3C 138

{{Short description|Quasar in the constellation Taurus}}

{{Infobox galaxy|name=3C 138|epoch=J2000.0|constellation name=Taurus|ra={{RA|05|21|09.886}}|dec={{DEC|+16|38|22.052}}|z=0.759000|h_radial_v=227,543 km/s|dist_ly=6.739 Gly|appmag_v=18.84|appmag_b=19.37|type=Opt. var, Sy 1.5|names=4C +16.12, PKS B0518+565, LEDA 2817552, DA 170, Cul 0518+565, NRAO 205|image=File:Image of 3C 138.jpg|caption=The quasar 3C 138.}}

3C 138 is a quasar{{Cite journal |last1=Cotton |first1=W. D. |last2=Dallacasa |first2=D. |last3=Fanti |first3=C. |last4=Fanti |first4=R. |last5=Foley |first5=A. R. |last6=Schilizzi |first6=R. T. |last7=Spencer |first7=R. E. |date=2003-11-17 |title=The Faraday screen near the nucleus of the CSS quasar 3C 138|url=http://dx.doi.org/10.1051/0004-6361:20030523 |journal=Astronomy & Astrophysics |volume=406 |issue=1 |pages=43–50 |doi=10.1051/0004-6361:20030523 |bibcode=2003A&A...406...43C |issn=0004-6361}} located in the constellation of Taurus. It has a redshift of (z) 0.76.{{Cite journal |last1=Geldzahler |first1=B. J. |last2=Fanti |first2=C. |last3=Fanti |first3=R |last4=Schilizzi |first4=R.T. |last5=Weiler |first5=K.W. |year=1984 |title=High resolution observations of the quasar 3C138|url=https://articles.adsabs.harvard.edu//full/1984A%26A...131..232G/0000235.000.html |journal=Astronomy & Astrophysics |volume=131 |issue=2 |pages=232–236 |bibcode=1984A&A...131..232G}} The radio spectrum of this source appears both compact and steep, making it a compact steep-spectrum radio quasar.{{Cite journal |last1=de Vries |first1=W. H. |last2=O'Dea |first2=C. P. |last3=Baum |first3=S. A. |last4=Sparks |first4=W. B. |last5=Biretta |first5=J. |last6=de Koff |first6=S. |last7=Golombek |first7=D. |last8=Lehnert |first8=M. D. |last9=Macchetto |first9=F. |last10=McCarthy |first10=P. |last11=Miley |first11=G. K. |date=1996-09-30 |title=Hubble Space Telescope Imaging of Compact Steep Spectrum Radio Sources |url=https://iopscience.iop.org/article/10.1086/313001/fulltext/ |journal=The Astrophysical Journal Supplement Series |volume=110 |issue=2 |pages=191–211 |doi=10.1086/313001 |issn=0067-0049}}{{Cite journal |last1=Dallacasa |first1=D. |last2=Cotton |first2=W.D. |last3=Fanti |first3=C. |last4=Foley |first4=A.R. |last5=Schilizzi |first5=R.T. |title=5 GHz VLBI polarization of 3C 138. |url=https://articles.adsabs.harvard.edu//full/1995A%26A...299..671D/0000671.000.html |journal=Astronomy & Astrophysics |date=1995 |volume=299 |pages=671–673 |bibcode=1995A&A...299..671D}}{{Cite journal |last1=Shen |first1=Zhi-Qiang |last2=Shang |first2=Lin-Lin |last3=Jiang |first3=Dong-Rong |last4=Cai |first4=Hong-Bing |last5=Chen |first5=Xi |date=2006-12-25 |title=Phase-Reference VLBI Observations of the Compact Steep-Spectrum Source 3C 138 |url=http://dx.doi.org/10.1093/pasj/58.6.1033 |journal=Publications of the Astronomical Society of Japan |volume=58 |issue=6 |pages=1033–1037 |doi=10.1093/pasj/58.6.1033 |issn=2053-051X|arxiv=astro-ph/0611614 }} It is also one of the few 3C objects showing a defined and turn-over in its electromagnetic spectrum at low frequencies.

