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Astronomical & Astrophysical Transactions

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Leo II Group: decoupled cores of NGC 3607 and NGC3608
V. L. Afanasiev a; O. K. Silchenko bc a Special Astrophysical Observatory, Nizhnij Arkhyz, Russia b Sternberg Astronomical Institute, Moscow, Russia c UK Astronomy Data Centre, Guest Investigator,

Online Publication Date: 01 August 2007 To cite this Article: Afanasiev, V. L. and Silchenko, O. K. (2007) 'Leo II Group: decoupled cores of NGC 3607 and NGC3608', Astronomical & Astrophysical Transactions, 26:4, 311 - 337 To link to this article: DOI: 10.1080/10556790701553524 URL: http://dx.doi.org/10.1080/10556790701553524

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Astronomical and Astrophysical Transactions Vol. 26, Nos. 4­5, August­October 2007, 311­337
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Leo II Group: decoupled cores of NGC 3607 and NGC3608
V. L. AFANASIEV* and O. K. SILCHENKO§
Special Astrophysical Observatory, Nizhnij Arkhyz, 369167, Russia Sternberg Astronomical Institute, University av. 13, Moscow 119991, Russia §UK Astronomy Data Centre, Guest Investigator
(Received 20 June 2007) The kinematics, structure, and stellar population properties in the centres of two brightest early-type galaxies of the Leo II group, NGC 3607 and NGC 3608, are studied by means of integral-field spectroscopy. The kinematically distinct areas in the centres of these galaxies, with radii of 6 and 5 , respectively, are found also to be chemically distinct: they are characterized by enhanced magnesiumline strength. However, no stellar age differences have been found between the decoupled cores and their outskirts. An analysis of two-dimensional line-of-sight velocity fields reveals systematic turns of kinematical major axes near the nuclei of both galaxies; in NGC 3608 the ionized gas rotates perpendicular to the stellar component rotation. When taking into account some morphological features, it is concluded that both NGC 3607 and NGC 3608 have large triaxial stellar spheroids. it is argued that the magnesium-enhanced cores are not circumnuclear disks; instead they resemble rather compact triaxial structures that force the formation of polar disks around them ­ a gaseous one in NGC 3608 and a stellar-gaseous one in NGC 3607; in the latter, star formation is perhaps still proceeding. Keywords: Galaxies: individual: NGC 3607 ­ Galaxies: individual: NGC 3608 ­ Galaxies: nuclei ­ Galaxies: stellar content ­ Galaxies: kinematics & dynamics ­ Galaxies: evolution

1. Introduction There exist now certain contradictions between a homogeneous red-colour appearance of nearby early-type galaxies, implying a rather brief ancient epoch of the main star formation, and prescriptions of the hierarchical concept of galaxy formation requiring quite recent merger events followed by some secondary (nuclear) star formation. Galaxy groups represent perhaps the best places where signatures of the external-driven secular evolution in early-type galaxies may be found because dense environments and moderate (with respect to clusters') velocity dispersions are favourable for mergers and tidal interactions. Recently we have considered Leo I group and its members NGC 3379, NGC 3384, and NGC 3368 [1]. A combined analysis of the stellar and gaseous kinematics and of stellar population properties in the centres of galaxies has revealed signatures of synchronous secular
*Corresponding author. Email: vafan@sao.ru

Astronomical and Astrophysical Transactions ISSN 1055-6796 print/ISSN 1476-3540 online © 2007 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/10556790701553524


