spiral galaxy
Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''Piscis Austrinus. It is located at a distance of about 110 million
light year
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
s from
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, which, given its apparent dimensions, means that NGC 7172 is about 100,000 light years across. It was discovered by
John Herschel
Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor, experimental photographer who invented the blueprint and did botanical wor ...
on September 23, 1834.
Characteristics
NGC 7172 is a spiral galaxy that is seen edge-on. A thick dark dust lane runs across the galaxy, obscuring the nucleus of the galaxy. The galaxy appears in deep photographs to be tidally distorted, and a diffuse tail is extending towards the north-west.
Nucleus
The nucleus of NGC 7172 was originally considered to be normal, but later observations revealed that it emitted X-rays and was a powerful
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
source, which exhibited variation. These findings indicated that NGC 7172 has an active galactic nucleus which is obstructed in optical wavelengths. The nucleus was classified as a type 2
Seyfert galaxy
Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars. They have quasar-like nuclei (very luminous, distant and bright sources of electromagnetic radiation) with very high surface brightnesses whose spectra ...
in the optical, but observations in other wavelengths suggest it is type 1 Seyfert galaxy. The source of activity of the nucleus is an accretion disk around a supermassive black hole. The mass of the supermassive black hole in the centre of NGC 7172 is estimated to be based on the MBH–σ⋆ relation or based on the X-ray scaling method.
NGC 7172 has been found to exhibit variability in X-rays. As observed by the Advanced Satellite for Cosmology and Astrophysics (ASCA), the galaxy exhibited short term variability of about 30% in the time scale of hours, which is common for Seyfert I galaxies. It also exhibits long term variability, as it dimmed three to four times between 1995 and 1996. This was also observed by
BeppoSAX
BeppoSAX was an Italian–Dutch satellite for X-ray astronomy which played a crucial role in resolving the origin of gamma-ray bursts (GRBs), the most energetic events known in the universe. It was the first X-ray mission capable of simultaneous ...
. The FeKα line appeared constant when observed with BeppoSAX, but appeared to variate by a factor of two by ASCA, and thus it is associated with an
accretion disk
An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other fo ...
. The hard X-ray spectrum appears similar to that of a Seyfert I galaxy, indicating that the nucleus is seen though a Compton-thin absorber.
In infrared observations of the nucleus dominates the spectrum of the active nucleus, with 3.4-μm carbonaceous dust absorption detected, but no 3.3-μm PAH emission, indicating that the nucleus is obstructed. A strong obsurption feature is observed, probably the 9.7-μm silicate dust
absorption line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...
. X-rays and mid-infrared have similar absorption columns. It is possible that an outflow towards the southwest is detected in radiowaves.
Nearby galaxies
NGC 7172 is part of the Hickson Compact Group 90. NGC 7172 lies 6 arcminutes north of the core of the compact group, which is comprised by the elliptical galaxies NGC 7173, and NGC 7174, and disturbed spiral galaxy NGC 7176; these three galaxies lie within 6 arcminutes from each other.
The compact group is surrounded by a more loose group, with 19 galaxies of similar
redshift
In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in f ...
within 1.5 degrees from the core group. Garcia identified as members of this group the galaxies NGC 7154, ESO 404- 12, NGC 7163, ESO 466- 36, ESO 466- 46, ESO 404- 27, NGC 7187, IC 5156, ESO 404- 39, and ESO 466- 51. Other nearby galaxies include NGC 7135 and its group, NGC 7204, and NGC 7208.