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The Astrophysical Journal, 769:105 (10pp), 2013 June 1
C

doi:10.1088/0004-637X/769/2/105

2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

LENTICULAR GALAXY IC 719: CURRENT BUILDING OF THE COUNTERROTATING LARGE-SCALE STELLAR DISK
Sternberg Astronomical Institute, M. V. Lomonosov Moscow State University, Moscow 119992, Russia; katkov.ivan@gmail.com, olga@sai.msu.su 2 Isaac Newton Institute, Moscow Branch, Chile Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnii Arkhyz, Karachaevo-Cherkesskaya Republic 369167, Russia; vafan@sao.ru Received 2012 December 24; accepted 2013 April 8; published 2013 May 10
1

Ivan Yu. Katkov1 , Olga K. Sil'chenko1 ,2 , and Victor L. Afanasiev3

3

ABSTRACT We have obtained and analyzed long-slit spectral data for the lenticular galaxy IC 719. In this gas-rich S0 galaxy, its large-scale gaseous disk counterrotates the global stellar disk. Moreover, in the IC 719 disk, we have detected a secondary stellar component corotating the ionized gas. By using emission line intensity ratios, we have proven the gas excitation by young stars and thus claim current star formation, the most intense in a ring-like zone at a radius of 10 (1.4 kpc). The oxygen abundance of the gas in the star-forming ring is about half of the solar abundance. Since the stellar disk remains dynamically cool, we conclude that smooth prolonged accretion of the external gas from a neighboring galaxy provides the current building of the thin large-scale stellar disk. Key words: galaxies: elliptical and lenticular, cD ­ galaxies: evolution ­ galaxies: individual (IC 719) ­ galaxies: ISM ­ galaxies: kinematics and dynamics Online-only material: color figures

1. INTRODUCTION Lenticular galaxies represent a rather common morphological type in the nearby universe: they constitute up to 15% of all galaxies in the field (Naim et al. 1995), being the second most abundant after spirals. Though they are more frequent in clusters, there is also a considerable population of isolated lenticulars (Sulentic et al. 2006) or of lenticulars in isolated pairs and triplets. Lenticulars are thought to lack gas in general; however, only considering the field, cold neutral hydrogen is found in up to 25%­45% of all lenticulars, in contrast with cluster members (Chamaraux et al. 1987; Grossi et al. 2009; Serra et al. 2012). The difference in the gas detection frequency depending on environment density can be explained by numerous mechanisms of gas removal from the large-scale disks which are effective only in dense environments. However, this scenario cannot explain the difference in gas kinematics depending on environment density: in the cases when the gas is found in cluster lenticulars, its rotation usually matches the rotation of the stellar components, while in the field S0s the gas kinematics is very often decoupled from that of the stellar components (Davis et al. 2011). It is likely that the main difference related to the environments must be looked for in the area of the gas acquisition conditions, and not in the area of its subsequent dynamical evolution. Davis et al. (2011) suggest that the gas in cluster lenticulars is provided by the mass loss of the stellar components, and so comes from an intrinsic source. Field lenticulars are evidently fed by external accretion, but the sources of the external accretion are not often recognized unambiguously. There are two main mechanisms that have been suggested by authors--minor mergers or cold accretion from large-scale filaments--and to choose between them we need particular observational investigations "in depth." Thus every new S0 galaxy with a significant kinematically decoupled gas component attracts the close attention of investigators.
Based on observations collected with the 6 m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences, which is operated under the financial support of the Science Department of Russia (registration number 01-43).

