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The Galactic Bulge:
Implications for the Galactic Center
R. Michael Rich
UCLA
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In order to understand the center, we must understand the context
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Pampaneira
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The context matters
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Important Scales:
Galactic Nuclear Star Cluster: 5 pc
Nuclear stellar disk: 100 pc
Galactic bulge 100-1000 pc
Nuclear Disk: Nogueres-Lara review;
Nieuwmunster talks
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Age, abundance, kinematics, structure
Metal rich bulge is strongly barred
And shows rotation
The Blanco DECam Bulge Survey: A Rubin Observatory Pathfinder�
R. Michael Rich (UCLA), Co-PI
Christian Johnson, STScI, Co-PI, Tommaso Marchetti (ESO), Kathy Vivas (CTIO,NoirLab), Scott Michael & Mike Young (Indiana University), Andrea Kunder (St. Martin’s U), Tomasso Marchetti (ESO) Andreas Koch (ZAH), Will Clarskon U. Mich Dearborn, Caty Pilachowski (Indiana University), Ortwin Gerhard (MPIA), Iulia Simion, Juntai Shen (Shanghai Observatory), Roberto de Propris ( FINCA), Annie Robin (Besancon), Mario Soto
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Blanco DECam Bulge Survey (BDBS )
LSST pathfinder (Rubin)
BDBS Science: ugrizY colors superior to JHK for metallicities- more sensitive
Explore metallicity distribution,
age controversy over the whole of the bulge
Use Gaia to explore abundance/kinematics correlations
Bulge globular clusters, tidal tails
Matching to Galex and other surveys
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BDBS Fields include Sgr dwarf, disk fields
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BDBS survey spans most of low
extinction Southern Galactic bulge,
extends beyond VVV footprint
Dereddened region from Simion et al. 2017 (1x1 arcmin)- Left
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Project begins with 50 TB of data taken of 3 observing seasons, in varying photometric conditions.
Leads photometric calibration and reduction using daophot+allstar psf photometry- but over this massive dataset.
Calibration and data reduction
Undertaken by a single person-
Christian Johnson (STScI)
Also finds (u-i)o vs [Fe/H] correlation
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Calibrated, dereddened CMDs over whole bulge
Rich, Johnson et al. 2020
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Calibrate using red clump stars with measured abundances.
Matching to existing Ca triplet abundances
(Zoccali et al. 2017) gives u-i vs [Fe/H]
Johnson, Rich et al. (BDBS II) 2020
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Another comparison with Zoccali et al. 2017
Note that we confirm a multimodal abundance distribution
Millions of metallicities from u-i photometry
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By physical
Distance
Above
The plane
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Johnson,Rich 22
Metal rich component metallicity constant
Metal poor component present in the plane
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Rich +2020, Johnson, Rich+ 2020, 2022
Metal rich red clump stars concentrate to the plane; no
Metallicity variation radially.
Age Controversy
Proper motion cleaned
color-magnitude diagrams using HST
Require greater ages of 10 Gyr
Clarkson et al. 2008
Clarkson et al. (2008, 2011)
Renzini+18
Also asserts old
Bulge based on luminosity functions
Bensby +17 find
Dramatically younger ages
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Bensby et al. 2017 usese log g, log Teff , [Fe/H] measured from high
resolution spectra of microlensed bulge stars-significant int age pop
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Foreground –
Cleaned full bulge
CMD
Shows no
Extensive population
<1 Gyr Tour de Force
Marchetti et al. 2022 A&A
>100M Gaia matches
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Re-derive ages from Bensby+17 with new isochrones
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Joyce, Johnson, Marchetti, Rich et al. 2023
Bar?
Thick
Disk?
Ages:
New
Result:
We find very few stars < 5 Gyr
10+/-3 Gyr
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Rich +2020, Johnson, Rich+ 2020, 2022
Metal rich red clump stars concentrate to the plane; no
Metallicity variation radially.
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New Kinematics Gaia + Red clump [Fe/H]
Marchetti et al. 2023 in prep.
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Nandakumar et al. 18, 22
Compare to bulge at ~100pc
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Relating the nucleus to the bulge
Nucleus has an old population (RR ab stars) < 18% (Hui, Schodel+2017)
Do+2015
Metal poor +
metal rich
Rich+2017 highres
Thorsbro+2020
Bayesian Modeling gives age 5+/- 3 Gyr Chen,Do+22
Feldmeier-Krause+17 inner 4pc
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Thorsbro et al. 2020
Thorsbro+ 2023 in prep;poster
Bentley et al. 2023 dSph stars?
High alphas (Type II Sne)
.. Other bulge surveys find no evidence for dSph contribution
Johnson+2014
Composition inner pc
Si
Ca new
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Johnson et al. 2014 Bulge has few if any dSpH
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Queiroz 2020
Did the Bentley Stars arise from the Old Thin Disk?
Massive Clusters as 109 Msun “bulge building blocks”?
that are ~107 Msun today NO IMBH CONFIRMED IN ANY GAL CLUSTER
Terzan 5, Liller 1 - Both ~ 10^6 Msun Both have age, abundance ranges!
NSC is ~6e6 Msun (Fritz et al. 2014)
Is the central cluster a remnant “building block”?
Origlia, Rich +11
Terzan 5
Ferraro+2016
Terzan 5
M31-G1 2e4Msun IMBH
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Ferraro+2016
Terzan 5
Liller 1
Ferrar+ 2020, 2023
Age, abundance range
2e6 Msun, 34 msec pulsars
2e6 Msun
Origin of the NSC still unclear
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Conclusions
The abundance distribution of the central pc is similar to the bulge and inner bulge
But, NSC has elevated alphas at high Fe/H-not seen elsewhere
Precise age distribution of the NSC is an open, important question. (very old, very young stars; bulk of stars may be ~5 Gyr) MSTO? EELT?
Similarities, differences vs. Ter 5, Liller 1 but no BH in any other cluster
Many aspects of SgrA* cluster remain unique
GRACIAS!