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evolution: inner rotation axes are aligned, despite the different orientations of the outer galactic bodies, and the mean stellar age estimates are evidence for quasi-simultaneous star formation bursts about 3 Gyr ago. However, the Leo I group is unique in some sense because it possesses a supergiant intergalactic H I cloud of 1.7 · 109 M [2], [3]; its shape is a clumpy ring with a radius of 100 kpc encircling the galaxy pair NGC 3379/NGC 3384, and just the spatial orientation of this ring defines the alignment of the inner rotation axes of the three galaxies. So it seems probable that the secular evolution of the Leo I early-type galaxies is governed by the tidal interaction of every galaxy with the intergalactic H I ring and not by the tidal interactions between the galaxies. Now we are going to study central early-type galaxies of the Leo II group lacking considerable masses of neutral hydrogen. It is interesting to look for signatures of the secular evolution in the circumnuclear parts of these galaxies. The Leo II group contains 16 galaxies brighter than BT 16, according to [4]. Of those, only five are of early type, namely, S0 or ellipticals; and three early-type galaxies, NGC 3605 (compact dwarf elliptical), NGC 3607, and NGC 3608 are located in the very centre of the group within a 60 kpc area. The velocity dispersion of the whole group is rather moderate, 417 km/s according to [5]. The main characteristics of the galaxies to be considered in the present paper are given in table 1. NGC 3607 is a giant lenticular galaxy located in the centre of the Leo II group. Hot X-ray gas is detected inside this galaxy [6], as well as around it [7] ­ obviously, all the central galaxies of the group are embedded into a common envelope of hot gas. A new X-ray map of the group in the atlas [9] demonstrates two maxima of the hot gas concentration ­ one at NGC 3607 and another at NGC 3608 ­ which forces authors to state a recent merger history of the Leo II group. Also NGC 3607, though of early type, is known to have a rather extended central ionized-gas disk ([10], [11]) with a radius of 15 (1.5 kpc) containing a broad dust ring between R = 8.4 and R = 13.2 [12]. A central structure of the neighbouring low-luminosity elliptical galaxy NGC 3608 is also very interesting: it has a counterrotating core. Basing on a long-slit cross-section taken along the major axis of NGC 3608 it is noted by [13] that at approximately R = 10 (1.1 kpc) the stellar rotation changes its sense; the maximum rotation velocity of the inner core, achieved at R 4 , was, however, rather low, of 15 km/s. In [14] high-resolution images are inspected that have been obtained with the WFPC2/HST through two filters, F555W and F814W, for the sample of elliptical galaxies with kinematically decoupled cores, and particularly in NGC 3608 no evidence has been found for

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Table 1. Global parameters of the galaxies. NGC Type (NED ) R25 , kpc (LEDA2 ) 0 BT (RC33 ) MB (LEDA) (B - V)0 (RC3) T (U - B)0 (RC3) T Vr (NED), km·s-1 Distance, Mpc Inclination (LEDA) PAphot (LEDA) ,km·s-1 (LEDA)
1 2 3 4

3607
1

3608


SA(s)0 15.2 10.79 -20.00 0.92 0.49 935 34 120 224

E2 10.5 11.69 -19.74 0.93 0.40 1253 23
4

52 80 192

NASA/IPAC Extragalactic Database. Lyon-Meudon Extragalactic Database. Third Reference Catalogue of Bright Galaxies. [8].


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a circumnuclear stellar disk presence. In this work it was claimed that the colour V - I is very homogeneous over the central parts of the sample galaxies, the kinematically decoupled cores being indistinctive from the surrounding stellar bodies; in the particular case of NGC 3608, however, we suspect a break of V - I by less than 0.1 mag at R 4 from their figure 1d. Curiously, though neither [14] nor other investigators find any fine morphological substructure in NGC 3608, the global properties of the galaxy look unusual: despite its low luminosity, it is a slow rotator [13], it is boxy [15], and it has a shallow core profile [14] ­ a classic set of properties of a giant elliptical. A general view of both galaxies is presented by figures 1 and 2 where three various fields of view are outlined: the large-scale view of the galaxies, the HST/PC-frame field, and the central regions observed by means of 2D spectroscopy. The layout of the paper is as follows. We report our observations and other data which we use in section 2. The radial variations of the stellar population properties are analysed in section 3, and in section 4 2D velocity fields obtained by means of 2D spectroscopy for the central parts of NGC 3607 and NGC 3608 are presented. Section 5 provides a discussion and our conclusions.

