The Globular Clusters system of M31

1. The Globular Clusters system of M31 Comparison with the Milky Way / A bridge to extragalactic GC systems La nostra galassia gemella: entro un fattore 2-3 in massa Il sistema di GC in una galassia gigante meglio studiato, dopo la MW La sola galassia per la quale HST ci permette di risolvere i GC in stelle come nella MW ma da terra li vediamo ~puntiformi come nelle galassie lontane 4) Differenze e similarità: funzione di luminosità, distribuzione di metallicità, cinematica in funzione della metallicità, ammassi esterni 5) Come ti studio un sistema di ammassi extragalattico: contaminatori, metodi di identificazione e conferma, colori integrati, spettri integrati.

2. Studying an extragalactic GC system:methods and problems Identify candidates: by shape (nearly stellar but extended; round; etc.), by color … 2. Verify the nature against possible contaminants: spectra (radial velocity, spectral type …) and, ultimately, resolve into stars (feasible only within the LG, with current instrumentation). 3. Colors and spectral indices to infer ages and metallicities

3. Spectroscopic Surveys of Candidate M31 GCs How to tell a genuine cluster from a contaminant? • Cosmologic recession velocity ==> background galaxy • Vr around 0 km/s foreground Galactic star • Right Vr but EL ==> HII region • If Vr<-150 km/s ==> cluster • If -150<Vr<200 km/s and • extended ==> cluster • ● Resolved into stars ==> cluster

4. Are they similar to their MW counterparts? CMDs with HST RR Lyr light curves in B514 from HST Contreras et al. 2008 M31 GCs show the same variety of HB morphologies as their MW counterparts. Only an external corona is currently resolved into stars with HST: the typical CMD reaches just below the HB Rich et al. 2005 The only case in which the TO level has been reached: 100 HST orbits Brown et al. 2004

5. ECs GCs Endemic Clusters Species Extended Clusters: old & metal poor with 5×rh Bright clusters at very large distances from the Galactic center: Only one counterpart in the MW (NGC2419)‏

6. Endemic Clusters Species Massive Young Clusters They lie in the disc and have disc kinematics Age < 1 Gyr Mass > 104 M⊙ No counterpart in the MW but… A handful (12) of clusters with Red Super Giants are being discovered in the IR in the MW disc (Messineo et al. 2009). All of them have age< 20 Myr, while all MYC in our M31 sample have age > 25 Myr and most of them have age > 50 Myr

7. Fusi Pecci et al. 2005 How many? Luminosity Function • A factor ≃ 3 more clusters, at least • Note: the peaks of the distributions coincide, this is the basis for the use of GC LFs as distance indicators • The M31 sample may be seriously incomplete for MV>-5 Old clusters have the same color Distribution. Young massive clusters seems relatively frequent in M31 and are missing in the MW but this may be (partly) due to selection effects as we are immersed in the MW Disc, while we see the Disc of M31 from “above”

8. Spectra & Spectral indices The sensitivity to age and metallicity is not better than that of integrated colors BUT They are reddening free You can take advantage of the different sensitivity of different indices to disentangle age and metallicity 1, 4, 13 Gyr @ Z⊙ [Fe/H]= -1.7, -0.4, +0.4 @ 10 Gyr F I X E D M E T A L L I C I T Y F I X E D A G E

9. Hβ Δ Δ Hβ 8 2 15 Spectra & Spectral indices An example: At fixed amplitude of the Balmer jump Δ Differences in Hβ traces differences in ages, for ages > 1Gyr Large values of Hβ (> 3.5 Å) always indicate young ages

10. Hβ Δ Δ Hβ Spectra & Spectral indices Another similar example: Hβ vs Mg

11. Spectra & Spectral indices Another example: A metallicity scale for old GCs based on Lick indices Brodie & Huchra 1990 Galleti et al. 2009

12. Spectra & Spectral indices Compare metallicity from Lick indices vs. from CMDs: we can do this only in M31

13. Are they similar to their MW counterparts? Yes Fundamental Plane Relations: Djorgovski et al. 1997 × = MW clusters ; ⦁ = M31 clusters No Metallicity Distribution: Galleti et al. 2009

14. Rotation as a function of metal content MW custers Zinn 1985 Only clusters with [Fe/H]>-0.8 Rotate The net rotation of the metal-poor clusters is consistent with Zero M31 clusters: the ratio of dispersion to rotational velocity increases with decreasing metallicity But the rotation pattern is present at any metallicity.

15. Rotation as a function of metal content M31 clusters: if one limits to clusters near the major axis the rotation is seen much clearly; projection smears out the signal

16. The most widely used catalogue of M31 GCs has been assembled and is manteined at the Bologna observatory:RBC Lists more than two-thousands clusters, candidate clusters, previously believed candidates that now have a different classifications Includes positions, optical, NIR and UV colors, velocities, spectral indices, metallicities… 87 citations since 2004 [self-citations excluded] ~600 web contacts per year from all over the world