Summer Research Fellowship Programme of India's Science Academies

Studying the nature of the galaxies in the Arp catalogue

Rishabh Nakra

Guru Nanak Dev University, Scf 54, UT Market Grand Trunk Road, Off, NH 1, Amritsar, Punjab 143005

Mousumi Das

Indian Institute of Astrophysics, Sarjapur Main Road, 2nd Block, Koramangala, Bengaluru, Karnataka 560034


The ARP catalogue of peculiar galaxies is one of the most well known samples of peculiar galaxies in the local Universe. It was catalogued by Halton Arp in 1966. The atlas of these galaxies was originally published by California Institute of Technology. It consists of 338 images of interacting and tidally distorted galaxies, some of which are spectacular starburst galaxies and others small groups of active galaxies. This catalogue also includes some of the most prominent galaxies in the universe such as the Pinwheel galaxy, Whirlpool galaxy, M87, M77, M90, Tadpole galaxy, Mice galaxies, the Antennae galaxies and the Cigar galaxy among others. Because little was known about the physical processes that give the galaxies their peculiar shape, Arp catalogued them according to their physical appearance. Today, these physical processes are well understood and we can categorize these galaxies in many more ways. In this study we revisit this sample to see what fraction are isolated galaxies, tidally distorted galaxies, merging galaxies, active galactic nuclei and multiple nuclei systems. Apart from defining their nature, we will also try to estimate how their nuclear velocity dispersions are changing with nuclear separations, how their UV emission changes with morphology and what fraction are bright at radio wavelengths. Our results will tell us how galaxies change in brightness over different wavelengths during interactions and mergers.

Keywords: interacting, merging, starbursts, starformings, AGNs, tidal dwarfs


 AGNActive Galactic Nucleus 
IC Index Catalogue 
NED NASA/IPAC Extragalactic Database 
 NGCNew General Catalogue
SDSS Sloan Digital Sky Survey 
 SFRStar Formation Rate 
 UGCUppsala General Catalogue 


The Hubble classification scheme, which is widely used in extragalactic astronomy, classifies the galaxies into three main categories on the basis of their shape. The three categories are: elliptical, lenticular and spiral. The fourth category belongs to irregular galaxies. However, in the later half of the twentieth century, certain pecular galaxies were catalogued by astronomers such as Vorontsov-Velyaminov, A.G. Wilson, E. Herzog, Thornton Page, W.W. Morgan, F. Zwicky, C. Kowal, and G. Reaves. It was clear that not all the galaxies fit into the Hubble sequence. In 1966, using the 200 inch telescope at Palomar Observatory, ​Halton Arp, 1966​ published his catalogue of 338 peculiar objects. This catalogue, now known as the Arp Catalogue of Peculiar Galaxies, also covers the fainter peculiar galaxies that were not catalogued in the previous surveys. These galaxies were divided into categories based on their physical appearence. Now we have fair knowledge of the physical processes that give these galaxies their peculair shape. Using the data from the Sloan Digital Sky Survey (SDSS), we can study the nature of Arp galaxies and categorize them further into single and multiple Active Galactic Nuclei (AGN), starburst galaxies, single H II galaxies etc. In this project, we aim to study the nature of these galaxies.


In 1926, Edwin Hubble invented a morphological classification scheme for galaxies, known as the Hubble sequence. Under this, the galaxies were classified on the basis of their visual appearence into three broad categories: elliptical, lenticular and spiral. A fourth class contains galaxies with irregular appearence. Hubble's classification scheme was widely used by the astronomers in the first half of the twentieth century. As more and more patches of the sky were surveyed, astronomers catalogued various peculiar galaxies that did not fall under Hubble's classification scheme. One of the most important catalogue of such peculiar galaxies is the Arp catalogue, named after Halton Arp. Most of the 338 images of this catalogue were taken by 200 inch telescope at the Palomar Observatory, California. Arp categorized these images according to their visual appearence as the physical processes responsible for the peculiar nature of these galaxies were not understood. There were six major categories in this catalogue:

Major Categories In Arp Catalogue
Arp NumberClassification
 1-101Largest class involving spiral galaxies
102-145Elliptical and elliptical like galaxies 
146-268Neither elliptical nor spiral
 269-327 Double galaxies
332-338 Miscellaneous 

Today, the physical processes behind these pecularities are well known. With the use of modern telescopes and surveys, it has become possible to further categorize the galaxies according to their spectrum. A lot of these galaxies are interacting galaxies, starforming galaxies, AGNs, starburst galaxies and galaxy merger remnants. The Arp catalogue is a rich source of studying the physics of peculiar galaxies.

