Editing Galactic bulge (section)

== Central compact mass ==
[[File:A tiny galaxy with a big heart.jpg|thumb|ESO 495-21 may host a supermassive black hole, an unusual feature for a galaxy of its size.<ref>{{cite web |title=Hubble Observes Tiny Galaxy with Big Heart |url=https://www.spacetelescope.org/news/heic1911/ |website=www.spacetelescope.org |access-date=17 June 2019 |language=en}}</ref>]]

Most bulges and pseudo-bulges are thought to host a central relativistic compact mass,
which is traditionally assumed to be a [[supermassive black hole]]. Such black holes by definition cannot be observed directly (light cannot escape them), but various pieces of evidence suggest their existence, both in the bulges of spiral galaxies and in the centers of ellipticals. The masses of the black holes correlate tightly with bulge properties.
The [[M–sigma relation]] relates black hole mass to the velocity dispersion of bulge stars,<ref name="ferrarese2000">
{{cite journal
   | author1 = Ferrarese, L.
   | author-link = Laura Ferrarese
   | author2 = Merritt, D.
   | author2-link = David Merritt
   | title = A Fundamental Relation between Supermassive Black Holes and Their Host Galaxies
   | year = 2000
   | journal = The Astrophysical Journal Letters
   | volume = 539
   | issue = 1
   | pages = L9–L12
   | doi = 10.1086/312838
   | arxiv = astro-ph/0006053
   | url = http://esoads.eso.org/abs/2000ApJ...539L...9F
   | bibcode = 2000ApJ...539L...9F
| s2cid = 6508110
 }}
</ref><ref name="xiao2011">
{{cite journal
   | last1 = Xiao | first1 = T.
   | last2 = Barth | first2 = A. J.
   | last3 = Greene | first3 = J. E.
   | last4 = Ho | first4 = L. C.
   | last5 = Bentz | first5 = M. C.
   | last6 = Ludwig | first6 = R. R.
   | last7 = Jiang | first7 = Y.
   | title = Exploring the Low-mass End of the M $_BH$-$\sigma$$_*$ Relation with Active Galaxies
   | year = 2011
   | journal = [[The Astrophysical Journal]]
   | volume = 739
   | issue = 1
   | pages = 28
   | doi = 10.1088/0004-637X/739/1/28
   | arxiv = 1106.6232
   | url = http://esoads.eso.org/abs/2011ApJ...739...28X
   | bibcode = 2011ApJ...739...28X
| s2cid = 118444825
 }}
</ref> 
while other correlations involve the total stellar mass or luminosity of the bulge,<ref name="magorrian1998">
{{cite journal
   | last1 = Magorrian | first1 = J.
   | last2 = Tremaine | first2 = S.
   | last3 = Richstone | first3 = D.
   | last4 = Bender | first4 = R.
   | last5 = Bower | first5 = G.
   | last6 = Dressler | first6 = A.
   | last7 = Faber | first7 = S. M.
   | last8 = Gebhardt | first8 = K.
   | last9 = Green | first9 = R.
   | last10 = Grillmair | first10 = C.
   | last11 = Kormendy | first11 = J.
   | last12 = Lauer | first12 = T.
   | title = The Demography of Massive Dark Objects in Galaxy Centers
   | year = 1998
   | journal = [[The Astronomical Journal]]
   | volume = 115
   | issue = 6
   | pages = 2285–2305
   | doi = 10.1086/300353
   | arxiv = astro-ph/9708072
   | url = http://esoads.eso.org/abs/1998AJ....115.2285M
   | bibcode = 1998AJ....115.2285M
| s2cid = 17256372
 }}
</ref><ref name="haering2004">
{{cite journal
   | last1 = Häring | first1 = N.
   | last2 = Rix | first2 = H.-W.
   | title = On the Black Hole Mass-Bulge Mass Relation
   | year = 2004
   | journal = The Astrophysical Journal Letters
   | volume = 604
   | issue = 2
   | pages = L89–L92
   | doi = 10.1086/383567
   | arxiv = astro-ph/0402376
   | url = http://esoads.eso.org/abs/2004ApJ...604L..89H
   | bibcode = 2004ApJ...604L..89H
| s2cid = 119431361
 }}
</ref><ref>Giulia A.D. Savorgnan, et al. (2016), [http://adsabs.harvard.edu/abs/2016ApJ...817...21S Supermassive Black Holes and Their Host Spheroids. II. The Red and Blue Sequence in the M<sub>BH</sub>-M<sub>*,sph</sub> Diagram]</ref> 
the central concentration of stars in the bulge,<ref>Graham et al. (2001), [http://adsabs.harvard.edu/abs/2001ApJ...563L..11G A Correlation between Galaxy Light Concentration and Supermassive Black Hole Mass]</ref> 
the richness of the [[globular cluster]] system orbiting in the galaxy's far outskirts,<ref name="spitler2009">
{{cite journal
   | last1 = Spitler | first1 = L. R.
   | last2 = Forbes | first2 = D. A.
   | title = A new method for estimating dark matter halo masses using globular cluster systems
   | year = 2009
   | journal = [[Monthly Notices of the Royal Astronomical Society]]
   | volume = 392
   | issue = 1
   | pages = L1–L5
   | doi = 10.1111/j.1745-3933.2008.00567.x
   | doi-access = free
 | arxiv = 0809.5057
   | bibcode = 2009MNRAS.392L...1S
| s2cid = 16818778
 }}
</ref><ref name="sadoun2012">
{{cite journal
   | last1 = Sadoun | first1 = R.
   | last2 = Colin | first2 = J.
   | title = M<sub>BH</sub>–σ relation between supermassive black holes and the velocity dispersion of globular cluster systems
   | year = 2012
   | journal = [[Monthly Notices of the Royal Astronomical Society]]
   | volume = 426
   | issue = 1
   | pages = L51–L55
   | doi = 10.1111/j.1745-3933.2012.01321.x
   | doi-access = free
 | arxiv = 1204.0144
   | bibcode = 2012MNRAS.426L..51S
| s2cid = 117185846
 }}
</ref>
and the winding angle of the spiral arms.<ref>Seigar, M., et al. (2008), [http://adsabs.harvard.edu/abs/2008ApJ...678L..93S Discovery of a Relationship between Spiral Arm Morphology and Supermassive Black Hole Mass in Disk Galaxies]</ref>

Until recently it was thought that one could not have a supermassive black hole without a surrounding bulge.
Galaxies hosting supermassive black holes without accompanying bulges have now been observed.<ref name="kormendy2010" /><ref>[http://www.space.com/scienceastronomy/080110-aas-fat-black-holes.html SPACE.com - Even Thin Galaxies Pack Hefty Black Holes<!-- Bot generated title -->]
</ref><ref name="simmons2017">
{{cite journal
   | last1 = Simmons | first1 = B. D.
   | last2 = Smethurst | first2 = R. J.
   | last3 = Lintott | first3 = C.
   | title = Supermassive black holes in disk-dominated galaxies outgrow their bulges and co-evolve with their host galaxies
   | year = 2017
   | journal = [[Monthly Notices of the Royal Astronomical Society]]
   | volume = 470
   | issue = 2
   | pages = 1559–1569
   | arxiv = 1705.10793
   | url = http://esoads.eso.org/abs/2017arXiv170510793S
   | bibcode = 2017MNRAS.470.1559S
   | doi=10.1093/mnras/stx1340
| doi-access = free
 }}
</ref>
The implication is that the bulge environment is not strictly essential to the initial seeding and growth of massive black holes.

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