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
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
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R R R µ µ –50 –50 –50 µ
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16 0.8 10 0 ) 2 c
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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 < subtracted from the observed spectrum. 1 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] ,
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). The distribution of the central val- ues of HB and [MgFe] indices (left panel) and
0 0 0 Although the correlations are not statisti- 2 4 6 8 10 –0.6 –0.4 –0.2 0.0 0.2 0.4 –0.4 –0.2 0.0 0.20.4 cally very strong, the elliptical and S0 Age (Gyr) [Z/H] (dex) [/Fe] (dex) galaxies (T < 0, where T is the numerical Figure 4. Distribution of age (left panel), metallicity RC3 galaxy type) have bulges older and (central panel) and A /Fe enhancement (right panel) more metal-rich than the spirals (T > 0) for the central regions of the sample galaxies. The in the central region. Most of the sample solid line in the right panel represents a Gaussian bulges display solar A /Fe enhancements centred at the median value [A /Fe] = 0.07 dex of the distribution. Its width, S = 0.11, is approximated by with the median of the distribution at the value containing 68 per cent of the objects of the [A /Fe] = 0.07 dex (Figure 4, right panel). distribution.
The Messenger 133 – September 2008 33 Astronomical Science Morelli L. et al., Stellar Populations of Bulges of Disc Galaxies in Clusters
Age is mildly correlated with velocity dis- 7 7 7 persion, and A /Fe enhancement. We conclude that the more massive bulges of our sample galaxies are older, more metal-rich and characterised by rapid 6 6 6 star formation. Med = 0.4 Med = –0.15 Med = 0.00 = 1.1 = 0.15 = 0.09 5 5 5 Age, metallicity and A /Fe enhancement: gradients
Different formation scenarios predict dif- 4 4 4 ferent radial trends of age, metallicity, and
A /Fe enhancement. Therefore the radial N N N gradients of the properties of the stellar populations of bulges are a key piece of 3 3 3 information to understand the processes of their formation and evolution. In the monolithic collapse scenario, gas dissipa- tion towards the galaxy centre, with sub- 2 2 2 sequent occurrence of star formation and galactic winds, produce a steep metallic- ity gradient. A strong gradient in /Fe A 1 1 1 enhancement is expected too. The pre- dictions for bulges forming through long time-scale processes, such as dissipa- tionless secular evolution, are more con- 0 0 0 tradictory. In the latter scenario the –2 024 6 8 10 12 –0.4 –0.2 0.0 0.20.4 –0.4 –0.2 0.0 0.20.4 bulge is formed by redistribution of disc ∆Age (Gyr) ∆[Z/H] (dex) ∆[/Fe] (dex) stars. The gradients possibly present in the progenitor disc could be either ampli- Figure 5. Distribution of the gradients of age (left of the distribution. The width of the distributions, S , fied, since the resulting bulge has a panel), metallicity (central panel) and A /Fe enhance- are approximated by the value containing 68 per ment (right panel) within radius rbd at which bulge cent of the objects and are also listed. The green smaller scalelength than the progenitor, and disc give equal brightness contributions for the and blue arrows show the average gradient found or erased as a consequence of disc heat- sample galaxies. Dashed line represents the median for early-type galaxies and bulges by Mehlert et ing ( Moorthy & Holtzman, 2006). of the distribution and its value is listed. Solid line al. (2003) and Jablonka et al. (2007), respectively. represents a Gaussian centred at the median value An issue in measuring the gradients of age, metallicity, and [A /Fe] in bulges of age, metallicity, and A /Fe enhance- and gradient of A /Fe enhancement, while could be the contamination of their stellar ment following Thomas et al. (2003) and the central value and gradient of metallic- population by the light coming from are shown in Figure 5. ity are correlated. All these hints suggest the underlying disc stellar component. that a pure dissipative collapse is not able This effect is negligible in the galaxy cen- Most of the sample galaxies show no to explain formation of bulges and that tre but it could increase going to the gradient in age with the median of the other phenomena like mergers or acquisi- outer regions of the bulge, where the light distribution at 0.4 Gyr. Negative gradients tion events need to be invoked. starts to be