Some Notable Arp Galaxies

There are many galaxies in this catalogue that are more notable than the others. Some of them are listed in the table below.

Some Notable Arp Galaxies
Arp Number Common NameNotes
 26Pinwheel Galaxy Spiral Galaxy
37 Messier 77Radio Galaxy
85 Whirlpool GalaxyInteracting Galaxy
116Messier 60 Colliding Galaxies
152 Virgo A Elliptical Galaxy
 188Tadpole Galaxy Galaxy Finishing Merging 
242 Mice Galaxies  Colliding Galaxies
244 Antennae Galaxies  Colliding Galaxies
319  Part of Stephen's QuintetGalaxy In Colliding Group 
 337 Cigar GalaxyStarburst Galaxy 


The Sloan Digital Sky Survey (SDSS) is a multi spectral imaging and spectroscopic redshift survey that uses a 2.5 m wide angle optical telescope at Apache point observatory in New Mexico, USA. Images are taken using a photometric system of five filters (u,r,z,i and g). The telescope's imaging camera is made up of 30 CCD chips. The chips are arranged in 5 rows of 6 chips. Each row has a different optical filter with average wavelengths of 355.1, 468.6, 616.5, 748.1 and 893.1 nm, with 95% completeness in typical seeing to magnitudes of 22.0, 22.2, 22.2, 21.3, and 20.5, for u, g, r, i, z respectively These images are then processed to produce lists of objects. Using the spectroscopic data, stars, galaxies and quasars are also selected for spectroscopy. In this project, we have studied the Arp galaxies surveyed by SDSS. The spectrum of these galaxies provides an important insight to reclassify them as AGNs, starburst and starforming galaxies. The data of nuclear velocity dispersion of galaxies and the images in different band are also taken from the SDSS.


The first objective of the project is to look up at the Arp galaxies in the SDSS and classify according to their apparent and physical features.

The Apparent Classification

  • Single Galaxies
  • Interacting Galaxies
  • Merging Galaxies

Single galaxies

The catalogue contains many single galaxies that are not physically interacting with other systems. Some of these galaxies have close neighbours in the frame but using the redshift of these galaxies, we can determine if they are the background/foreground galaxies. These single galaxies are of many types. Most of them are starforming and starburst galaxies.

Single Galaxies
 Arp Number Common Name NED Classification
1 UGC 2857 SA(s)c
2 UGC10310  SB(s)m
5 NGC 3664 SB(s)m pec
6 NGC 2537  SB(s)m pec
10 UGC 1775 S
12 NGC 2608 SB(s)b?
27 NGC 3631SA(s)c
36 UGC 8548SB
37M 77 (R)SA(rs)b
43IC 607 SB(rs)bc
44 IC 609SAB(rs)bc pec
 49  NGC 5665SAB(rs)c pec
152 M 87cD0-1 pec
155 UGC 5184 SBb
165NGC 2418 E
188  Tadpole GalaxySB(s)c pec
189  NGC 4651SA(rs)c
203  NGC 3712SB
215  NGC 2782 SAB(rs)a pec
217  NGC 3310SAB(r)bc pec
233 UGC 5720 Sb
234 NGC 3738Im
235  NGC 14(R)IB(s)m pec
263NGC 3239 IB(s)m pec
264 NGC 3104  IAB(s)m
267  UGC 5764 IB(s)m
334  UGC 8498Sb
335 NGC 3509 SA(s)bc pec
337 Cigar GalaxyI0 edge-on

Some Notable Single Galaxies

  • Arp 152: A supergiant elliptical galaxy with a SMBH at its center, the first one to be imaged by the Event Horizon Telescope.
    Hubble Space Telescope of M87 (Arp 152)
    • Arp 188: Also known as the tadpole galaxy, it is a result of a merger and has a very long characteristic tidal tail of gas.Arp 189: Also known as the umbrella galaxy, it contains a strange structure that extends from its disk to the east.
    Tadpole Galaxy.JPG
      The Tadpole Galaxy Showing a Long Tidal Tail
      • Arp 189: Also known as NGC 4651, this galaxy is peculiar because of an umbrella shaped structure that extends from its disk to the east and is composed of stelar streams. It is the remnant of a tidal interaction of a small galaxy with NGC 4651.
      189 Arp.JPG
        Arp 189

        Interacting galaxies

        Most of the galaxies in the Arp catalogue are either interacting galaxies or merging galaxies. The interacting galaxies are the ones whose gravitational fields have caused a disturbance in each other. One of the most common type is a satellite galaxy disturbing the larger galaxy's spiral arm. The Arp catalogue contains interacting galaxies of all levels. While some are very far away from each other and are only connected with very faint HII tails, some galaxies are on the extent of becoming one after merging. The interacting events can trigger star formation and in some cases, the galaxies can turn into starburst galaxies. In few cases, the tidal interaction between the galaxies can lead to the formation of Tidal Dwarf Galaxies (TDGs). The colliding galaxies must be gas rich in order to support the formation of a TDG. The table below contains the list of interacting galaxies in the Arp catalogue. Some of the companion galaxies are foreground/background galaxies which can be determined from their redshifts.

