R. Michael Rich: New Results on the Galactic Bulge and Nuclear Star Cluster*

The Nuclear Star Cluster and Environs
R. Michael Rich (UCLA), Nils Ryde (Lund University), Brian Thorsbro (Lund University),
Mathias Schultheis (Obs. de Cote d’Azur), Livia Origlia (INAF-Bologna), Tobias Fritz (IAC),
Sotiris Chatzopoulos (Athens), Govand Nandakumar (ANU-RSAA)

A Richer Universe 2019 May
1. What is the nuclear star cluster and how is it
related to the bulge?
2. Brief history of metallicity studies of the
Galactic bulge
3. Spectroscopy of the nuclear cluster
4. Extreme Sc, V, Y abundances
5. Possible solutions

A nuclear star cluster adds light in excess of disk/bulge
profile

image: M. Schultheis

Formation Scenarios for nuclear star clusters
1. In situ formation 2. Infall of seed star cluster(s)
e.g. Tremaine et al. 2915, Capuzzo-Docetta 1993,
Lotz et al. 2004, Agarwal & Miolsavjevic 2011,
Gnedin et al. 2013, …
How does the NSC relate
to the bulge and other
populations? In situ star
formation and chemical
evolution? Unique
signatures?
Relationship to central
black hole?
Only G1 in M31 has an IMBH
(Gebhardt, Rich & Ho 2005)
Very few bulge globular clusters at
present epoch have [Fe/H]~ Solar
Young populations in the NSC a
problem: How to form young stars in
tidal field of SgrA* “paradox of
youth” Ghez (2008).
Milosavljevic & Merritt 2004, Schinnerer et al.
2008, Trippe et al. 2008, Schodel et al. 2009, Seth et
al. 2009

Milky Way NSC in context
2MASS
Wyse et al. (1997)
Oph SFR (foreground)
Sgr dSph
(28 kpc)
Courtesy J. Fulbright
1kpc

Image: Galactic Nucleus project, R. Schodel
Nogureres-Lara et al. 2014
reff=4.2 pc

Launhardt et al. 2002 NSC context

slide: M. Schultheis

Galactic bulge age controversy
Bensby et al. + Cohen 2013
Bulge dwarfs
brightened by
microlensing
[Fe/H] , log g
Teff
Age from the
HR diagram
25% “young??”

bulge age constraint from PM separation
~99% of bulge older than 5Gyr; pure 10+ Gyr likely (Clarkson+ 08, 11);
Brown et al. 2010; 2016 (HST Treasury survey)

Age of the old population near GC
HST/NICMOS finds old population present in inner 50 pc

Nogueres-Lara et al. 2018 fit 13 Gyr population to the luminosity
function; Schoedel et al. 2017 find RR Lyrae in central few pc
Confirms Pfuhl et al. 2011
Most of mass appears to be old

RR Lyrae in the NSC
Dong et al. 2017

Matteucci & Brocato (1990) early, rapid
formation of bulge (originally also presented by Tinsley 1978;
Searle & Sargent 1972
Matteucci & Brocato 1990
Massive star SNe (alpha elements)
Type I Sne
Fe peak
Fast
slow
IMF

Does the inner 200 pc have dramatic structure change
with Fe/H]?
Zoccali + 17
Barbuy, Chiappini, Gerhard 2017
[Fe/H]
<0??
[Fe/H]
>0 ??No vertex deviation change across
Solar metallicity

Chemistry supports early, rapid enrichment, minimal infall
These are not thick disk trends (See also McWilliam et al. 2016; Zasowski +
2018)
Johnson, Rich et al. 2014
O
Na
Mg
Al
Si
Ca
Cr
Co
Ni
Cu

What fraction of bulge from dissolved clusters?
Maybe not so much based on Al. Schiavon et al. 2017 revisited
Johnson et al. 2014
Schiavon et al. 2017
C. Johnson 2018 in prep.

