IC 1613

IC 1613 (object 1613 in the Index Catalogues (IC), also known as Caldwell 51) is an irregular dwarf galaxy, visible in the constellation Cetus near the star 26 Ceti.^[6] It was discovered in 1906 by Max Wolf,^[7] and is approaching Earth at 234 km/s.

IC 1613
IC 1613 captured with the VLT's OmegaCAM.[1]
Observation data (J2000 epoch)
Constellation	Cetus
Right ascension	01h 04m 47.8s[2]
Declination	+02° 07′ 04″[2]
Redshift	−234±1 km/s[2]
Distance	2.38±0.07 Mly (730±20 kpc)[3][4]
Group or cluster	Local Group
Apparent magnitude (V)	9.9[2]
Absolute magnitude (V)	−15.2[5]
Characteristics
Type	IB(s)m[2]
Mass	(1.1±0.2)×108[5] M☉
Apparent size (V)	16.2′ × 14.5′[2]
Other designations
UGC 668,[2] DDO 8,[2] PGC 3844,[2] Caldwell 51

The sky around IC 1613, with 26 Ceti towards the bottom of the frame (south) and 29 Ceti to the left (east)

IC 1613 is a member of the Local Group.^[8] It has played an important role in the calibration of the Cepheid variable period-luminosity relation for estimating distances.^[9] Other than the Magellanic Clouds, it is one of the few Local Group dwarf irregular galaxy where RR Lyrae-type variables have been observed; this factor, along with an unusually low abundance of interstellar dust both within IC 1613 and along the line of sight enable especially accurate distance estimates.^[8][10]

In 1999, Cole et al.^[8] used the Hubble Space Telescope to find that the dominant population of this galaxy has an age of ~7 Gyr. Using its Hess diagram, they found that its evolutionary history may be similar to that of the Pegasus Dwarf Irregular Galaxy. Both galaxies are classified as Ir V in the DDO system. Also in 1999, Antonello et al. found five cepheids of Population II in IC 1613, giving self-evident support for the existence of a very old stellar population component of IC 1613. In 1999, King, Modjaz, & Li discovered the first nova ever detected in IC 1613.^[11]

Hubble image of the central part of IC 1613, showing the faint galaxies of a background galaxy cluster.

IC 1613 contains a WO star known as DR1, which might be the only Wolf–Rayet star in the galaxy,^[12][13] although a candidate WC+O binary, SPIRITS14bqe, has been found.^[14] A luminous blue variable candidate^[15] has been proposed but a revised classification led to a B2.5 III type.^[16] Moreover, a rich population of OB-type^[16] stars and OB associations^[17] has been discovered.

There are many faint galaxies close to IC 1613, 14 of which are catalogued as members of a yet-unnamed galaxy cluster located at z≈0.20.^[18]