        List of Interacting Galaxies
         Arp NumberCommon Name Companion Comments 
        11 UGC 717 UGC 719 Split Arm
        30 UGC 10832 NGC 6365Both starforming galaxies
        32UGC 10770  -Tidal tails look like spiral arms 
        34NGC 4615 NGC 4614, NGC 4613 Galaxy triplet with two starforming galaxies 
        40  IC 4271SDSS J132921.33+372434.9 Spiral with low surface brightness companion. 
        45UGC 9178VV 2bStarburst galaxy
        47 VV 435 MCG+03-38-014 Spiral with low surface brightness companion
        63  NGC 2944 NED01 NGC 2944 NED02 Starburst galaxy
        64UGC 9503  UGC 9503 NOTES02 Starforming Galaxy
        71 NGC 6045 2MASX J16051033+1745298 Galaxies part of cluster 
        72 NGC 5994  NGC 5996Both are starburst galaxies 
        73IC 1222 SDSS J163514.15+461232.6 IC 1222 is starburst galaxy 
        78  NGC 772NGC 770  A tidal arm has started to emerge
        82  NGC 2535 NGC 2536 Starburst (NGC 2535) interacting with star forming
        83NGC 2799, NGC 3800 - NGC 3799 is AGN
        84  NGC 5394, NGC 5395 - Connecting arms
        85 Whirlpool Galaxy  NGC 5195 M51 with Seyfert II nucleus
        87  NGC 3808A, NGC 3808B -  One is edge on barred galaxy
        89NGC 2648 [BKD2008] WR 561 Interacting with a starburst galaxy 
        91  NGC 5953, NGC 5954- Tail of starburst (NGC 5953) heading towards the nucleus 
        92NGC 7603 PGC 71041 Spiral Seyfert interacting with elliptical  
        95 IC 4461, IC 4462 - Starforming galaxies 
        97  VV 013a VV 013b Starburst interacting with long tidal tail
        101 UGC 10184, UGC 10169  -Long tidal tail
        104  NGC 5216, NGC 5214 -A bridge of gas between the interacting galaxies
        105  NGC 3561 PGC 33992 Also known as "The Guitar". Interaction of a starburst and AGN
        106  NGC 4211 UGC 7277 SInteraction of starburst and AGN. Close nuclei. 
        107  UGC 5984 SDSS J105218.49+300420.9Galaxies are in a process of colliding and merging. 
        111  NGC 5421 - Galaxy group
        115 UGC 6678 MCG+05-28-021Galaxy triplet with two eliipticals and one spiral 
        116  NGC 4647, M 60 -Extremely bright elliptical (M 60) with a spiral (NGC 4647) 
        117  IC 982, IC 983 - The spiral galaxy is extremely large
        118  NGC 1141, NGC 1142-  NGC 1142 is off centered an Type II AGN 
        120 NGC 4435, NGC 4438 - Also known as Eyes Galaxy
        124  NGC 6361 MCG+10-25-003Elliptical and spiral almost at same distance 
        125 UGC 10491 - Galaxy pair. 
        127 NGC 191, IC 1563- NGC 191 is a spiral galaxy interacting with IC 1563, a lenticular galaxy
        132  Arp 32 - Very mild interaction
        133 NGC 541 NGC 543, NGC 547 The jets from NGC 541 have triggered a star formation region nearby, known as Minkowski's object
        134  M 49 UGC 7636 Giant elliptical gravitationally interacting with irregular dwarf galaxy
        138 NGC 4015 SDSS J115842.99+250233.4Elliptical interacting with edge on spiral 
        140  NGC 274, NGC 275- Severe interaction
        142 NGC 2936, NGC 2937, UGC 5130 - NGC 2936 is in penguin shape and is interacting with NGC 2937
        143  NGC 2444, NGC 2445 - Ring system of galaxies
        147 IC 298 -Interacting pair of ring galaxies
        149IC 803  LEDA 215034Jets emitted due to interaction 
        168  M 32 Andromeda Galaxy Dwarf elliptical in interaction with Andromeda (M 31) 
        169  NGC 7236, NGC 7237, NGC 7237C-  Galaxy triplet
        171  NGC 5718, IC 1042 - Nuclei are fairly close
        172 IC 1178, IC 1181 - Close nuclei
