{{Short description|Seyfert 2 galaxy and quasar in the constellation Coma Berenices}} {{Infobox galaxy|name=[[Parkes Catalogue of Radio Sources|PKS]] 1155+251|constellation name=[[Coma Berenices]]|epoch=[[J2000.0]]|ra={{RA|11|58|25.78}}<ref name="ned" />|dec={{DEC|+24|50|17.96}}<ref name="ned" />|z=0.202556<ref name="ned" />|dist_ly=2.554 [[light year|Gly]]|h_radial_v=60,725 km/s ± 8<ref name="ned" />|appmag_v=19.40|type=FSRQ<ref name="ned" />|size=~{{convert|87.78|kpc|ly|abbr=on|lk=on|order=flip}} (estimated){{r|ned}}|names=[[Seventh Cambridge Survey|7C]] 1155+2206, VIPS 0500, [[Lyon-Meudon Extragalactic Database|LEDA]] 1717805, [[Sloan Digital Sky Survey|SDSS]] J115825.79+245018.0, [[NRAO VLA Sky Survey|NVSS]] J115825+245017, OCARS 1155+251, JVAS J1158+2450<ref name="ned" />|image=File:Image of PKS 1155+251.png|caption=[[Dark Energy Spectroscopic Instrument|DESI Legacy Surveys]] image of PKS 1151+251}}
'''PKS 1155+251''' is a [[Seyfert galaxy|Seyfert type 2 galaxy]] and also a [[quasar]]<ref name=":0">{{Cite journal |last1=Yang |first1=Xiaolong |last2=Liu |first2=Xiang |last3=Yang |first3=Jun |last4=Mi |first4=Ligong |last5=Cui |first5=Lang |last6=An |first6=Tao |last7=Hong |first7=Xiaoyu |last8=Ho |first8=Luis C. |date=2017-07-13 |title=VLBA 24 and 43 GHz observations of massive binary black hole candidate PKS 1155 + 251 |journal=Monthly Notices of the Royal Astronomical Society |volume=471 |issue=2 |pages=1873–1878 |doi=10.1093/mnras/stx1743 |doi-access=free |issn=0035-8711|arxiv=1707.03121 }}</ref><ref>{{Cite web |title=MOJAVE Sample: 1155+251 |url=https://www.cv.nrao.edu/MOJAVE/sourcepages/1155+251.shtml |access-date=2025-12-19 |website=www.cv.nrao.edu}}</ref> located in the constellation of [[Coma Berenices]]. The [[redshift]] of the object is (z) 0.202<ref name="ned">{{Cite web |title=NED Search results for PKS 1155+251 |url=https://ned.ipac.caltech.edu/byname?objname=PKS+1155+251&hconst=67.8&omegam=0.308&omegav=0.692&wmap=4&corr_z=1 |access-date=2025-12-19 |website=NASA/IPAC Extragalactic Database}}</ref> and it was first discovered as an [[astronomical radio source]] by [[Astronomer|astronomers]] whom they identified it with a galaxy counterpart in 1974.<ref>{{Cite journal |last1=Hoskins |first1=D. G. |last2=Murdoch |first2=H. S. |last3=Adgie |first3=R. L. |last4=Crowther |first4=J. H. |last5=Gent |first5=H. |date=February 1974 |title=Optical identifications for 45 radio sources based on accurate positions |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=166 |issue=2 |pages=235–248 |doi=10.1093/mnras/166.2.235 |bibcode=1974MNRAS.166..235H |doi-access=free |issn=0035-8711}}</ref> This object has also been classified as a compact symmetrical object (CSO) in literature.<ref name=":1">{{Cite journal |last1=Tremblay |first1=S. E. |last2=Taylor |first2=G. B. |last3=Helmboldt |first3=J. F. |last4=Fassnacht |first4=C. D. |last5=Pearson |first5=T. J. |date=September 2008 |title=A Shrinking Compact Symmetric Object: J11584+2450? |url=https://iopscience.iop.org/article/10.1086/590377/fulltext/ |journal=The Astrophysical Journal |volume=684 |issue=1 |pages=153–159 |doi=10.1086/590377 |arxiv=0806.3955 |bibcode=2008ApJ...684..153T |issn=0004-637X}}</ref><ref name=":2">{{Cite journal |last1=Tremblay |first1=S. E. |last2=Taylor |first2=G. B. |last3=Helmboldt |first3=J. F. |last4=Fassnacht |first4=C. D. |last5=Romani |first5=R. W. |date=February 2009 |title=Identifying Compact Symmetric Objects from the VLBA Imaging and Polarization Survey |journal=Astronomische Nachrichten |language=en |volume=330 |issue=2 |pages=206 |doi=10.1002/asna.200811157 |arxiv=0905.2982 |bibcode=2009AN....330..206T |issn=0004-6337}}</ref><ref>{{Cite journal |last1=Readhead |first1=A. C. S |last2=Ravi |first2=V. |last3=Blandford |first3=R. D. |last4=Sullivan |first4=A. G. |last5=Somalwar |first5=J. |last6=Begelman |first6=M. C. |last7=Birkinshaw |first7=M. |last8=Liodakis |first8=I. |last9=Lister |first9=M. L. |last10=Pearson |first10=T. J. |last11=Taylor |first11=G. B. |last12=Wilkinson |first12=P. N. |last13=Globus |first13=N. |last14=Kiehlmann |first14=S. |last15=Lawrence |first15=C. R. |date=2024-01-31 |title=Compact Symmetric Objects. III. Evolution of the High-luminosity Branch and a Possible Connection with Tidal Disruption Events |journal=The Astrophysical Journal |volume=961 |issue=2 |pages=242 |doi=10.3847/1538-4357/ad0c55 |arxiv=2303.11361 |bibcode=2024ApJ...961..242R |doi-access=free |issn=0004-637X}}</ref>