Though small amounts of kinematically decoupled gas are common in lenticular galaxies settling in low-density environments (Bertola et al. 1992), large counterrotating gaseous disks are rare, and gas-rich S0 galaxies are hardly seen at all. In the (R)SA(rs)0+ galaxy NGC 3626 (Ciri et al. 1995; Garcia-Burillo et al. 1998; Haynes et al. 2000), located at the periphery of a massive X-ray-detected group, all the gas found, including ionized, molecular, and neutral species, counterrotates the stellar component. In unbarred galaxies and group members--in the SA0/a-galaxy NGC 3593 (Bertola et al. 1996) and in the SA(r)0+ galaxy NGC 4138 (Thakar et al. 1997)--their counterrotating gas is already partly processed into stars, so these galaxies demonstrate two counterrotating stellar disks, one of which rotates together with gas. In the SA0 NGC 1596 (Chung et al. 2006), the outer gas, freshly accreted from the neighboring NGC 1602, counterrotates the inner part of the lenticular galaxy: the gas radial inflow in this unbarred galaxy is perhaps ineffective. To this list we have recently added two more lenticular galaxies with extended counterrotating gas, NGC 2551 and NGC 5631 (Sil'chenko et al. 2009); in the former, GALEX also detects a broad ring of star formation in the disk-dominated area, and in the latter we found two counterrotating stellar components. Interestingly, all of the galaxies mentioned above are unbarred, though some possess rings and some reveal signatures of oval distortion (Afanasiev & Sil'chenko 2002; Sil'chenko et al. 2010) which may be traces of past interaction. In the present paper, we add one more interesting object to the small list of S0s with extended counterrotating gas and to the even smaller list of S0s with two counterrotating stellar components (Kuijken et al. 1996). We have undertaken an analysis of various spectral data for the lenticular galaxy IC 719. The galaxy is of intermediate luminosity and constitutes an isolated non-interacting pair with IC 718--a late-type galaxy of similar luminosity. The general characteristics of IC 719 are presented in Table 1, and the general view can be seen in Figure 1. Though the galaxy is classified as early-type in all catalogs and databases, one can be assured that it is almost bulgeless: its surface brightness profiles represent a pure quasiexponential disk (Figure 1). The galaxy is rather gas-rich for 1


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Figure 1. S0-galaxy IC 719: top left--a K-band image from the UKIDSS DR8; top right--residuals between the K-band image and the outward exponential disk model; bottom--the azimuthally averaged surface brightness profiles calculated from the SDSS g -,r -,i -band images and the UKIDSS K-image. (A color version of this figure is available in the online journal.) Table 1 Global Parameters of the Galaxies Galaxy Type (NEDa ) R25 , kpc (NED+RC3b ) 0 BT (LEDAc ) MB (LEDA) MK (ATLAS-3D) Vr (NED) Distance, Mpc (ATLAS-3D) Inclination (LEDA) PAphot (LEDA) Vrot sin i , km s-1 , (LEDA, H i) , km s-1 , (LEDA) MH i , 109 M d MH2 , 108 M e IC 719 S0? 5.5 13.66 -18.6 -22.7 1860 km s-1 29.4 90 52 114.4 ± 6.6 121 0.54 1.8

Notes. a NASA/IPAC Extragalactic Database. b Third Reference Catalogue of Bright Galaxies. c Lyon-Meudon Extragalactic Database. d Grossi et al. (2009), Serra et al. (2012). e Young et al. (2011).

the S0 type; moreover, neutral H i and molecular gas are both detected. The galaxy was included in the sample of the survey ATLAS-3D (Cappellari et al. 2011) and observed with the integral-field spectrograph SAURON (Bacon et al. 2001). As a result, the counterrotation of the gaseous and stellar components 2

has been detected for its central part, R < 20 . Meanwhile, the H i disk of IC 719 is very extended--much more extended than even its stellar disk (Grossi et al. 2009); however, the large-scale velocity field of the H i is unknown. It would be interesting to probe the sense of gas rotation beyond the SAURON field of view, up to the optical border of the stellar disk--it is an opportunity to estimate the space scale of external gas accretion. By measuring the oxygen abundance in the extended gaseous disk, we can also choose between various possible accretion sources. Finally, the last, but not the least, aim of our analysis is a search for the secondary stellar component which may be coupled with the counterrotating gas: the detailed star formation history in the counterrotating gaseous disk is probably coupled with the accretion history and so allows us to reconstruct the accretion history and to identify its source. We have fulfilled long-slit spectral observations of IC 719 and now present their results. We also involve panoramic spectroscopic data from the ATLAS-3D survey (Cappellari et al. 2011) into our analysis. The layout of the paper is as follows: Section 2 describes our observations and data reduction. Section 3 is dedicated to the description of our analysis approaches. In Section 4, we discuss the results on kinematics of the stellar and gaseous components and their chemical compositions. Finally, in Section 5, we present our conclusions. 2. OBSERVATIONS AND DATA REDUCTION We used two data sets obtained from long-slit and integralfield spectroscopy. The long-slit spectroscopic observations


