View source for Draco Dwarf

{{Short description|Spheroidal galaxy in the constellation Draco}}
{{Infobox Galaxy
| image = Draco Dwarf Spheroidal (2024-017).jpg
| image_size = 250px
| caption = Field image of the Draco dwarf spheroidal galaxy, by the [[Digitized Sky Survey]]
| credit = 
| name = Draco Dwarf
| epoch = [[Epoch (astronomy)#Julian years and J2000|J2000]]
| type = E pec<ref name="ned" />
| ra = {{RA|17|20|12.4}}<ref name="ned" />
| dec = {{DEC|+57|54|55}}<ref name="ned" />
| dist_ly = [[1 E21 m|260 ± 30]] [[light-year|kly]] (75.4 [[parsec|kpc]])<ref name="karachentsevetal2004" /><ref name="Karachentsevetal2006" /><ref name="Vitral+24">{{cite journal|last=Vitral|first=E.|display-authors=etal|title=HSTPROMO Internal Proper-motion Kinematics of Dwarf Spheroidal Galaxies. I. Velocity Anisotropy and Dark Matter Cusp Slope of Draco|journal=The Astrophysical Journal|date=2024|volume=970|issue=1|page=1 |doi=10.3847/1538-4357/ad571c|doi-access=free |arxiv =2407.07769|bibcode = 2024ApJ...970....1V}}</ref>
| z = −292 ± 21 km/[[second|s]]<ref name="ned" />
| appmag_v = 10.9<ref name="ned" />
| mass = {{val|1.2|e=8|u=}}<ref name="Vitral+24" />{{Ref_label|E|e|none}}
| size_v = 35.5{{prime}} × 24.5{{prime}}<ref name="ned" />
| constellation name = [[Draco (constellation)|Draco]]
| notes = Highest known dark matter concentrated object
| names = Draco Dwarf Spheroidal,<ref name="ned" /> Draco&nbsp;dSph,<ref name = "Fariaetal2007" /> [[Uppsala General Catalogue|UGC]]&nbsp;10822,<ref name="ned" />
[[Principal Galaxies Catalogue|PGC]]&nbsp;60095,<ref name="ned" /> [[David Dunlap Observatory Catalogue|DDO]]&nbsp;208,<ref name="ned" />
}}

The '''Draco Dwarf''' is a [[dwarf spheroidal galaxy|spheroidal galaxy]] which was discovered by [[Albert George Wilson]] of [[Lowell Observatory]] in 1954 on photographic plates of the [[National Geographic Society|National Geographic Society's]] [[Palomar Observatory Sky Survey]] (POSS).<ref name="Wilson1955">{{citation
 | title = Sculptor-Type Systems in the Local Group of Galaxies
 | last1 = Wilson
 | first1 = Albert George
 | author-link1 = Albert George Wilson
 | journal = Publications of the Astronomical Society of the Pacific
 | volume = 67
 | issue = 394
 | pages = 27–29
 | date = February 1955
 | bibcode = 1955PASP...67...27W
 | doi = 10.1086/126754
}}</ref>  It is part of the [[Local Group]] and a [[satellite galaxy]] of the [[Milky Way]] galaxy.  The Draco Dwarf is situated in the direction of the [[Draco (constellation)|Draco]] [[Constellation]] at 34.6°<ref name="Hodge1964" /> above the [[galactic plane]].

==Characteristics==
[[Paul W. Hodge]] analyzed the distribution of its stars in 1964 and concluded that its [[ellipticity]] was 0.29 ± 0.04.<ref name="Hodge1964">{{citation
 | title = Distribution of stars in the Draco dwarf galaxy
 | last1 = Hodge
 | first1 = Paul W.
 | author-link1 = Paul W. Hodge
 | date = December 1964
 | journal = The Astronomical Journal
 | volume = 69
 | bibcode = 1964AJ.....69..853H
 | pages = 853
 | doi = 10.1086/109360
| doi-access = free
 }}</ref> 
Recent studies have indicated that the galaxy may potentially hold large amounts of [[dark matter]].<ref name="Tyler2002">{{citation
 | title = Particle dark matter constraints from the Draco dwarf galaxy
 | last1 = Tyler
 | first1 = Craig
 | volume = 66
 | issue = 2
 | date = July 2002
 | bibcode = 2002PhRvD..66b3509T
 | doi = 10.1103/PhysRevD.66.023509
 | pages = 023509
 | journal = Physical Review D
|arxiv = astro-ph/0203242 | s2cid = 18393976
 }}</ref> Having an absolute magnitude of −8.6{{Ref_label|C|c|none}} and a total luminosity of only {{Solar luminosity|link=y|2{{e|5}}}}, it is one of the faintest companions to our Milky Way.<ref name = "Fariaetal2007">{{citation
 | title = The usage of Strömgren photometry in studies of local group dwarf spheroidal galaxies. Application to Draco: a new catalogue of Draco members and a study of the metallicity distribution function and radial gradients
 | last1 = Faria
 | first1 = D.
 | last2 = Feltzing
 | first2 = S.|author2-link= Sofia Feltzing
 | last3 = Lundström
 | first3 = I.
 | last4 = Gilmore
 | first4 = G.
 | last5 = Wahlgren
 | first5 = G. M.
 | last6 = Ardeberg
 | first6 = A.
 | last7 = Linde
 | first7 = P.
 | journal = Astronomy and Astrophysics
 | volume = 465
 | issue = 2
 | date = April 2, 2007
 | pages = 357–373
 | bibcode = 2007A&A...465..357F
 | doi = 10.1051/0004-6361:20065244
|arxiv = astro-ph/0611883 | s2cid = 55281148
 }}</ref>