== Description == PKS 1155+251 is described as a radio-loud quasar.<ref name=":0" /> The source is mainly compact, with a radio core containing a flat [[radio spectrum]], which is positioned between the lobes of a steep spectrum.<ref name=":2" /> Radio imaging made with the [[Very Long Baseline Array]] (VLBA) at 15 [[Hertz|GHz]] frequencies found an unresolved component which is surrounded by [[Radio wave|radio emission]] towards both directions. An eastern spur feature was discovered from the northern emission component, which goes in the opposite direction from most of the diffused emission. Evidence also suggests the source is shrinking as its hotspot features retreat towards the core position.<ref name=":1" /> The angular size is estimated to be 46.0 milliarcseconds in extent with a [[Turnover number|turnover frequency]] of 2.0 GHz.<ref>{{Cite journal |last1=Kiehlmann |first1=S. |last2=Lister |first2=M. L. |last3=Readhead |first3=A. C. S |last4=Liodakis |first4=I. |last5=O’Neill |first5=Sandra |last6=Pearson |first6=T. J. |last7=Sheldahl |first7=Evan |last8=Siemiginowska |first8=Aneta |last9=Tassis |first9=K. |last10=Taylor |first10=G. B. |last11=Wilkinson |first11=P. N. |date=2024-01-31 |title=Compact Symmetric Objects. I. Toward a Comprehensive Bona Fide Catalog |journal=The Astrophysical Journal |volume=961 |issue=2 |pages=240 |doi=10.3847/1538-4357/ad0c56 |arxiv=2303.11357 |bibcode=2024ApJ...961..240K |doi-access=free |issn=0004-637X}}</ref>
Imaging made with [[Very-long-baseline interferometry|Very Long Baseline Interferometry]] (VLBI) suggested the radio structure is very complicated with evidence suggesting the northeastern feature is further divided into two narrow [[Astrophysical jet|jet]] features.<ref>{{Cite journal |last1=Kellermann |first1=K. I. |last2=Vermeulen |first2=R. C. |last3=Zensus |first3=J. A. |last4=Cohen |first4=M. H. |date=April 1998 |title=Sub-Milliarcsecond Imaging of Quasars and Active Galactic Nuclei |journal=The Astronomical Journal |language=en |volume=115 |issue=4 |pages=1295–1318 |doi=10.1086/300308 |arxiv=astro-ph/9801010 |bibcode=1998AJ....115.1295K |issn=0004-6256}}</ref> Further observations made with the VLBA at both 24 and 43 GHz [[Frequency|frequencies]] have located the southern complex region. When imaged, it shows the presence of hotspot and blob features that have an [[Orientation (geometry)|orientation]] from north to west. Further evidence found the component on the central side is unresolved, while the southern component is extremely bright, with an approximate [[brightness temperature]] of 10<sup>9</sup> [[Kelvin]]. The measured spectral indexes for both components are around 0.36 ± 0.03 and -0.38 ± 0.03.<ref name=":0" /> Two [[Supermassive black hole|supermassive black holes]] are suggested to lie inside the center of the galaxy, with the estimated separation of only 3.5 [[Minute and second of arc|milliarcseconds]] from each other.<ref name=":0" /><ref>{{Cite journal |last1=Yang |first1=Xiaolong |last2=Liu |first2=Xiang |date=February 2016 |title=May PKS 1155+251 be the habitat of a binary black hole? |url=https://www.cambridge.org/core/journals/proceedings-of-the-international-astronomical-union/article/may-pks-1155251-be-the-habitat-of-a-binary-black-hole/A73EB109539110489D8AEED09E5F10E0 |journal=Star Clusters and Black Holes in Galaxies Across Cosmic Time |language=en |volume=312 |pages=71–72 |doi=10.1017/S1743921315007528 |issn=1743-9221}}</ref><ref>{{Cite web |title=Research Progress in Massive Binary Black Holes Candidate VLBI Observation--Xinjiang Astronomical Observatory |url=http://english.xao.cas.cn/news/202407/t20240729_677911.html |access-date=2025-12-19 |website=english.xao.cas.cn}}</ref>
== References == <references />
== External links == *[https://simbad.cds.unistra.fr/simbad/sim-id?bibyear1=1850&bibyear2=%24currentYear&submit=Display&Ident=%401862482&Name=ICRF+J115825.7%2B245017&bibdisplay=refsum&bibyear1=1850&bibyear2=%24currentYear#lab_bib PKS 1155+251 on SIMBAD]
{{Coma Berenices}} [[Category:Seyfert galaxies]] [[Category:Quasars]] [[Category:Coma Berenices]] [[Category:Active galaxies]] [[Category:Black holes]] [[Category:LEDA objects|1717805]] [[Category:Astronomical objects discovered in 1974]]