Figure 1. The photometric maps for NGC 3607: left ­ large-scale image through the filter V (Haute-Provence data from the HYPERLEDA database), intensity is linearly gray-scaled, middle ­ the HST/PC frame through the filter F547M, the same, right ­ the area observed with the Multi-Pupil Fiber Spectrograph, with the uncalibrated colour distribution obtained by dividing the HST/PC frames through the filters F814W and F555W by each other and taking 2.5 logarithms, darker means redder.

Figure 2. The photometric maps for NGC 3608: left ­ a large-scale red image from DSS, intensity is linearly gray-scaled, middle ­ the HST/PC frame through the filter F555W, the same, right ­ the area observed with the Multi-Pupil Fiber Spectrograph, with the uncalibrated colour distribution obtained by dividing the HST/PC frames through the filters F814W and F555W by each other and taking 2.5 logarithms, darker means redder.


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2.

Observations and data reduction

The spectral data analysed in this work are obtained using two different integral-field spectrographs. Integral-field spectroscopy is a rather new approach which was first proposed by Professor G. Courtes some 25 years ago ­ for a description of the idea behind the instrument see [16]. It allows one to obtain simultaneously a set of spectra over a wide spectral range from an extended area of the sky, for example, from the central part of a galaxy. In the spectrographs which we use, a 2D array of microlenses provides a set of micropupils which are put onto the spectrograph, so after having reduced the full set of spectra corresponding to the individual spatial elements, we obtain a list of fluxes in continuum and in emission lines, of line-of-sight velocities, both for stars and ionized gas, and of absorption-line equivalent widths, which are usually expressed as indices in the well-formulated Lick system [17]. This list can be transformed into 2D maps of the above mentioned characteristics for the central part of a galaxy. Besides the panoramic view benefits, such an approach gives a unique opportunity to overlay various 2D distributions over each other without any difficulties with positioning. In this work we use data from two 2D spectrographs: the fibre-lens Multi-pupil Fiber Spectrograph (MPFS) at the 6 m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS) and the international Tiger-mode SAURON at the 4.2 m William Herschel Telescope at La Palma. SAURON is a private spectrograph, but its data are available after expiration to everybody, from the Isaac Newton Group Archive in the UK Astronomy Data centre. The last variant of the MPFS became operational in the prime focus of the 6 m telescope in 1998: (http://www.sao.ru/hq/lsfvo/devices/mpfs/mpfs_main.html) [18]. With respect to the previous variant, in the new MPFS the field of view is increased and the common spectral range is larger due to the use of fibres: they transmit light from 16 â 15 square elements of the galaxy image to the slit of the spectrograph together with an additional 16 fibres that transmit the sky background light taken a few arcminutes apart from the galaxy, so that separate sky exposures are not necessary now. The size of one spatial element was approximately 1 â 1 ; a CCD TK 1024 â 1024 detector was used before 2003. The reciprocal dispersion is 1.35 å per pixel, with a spectral resolution of 5 å rather stable over the field of view. To calibrate the wavelength scale, we expose separately a spectrum of the hollow cathode lamp filled with helium, neon, and argon; an internal accuracy of linearization was typically 0.25 å in the green and 0.1 å in the red, and additionally we checked the accuracy and for the absence of systematic velocity shift by measuring strong emission lines of the night sky [OI]5577 and [OI]6300. We obtain MPFS data in two spectral ranges, the green one, 4300­5600 å, and the red one, 5900­7200 å. The green spectra are used to calculate the Lick indices H , Mgb, Fe5270, and Fe5335, which are suitable to determine metallicity, age, and Mg/Fe ratio of old stellar populations [19]. To calibrate the new MPFS index system onto the standard Lick system, we have observed 15 stars from the list of [17] during four observational runs and calculated the linear regression formulae to transform our index measurements into the Lick system; the rms scatter of points near the linear dependencies are about 0.2 å for all four indices under consideration, thus being within the observational errors of [17]. To correct the index measurements for the stellar velocity dispersion which is usually substantially non-zero in the centres of early-type galaxies, we have smoothed the spectrum of the standard star, HD 97907, by a set of running Gaussians of various widths; the derived dependencies of index corrections on were approximated by polynomials of 4th order and applied to the measured index values before their calibrations into the Lick system. Due to surface brightness differences between the circumnuclear and outer part of the bulges, the accuracy of the measured indices falls from 0.1 å at the centre to more than 0.5 å at the edges of the multi-pupil frame. To support a constant level of