dominated by the disc com- of metallicity were observed and the ponent. In order to reduce the impact of number distribution shows a clear peak disc contamination and extend as much at [Z/H] = −0.15 dex. The presence of Pseudo-bulges as possible the region in which gradients a negative gradient in the metallicity radial were derived, we mapped them inside profile favours a scenario with bulge for- Classical bulges are similar to low-lumi- rbd, the radius where the bulge and disc mation via dissipative collapse. Dissipa- nosity ellipticals and are thought to be give the same contribution to the total tive collapse implies strong inside-out formed by mergers and rapid collapse. surface brightness. For each galaxy, we formation that should give rise to a nega- Pseudo-bulges are disc or bar compo- derived the Mg2, HB , and
34 The Messenger 133 – September 2008 3.5 –22 n < 2 n < 2
n > 2 2 n > 2 9
3.0 2 1 NGC 7515 IC 5267 C 5 G 2 N 4 –20 4 NGC 1351 1
2.5 4
- NGC 7557 C
4 NGC 7631 G 8 5 N NGC 1425 ) O IC 5309 NGC 1366 g 0 2.0 S 0 a E NGC 7531 / 5 1 ESO 358-50 - x m 5 ( a –18 8
3 7 m 5 5 R 1 6 1 V NGC 7643
3 2 5
M 1.5 1 C 5 7 3 O
7 G 5 S C C 5 I 7 N E
5 G 9 6 7 C 0 N
ESO 548-44 6 G 3 3 1.0 C IC 1993 1 1 5 N
NGC 1292 G 3 –16 C 6 C N I 3 7 G
4 3 N 6 C 9 7
0.5 G 9 1 C N
G C I N 0.0 –14 0.0 0.10.2 0.30.4 0.5 0.6 0.7 1.421.61.8 .0 2.22.4 log
Figure 6. The location (left panel) of the sample only by rotation. The location (right panel) of the bulges in the (Vmax/S 0, E ) plane. Filled and open sample bulges with respect to the Faber-Jackson circles correspond to bulges with Sèrsic index n ≤ 2 relation by Forbes & Ponman (1999, blue dashed and n > 2, respectively. The continuous line corre- line). Filled and open circles correspond to bulges sponds to oblate-spheroidal systems that have iso- with Sèrsic index n ≤ 2 and n > 2, respectively and tropic velocity dispersions and that are flattened the linear fit is shown (red continuous line).
Kormendy & Kennicutt (2004). The more or equivalently a higher luminosity, with Jablonka, P., Gorgas, J. & Goudfrooij, P. 2007, A&A, characteristics applied, the safer the respect to their early-type counterparts 474, 763 Kormendy, J. & Kennicutt, R. C. 2004, ARA&A, 42, classification becomes. Pseudo-bulges (Figure 6, right panel). According to the 603 are expected to be more rotation-domi- prescriptions by Kormendy & Kennicutt Mehlert, D., et al. 2003, A&A, 407, 423 nated than classical bulges, which are (2004), the bulge of NGC 1292 is the most Méndez-Abreu, J., et al. 2008, A&A, 478, 353 more rotation-dominated than giant ellip- reliable pseudo-bulge in our sample. In- Moorthy, B. K. & Holtzman, J. A. 2006, MNRAS, 371, 583 tical galaxies. We measured the maxi- formation about its stellar population Thomas, D., Maraston, C. & Bender, R. 2003, mum rotation velocity Vmax within rbd from gives more constraints on its nature and MNRAS, 339, 897 the stellar velocity curve and the cen- formation process. In fact, the NGC 1292 tral velocity dispersion S 0 from the veloc- bulge population has an intermediate ity dispersion profile. For each galaxy we age and low metal content. The A /Fe derived the ratio Vmax/S 0. In Figure 6 (left enhancement is the lowest in our sample panel) we compare it to the value pre- suggesting a prolonged star-formation dicted for an oblate spheroid with an iso- history. The presence of emission lines in tropic velocity dispersion and the same the spectrum shows that star formation observed ellipticity (Binney & Tremaine, is still ongoing. These properties are con- 1987). sistent with a slow build-up of the bulge of NGC 1292 within a scenario of secular Another defining characteristic of pseudo evolution. bulges are their position on the Faber- Jackson relation. The pseudo-bulges fall above the Faber-Jackson correlation References between the luminosity of the elliptical Aguerri, J. A. L., Balcells, M. & Peletier, R. F. 2001, galaxies and early-type bulges and their A&A, 367, 428 central velocity dispersion (Kormendy & Bender, R., Saglia, R. P. & Gerhard, O. E. 1994, Kennicutt, 2004). Sample bulges, ex- MNRAS, 269, 785 cept for ESO 358-50 and NGC 1292, Binney, J. & Tremaine, S. 1987, Galactic Dynamics, (Princeton, New Jersey: Princeton University are consistent with the R-band Faber- Press), 747 Jackson relation we built from Forbes & Forbes, D. A. & Ponman, T. J. 1999, MNRAS, 309, Ponman (1999, L t S 3.92). They are char- 623 acterised by a lower velocity dispersion, Gilmore, G. & Wyse, R. F. G. 1998, AJ, 116, 748
The Messenger 133 – September 2008 35