous galaxies. Acknowledgements This work was supported by DFG­RFBR grant 06­02­04017.


I 4gor D. Karachentsev1, Valentina E. Karachentseva2, Walter K. Huchtmeier3 , Dmitry I. Makarov1, and Serafim S. Kaisin1 References
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Probe of dark galaxies via disturbed/ lopsided isolated galaxies

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Figure 1. BTA images of the eight disturb ed isolated galaxies seen in different optical bands. Enlarged central parts of them are given at right side. East is left and North is up.


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Probe of dark galaxies via disturbed/ lopsided isolated galaxies

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Probe of dark galaxies via disturbed/ lopsided isolated galaxies

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