The Astrophysical Journal, 769:105 (10pp), 2013 June 1

Katkov, Sil'chenko, & Afanasiev

were made with a new universal spectrograph SCORPIO-2 (Afanasiev & Moiseev 2011) at the prime focus of the Russian 6 m Bol'shoj teleskop azimutal'nyj telescope operated by the Special Astrophysical Observatory, at the Russian Academy of Sciences. IC 719 was observed in 2011 November with the 1 slit aligned along the major axis with total exposure times of 7 â 1200 s. The median atmosphere seeing FWHM during these observations was 1.5 arcsec. We used the VPHG1200 grism which provided an intermediate spectral resolution FWHM 3.5 å in a wavelength region from 4300 to 7300 å. This spectral range included a set of strong absorption and emission features, making it suitable to study both internal stellar and gaseous kinematics and the stellar populations of the galaxy. The slit was 6 in length, which provided the possibility of using the edge spectra to evaluate the sky background. The CCD chip E2V CCD42-90, with a format of 2048â4600, used in the 1â2 binning mode provided a spatial scale of 0. 357 pixel-1 and a spectral sampling of 0.84 å pixel-1 . The preliminary data reduction was identical to that applied to the lenticular galaxy NGC 7743 described in Katkov et al. (2011b). Briefly, the primary data reduction was comprised of bias subtraction, flat-fielding, cosmic ray hit removal, and building the wavelength solution using the He-Ne-Ar arc-line spectra. To subtract the sky background, we invented a rather sophisticated approach. We constructed the spectral line-spread function (LSF) model varied along and across the wavelength direction by using the twilight spectrum (Katkov & Chilingarian 2011). The final stages of the long-slit spectra reduction were night sky spectrum subtraction taking into account the LSF variations, linearization, and accounting for spectral sensitivity variation using the spectrum of a spectrophotometrical standard star. The error frames were computed using the photon statistics and processed through the same reduction steps as the data. We also used the data obtained with the integral-field spectrograph SAURON (Bacon et al. 2001) mounted at the 4.2 m William Hershel Telescope, La Palma, in the frame of the ATLAS-3D survey (Cappellari et al. 2011). The raw science and calibration exposures were retrieved from the open Isaac Newton Group Archive of the Cambridge Astronomical Data Center. The field of view of the spectrograph is 44 â 38 spatial elements (spaxels) at a 0. 94 scale per spaxel. The SAURON spectral range is 4800­5350 å with spectral resolution FWHM of 4 å. For details of how we reduced the SAURON raw data, see Sil'chenko & Chilingarian (2011). 3. INTERNAL KINEMATICS AND STELLAR POPULATIONS 3.1. nbursts Fitting We derived the parameters of the internal stellar kinematics and the stellar population properties of IC 719 by fitting highresolution PEGASE.HR (Le Borgne et al. 2004) single (simple) stellar population (SSP) models, which were computed with the Salpeter stellar initial mass function, to our spectra by using the nbursts full spectral fitting technique (Chilingarian et al. 2007a, 2007b). Before the main minimization loop, a grid of the SSP spectra with a fixed set of ages and metallicities is convolved with the instrumental response of the spectrograph (LSF). The nonlinear least-square fitting algorithm is applied to select a template spectrum from the grid of the SSP models by specifying its luminosity-weighted ("SSP-equivalent") age t and metallicity [Z/H]. Then we convolve the template spectrum with a line-of-sight velocity distribution (LOSVD) approximated by 3