Draco Dwarf contains many [[red giant branch]] (RGB) stars; five [[carbon star]]s have been identified in Draco Dwarf and four likely [[asymptotic giant branch]] (AGB) stars have been detected.<ref name = "Fariaetal2007" />

The Draco Dwarf is estimated to be 80 ± 10 kpc<ref name="karachentsevetal2004">{{Citation
  | author=I. D. Karachentsev
  | author2=V. E. Karachentseva
  | author3=W. K. Hutchmeier
  | author4=D. I. Makarov
  | title=A Catalog of Neighboring Galaxies
  | journal=Astronomical Journal
  | date=2004
  | volume=127
  | issue=4
  | pages=2031–2068
  | bibcode=2004AJ....127.2031K
  | doi=10.1086/382905
  | postscript=.| doi-access=free
  }}</ref><ref name="Karachentsevetal2006">{{Citation
  | author=Karachentsev, I. D.
  | author2=Kashibadze, O. G.
  | title=Masses of the local group and of the M81 group estimated from distortions in the local velocity field
  | journal=Astrophysics
  | date=2006
  | volume=49
  | issue=1
  | pages=3–18
  | bibcode=2006Ap.....49....3K
  | doi=10.1007/s10511-006-0002-6
  | s2cid=120973010
 | postscript=. }}</ref> from [[Earth]] and span a distance of 830 ± 100 × 570 ± 70 pc.{{Ref_label|D|d|none}}

==RR Lyrae==
In 1961, [[Walter Baade]] and [[Henrietta H. Swope]] studied Draco Dwarf and discovered over 260 [[variable star|variables]], of the 138 in the cluster's center, all but five were determined to be [[RR Lyrae variables]].<ref name="Baade1961">{{citation
 | last1 = Baade
 | first1 = Walter
 | author-link1 = Walter Baade
 | last2 = Swope
 | first2 = Henrietta H.
 | author-link2 = Henrietta H. Swope
 | title = The Draco system, a dwarf galaxy
 | journal = [[The Astronomical Journal]]
 | volume = 66
 | pages = 300–347
 | date = September 1961
 | bibcode = 1961AJ.....66..300B
 | doi = 10.1086/108431
}}</ref>  From this work a RR Lyrae derived [[distance modulus]] of 19.55{{Ref_label|A|a|none}}  is found which implies a distance of 81 kpc.{{Ref_label|B|b|none}}

==Metallicity==
The Draco Dwarf contains primarily an [[metallicity|old population]] of stars and insignificant amounts of [[interstellar matter]] (being basically dust free).<ref name = "Fariaetal2007" />  From 75% to 90% of its stars formed more than ~10 Gyr ago followed by a low rate of formation with a small burst of star formation around 2–3 Gyr ago.<ref name="Aparicioetal2001">{{citation
 | title = The Star Formation History and Morphological Evolution of the Draco Dwarf Spheroidal Galaxy
 | last1 = Aparicio
 | first1 = Antonio
 | last2 = Carrera
 | first2 = Ricardo
 | last3 = Martínez-Delgado
 | first3 = David
 | date = November 2001
 | journal = The Astronomical Journal
 | volume = 122
 | issue = 5
 | pages = 2524–2537
 | bibcode = 2001AJ....122.2524A
 | doi = 10.1086/323535
|arxiv = astro-ph/0108159 | s2cid = 17088564
 }}</ref> It has a single [[Gaussian distribution]] with average metallicity of [Fe/H] = &minus;1.74 dex with a [[standard deviation]] (sigma/σ) of 0.24 dex and a small tail of metal-rich stars.<ref name = "Fariaetal2007" />  The central region of Draco Dwarf exhibits a concentration of more metal-rich stars there being more centrally concentrated red [[horizontal branch]] stars than blue horizontal branch stars.<ref name = "Fariaetal2007" />

