NGC 1553

Lore & Background

NGC 1553 was discovered by British astronomer John Herschel on December 5, 1834 using an 18.7 inch reflector. It is an early type galaxy with a luminosity of 4 L☉. It has been detected in H I but has an H I mass to B-band luminosity ratio of less than 0.01. Its ultraviolet spectrum shows weak flux under the main-sequence turn off at around 2,400 A°, characteristic of old intermediate populations of quiescent lenticular galaxies. The galaxy has an especially well developed lens component of nearly constant surface brightness, about 36″ across, found between the bulge and the exponential disk, similar to that in NGC 3945; its inner parts are very hot. NGC 1553 has associated cool gas and dust, is visible in infrared, and is a weak radio source. Hubble observations in 2000 revealed an inner torus-like dust lane about 3″ across at the galaxy's center. Chandra X-ray imaging shows diffuse hot gas making up 70% of the emissions, dotted with many point-like sources (low-mass X-ray binaries) making up the rest. The brightest point source is coincident with the galaxy's optical nucleus, and its luminosity and spectrum suggest it is an obscured central active galactic nucleus (AGN). Globular clusters have been resolved by the Hubble Space Telescope, with an estimated 600 globulars; like Messier 87, there is a central deficit of globulars with respect to the underlying luminosity. NGC 1553 has a roughly symmetrical X-ray spiral feature not detected in visible light, with a possible 10″ inner bar in the diffuse emission near the center, probably due to adiabatic or shock compression of ambient gas perhaps due to interaction with the radio source. Hα emission also has spiral structure surprisingly similar to the X-ray spiral.

Reader's Guide

NGC 1553 is notable as a prototypical lenticular galaxy and the second brightest member of the Dorado Group. Its interaction with NGC 1549 appears to be in an early stage, visible optically through faint irregular shells of emission and a curious jet on the northwest side. The galaxy's X-ray emissions, dominated by diffuse hot gas and dotted with low-mass X-ray binaries, reflect an active past involving binary star systems of black holes and neutron stars, most located in globular clusters. The presence of an obscured central AGN adds to its significance as a site of ongoing nuclear activity. The X-ray spiral feature, not seen in visible light, and the similar Hα spiral structure provide clues to gas dynamics possibly linked to the radio source. With an estimated 600 globular clusters and a central deficit similar to Messier 87, NGC 1553 offers insights into the formation and evolution of lenticular galaxies and their globular cluster systems. Its well-developed lens component and hot inner parts further characterize it as a key object for studying the structure of early-type galaxies.

Did You Know?

• NGC 1553 forms an interacting pair with elliptical galaxy NGC 1549, which lies 11.8 arcminutes away.
• The galaxy has an estimated 600 globular clusters, with a central deficit similar to Messier 87.
• Chandra X-ray imaging shows that 70% of its emissions come from diffuse hot gas.