the Gauss­Hermite parameterization up to the fourth moment, i.e., v , , h3 , and h4 (van der Marel & Franx 1993). Multiplicative Legendre polynomials are also included to take into account possible internal dust reddening and residual spectrum slope variations due to the errors in the assumed instrument spectral response. Ionized-gas emission lines and traces of the subtracted strong airglow lines do not affect the solution due to masking of the narrow 20 å wide regions around them. Moreover, excluding age-sensitive Balmer lines from the fit neither biases age estimates nor significantly degrades the quality of the age determination (see details in Chilingarian 2009 and Appendix A2 in Chilingarian et al. 2007a). In order to achieve the required signal-to-noise ratio (S/N) per spatial bin, we performed adaptive binning of the long-slit spectra as well as of the integral-field spectra by using the Voronoi-tessellation scheme (Cappellari & Copin 2003). Figure 2 shows the parameters of the stellar kinematics for IC 719 derived in such a way. The uncertainties of the parameters derived were returned by the minimization procedure using flux error propagation through all data reduction steps. Chilingarian et al. (2007a) performed some Monte Carlo simulations for the nbursts fitting technique and demonstrated the consistency between the uncertainties of the parameters returned by the minimization procedure and the real error distributions. By applying this approach, we found that the stellar velocity dispersion demonstrates two off-centered maxima and high absolute values of h3 , h4 ,upto2.5 (seeFigure 2). Such values of h3 , h4 correspond to a strongly asymmetric stellar LOSVD. In order to analyze in detail the stellar LOSVD, we then probed a non-parametric recovery technique that does not require a priori knowledge of the LOSVD shape. 3.2. Non-parametric LOSVD The only assumption we used is that an observed galaxy spectrum logarithmically rebinned in the wavelength domain can be represented by the convolution of a velocity distribution L and a typical stellar (template) spectrum. We used the SSP model from the nbursts fitting, pre-convolved with the LSF output, as a template spectrum. The convolution can be considered as a linear inverse problem whose solution (vector L) can be estimated by the least-squares method. The solution is very sensitive to the noise in the data. Hence, we chose to regularize the problem by requiring the LOSVD to be smooth. In order to do so, we choose the smoothing regularizing matrix operator in the following form: P (L) = LT · DiT · Di · L, where matrix Di is the i th-order difference operator. For details, see chapter 19 of Press et al. (2007). The regularized inverse problem, where the function to be minimized is given by = 2 + P (L), can be expressed as a linear matrix equation and solved by the BVLS algorithm (Lawson & Hanson 1995) which allows us to constrain the solution by positive values. We performed numerous tests by varying the order of the difference operator and penalization coefficient , and concluded that the optimal values are i = 5 and = 0.01e . The choice of equal to e will tend to make both parts of the minimization function to have comparable weights. The technique used here was applied to study the stellar velocity distribution in NGC 524 (Katkov et al. 2011a) and is similar to that developed by Kuijken & Merrifield (1993). The stellar LOSVD derived from the long-slit data for IC 719 is presented in Figure 3 as a position­velocity diagram. The Gauss­Hermite representation of the stellar line of sight (LOS) recovered under the one-component approach is also shown in Figure 3. One can see that there is no acceptable agreement


The Astrophysical Journal, 769:105 (10pp), 2013 June 1

Katkov, Sil'chenko, & Afanasiev
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Figure 2. Results of the analysis of the long-slit spectrum for IC 719: top left--stellar LOS velocity profile; bottom left--stellar velocity dispersion profile; top and bottom right--Hermite coefficients h3 , h4 . The binning is made along the slit to maintain S/N = 15 pixel-1 .
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Figure 3. View of the stellar LOSVD for IC 719 obtained by the non-parametric technique. Left--stellar-position­velocity (PV) diagram for the long-slit spectrum. The black and blue circles mark the position of Gaussian centers for two-Gaussian decomposition. Right--cross-sections of the PV diagram at the different radial distances (r = -20, -5, 0, 10 arcsec). The solid black line represents the recovered stellar LOS at given distances, the red line represents the model of the sum of two separate Gaussians (green and blue lines). The thin black lines correspond to the Gauss­Hermite approximation of the stellar LOS recovered under the one-component approach using the nbursts fitting technique (see Section 3.1). (A color version of this figure is available in the online journal.)

between the non-parametric approach and the Gauss­Hermite representation. Therefore, the Gauss­Hermite parameterization cannot be applied to the complex LOSVD where two comparable peaks have a significant velocity separation. 3.3. Two-component Fitting Figure 3 demonstrates that the stellar LOSVD for IC 719 has a complex two-peaked structure. We have decomposed the stellar LOSVD derived by the non-parametric technique into two separate Gaussians in order to evaluate the LOS velocity and velocity dispersion of every component. The centers of the 4

