Il Sistema degli Ammassi Globulari nella galassia di Andromeda

Flavio Fusi PecciINAF Osservatorio Astronomico

Bologna

M 31 Galassia di Andromeda (spirale)

Il Gruppo Locale

(Eva Grebel)

La galassia nana del Sagittario

La galassia ellittica nana del Sagittario orbita attorno alla Via lattea ogni milione di anni. In un lontano futuro verrà “mangiata” dalla nostra galassia unendosi ad essa:

Alcune caratteristiche principali:Costellazione: SagittarioDistanza: 88 000 a.l.Magnitudine visuale: 4,5Diametro: 10 000 a.lTipo: dE

Martinez-Delgado et al 2010 AJ. 140, 962

AW McConnachie et al. Nature 461, 66-69 (2009)

Figure 1 from PAndAS' Progeny:

Richardson et al. 2011 ApJ 732 76

Figure 3 Evidence for an Accretion Origin for the Outer Halo Globular Cluster System of M31

A. D. Mackey et al. 2010 ApJ 717 L11

Different stellar populations in different structures: metallicity tagging

M31 and the Milky Way are running towards each other at ~100km/sand they will finally collide in ~4 billion years

(It is likely the sun will be flung into a new region of our galaxy, but our Earth and solar system are in no danger of being destroyed!!!)

GALAXY FORMATION MECHANISM

• SINGLE, LARGE SCALE COLLAPSE

! < 109 yrs ~ 2 – 5 x 108 yrs

Eggen, Lynden Bell, Sandage 1962

• RANDOM MERGERS OF FRAGMENTED GAS CLOUDS

! ~ 2 – 4 x 109 yrs (… 1010 yrs?)

Searle, Zinn 1978

• MULTI-PHASE IN SITU FORMATION

Forbes, Bropie, Grillmair 1997• “CONTINUOUS” MERGERS

Zepf, Ashman 1993• TIDAL STRIPPING + CAPTURE

Cotè, Marzre, West 1992

GCs useful tools to study f.i.:

• Stellar evolution • Stellar dynamics• Distance indicators• Peculiar objects (variables, binaries, blue

stragglers, millesecond pulsars, etc.)• Templates of stellar populations• Parent galaxy formation, evolution, masses, etc. • Constraints on cosmic age, helium abundance, etc.

• etc. etc.

Come “appaiono” i GC’sman mano che si allontanano ?

• Diametro apparente e risoluzione in stelle sempre minore…

• Magnitudine integrata apparente sempre più debole….• Colori integrati diversi …. Metallicità ed età diverse..• Velocità radiale … compatibile con galassia parente..

• Galassia: (m-M) = 10-18 mag diam =50’-10’• Nubi di Mag: (m-M) = 18-20 mag diam =10’-2’• Andromeda: (m-M) = 24-25mag diam =20’’ - 2’’• Bordi Gruppo Locale: (m-M) = 27 mag diam=2’’-0.5’’• A (m-M) > 28…. 30… sono “uguali” alle stelle, hanno la

stessa “identica” Point Spread Function… e “… allora sono guai !” … come dice Edoardo Bennato

1976

INIZIO DELLA STORIA …

STUDIO DEL SISTEMA DI AMMASSI GLOBULARI IN M31

… A LOIANO

Telescopio Cassini 152 cm Loiano

• Circa 150 lastre

• 1- 2 ore di posa

• Seeing < 1",5 spesso ! 1"

• V > 20

25 x 25 cm

72'

scala 17"/mm

in totale un centinaio di lastre a grande campo BUONE (e centinaia di frames CCD post-1983)

oltre 350 GC selezionati da un campione di oltre 1000 candidati,

in un campo di 3° x 3° (~30 kpc)

oltre 30 kpc i GC previsti sono poche unità, ma si devono ispezionare molte migliaia di immagini (~1500/sqdeg)!

astrometria (<rms> ⇒ " ± 0,5" # ± 0,3")

1. Ricerca e selezione

2. Fotometria multicolore (FUV + UBVRIJHK)3. Morfologia

4. Spettroscopia

5. Identificazioni sorgenti X

6. Osservazioni HST

• 152 cm Loiano

• 2m Schmidt Tautenburg

• Palomar Schmidt

• 4m Kitt Peak

• Schmidt Campo Imperatore

• TNG

• LBT

• EINSTEIN

Fotometria di stelle singole nel GC’s

•…. dal 1993 in avanti HST

•… UV con GALEX

G1- ammasso globulare nella Galassia di Andromeda

Telescopio HST

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…

~600 web contacts per year from all over the world

Studying an extragalactic GC system:methods and problems

Identify candidates: by shape (nearly stellar but extended; round; etc.), by color …

Verify the nature against possible contaminants: spectra (radial velocity, spectral type …) and, ultimately, resolve into stars (feasible only within the LG, with current instrumentation).

Colors and spectral indices to infer ages and metallicities

Fusi Pecci et al. 2005

Spectroscopic Surveys of Candidate M31 GCs

How to discriminate a genuine cluster from a contaminant?