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index accuracy along the radii when analysing the stellar population properties, we co-add the individual spectra within circular rings centred onto a nucleus; the typical statistical error of the azimuthally averaged indices is 0.1 å. The green-range 2D spectroscopic observations are also used to cross-correlate galactic elementary spectra with a spectrum of a template star, usually of G8III­K3III spectral type, to obtain in such a way a line-of-sight velocity field for the stellar component and a map of stellar velocity dispersion. The cross-correlation peaks are fitted by Gaussians; the benefits of this approach as opposed to the more popular Fourier Correlation Quotient (FCQ) are thoroughly discussed by [20]. The red spectral range contains strong emission lines H and [NII]6583, so it is used to derive line-of-sight velocity fields for the ionized gas, mostly by calculating emission-line baricentre positions; in these particular galaxies the emission line [NII]6583 is the strongest, and we present only its measurements. The accuracy of elementary velocity measurements, both for stars and ionized gas, is about 10 km/s. The second 2D spectrograph which data we use in this work is a new instrument, SAURON, operated at the 4.2 m William Herschel Telescope (WHT) on La Palma ­ for its detailed description see [21]. We have taken the data for NGC 3608 from the open ING Archive of the UK Astronomy Data Centre. Briefly, the field of view of this instrument is 41 â 33 with the spatial element size of 0.94 â 0.94 . The sky background taken 2 arcminutes from the centre of the galaxy is exposed simultaneously with the target. The fixed spectral range is 4800­5400 å, the reciprocal dispersion is 1.11 å­1.21 å varying from the left to the right edge of the frame, and the spectral resolution is about of 4 å. The comparison spectrum is that of neon, and the linearization is made by a polynomial of the 2nd order with an accuracy of 0.07 å. The index system is checked by using stars from the list of [17] which have been observed during the same observational run. The regressions fitting the index system calibration of the February-1999 run when NGC 3608 has been observed are shown in an earlier paper [22]. The relations between instrumental and standard-system indices are very close to the y = x relation so no corrections are needed to calibrate them into the standard Lick system. The stellar velocity dispersion effect was corrected in the same manner as for the MPFS data. Whereas to prepare the azimuthally averaged index profiles from the MPFS data, we co-added spectra in the rings, to prepare the SAURON azimuthally-averaged index data which have higher signal-to-noise ratios, we averaged the measured individual-element indices over the same rings so attached error bars are the formal rms errors of the means ­ they are all below 0.03 å. The full list of the exposures made for NGC 3607 and NGC 3608 with the two 2D spectrographs is given in table 2. The seeing was estimated by 2D-Gauss fitting of continuum images of the template stars which were observed the same nights as the galaxies under consideration. To analyse the structure of the galaxies and to refine a kinematical analysis, for both galaxies we have retrieved the WFPC2/HST image data from the HST Archive. NGC 3607 was observed in the frame of the program of A. Phillips (ID 5999) on nuclei of S0s, and NGC 3608 was observed in the frame of the program of M. Franx on kinematically decoupled nuclei (ID 5454). For NGC 3607 we have also used the large-scale images taken from the database
Table 2. 2D spectroscopy of the galaxies studied. Seeing (FW H M ) 2 2 2 1 .3 .8 .8 .4