Gaussians are overplotted at the position­velocity diagram as black and blue circles (Figure 3). In order to study the stellar populations of the counterrotating disks, we separated their contributions into the observed spectrum by using an advanced implementation of the nbursts full spectral fitting technique. In general, this approach is similar to that described in Section 3.1, but now we use a two-component model where the optimal template consists of a linear combination of two SSPs characterized by their ages and metallicities, each of them convolved with its own Gaussian-shaped LOSVD. The relative luminosity contribution of one of the components, p, is included in the parameter set of the minimization loop. The


The Astrophysical Journal, 769:105 (10pp), 2013 June 1

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Figure 4. Galaxy spectrum in the spatial bin at 7­8 arcsec southwest from the galaxy center extracted from the long-slit (top and middle panels) and IFU data set (bottom panel). The data are shown in black, and the red line shows the best-fit model which is the sum of the two stellar components (purple and black). Residuals and noise level are also shown. The gray vertical stripes correspond to masked narrow regions around emission lines which are excluded from the fitting procedure. (A color version of this figure is available in the online journal.)

contribution of the other component is equal to 1 - p.The multiplicative polynomial continuum factor is similar for both components. By undertaking the re-analysis of the long-slit data, we have decided to keep the velocity dispersions equal to the values that were derived from the decomposition of the non-parametric LOSVD into two Gaussians. The range of the stellar velocity dispersions obtained for both of the Gaussian components is rather narrow, = 60­80 km s-1 , so during the integral field unit (IFU) data analysis we fixed velocity dispersions of the components to be equal to 70 km s-1 . This is why there are no error bars 5

at the velocity dispersion profiles in Figure 5 and no velocity dispersion maps in Figure 6. If we take the dispersions of both components as free parameters, then the velocity profiles and the profiles of the stellar population properties look more noisy due to the additional degree of freedom. In Figure 4, we demonstrate the quality of our spectral fitting results by plotting the observed and model spectra in the same, arbitrarily selected spatial bin for two data sets--our long-slit data and the SAURON IFU data. The kinematical and stellar population parameters of the stellar components separated in such a way, as well as the


The Astrophysical Journal, 769:105 (10pp), 2013 June 1
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Katkov, Sil'chenko, & Afanasiev
10 8 Relative weights Light profile -20 -10 0 10 20 Radial distances, arcsec 30 6 5 4 3 2 1.0 0.8 0.6 0.4 0.2 1 0.4 0.2 0.0 -0.2 -0.4 -0.6 0.0

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Figure 5. Results of the two-component analysis of the long-slit spectrum for IC 719: top left--LOS velocity profiles for both the stars (black and blue dots) and for the ionized gas (pink triangles for the [N ii] 6583); bottom left--stellar velocity dispersion profiles; top middle--SSP-equivalent age profiles for two stellar components; bottom middle--metallicity profiles for two stellar components; top right--relative contribution of components; bottom right--open diamonds show the light profile which was obtained by integration spectra along long-slit and closed symbols correspond to light profiles for each component. The binning is made along the slit to maintain S/N = 20 pixel-1 . The gray lines in the left panel correspond to SAURON data extracted along major axis. (A color version of this figure is available in the online journal.)

relative luminosity contributions and light profile decomposition results, are shown in Figure 5 for the long-slit data and in Figure 6 for the SAURON data. We have succeeded in extracting two stellar components counterrotating with similar velocities about 200 · sin i km s-1 . The component that corotates the ionized gas (see below) gives a lower contribution to the disk surface brightness at R > 10 than its counterpart, so we treat it as a "secondary" stellar component; in general, it looks somewhat younger (Tsec 2.5 Gyr) and more metal-rich ([Z/H]sec -0.1 to 0.0 dex) than the other one (Tmain 4Gyr, [Z/H]main -0.3 to -0.2 dex). Two derived counterrotating stellar components seem to be dynamically cold disk structures, with their ratio of the maximum rotation velocity to velocity dispersion Vrot / 3 for each component. The identification of the large-scale stellar structures in IC 719 as the cold flat disks is supported by surface photometry. We extracted azimuthally averaged surface brightness profiles in the Sloan Digital Sky Survey (SDSS) g, r, i bands and in the UKIDSS K band using the IRAF task ellipse (Jedrzejewski 1987; see Figure 1) and found that the profiles are quasi-exponential with a compact nuclear concentration and no presence of a massive bulge. At the radii of r = 7 ­10 , one can see an excess of the surface brightness, which apparently corresponds to the concentration of the secondary stellar component. This luminosity excess looks like a ring structure at the bivariate picture of the residuals between the K-band image and the exponential disk model constructed over the outer regions (r > 20 ) of the galaxy by using the GALFIT package (Peng et al. 2002). 3.4. Ionized Gas An emission-line spectrum of every spatial bin was obtained by subtracting the stellar contribution (i.e., the best-fitting stellar population model) from the observed spectrum. This step provided a pure emission spectrum uncontaminated by absorption lines of the stellar component that is especially important for the Balmer lines. Then we fitted emission lines with Gaussians pre-convolved with the instrumental LSF in 6

