MNRAS 000, 1–11 (2016) Preprint 25 July 2021 Compiled using MNRAS LATEX style file v3.0 Tidal bridge and tidal dwarf candidates in the interacting system Arp194 Anatoly V. Zasov1,2⋆, Anna S. Saburova1, Oleg V. Egorov1, Viktor L. Afanasiev3 1 Sternberg Astronomical Institute, Moscow M.V. Lomonosov State University, Universitetskij pr., 13, Moscow, 119991, Russia 2 Faculty of Physics, Moscow M.V. Lomonosov State University, Leninskie gory 1, Moscow, 119991, Russia 3 Special Astrophysical Observatory, Russian Academy of Sciences, Nizhniy Arkhyz, Karachai-Cherkessian Republic 357147, Russia 25 July 2021 ABSTRACT Arp194 is a system of recently collided galaxies, where the southern galaxy (S) passed throughthe gaseousdisc of the northerngalaxy(N) which in turn consists of two close compo- nents. This system is of special interest due to the presence of regions of active star-formation in the bridge between galaxies, the brightest of which (the region A) has a size of at least 4 kpc. We obtained three spectral slices of the system for different slit positions at the 6-m tele- scope of SAO RAS. We estimated the radial distribution of line-of-sight velocity and velocity dispersion as well as the intensities of emission lines and oxygen abundance 12 + log(O/H). The gas in the bridge is only partially mixed chemically and spatially: we observe the O/H gradient with the galactocentric distances both from S and N galaxies and a high dispersion of O/H in the outskirts of N-galaxy. Velocity dispersion of the emission-line gas is the low- est in the star-forming sites of the bridge and exceeds 50-70 km/s in the disturbed region of N-galaxy. Based on the SDSS photometrical data and our kinematical profiles we measured the masses of stellar population and the dynamical masses of individual objects. We confirm that the region A is the gravitationally bound tidal dwarf with the age of 107 108 yr, which is falling onto the parent S- galaxy. There is no evidence of the significant− amount of dark matter in this dwarf galaxy. Key words: galaxies: individual: Arp194, galaxies: kinematics and dynamics, galaxies: evo- lution, galaxies: interactions, galaxies: star formation dark matter 1 INTRODUCTION are usually observed either in long tidal tails or inside the galac- tic discs, but not in the bridge between galaxies. Among the Arp Arp 194 = VV126 is a tightly interacting system containing two galaxies the extended islands of star-formation between the closely main galaxies with active star-formation – southern (S) and north- spaced systems are noticeable only in Arp269, Arp270 and, proba- ern (N) ones, separated by 30 kpc (in projection). N-galaxy, in ∼ bly, in Arp59 where the interaction involves several galaxies. arXiv:1607.08761v1 [astro-ph.GA] 29 Jul 2016 turn, consists of two apparently merging galaxies (Na-component possessing the distinct spiral arms and the less noticeable and red- The location of individual objects and their designation der component Nb, projected onto the eastern spiral arm of Na- adopted in the current paper as well as the positions of the slit are galaxy). The peripheral structure of the galaxies, especially of the shown in Fig. 1 superimposed on the HST composite colour image. northern ones, is perturbed by the recent close approach or direct The mean system velocity of the components of Arp194 is 1 collision of N and S-galaxies. The best high-resolution images of close to 10500 km s− , the difference between the central velocities 1 this system may be found in the HST archive data.1. of S- and N- galaxies does not exceed 30–40 km s− . According Arp194 is of special interest due to the presence of the chain of to Bushouse & Stanford (1992) their stellar magnitudes in K band star-formation regions between the galaxies which seldom occurs are 12.7 (S) and 12.1 (N), which correspond to the luminosities in interacting systems. Indeed, if galaxies retained their integrality 3.6 1010(S ) and 6.3 1010 (N) in solar units adopting the distance · · 1 1 after the close passage, the extended regions containing young stars 144 Mpc (H0 = 73 km s− Mpc− ). Hence their NIR luminosities and consequently the stellar masses are comparable to that of the Galaxy. ⋆ E-mail: [email protected] The first spectral observations of the system were conducted 1 The color image may be found at the HST web-page: at 6-m telescope BTA at the dawn of its operation by Karachent- http://hubblesite.org/gallery/album/galaxy/interacting/pr2009018a sev and Zasov in the end of 70-s using the UAGS spectrograph c 2016 The Authors 2 A. Zasov et al. 57m56.8s PA=138o 57m55.2s Arp 194 Nb Arp 194 Na 57m53.6s PA=115o