P1254413668YZGMX

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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.

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Studying an extragalactic GC systemmethods 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

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Spectroscopic Surveys of Candidate M31 GCs

• How to tell a genuine cluster from a contaminant?
• Cosmologic recession velocity gt background
  galaxy
• Vr around 0 km/s foreground Galactic star
• Right Vr but EL gt HII region
• If Vrlt-150 km/s gt cluster
• If -150ltVrlt200 km/s and
• extended gt cluster
• ? Resolved into stars gt cluster

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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
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Endemic Clusters Species
Extended Clusters old metal poor with 5rh
Bright clusters at very large distances from
the Galactic center Only one counterpart in the
MW (NGC2419)?
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Endemic Clusters Species
Massive Young Clusters They lie in the disc and
have disc kinematics Age lt 1 Gyr Mass gt 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 agelt 20
Myr, while all MYC in our M31 sample have age gt
25 Myr and most of them have age gt 50 Myr
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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
  MVgt-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
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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 _at_ Z?
Fe/H -1.7, -0.4, 0.4 _at_ 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
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Spectra Spectral indices
An example At fixed amplitude of the Balmer jump
? Differences in Hß traces differences in ages,
for ages gt 1Gyr Large values of Hß (gt 3.5 Å)
always indicate young ages
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Spectra Spectral indices
Another similar example Hß vs Mg
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Spectra Spectral indices
Another example A metallicity scale for old GCs
based on Lick indices
Brodie Huchra 1990
Galleti et al. 2009
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Spectra Spectral indices
Compare metallicity from Lick indices vs. from
CMDs we can do this only in M31
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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
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Rotation as a function of metal content
MW custers Zinn 1985 Only clusters with
Fe/Hgt-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.
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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
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The most widely used catalogue of M31 GCs has
been assembled and is manteined at the Bologna
observatoryRBC
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
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