order to determine the LOS velocities of the ionized gas and emission-line fluxes. The LOS velocity profiles and the maps of the velocities and fluxes of the emission lines are shown in Figures 5 and 7, respectively. The line fluxes have been corrected for the internal interstellar extinction as well as for the Galactic extinction according to Schlegel et al. (1998). The color excess E (B - V ) corresponding to the internal dust reddening was determined from the Balmer decrement using the theoretical line ratios F (H )/F (H ) = 2.87 for the electron temperature Te = 10,000 K (Osterbrock & Ferland 2006) and the parameterized extinction curve (Fitzpatrick 1999). We plotted our measurements of emission lines onto the classical excitation-type BPT (Baldwin, Phillips, & Terlevich) diagnostic diagrams (Baldwin et al. 1981) to identify the gas ionization source (Figure 8). One can see that all measurements are located in the area corresponding to photoionization by hot stars, except a few central points that correspond to shock-like excitation or to that powered by a weak (LINER) active galactic nucleus (AGN). Due to the wide spectral range of the SCORPIO long-slit spectra covering a set of strong emission lines (H , [O iii] 5007, H , [N ii] 6548,6583, [S ii] 6717,6730), we are able to determine the oxygen and nitrogen abundances of the ionized gas. To do this, we used the so-called NS-calibration method (Pilyugin & Mattsson 2011; based on the emission lines of nitrogen (N) and sulfur (S)) which does not require [O ii] 3727 + 3729 lines. This NS calibration uses strong emission lines O++ , N+ , S+ and is based on the spectra of H ii regions with measured electron temperatures as a calibration data set. In order to compare the oxygen and nitrogen abundances of the ionized gas with the metallicity of the stellar component, we plot all the radial profiles of the abundances in Figure 9. 4. DISCUSSION The LOSVD of IC 719 can be easily split by eye into two counterrotating stellar components (Figure 3). Thus, for this


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Figure 6. Results of the analysis of the SAURON spectra for IC 719: the left column shows the results of fitting by a single stellar component, and the center and right columns show the two-component fitting results. From top to bottom, the LOS velocity field, the SSP-equivalent age map, and the metallicity map are presented. The bottom row consists of the stellar velocity dispersion field for one-component stellar fitting (left column) and the maps of the relative contribution of every stellar component (middle and right columns). Note that the color bar has a different range for the velocity fields in the left column than in the other columns. (A color version of this figure is available in the online journal.)

Figure 7. Results of the emission-line component analysis by using the SAURON data: left--the ionized gas velocity field constructed using the H line; middle--the intensity map for the emission line H ; right--the intensity map for the emission line [O iii] 5007. (A color version of this figure is available in the online journal.)

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Katkov, Sil'chenko, & Afanasiev

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Figure 8. Excitation diagnostic diagrams comparing the emission-line ratios: [N ii]/H vs. [O iii]/H (left), [S ii]/H vs. [O iii]/H (mid), [O i]/H vs. [O iii]/H (right). The points show the extracted emission-line ratios from the long-slit spectra for IC 719. The color of the points corresponds to the distance from the galaxy center. The distribution of the measurements of the line ratios for galaxies from the SDSS survey with high signal-to-noise ratios (S/N > 3 in every line) are shown by gray color. The black curves, which separate the areas with the AGN/LINER excitations from areas with the star-formation-induced excitation, are taken from Kauffmann et al. (2003; dash-dotted curve) and from Kewley et al. (2006; solid curve). (A color version of this figure is available in the online journal.)