C PA=95o B 57m52.0s A 57m50.4s Arp 194 S 11h57m48.8s +36o21m12s 21m36s 22m00s 22m24s 22m48s Figure 1. The HST composite colour image in BVI bands with overplotted positions of the slits and regions of star-formation. and the image intensifier with photographic registration of spec- system. In other words, they resulted from the star-burst in the cold tra. The spectrograms were reduced by V. Metlov and published in gas lost by galaxies during the collision. The absence of noticeable Metlov (1980). The distribution of line-of-sight velocities of emis- radiation in J and K-bands speaks in favour of young age of these sion gas along the three spectral slices obtained in the cited paper emission bright regions. Marziani et al. (2003) estimated the age of unveiled the complexity of motions of gas and the presence of gi- the brightest stellar island A which is close to S-galaxy (see Fig.1): ant Hii regions with absolute stellar magnitudes of -15 – -17. A T = 7 107 yr and concluded that it falls into the central part of the total dynamical mass of the system according to Metlov (1980) ex- galaxy,· which is in a good agreement with the conclusions of the ceeds 1011 M . A comparison of the velocity estimates with our current paper. ⊙ measurements (this work) revealed a significant difference of some The goal of current paper is the study of the individual star- 1 local values (up to several tens of km s− ), mostly in the regions of forming regions on the periphery of the galaxies and between them, strong emission. However, the overall manner of distribution of gas which could be considered as candidates to the tidal dwarfs parallel velocity along the slits from Metlov (1980) agrees with the later with the analysis of dynamics and chemical composition of gas in estimates. the system. More elaborate study of the system Arp194 was carried out The paper is organized as follows: in Sec. 2 we present the by Marziani et al. (2003) using the results of spectral and photo- results of our long-slit spectral observations of Arp194 conducted metrical observations at the 2.1 m telescope. These authors ob- at BTA, including the description of data reduction and the kine- tained the spectral slices for two slit orientations and concluded matic and chemical abundance radial profiles. Sections 3 and 4 are that the unusual morphology of the system is related to the passage devoted to the analysis of the obtained data and present general re- of the more compact southern galaxy through the disc of the north- sults. In these sections we also involve the photometric data from ern one several hundred Myr ago, which caused the formation of SDSS. the sequence of bright emission islands between the galaxies and the apparently expanding ring structure around the northern galaxy (a collisional ring) which contains gas and star-forming regions. However, the detailed image of the system obtained later with HST 2 OBSERVATIONS demonstrates more complex picture: the arc of star-forming regions 2.1 Data reduction surrounding the system from the north side almost coincides with the spiral arm of Na-galaxy which is deformed and highly elon- We performed the long-slit spectral observations of gated to the west, at the opposite side there are no large-scale struc- Arp194 in 2013-2016 using the spectrograph SCORPIO-2 tures that could be attributed to the ring. Marziani et al. (2003) (Afanasiev & Moiseev 2011) mounted at the prime focus of the gave the convincing reasoning that the emission regions between 6-m Russian telescope BTA at Special Astrophysical Observatory the galaxies represent the ‘blobs of stripped gas due to the inter- of the Russian Academy of Sciences (SAO RAS). Three different penetrating encounter’ of southern and northern components of the positions of the slit were used, so that each one crossed a certain MNRAS 000, 1–11 (2016) Tidal bridge and tidal dwarf candidates in the interacting system Arp194 3 5 o Table 1. Log of observations 4 PA=115 3 Slit PA Date Exposure time Seeing 2 Rel. Flux (o) (s) (arcsec) 1 0 115 10.02.2013 3600 1.5 10 PA=138o 138 24.03.2015 1800 4.2 8 95 09.03.2016 900 1.2 6 Rel. Flux 4 2 1.0 o compact object of the system. The slit with positional angle 0.8 PA=95 PA=115◦ is close to that used by Marziani et al. (2003) with 0.6 PA=118◦, but in our case the slit width is more than twice 0.4 Rel. Flux narrower (about 1”). The log of the observations is given in Table 0.2 1. The positions of the slits are shown in Fig. 1. We used the grism 0.0 4500 5000 5500 6000 6500 7000 VPHG1200@540 which covers the spectral range 3600-7070 Wavelength 1 Å and has a dispersion of 0.87 Å pixel− .