NGC 5548

NGC 5548 is a barred spiral galaxy in Boötes whose active galactic nucleus varies in brightness and shows Seyfert 1 characteristics.

Active galactic nucleus: A compact, luminous galaxy center powered by accretion onto a supermassive black hole. NGC 5548’s variable central engine is a well-studied example of this phenomenon.
Reverberation mapping: A method that maps active galactic nuclei by timing delays between continuum changes and emission-line responses. NGC 5548’s variability has enabled detailed measurements of its central regions.
AGN STORM: A multiwavelength monitoring campaign that tracked NGC 5548’s changing active nucleus in 2014. This campaign revealed unusual absorption and delays in the galaxy’s emission.
Barred spiral galaxy: A spiral galaxy whose central stars form an elongated bar from which spiral arms extend. NGC 5548’s host galaxy has this large-scale structure around its active center.
Seyfert galaxy: A galaxy whose bright nucleus emits strong radiation and distinctive spectral lines. NGC 5548 belongs to this class of active galaxies.
Continuum reverberation mapping: A technique that infers accretion-disk sizes from time delays among continuum variations at different wavelengths. Monitoring NGC 5548 across wavelengths tests how its disk responds to changing illumination.
BLR Holiday: An unexpected weakening of broad emission lines observed in NGC 5548 during its 2014 monitoring campaign. The episode showed that changes in the ionizing continuum alone could not explain every line variation.
Boötes: A northern constellation containing the bright star Arcturus and many galaxies. NGC 5548 lies in this constellation’s direction.
Seyfert 1 galaxy: A Seyfert galaxy whose spectrum shows broad permitted emission lines as well as narrow lines. Its broad-line spectrum establishes the specific Seyfert subtype associated with NGC 5548.
Virial mass estimation: A method for estimating a black hole’s mass from gas velocity and its distance from the black hole. Its broad-line variability provides the velocity and distance estimates needed for this calculation.

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