Astronomical Science Stellar Populations of Bulges of Disc Galaxies in Clusters Lorenzo Morelli1, 5 brightness radial profile of large bulges is assembly. We present a photometric and Emanuela Pompei 2 well described by the de Vaucouleurs spectroscopic study of the bulge-domi- Alessandro Pizzella1 law, although this law can be drastically nated region of a sample of spiral galax- Jairo Méndez-Abreu1, 3 changed taking into account the small- ies in clusters. Our aim is to estimate the Enrico Maria Corsini1 scale inner structures, smoothed by the age and metallicity of the stellar popula- Lodovico Coccato 4 seeing in ground-based observations. tion and the efficiency and timescale of Roberto Saglia 4 Some bulges are rotationally-flattened the last episode of star formation in order Marc Sarzi 6 oblate spheroids with little or no anisot- to disentangle early rapid assembly from Francesco Bertola1 ropy. But, the intrinsic shape of a large late slow growth of bulges. fraction of early-type bulges is triaxial, as shown by the isophotal misalignment 1 Dipartimento di Astronomia, Università with respect to their host discs and non- Sample, photometry, and spectroscopy di Padova, Italy circular gas motions. The bulk of their 2 ESO stellar population formed between red- In order to simplify the interpretation of 3 INAF-Osservatorio Astronomico di shifts 3 and 5 (~ 12 Gyr ago) over a short the results, we selected a sample of disc Padova, Italy timescale. The enrichment of the inter- galaxies, which do not show any mor- 4 Max-Planck-Institut für Extraterres- stellar medium is strongly related to the phological signature of having undergone trische Physik, Garching, Germany time delay between type II and type Ia a recent interaction event. All the ob- 5 Department of Astronomy, Pontificia supernovae, which contributed most of served galaxies are classified as non- Universidad Católica de Chile, Santiago, the A elements and iron, respectively. barred or weakly barred galaxies. They Chile are bright (BT ≤ 13.5) and nearby (D < 6 Centre for Astrophysics Research, On the contrary, the bulges of late-type 50 Mpc) lenticulars and spirals with a University of Hertfordshire, Hatfield, spiral galaxies are reminiscent of discs. low-to-intermediate inclination (i ≤ 65˚ ). United Kingdom They are flat components with exponen- Twelve of them were identified as mem- tial surface brightness radial profiles bers of the Fornax, Eridanus and and rotate as fast as discs. Moreover, the Pegasus clusters and a further two are Photometry and long-slit spectroscopy stellar population in late-type bulges members of the NGC 7582 group. are presented for 14 S0 and spiral is younger than in early-type bulges. They galaxies of the Fornax, Eridanus and appear to have lower metallicity and The photometric and spectroscopic ob- Pegasus clusters and the NGC 7582 lower A /Fe enhancement with respect to servations of the sample galaxies were group. The age, metallicity and A /Fe early-type galaxies. carried out in three runs at ESO La Silla enhancement of the stellar population in in 2002 (run 1), 2003 (run 2) and 2005 the centres and their gradients are In the current paradigm, early-type (run 3). We imaged the galaxies with obtained using stellar population mod- bulges were formed by rapid collapse the Bessel R-band filter. In runs 1 and 2 els with variable element abundance and merging events, while late-type spectra were taken at the 3.6-m tele- ratios. Most of the sample bulges dis- bulges have been slowly assembled by scope with EFOSC2; in run 3 spectra play solar A /Fe enhancement, no gra- internal and environmental secular were obtained with EMMI on the NTT in dient in age, and a negative gradient of processes (Kormendy & Kennicutt, 2004). red medium-dispersion mode. metallicity. One of the bulges, that of But many questions are still open. For NGC 1292, is a pseudobulge and the instance, the monolithic collapse scenario In order to derive the photometric param- properties of its stellar population are cannot explain the presence in bulges of eters of the bulge and disc, we fitted iter- consistent with a slow build-up within a kinematically-decoupled components. atively a model of the surface brightness scenario of secular evolution. Moreover, the environment plays a role in to the pixels of the galaxy image using a defining the