BRAVA survey confirms stellar population dominated by
a bar

Modeling the Milky Way Bulge
Shen, Rich et al. 2010
N-body model of the
Galactic bulge matches the
BRAVA data extremely well
in almost all aspects:
– b = -4o major axis
– b = -8o degree major axis
– l = 0o degree minor axis
– Surface density
– Shen, J., RMR, Kormendy et al
2010, ApJL
– Bar dominates: no
bimodality in [Fe/H]

Background
Rich and Livia Origlia pursued IR spectroscopy of
the bulge globular clusters and field using nirspec
on Keck II (2001-2017), primarily in H band – over
30 papers
Pathfinder early work showed feasibility of
APOGEE spectroscopic survey
Nils Ryde proposes collaboration in 2013
Abundance determination analagous to building an
instrument

Central cluster
• ~globular cluster mass (M<100 arcsec is 8x106 Msun)
(Chatzopoulos et al. 2015; Fritz et al. 2016)
• Two populations: red giants with core, young stars
with cusp
• RR Lyrae suggest old stellar population; Pfuhl et al.
find substantial underlying old population
• Do et al. 2015 report extremely metal rich population,
some metal poor giants
• GC supergiants Solar metallicity, alpha enhanced
(Cunha et al. 2007)
• Paradox of Youth is always there – at time BH is
formed, and at present day.

Do et al. 2015 Gemini/NIFS R=5000; Bayesian constraints on
log g, Teff, [Fe/H] (Kerzendorf Starkit) AO corrected study of central pc.
claim new metal poor giants in Galactic center,
Histogram below does not include numerous metal rich
stars with [Fe/H]>+0.5, to +0.96. Spectra appear to have very strong
lines: Metallicity? Temperature? Molecules?

VLT/KMOS R=4500; Bayesian;
high metallicity
Feldmeier-Kraus et al. 2016

Metal Poor Giant near Galactic Center
Ryde, Fritz, Rich et al. 2016 ApJ 831, 40
[Fe/H]=-1; alpha enhanced
Use Keck/nirspec to explore actual metallicity

Do et al. 15
New metal poor giant has log g=0.5
Do “giants”
have log g>3!

New metallicity distribution in the Galactic Center
Rich et al. 2017, AJ,
• High spectral resolution, R > 23,000, necessary for retrieving detailed abundances. R
= 23,000 — 60,000
• Evolved red giants: 3500-4500 K, 0.5<logg<2.5; avoiding supergiants
• Giants in |b| < 3o observed VLT/CRIRES, VLT/ISAAC, NTT/SOFI
• Giants in the NSC with KECK/nirspec, and VLT/SINFONI
• Determine metallicities [Fe/H], CNO, F, Sc, Na,
Al, and the alpha elements Mg, Si, S, Ca, and Ti
• Difficult spectra: faint cool stars, lot of molecules,
new wavelength region
• Careful analysis required and optimized
observations to minimize systematics

• Spectra calculated for spherical, 1D, LTE MARCS model atmospheres (Gustafsson et al.
2008) for the M giants (3500-4500 K, 0.5<logg<2.5)
• Accurate stellar parameters important (Teff, logg, [Fe/H], microturb., [alpha/Fe])
• Synthetic spectra in LTE and spherical geometry at R=200,000. Convolved to R=23,000.
• Careful line-by-line chi2-fitting in SME (Valenti & Piskunov et al. 2012). New line-list
for the K band needed (Thorsbro et al. 2018) & CN molecule list
by Sneden et al. 2015
• Metal rich stars are very difficult! Fe lines are blended with weak molecule lines;
continuum extremely hard to define.
Spectral analysis
(Ryde, Thorsbro leads)

Example: M giant GC7688 with KECK/nirspec :
3900 K, logg=1.5, [Fe/H]=+0.1;
40 min. for K = 11.0, R = 23000; SNR = 100
Example spectrum:

Teff diagnostic CO band-heads at low resolution (VLT/ISAAC,
NTT/SOFI);
empirical relations and model independent; this method would
work for NIFS R=5,000 spectra
12CO (2-0) & (3-1) bands
Teff correlates with CO(2 − 0)
and CO(3 − 1) bands with σ(T)
= 100 K. Indep of [Fe/H],
Resolution
Schultheis et al. A&A (2016)