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Date 29 09 09 20 Apr Mar Mar Feb 01 02 02 99

NGC 3607 3607 3608 3608

Exposure 45 45 60 120 min min min min

Configuration MPFS+CCD 1 MPFS+CCD 1 MPFS+CCD 1 SAURON+CCD k k k 2 â 1k â 1k â 1k k â 4k 16 16 16 33

Field â â â â 15 15 15 41

Spectral range 4200­5600 5800­7200 5800­7200 4800­5400 å å å å


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V. L. Afanasiev and O. K. Silchenko Table 3. Photometric data on NGC 3607 and NGC 3608. Telescope WFPC2/HST WFPC2/HST WFPC2/HST WFPC2/HST WFPC2/HST 1.2-OHP 1.2-OHP 1.2-OHP 1.2-OHP Filter F555W F814W F547M F555W F814W B V R I Exposure 500 230 260 160 160 300 120 120 120 s s s s s s s s s Seeing 0 .1 0 .1 0 .1 0.1 0.1 6 .3 6.0 6.0 5.7 Scale 0 0 0 0 0 0 0 0 0 . . . . . . . . . 045 045 045 045 045 686 686 686 686

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Date 7May 1994 7May 1994 6Nov 1994 27Jan 1996 27Jan 1996 6Feb 1999 6Feb 1999 6Feb 1999 6Feb 1999

Galaxy NGC NGC NGC NGC NGC NGC NGC NGC NGC 3608 3608 3607 3607 3607 3607 3607 3607 3607

HYPERCAT/FITS Archive (PI Ph. Prugniel). The details of the photometric observations are given in table 3. All the data, spectral and photometric, except the data obtained with the MPFS, have been reduced with the software produced by Dr V.V. Vlasyuk in the Special Astrophysical Observatory [23]. Primary reduction of the data obtained with the MPFS was done within IDL with a software created by one of the authors (VLA). The Lick indices were calculated with our own FORTRAN program as well as by using the FORTRAN program of Dr A. Vazdekis which provides also the index error calculation.

3. Chemically decoupled cores in NGC 3607 and NGC 3608 3.1 NGC 3607 Figure 3 presents index maps for the central part of NGC 3607 constructed from the MPFS 2D spectral data. The Mgb map reveals a certain presence of a chemically decoupled core; this finding is not very unexpected because NGC 3607 was earlier listed by us as a good candidate for possessing a chemically distinct nucleus [24]. Interestingly, the Fe (Fe5270 + Fe5335)/2 and H absorption-line index maps look rather homogeneous and do not demonstrate any sharp features. Perhaps, one can note a very shallow unresolved Fe minimum near the photometric centre of the galaxy and similarly shallow extended H minimum aligned with the major axis of the isophotes; the latter feature may be associated to the circumnuclear ionized-gas disk mentioned in the Introduction and probably results from the emission contamination of the H absorption line. However, both Fe (Fe5270 + Fe5335)/2 and H absorption line index minima are marginal, and the Mgb peak is much more prominent than the other details. It seems to be resolved; moreover, it seems to be elongated along the minor axis of the isophotes. To clarify the orientation of the magnesium-enhanced core, we have smoothed strongly the Mgb map and have drawn isolines of the smoothed Mgb distribution (figure 3, top right). Indeed, the magnesium-enhanced core is elongated in PA 46 though the Mgb isolines do not look quite symmetric ovals. The orientation of the magnesium-enhanced area hints at some stellar substructure, which may be seen with its alignment along the minor axis of the isophotes, some kind of minibar. But in fact, the Mgb-index distribution morphology alone is not enough to draw conclusions about the nature of the magnesium-enhanced core: the visible elongation of the magnesium-enhanced core may be a result of a magnesium depression along the major axis, similar to that of the H index, which may be caused by a very young stellar disk aligned with the major axis. To draw definite conclusions, further analysis of the stellar population age in an area of the magnesium-enhanced core is needed.