Cosmologic recession velocity ==> background galaxy

Vr around 0 km/s foreground Galactic star

Right Vr, emission lines ==> HII region

If Vr <-150 km/s ==> cluster

If -150<Vr<200 km/s and extended ==> cluster

Resolved into stars ==> cluster

Are they similar to their MW counterparts? CMDs with HST

Only an external corona is currently resolved into stars with HST: the typical CMD reaches just below the HBRich et al. 2005

M31 GCs show the same variety of HB morphologies as their MW counterparts.

The only case in which the TO level has been reached: 100 HST orbitsBrown et al. 2004

RR Lyr light curves in B514 from HST

Contreras et al. 2008

Endemic Clusters Species

Bright clusters at very large distances from theGalactic center:Only one counterpart in the MW (NGC2419)!

ECs

GCs

Extended Clusters: old & metal poor with 5"rh

Endemic Clusters Species

Massive YoungClusters

They lie in thedisc and have disc kinematics

Age < 1 Gyr

Mass > 104 M⊙

No counterpartin 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 mostof them have age > 50 Myr

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 weare immersed in the MW Disc, while we see the Disc of M31 from “above”

Fusi Pecci et al. 2005

Spectra & Spectral indices

1, 4, 13 Gyr @ Z⊙ [Fe/H]= -1.7, -0.4, +0.4 @ 10 Gyr

FIXED

METALLICITY

FIXED

AGE

The sensitivity to age and metallicity is not better than that of integrated colors

BUT 1.They are reddening free2.One can take advantage of the different sensitivity of different indices to disentangle age and metallicity

Spectra & Spectral indices

H!

H!

"

"

An example:At fixed amplitude of the Balmer jump #Differences in H$ traces differences in

ages, for ages > 1Gyr

Large values of H$ (> 3.5 Å) always indicate young ages

Fusi Pecci et al. 2005

15

8 2

Spectra & Spectral indices

Hβ

Hβ

Δ

Δ

Another similar example:Hβ vs Mg

Spectra & Spectral indices

Another example:A metallicity scale for old GCs based on

Lick indices

Galleti et al. 2009

Brodie & Huchra 1990

Perina et al. 2009, A&A 507, 1375

Spectra & Spectral indices

Compare metallicity from Lick indices vs. from CMDs:

we can do this only in M31Galleti et al. 2009

Are they similar to their MW counterparts?

Metallicity Distribution:Galleti et al. 2009

Yes

No

Rotation as a function of metal content

MW GC’sZinn 1985

Only clusters with [Fe/H]>-0.8Rotate

The net rotation of the metal-poor clusters is consistent with Zero

M31 GC’s: the ratio of dispersion to rotational velocity increases with decreasing metallicityBut the rotation pattern is present at any metallicity.

Galleti et al. 2009

Rotation as a function of metal content

The horizontal branch luminosity vs. metallicity of M 31 globular clusters

A&A 544, A155 (2012) L. Federici - C. Cacciari - M. Bellazzini - F. Fusi

Pecci – S. Galleti - S. Perina

The horizontal branch morphology of M 31 globular clusters.

Extreme second parameter effect in outer halo clusters

A&A 546, A31 (2012)

S. Perina - M. Bellazzini – A. Buzzoni - C. Cacciari

45

Not all GCs were created simple…from the “classical” ω Cen …

Norris et al. 1996

Bedin et al. 2004Ferraro et al. 2004

Lee et al. 1999

B-V

V

46

Not all GCs were created simple…to M54, NGC 2808, M22, etc …

NGC2808: Piotto et al. 2005

M54: Siegel et al. 2007M22: courtesy A.Milone

STRIKING examples in MW GCs

Rich et al. 1997

NGC 6388

ZYt

Major contributors to …

FUV

NUV

NUV

Conclusion• As shown for NGC 6441 & 6388, there is a quite

wide group of metal-rich GCs in M31 which share the UV-excess + BHB anomaly

• Most of these objects are among the brightest metal-rich GCs in M31, brighter (i.e. more massive) than ω Cen in the MW

• Very Bright (i.e. very massive) metal-rich GC’s in M31 (probably) contain multi-populations (possibly) He-enriched at various levels (even quite high) as found in ω Cen in the MW

Helium-rich Globular clusters?Helium-rich Globular clusters?

Rey et al (2007) (adapted)

X

M87

M31

MW

ciao

Wilkinson & Evans (1999) used motions of 27 globularclusters and satellites : made a dynamical model of thehalo and estimated the total mass to be

Mtotal = 1.9 +3.6 - 1.7 x 1012 M

La massa della Via Lattea ....

Evans & Wilkinson (2000) used satellites and GCs in M31 to derive a lower limit on the mass of

1.2+3.6 -1.7 x 1012 M

for M31 - similar to the Galaxy, within the uncertainties

La massa di M31

For comparison, from least action arguments, the likely mass of the local group is 4-8 x 1012 M

( Peebles 1996, Schmoldt & Saha 1998)

Within the uncertainties, most of the mass in the Local Groupcould be in the two large spirals

La massa totale di MW + M31 ~ 3 x 1012 M