3C 138 is known to shown linear polarization at high degrees although the source of it remains a mystery. When correcting the Faraday rotation for this object, the electric field position angle is ~170°, indicating the magnetic field direction as ~ 80°. The source of 3C 138 is shown to emit gamma rays with a powerful flare detected in 2012.{{Cite journal |last1=Principe |first1=G |last2=Di Venere |first2=L |last3=Orienti |first3=M |last4=Migliori |first4=G |last5=D'Ammando |first5=F |last6=Mazziotta |first6=MN |date=November 2021 |title=Gamma-ray emission from young radio galaxies and quasars |url=https://academic.oup.com/mnras/article/507/3/4564/6353538 |journal=Monthly Notices of the Royal Astronomical Society |volume=507 |issue=3 |pages=4564–4563 |doi=10.1093/mnras/stab2357|doi-access=free |arxiv=2107.12963 }}

The radio structure of 3C 138 is unresolved at both 0.151 and 0.408 GHz. However, when detected through higher frequencies, it is revealed to be largely linear.{{Cite journal |last1=Akujor |first1=C. E. |last2=Spencer |first2=R. E. |last3=Zhang |first3=F. J. |last4=Fanti |first4=C. |last5=Ludke |first5=E. |last6=Garrington |first6=S. T. |date=1993-07-01 |title=3C 138 : multi-frequency observations of the suggested 'naked-jet' compact steep-spectrum source. |url=https://ui.adsabs.harvard.edu/abs/1993A&A...274..752A/abstract |journal=Astronomy and Astrophysics |volume=274 |pages=752–756 |bibcode=1993A&A...274..752A |issn=0004-6361}} The structure is then further divided into separate components consisting of a bright radio core and a radio jet{{Cite journal |last1=Axon |first1=D. J. |last2=Capetti |first2=A. |last3=Fanti |first3=R. |last4=Morganti |first4=R. |last5=Robinson |first5=A. |last6=Spencer |first6=R. |date=2000-04-07 |title=The Morphology of the Emission-Line Region Of Compact Steep-Spectrum Radio Sources |url=https://iopscience.iop.org/article/10.1086/316838/fulltext/ |journal=The Astronomical Journal |volume=120 |issue=5 |pages=2284–2299 |doi=10.1086/316838 |arxiv=astro-ph/0006355 |bibcode=2000AJ....120.2284A |issn=0004-6256}} which is polarized and showing a slight increase from 12 percent at 5 GHz to 14 percent at 15 GHz.{{Cite journal |last1=Ludke |first1=E. |last2=Garrington |first2=S. T. |last3=Spencer |first3=R. E. |last4=Akujor |first4=C. E. |last5=Muxlow |first5=T. W. B. |last6=Sanghera |first6=H. S. |last7=Fanti |first7=C. |date=1998-09-01 |title=MERLIN polarization observations of compact steep-spectrum sources at 5 GHz |url=https://articles.adsabs.harvard.edu/pdf/1998MNRAS.299..467L |journal=Monthly Notices of the Royal Astronomical Society |volume=299 |issue=2 |pages=467–478 |doi=10.1046/j.1365-8711.1998.01843.x |doi-access=free |bibcode=1998MNRAS.299..467L |issn=0035-8711}} There are several extended knots present within the main jet emission region extending 400 milliarcseconds in a 65° position angle. A low surface-brightness counter-jet is located in an opposite direction from the region.{{Cite journal |last1=Shen |first1=Z.-Q. |last2=Jiang |first2=D. R. |last3=Kameno |first3=S. |last4=Chen |first4=Y. J. |date=2001-01-31 |title=Superluminal motion in a compact steep spectrum radio source 3C 138 |url=https://www.aanda.org/articles/aa/pdf/2001/16/aah2606.pdf |journal=Astronomy & Astrophysics |volume=370 |issue=1 |pages=65–69 |doi=10.1051/0004-6361:20010193 |arxiv=astro-ph/0102083 |bibcode=2001A&A...370...65S |issn=0004-6361}} Furthermore, 3C 138 has a compact radio lobe located east and a much fainter, diffused radio lobe located west.{{Cite journal |last1=Shen |first1=Z.-Q. |last2=Shang |first2=L.-L. |last3=Cai |first3=H.-B. |last4=Chen |first4=X. |last5=Jiang |first5=D. R. |last6=Chen |first6=Y.-J. |last7=Liu |first7=X. |last8=Yang |first8=R. |last9=Kameno |first9=S. |last10=Hirabayashi |first10=H. |date=2004-12-16 |title=The Center of Activity in the Compact Steep-Spectrum Superluminal Source 3C 138 |url=https://iopscience.iop.org/article/10.1086/428284/pdf |journal=The Astrophysical Journal |volume=622 |issue=2 |pages=811–815 |doi=10.1086/428284 |issn=0004-637X|arxiv=astro-ph/0412549 }}

3C 138 is one of the four primary calibrators used by the Very Large Array. The other three are 3C 48, 3C 147 and 3C 286. For all visibilities of other sources, they are calibrated via observed visibilities of one of the four calibrators.{{Cite web |last=Witz |first=Stephan W. |date=December 2015 |title=Calibrating the Flux Density Scale |url=https://science.nrao.edu/facilities/vla/docs/manuals/oss/performance/fdscale |access-date=2024-10-27 |website=National Radio Astronomy Observatory |language=en}}