Red Giants in the Nuclear Star Cluster
(confirmed by radial velocity and pm)

Metal Rich fraction jumps in NSC
Schultheis, Rich et al. 2019

72 M giants (CRIRES) from
Nandakumar et al. 2018 (b= -1o , -2o )
The GC may be more metal rich; needs investigation
GC

Lower alpha/Fe trend suggests a more thin-disk
like trend in Si, compared to the bulge… extended
SF history?
Nandakumar et al. 2018

Strong Sc Line
What about Sc, V?
Hyperfine splitting
Extreme sensitivity to
Teff, microturbulence
Lines do seem strong

However, an interesting problem…
Do et al. 2018 claim
extreme metallicities in
the nucleus..
Conclude big enhancements
of Sc, V, Y in the nucleus
But with Fe already high, huge
production of V, Y, Sc needed …
how?
Proximity to Sgr A*; some
stars in the loss cone. High
density might cause NS-NS
mergers? Binaries? Other
contributors to unusual
elements
Do et al. 2018
Do et al. 2018

V, Y, Sc have different nucleosynthetic origin

Do et al. 2018- calibrator star in NGC 6791 900K hotter

Possible Solution: Stars are cool, and V, Sc, Y have low
excitation potential, hyperfine splitting: The lines are
susceptible to anomalous strengthening for cool stars.
Thorsbro et al. 2018
Teff from APOGEE

Thorsbro et al . 2018
Trouble for Sc I (and also V, Y) compared to Fe
1.5 eV transition formed higher in the atmosphere more NLTE
𝑒−∆𝐸/𝑘𝑇 Boltzmann factor means 3 order of mag higher population
(1.5 eV line as opposed to Fe I at 3.5 and 6 eV)
Sc odd element, s electrons susceptible to hyperfine splitting

hfs is worse at low Teff, disappears> 4000K

Strong Teff dependence for Sc derived abundance

Even Solar neighborhood
stars have high Sc if they
are cool enough.

Terzan 5 107 Msun- GC analog?
Ferraro et al. 09 Nature

A Richer Universe 2019 MayOriglia, Rich et al. 2011; Massari et al. 2015
Analogy with Terzan 5? Only statistics can tell

Blanco DECam Bulge Survey
SDSS ugrizY Imaging survey of the Southern bulge
Rich (PI), C. Johnson (CfA; Co-PI), M. Young, S. Michael
(IU), W. Clarkson(U. Mich Dearborn, R. Ibata
(Strasbourg),K. Vivas(CTIO), A. Koch (Heidelberg), M.
Collins (Surrey) , N. Martin (Strasbourg), A. Robin
(Strasbourg), R. de Propris (FINCA), O. Gerhard (MPIA), J.
Shen (Shanghai)

Early bulge BDBS CMDs (C. Johnson)

BDBS- M22 (Johnson et al. 2019)

BDBS (left) Panstarrs (Right)
NGC 6522
C. Johnson

BDBS-M28
C. Johnson

New
cluster
FSR
1758

Conclusions (Nuclear star cluster)
1. No [Fe/H] near +1 in Galactic Center
2. No evidence of multimodality in [Fe/H] yet
3. Some metal poor giants [Fe/H]=-1 present
4. Extreme V, Sc, Y abundances due to hfs
5. overall distribution looks metal rich and bulge-
like
6. population looks overall bulge like but
presence of young population a mystery
7. more data needed (Xshooter survey under
analysis)

Conclusions
1. NSC has [Fe/H]~ Solar
2. alpha enhancement similar to bulge
3. Claims of extreme V, Sc, Y are spurious and
due to very low Teff
4. Need more data- Al-Mg anticorrelations? Ter 5
multimodality analog?
5. More abundances coming soon

Happy Birthday, Harvey
A very happy collaboration on
the white dwarf cooling sequence,
Fulbright Scholarship visit to UCLA
by Harvey

R. Michael Rich: New Results on the Galactic Bulge and Nuclear Star Cluster*

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R. Michael Rich: New Results on the Galactic Bulge and Nuclear Star Cluster*