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-0.4 -40 -30 -20 -10 0 10 Radial distances, arcsec 20 30

Figure 9. Radial profiles of gaseous (filled circles) and stellar abundances (crosses). The green and blue circles with error bars correspond to nitrogen and oxygen abundances subtracting the solar value, respectively. We adopted a solar nitrogen abundance 12 + log N/H to be 7.83 and a solar oxygen abundance 12 + log O/H to be 8.69 (Asplund et al. 2009). The abundances in the nucleus are unreliable due to a different gas excitation mechanism there. (A color version of this figure is available in the online journal.)

galaxy we have undertaken the spectrum fitting by two SSPs with different kinematical and stellar population parameters. The early analysis by the ATLAS-3D team that used the kinemetry approach involving a one-component stellar population with the unified kinematics returned the classification of IC 719 as a rotator, which was slow in the inner part. The galaxy was also classified as a so-called "2 " type, and was treated as a galaxy having two off-centered maxima of the stellar velocity dispersion (Krajnovic et al. 2011; Emsellem et al. 2011). In fact, when the correct approach is applied, including two kinematically (and evolutionarily) decoupled SSPs, we find two fast (counter-) rotating stellar components even in the innermost region (Figure 6). The off-centered spots of visibly high stellar velocity dispersion (or of a widened LOSVD) are in fact the locations nearest to the center where the two LOSVD branches start to separate. The gaseous component extracted from the SAURON data after the fitting by two SSPs rotates just as one of the stellar components; the surface intensity of the H emission demonstrates a ring-like distribution with a radius of about 10 (Figure 7). By applying our two-component fitting to the IC 719 longslit data obtained with SCORPIO (Figure 5), we have obtained 8

two counterrotating stellar components once again; one of the LOS stellar velocity profiles coincides perfectly with the gas velocity profile, while the other is mirroring. The amplitudes of both stellar LOS velocity profiles are similar. Beyond the central part of the galaxy, the stars corotating the ionized gas look somewhat younger (Tsec 2.5 Gyr) and more metal-rich ([Z/H]sec -0.1 to 0.0 dex) than the counterrotating component (Tmain 4 Gyr, [Z/H]main -0.3 to -0.2 dex). The BPT diagram diagnostics of the pure emission-line spectrum, which was obtained by subtracting the model stellar population spectrum from the observed one, has shown (Figure 8) the following picture: the gas excitation in the nucleus is shock-like or powered by a weak (LINER) AGN, but in the ring and beyond it the gas is ionized by young stars. After measuring the gas abundances by using the emission-line flux ratios and NS calibration relation from Pilyugin & Mattsson (2011), we find that in general the ionized gas metallicity is higher than the metallicity of the main stellar component and tends to be smaller or equal to the metallicity of the secondary component (see Figure 9). For the first time the technique of spectroscopic decomposition was presented by Coccato et al. (2011). They simultaneously measured both the kinematics and stellar population properties of the counterrotating stellar disks of NGC 5719 and found that the stellar component, which rotates in the same direction as the ionized gas, is younger, less rich in metals, more -enhanced, and less luminous than the main galaxy disk. Their investigation definitely confirmed the gas accretion scenario for the secondary stellar disk of NGC 5719. Later, this team studied stellar populations in two better known galaxies, NGC 3593 and NGC 4550, hosting counterrotating disks (Coccato et al. 2013). In both of the galaxies, the secondary stellar components are less massive, more metal-poor, and -enhanced than the main galaxy stellar disks, and corotate the ionized gas. These findings rule out an internal origin of the secondary stellar component and favor a gas accretion scenario. NGC 4550 was also studied by Johnston et al. (2013). They found that the age of the secondary disk is a bit younger than was found by Coccato et al. (2013). Both papers concluded that the most likely formation mechanism of the secondary stellar component in NGC 4550 is an unusual gas accretion history. What is the origin of the counterrotating gas in IC 719? A standard answer could be "merging" or "interaction." The chemistry of the gas that is not very metal-poor (at least within the stellar body of the galaxy) excludes the version