==Dark matter==
Recently, dwarf spheroidal galaxies have become key objects for the study of [[dark matter]].  The Draco Dwarf is one which has received specific attention.  [[Radial velocity]] computations of Draco have revealed a large internal velocity dispersion giving a mass to luminosity ratio of up to {{Solar mass|link=y|440}}/{{Solar luminosity|link=y}}, suggesting large amounts of dark matter.  It has been hypothesized that large velocity dispersions could be explained as ''tidal dwarfs'' (virtually unbound stellar streams from dwarf galaxies tidally disrupted in the Milky Way potential). However, Draco Dwarf's narrow [[horizontal branch]] width does not support this model.<ref name = "Mashchenkoetal2006">{{citation
 | title = Constraining Global Properties of the Draco Dwarf Spheroidal Galaxy
 | last1 = Mashchenko
 | first1 = Sergey
 | last2 = Sills
 | first2 = Alison
 | last3 = Couchman
 | first3 = H. M.
 | date = March 2006
 | journal = The Astrophysical Journal
 | volume = 640
 | issue = 1
 | pages = 252–269
 | bibcode = 2006ApJ...640..252M
 | doi = 10.1086/499940
|arxiv = astro-ph/0511567 | s2cid = 18708747
 }}</ref> This only leaves the dark matter explanation and makes Draco Dwarf the most dark matter dominated object known as of 2007.<ref name = "Fariaetal2007" />  The dark matter distribution within Draco Dwarf is at least nearly [[isothermal]].<ref name="Tyler2002" />

At large radii, radial velocity dispersion exhibit strange behavior.  One possible explanation for this would be the presence of more than one stellar population.  This suggests the need for further study of Draco Dwarf population's metallicity and ages and of dwarf spheroidals in general.<ref name = "Fariaetal2007" />

In 2024, a group of scientists using the [[Hubble Space Telescope]] measured [[proper motions]] of Draco with 18 years of data, making it the first [[dwarf galaxy]] to have its 3D velocity  dispersion profile radially resolved.<ref name="Vitral+24" /> The group of astronomers showed that Draco's [[dark matter]] distribution is in better agreement with the [[LCDM]] model, helping to alleviate the [[cusp-core problem]].<ref>{{cite web |title=NASA's Hubble Traces Dark Matter in Dwarf Galaxy Using Stellar Motions |url=https://science.nasa.gov/missions/hubble/nasas-hubble-traces-dark-matter-in-dwarf-galaxy-using-stellar-motions/ |date=11 July 2024 |access-date=3 July 2024}}</ref>

==Notes==
{{refbegin}}
<ol type="a">
<li>{{Note_label|A|a|none}}Assuming an absolute magnitude of +0.5 V for RR Lyrae the apparent modulus of the Draco Dwarf is 19.58 m−M.<ref name="Hodge1964" />  Using a reddening value towards Draco Dwarf of 0.03 ± 0.01<ref name = "Fariaetal2007" /> we get a true distance modulus of 19.55.</li>
<li>{{Note_label|B|b|none}} Using the distance modulus formula of 1{{e|(0.2 * 19.55 + 1)}} we get an RR Lyrae estimated distance of 81 kpc.</li>
<li>{{Note_label|C|c|none}}Apparent Magnitude of 10.9<ref name="ned">{{cite web
  | title=NASA/IPAC Extragalactic Database
  | work=Results for Draco Dwarf
  | url=http://nedwww.ipac.caltech.edu/
  | access-date=2008-04-08 }}</ref> – distance modulus of 19.52 (80 kpc) = −8.6</li>
<li>{{Note_label|D|d|none}}distance 80 ± 10 kpc × tan(diameter_angle = 35.5{{prime}} × 24.5{{prime}}<ref name="ned" />) = 830 ± 100 × 570 ± 70 pc diameter</li>
<li>{{Note_label|E|e|none}}The quoted total mass is the cumulative dark matter mass (much higher than the luminous mass) at 900 pc from the galaxy's center.</li>
</ol>
{{refend}}

==References==
{{Reflist|30em}}

==External links==
{{commonscat}}
* {{WikiSky|name=The Draco Dwarf}}

{{Milky Way}}
{{Sky|17|20|12.4|+|57|54|55|260000}}

[[Category:Dwarf spheroidal galaxies]]
[[Category:Peculiar galaxies]]
[[Category:Local Group]]
[[Category:Milky Way Subgroup]]
[[Category:Draco (constellation)]]
[[Category:UGC objects|10822]]
[[Category:Principal Galaxies Catalogue objects|60095]]
[[Category:Astronomical objects discovered in 1954|?]]