properties of galaxies. Re- non-linear least-squares minimisation. We cent studies of early-type galaxies in dif- adopted the technique for photometric The relative importance of dissipative ferent environments have shown that age, decomposition developed in GASP2D by collapse (Gilmore & Wyse, 1998), major metallicity, and A /Fe enhancement are Méndez-Abreu et al. (2008, see Figure 1). and minor merging events (Aguerri, more correlated with the total mass of the We measured the stellar kinematics from Balcells & Peletier, 2001), and redistribu- galaxy than local environment. the galaxy absorption features present tion of disc material due to the pres- in the wavelength range and centred on ence of a bar or environmental effects To investigate the formation and evolu- the Mg line triplet at 5 200 Å by applying (Kormendy & Kennicutt, 2004) drives tion of bulges, there are two possible the Fourier correlation quotient method the variety of observed properties in approaches: going back in redshift and (Bender et al., 1994). We also measured bulges. The bulges of lenticulars and looking at the evolution of galaxies the Mg, Fe, and HB line-strength indices early-type spirals are similar to low-lumi- through cosmic time; or analysing nearby from the flux-calibrated spectra. We indi- nosity elliptical galaxies and their photo- galaxies in detail to understand the cate the average iron index with <Fe> = metric and kinematic properties satisfy properties of their stellar population in (Fe5270 + Fe5335)/2, and the magne- the same fundamental plane (FP) cor- terms of the dominant mechanism at sium-iron index with [MgFe] = Mgb (0.72 relation found for ellipticals. The surface- the epochs of star formation and mass × Fe5270 + 0.28 × Fe5335) (see Fig- The Messenger 133 – September 2008 31 Astronomical Science Morelli L. et al., Stellar Populations of Bulges of Disc Galaxies in Clusters NGC 1351 Model Residuals 26.0 26.0 0.4 10 0 10 0 10 0 ) ) ) 2 2 2 c c c e e e ) 50 ) 50 ) 50 s s s c c c c c c e e e r r r s s s a a a c c c / / / r r r 20.5 20.5 0.0 g g g a a a ( ( ( 0 0 a 0 a a Y m Y m Y m ( ( ( R R R µ µ –50 –50 –50 µ 15.0 15.0 –0.4 –100 –500 50 –100 –500 50 –100 –500 50 X (arcsec) X (arcsec) X (arcsec) 14 1.0 150 16 0.8 10 0 ) 2 c e 18 0.6 s c r a A / 50 P g a m ( 20 0.4 R µ 0 22 0.2 0.4 0.4 0.4 0.2 0.2 A 0.2 P R / / µ 0.0 0.0 A 0.0 ∆ ∆ P –0.2 –0.2 ∆ –0.2 –0.4 –0.4 –0.4 0 20 40 60 80 10 0 0 20 40 60 80 10 0 0 20 40 60 80 10 0 R (arcsec) R (arcsec) R (arcsec) Figure 1 (above). Two-dimensional photometric de- NGC 1351 composition of a sample galaxy, NGC 1351. Upper r (kpc) panels (from left to right): Map of the observed, 00.5 1 1.5 2 modelled and residual (observed-modelled) surface- 4 brightness distribution of the galaxy. Lower panels ) 3 Å ( (from left to right): Ellipse-averaged radial profile 2 of surface-brightness, position angle, and ellipticity H 1 measured in the observed (dots with error-bars) 0 6 and modelled image (solid line). Residuals on the ob- ) Å 5 ( served model are shown in the bottom plots. ] 4 e F g 3 M [ 2 5 ure 2). The HB line-strength index was ) 4 Å ( measured from the resulting HB absorp- 3 > e tion line, after the emission line was F 2 < 1 subtracted from the observed spectrum. 6 ) Å ( 5 b g 4 Age, metallicity, and A /Fe enhancement: M 3 Figure 2. The line-strength indices central values 2 0.3 measured along the major axes of one ) g of the sample galaxies, NGC 1351. a 0.25 m ( From top to bottom: East-west folded From the central line-strength indices we 2 0.2 g radial profiles of HB , [MgFe] , <Fe>, derived the mean ages, total metallici- M 0.15 Mg , and Mg . Asterisks and dots refer ties and total /Fe enhancements of the 0.1 b 2 A 02468101214161820 22 24 to the two sides (east/west) of the gal- stellar populations in the centre of the r (arcsec) axy. 32 The Messenger 133 – September 2008 4.0 [/Fe] = 0.0 dex T < 0 4.0 = [/Fe] = 0.5 dex T > 0 3.0 1 Gyr 3.0 (Z/H) = 0.35 ) ) Å ( Å ( > ` ( 2 Gyr e F H Z (Z/H) = 0.00 / < 2.0 H 0 2.0 ) 3 = . 0 – 3 Gyr – 1 = 0 (Z/H) = –0.33 .3 ] 0 e . 5 (Z 5 Gyr F 0 0 / / = .3 H ( [ ] 0 0 ) Z e .5 = / F = 0 H (Z 10 Gyr / ] – ) [ e = 0 = /H F ] . 1.0 / e 1.0 3 0 ) ( 15 Gyr F . = Z [ / 3 0 / 0 0 H [ T < 0 . Age = 3 Gyr = 3 ) 5 = (Z/H) = –1.35 T > 0 0 Age = 12 Gyr .6 (Z/H) = –2.25 7 1.02.0 3.04.0 5.0 1.0 2.03.0 4.0 5.0 6.0 [MgFe] (Å) Mgb (Å) Figure 3 (above).