[Z/H], dex


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Katkov, Sil'chenko, & Afanasiev

that the extended counterrotating gaseous disk consists of the relic baryons that were never processed through the stars, so there is no reason to suspect cold accretion from largescale cosmological filaments. Meanwhile, IC 719 constitutes an isolated non-interacting pair with IC 718--a late-type galaxy of similar luminosity. Moreover, neutral hydrogen mapping reveals a huge gas cloud embedding both galaxies (Grossi et al. 2009; Serra et al. 2012). The H i disk of IC 719 extends up to 100 kpc from the galactic center, and this disk seems to be coplanar with the stellar disk of IC 719. Is it an interaction with gas flows directed from the late-type neighbor onto the early-type one? Merging with some third galaxy seems improbable: the high mass of the gas (at least 0.7 billion solar masses), together with the modest stellar mass of the galaxy, 6 â 1010 solar masses (see MK in Table 1), implies the necessity of merging galaxies with a mass ratio larger than 1:10; this means that the thin stellar disk could not survive during such merging (Walker et al. 1996; Thakar & Ryden 1996) unless very specific conditions were provided--intense hot halo gas cooling after the merger (Moster et al. 2012; meanwhile, X-ray halos are not reported around these galaxies) or strictly coplanar satellites orbiting before the merger. Meanwhile, both stellar disks of IC 719 are rather cold dynamically. However, it is evident that the inner ionized gas component having the same angular momentum has a strong connection with the secondary stellar disk, though star formation in the disk has started perhaps recently, because the bulk stellar populations in the large-scale stellar disk of the galaxy are of intermediate age, >2 Gyr. One can consider two accretion scenarios of the gas onto the galaxy. In the first scenario, the gas was acquired through a single accretion event but then two starbursts occurred. An early starburst gave rise to the formation of the secondary stellar component and the last one continues now. The second scenario consists of two accretion events, each with a subsequent starburst. We prefer the latter scenario of gas accretion because the former one is inconsistent with the gas metallicity that we measured. Indeed, under the first scenario, we expect that the metallicity of the self-enriched gas would be higher than the stellar metallicity of the secondary component, but Figure 9 shows the opposite relation. The second scenario allows the metallicity of the mixture of the metal-poor external gas with the inner enriched gaseous fraction to be below the metallicity of the secondary stellar component, which is consistent with our measurements. 5. CONCLUSIONS By adding our long-slit spectral observations to the panoramic spectroscopy with SAURON, we detected an extended counterrotating gaseous disk as well as a secondary stellar component corotating the ionized gas in the lenticular galaxy IC 719. The gas counterrotation can be traced up to the optical borders of the galaxies; moreover, the ionized-gas velocity profiles that we obtained are more extended than the stellar velocity profiles extracted from the same spectral data. The gas emission-line surface intensity demonstrates ring-like distributions within the disk of the galaxy; according to the BPT diagnostics from the line ratio confrontation, the gas in the rings is excited by young stars, so the extended disk of IC 719 possesses rather intense current star formation. We have demonstrated that the accretion history of the external gas onto IC 719 consists of two events, each with a subsequent starburst. In any case, the large-scale counterrotating stellar disk of this field lenticular galaxy is growing and in the near future promises 9

to become an analog of NGC 4550--the S0 galaxy that has two equal-mass counterrotating stellar disks (Rubin et al. 1992;Rix et al. 1992) and no current star formation (Crocker et al. 2011). This research is partly based on data obtained from the Isaac Newton Group Archive which is maintained as part of the CASU Astronomical Data Centre at the Institute of Astronomy, Cambridge. We made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. In this study, we used the UKIDSS DR8 image data available through the WFCAM science archive and SDSS DR9 data. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web site is http://www.sdss.org/. We thank the anonymous referee for useful comments and suggestions. We acknowledge the use of the HyperLeda database. This work was supported by the Ministry of Education and Science of the Russian Federation and Russian Foundation for Basic Research (projects Nos. 10-02-00062a and 12-02-00685) and Presidential grant No. MD-3288.2012.2. I.Y.K. is also grateful to Dmitry Zimin's nonprofit Dynasty Foundation. REFERENCES
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