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Figure 3. The MPFS index maps for NGC 3607; < Fe >(Fe5270+Fe5335)/2. The green (5000 å) continuum is overlaid by isophotes in three plots from four. At the right top the Mgb index distribution smoothed strongly with the 2D Gaussian of FW H M = 3.5 is plotted by isolines to show an orientation of the chemically decoupled structure; here the green continuum is gray-scaled.

To quantify peculiarities of the absorption-line index distributions in the centre of NGC 3607 seen in figure 3, we have calculated azimuthally averaged radial profiles of the indices and have compared them to the `zero-dimensional' and one-dimensional spectral data published in the literature [25], [26], and [27]) (figure 4). Though the long-slit data were taken along the major or minor axis of NGC 3607 and so were not obliged to agree exactly with our azimuthally averaged measurements, they confirm the overall shape of the index radial dependencies: a rather flat iron-line profile, an H profile with a shallow minimum in the centre, and a


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Figure 4. The MPFS azimuthally-averaged index profiles for NGC 3607 in comparison with the literature data. The minor-axis long-slit cross-section by [25] is plotted. The long-slit data taken along the major axis by [27] are also confronted to the major-axis index profiles simulated from our 2D index maps with the digital slit width of 2 . The central 4 -aperture measurements by [26] are plotted as connected squares at R = 0 - 2 .

magnesium-distinct core with the radius of 5 - 6 . A moderate, of 0.2 å­0.4 å, systematic discrepancy of our H and Mgb profiles with the data of [27] can also be noted. To check if these shifts may result from lack of azimuthal symmetry, we simulated one-dimensional major-axis cross-sections of our index maps and also plotted the simulated results in figure 4.


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Figure 5. `Index-index' < Fe > vs Mgb diagnostic diagram for the azimuthally averaged Lick indices in the centre of NGC 3607 taken along the radius with the step of 1 (open circles connected by a thin line, the nucleus being marked as `nuc'). The errors of the azimuthally averaged MPFS indices are about 0.1 å. The models for [Mg/Fe] = 0.0, +0.3, and +0.5 are plotted as a reference frame; the small signs connected by pointed, dashed, and point-dashed lines represent stellar population models of equal ages; the metallicities for the models are +0.67, +0.35, 0.0, -0.33, -1.35, and -2.25 if one takes the signs from top to bottom.

Indeed, the simulated major-axis Mgb profile has a much better agreement with the Fisher et al.'s data than the azimuthally averaged measurements. As for the H index profiles, the simulated major-axis profile and the azimuthally averaged one have coincided almost perfectly, implying an orientation of the ionized-gas disk far from edge-on, so we must conclude that the systematic shift of 0.2 å between our measurements and those of [25] and [27] exists. Meantime, the centred aperture measurement of [26], which may be treated as a Lick-system etalon, confirms our calibration. Due to progress in evolutionary population synthesis during recent years, we can now estimate mean stellar population characteristics by comparing different absorption-line indices with each other. Figure 5 presents an `iron-vs-magnesium' diagram where we compare our azimuthally averaged MPFS data for the central part of NGC 3607 with the models of [28], which are calculated for several values of magnesium-to-iron ratio. The loci of the models of various [Mg/Fe] are well separated on the diagram, so from inspecting figure 5 we can conclude that the magnesium-to-iron ratio in the centre of NGC 3607 is certainly above the solar one. But whereas in the circumnuclear region the [Mg/Fe] is between zero and +0.3, perhaps, closer to +0.2, the unresolved nucleus is outstanding with its [Mg/Fe] +0.4. We must note that, as figure 4 demonstrates, the previous Lick index measurements for the centre of NGC 3607 do not confirm this jump of [Mg/Fe] at R = 0 and imply rather constant ironindex behaviour along the radius. Since our estimates of the nuclear indices are confined to a single spatial element, we cannot insist that the low Fe5270 value in the unresolved nucleus is not a random artefact. So over the whole of the centre of NGC 3607 we assume the [Mg/Fe] ratio to be between +0.2 and +0.4.


