NGC 1097 - The Galaxy with the Longest known Optical Jets
NGC 1097, Arp 77 Constellation: Fornax, Galaxy type: SBbc (barred spiral)
RA = 2h 46m 18.9s Size: 9.3’ x 6.2’ Recession Velocity: 1,284 km/sec
DEC = -30° 16’ 21” m = + 10.2 Distance > 20 Mpc
In 1974, astronomer Halton Arp was sitting at a viewing machine scanning some deep plates in the southern sky when R.D. Wolstencroft walked up to him with a long exposure plate taken with the United Kingdom’s 48” Schmidt reflector of the barred spiral galaxy NGC 1097. Arp was asked what he thought of a faint luminous smear pointing away from the galaxy’s northern edge. He replied that it must be a plate defect since he had never seen anything like it. Later plates by Wolstencroft continued to show this “jet-like” feature emerging from the galaxy.
In 1975 about 1 year later, Dr. Arp had a 2 week night run on the new 4-meter reflector at Cerro Tololo in Chile by strict invitation by the Observatory’s director at the time, Victor Blanco. Arp’s main target was NGC 1097 and he took many photos of it. A total of 4 optical jets showed up all pointing away from the host galaxy, thanks to the masterful image processing by Arp’s colleague Jean Lorre. The jets are identified as JET 1, JET 2, JET 3, and JET 4. Figure 1 shows this image processed view of NGC 1097 with the jets. Traced backwards, all the jets intersected at the nucleus of NGC 1097. From direct examination of the high resolution photographs, Arp was able to estimate a) the age of the jets, b) their ejection velocity, and c) frequency of ejection. Figure 2 shows NGC 1097 with many H II regions dotting the spiral arms. The southern spiral arm shows a section of the H II regions “missing” at the point of where JET 3 interacts. JET 1 seems (Figure 2) to have separated the northern spiral arm. As the galaxy has rotated since the “ejection” Arp concluded from the 15° disruption angle of the northern spiral arm, that JET 1 is approximately 10 million years old.
The assumed distance of NGC 1097 is 20 Mpc (based upon a Hubble Constant of 65 km/sec/Mpc). JET 2’s extent of 9’ from the galaxy center has a linear amplitude of 52 kpc. Assuming a 10 million year lifetime, the ejection velocity must be on the order of 5,200 km/sec. The jets also exhibit a very low surface brightness as they are only seen on deep exposures. Dr. Arp has suggested that being so faint at an early age (10 million years) makes them transient, and probably a short lived phenomena similar to planetary nebula. Which is presumably why we don’t see many jets in other galaxies. Some other galaxies that have optical jets are M87, NGC 5128 (also known as Centaurus A) and quasar 3c 273.
In 1979 follow up observations of NGC 1097 was observed by the Einstein X-Ray satellite. A lot of X-Ray emission was evident on the north side of the galaxy. An object of m = +18 coincided with one bright patch of X-Ray emission - and this object turned out to be a quasar. Follow up observations in 1983 by Arp et. al. showed an even higher quasar density. Thirty-four quasars to m = +20 were discovered within 80 arc minutes of NGC 1097, far too many to be accounted for by random chance. (There are only 1000 quasars known across the entire sky!) And most of these quasars appear to be on the north side of the galaxy along the lines of the two northern jets! The redshifts of the quasars ranged from z = 0.34 to z = 3.09. Despite the overwhelming evidence of the association of these quasars with Arp’s ejection theories, the quasar density paper was almost rejected by the Astrophysical Journal. After a year and a half delay, it was published in the October 1984 issue.
The nature of the optical jets still remains a mystery as no conclusive observational proof (other than the unusually high quasar density) exists that the quasars (or some other material) have been ejected from NGC 1097. A recent paper by Ann Wherle, et. al. (Astronomical Journal) entitled “The Nature of the Optical “JETS” in the Spiral Galaxy NGC 1097” studied the colors of the optical jets. Although Wherle, et. al could not identify the composition of the jets by examining the B-V colors, their conclusions in the paper were based upon the process of elimination. Their observations were: 1) the jets showed no radio emission at certain frequencies, 2) that their colors were very blue, (indicating these colors are consistent with starburst formation seen in other galaxies) and 3) optically, the jets are inconsistent with the jet in the galaxy M87 in which something appears to have been ejected (The M87 jet is not continuous, but shows several knots). Wherle, et. al described that the jets in NGC 1097 were the result of some previous gravitational encounter with the companion galaxy NGC 1097A. The jets are thus merely tidal plumes from such an encounter, and the “X” pattern of these jets is due to the viewing angle. Wherle, et. al also mentioned in their 1997 paper that they have conducted a survey of other bright galaxies for similar “X” pattern jets. They have found none to date and thus the interacting galaxy “NGC 1097 with four optical jets is still a rara avis in the celestial menagerie.” Would any CCD imagers be willing to try and image the jets?
Figure 1. NGC 1097 image processed Figure 2. Notice the break in the northern spiral arm near Jet 1. to show the 4 optical jets Southern spiral arm near Jet 3 has a missing section of H II regions..
Finder chart for NGC 1097 in Fornax. Chart measures 35° square.
Arp, H., 1976, Ejection from the Spiral Galaxy NGC 1097, Astrophysical Journal Letters, 207, L147-L150.
Arp, H., Wolstencroft, R.D., He, X.T., 1984, Complete Quasar Search in the NGC 1097 Field, Astrophysical Journal, 285, p. 44-54.
Arp, H., 1996, Identification of X-Ray Sources < 1º from Seyfert Galaxies, Astronomy & Astrophysics, 319, p. 33-44.
Wehrle, A., Keel, W., Jones, D., 1997, The Nature of the Optical "Jets" in the Spiral Galaxy NGC 1097, Astronomical Journal, 114, p. 115