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Figure 6. `Index-index' H vs [MgFe], [MgFe] (Mgb < Fe >)1/2 , age-diagnostic diagram for the azimuthally averaged Lick indices in the centre of NGC 3607 taken along the radius with the step of 1 (open circles connected by a dot-dashed line, the nucleus being marked by `nuc'). The errors of the azimuthally averaged MPFS indices are about 0.1 å. The models for [Mg/Fe] = 0.0 and +0.3 are plotted as a reference frame; the small signs connected by solid and dashed lines represent stellar population models of equal ages; the metallicities for the models are +0.67, +0.35, 0.0, -0.33, -1.35, and -2.25, if one takes the signs from the right to the left.

Several investigators, e.g. [29] or [30], noted that in order to overcome an effect of the nonsolar magnesium-to-iron ratio when determining a mean age of stellar population, one must use a combined index [MgFe] (Mgb Fe )1/2 . So in figure 6 we confront [MgFe] to H ?? and compare our azimuthally averaged data for NGC 3607 with two sets of the models ­ for [Mg/Fe] = 0 and for [Mg/Fe] = +0.3. As promised by our choice of the metal-line index, the age estimates are robust to the varying choice of the model [Mg/Fe]; they are equal to about 10­12 Gyr. The nucleus is again outstanding, probably, due to the Balmer emission concentration in the very centre of the galaxy and obvious H emission contamination; but beyond the nucleus the age gradient along the radius is non-detectable: at the diagram `H , [MgFe]' the index variations occur along the model sequence of equal age, namely, of T = 10 ± 2 Gyr. We may suspect that the real mean age of the stellar population in the centre of NGC 3607 is lower than 12 Gyr, because the ionized-gas disk is known to extend up to R 15 ([10], [11]) and therefore the Balmer emission contaminates the absorption-line H index to some degree all the way. In the nucleus this effect must be especially strong, and we would try to correct it there by using measurements of the H -emission equivalent width by [31]. To correct the measured Lick index H for the emission H we use the well-known fact that an emission line H is always much stronger than H and an absorption line H is always weaker than H , so the equivalent width of the emission line H can be measured more precisely than that of H . The authors of [31] subtracted a pure-absorption template from the observed nuclear spectrum and obtained EW (H emis) = 2.03 å for NGC 3607; note here that in the nucleus of NGC 3608, another target galaxy, they saw only a marginal H emission, less that EW = 0.4 å. The well-established and minimum possible intensity ratio H/H is known for the case of the gas radiative excitation by OB-stars (`HII-region'-type excitation),


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2.5­2.7, and it is much larger for shock excitation. The authors of [31] classified the nuclear emission spectrum of NGC 3607 as a LINER. In [32] a large sample of integrated spectra of galaxies of various morphological types has been analysed, and it has been found that a good correlation EW (H emis) = 0.25EW (H emis) exists; just the relation we use to calculate EW (H emis), which is in fact the correction of the Lick index H for the emission. After it has been corrected for the emission contamination of the H index, the nucleus of NGC 3607 has settled to the age sequence of 12 Gyr in the figure 6, and any age difference between the nucleus and its outskirts has disappeared. We would like to stress that any significant age break is also absent at the border between the magnesium-enhanced core of NGC 3607 and its `bulge', at R = 5 - 6 . 3.2 NGC 3608 Figure 7 presents the index maps for NGC 3608, similar to those presented for NGC 3607 in figure 3, but calculated from the SAURON data, so instead of the combined iron index Fe (Fe5270 + Fe