         173UGC 9561 NVSS J145129+092005 UGC 9561 has very long counter tail 
         174NGC 3068 2MASX J09583805+2852155 Both are broadlines 
        175  IC 3481, IC 3481A, IC 3483 - Galaxy triplet
        178 NGC 5613, NGC 5614, NGC 5615  - Galaxy triplet
        191  UGC 6175 MCG+03-28-062 Interacting galaxy pair with counter tail
        196 Arp 196 - Minor interaction
        197  IGC 6503, IC 701 - The pair contains starforming galaxy
        201  UGC 224 MCG+00-02-019 Nuclei fairly close
        202  NGC 2719, NGC 2719 A - Interacting galaxy pair
        206 NGC 3432 UGC 5983 Intense star formation due to interaction
        208MCG+08-31-009, MCG+08-31-010 -  Interacting galaxy pair
        214 NGC 3718NGC 3729 Mild gravitational interaction
        218 MCG+03-40-057LEDA 1564508  Galaxy has a loop arm 
        232NGC 2911 NGC 2914 Peculiar S0 galaxy
        238  UGC 8335 - Extremely interacting pair
        239  NGC 5278, NGC 5279 - Connected arms. Extreme interaction.
        240 NGC 5257, NGC 5258 - Extreme interaction of starburst galaxies with connected arms
        241 UGC 9425  Segner's Wheel, VV 264bExtreme interaction 
        242 Mice Galaxies - Long tails of tidal interaction
        245 NGC 2992 NGC 2993 NGC 2992 is Seyfert galaxy in Hydra
         246 NGC 7837, NGC 7838 - Minor interaction
         247 IC 2338, IC 2339 - Interaction of two starbursts
         248 VV 035 - Interacting galaxy triplet
        255 UGC 5304 FIRST J095310.3+075224 Interacting galaxy pair 
        256 Arp 256 -One of the interacting galaxy is starburst
        260  UGC 7230 2MASX J12133956+1607334 Two Sc galaxies interacting with each other
        269  NGC 4485, NGC 4490 - Irregular galaxy interacting with spiral
        270 NGC 3395, NGC 3396  - Interacting galaxy pair
        271NGC 5426, NGC 5427  - Interacting spirals
        272 NGC 6050, IC 1179 - A third galaxy SDSSCGB 4240.3 is present between the connecting arms
        275 NGC 5679A, NGC 5679B, NGC 5679 C - Interacting galaxy triplet
        277  NGC 4809, NGC 4810 - Blue galaxies
        280  NGC 3769, NGC 3769A - Interacting galaxy pair
        282  NGC 169, NGC 169A - Barred spiral interacting with a smaller companion
         283 NGC 2798,NGC 2799-  Interacting galaxy pair
         285 NGC 2854, NGC 2856 - Galaxies are ver far but gravitationally interacting
         286 NGC 5560, NGC 5566, NGC 5569-  Interacting galaxy triplet
         287 NGC 2735, NGC 2735A-  Wind effect. Very bright core
        290  IC 195, IC 196 - Interacting galaxies with one showing wind effect
        293NGC 6285, NGC 6286 - Interacting galaxy pair
         294NGC 3786, NGC 3788  - Interacting galaxy pair
        297  NGC 5754, NGC 5752 - Interacting galaxy pair within a galaxy group
        301 UGC 6204, UGC 6207 - Interacting galaxy pair
         302 UGC 9618 - Gas rich spirals in their early stages of interaction
        305  NGC 4016, NGC 4017 - Galaxies very far away but at similar redshifts
        313 NGC 3994, NGC 3995-  Starburst (NGC 3995) interacting with starforming
        318  NGC 833, NGC 835, NGC 838 - Galaxy triplet
        320  Copeland's Septet - Group of interacting galaxies
        322 UGC 6527  - Interacting galaxy group
        323  Hickson 98 A-D - Chain of galaxies
        324 UGC 10143 - Chain of galaxies
        325 UGC 8613 LEDA 214126 Starburst (UGC 8613) interacting with broadline
        328 Hickson 72 - Interacting galaxy group

        Some Notable Interacting Galaxies

        • Arp 78: Arp 78 or NGC 772 is an unbarred spiral galaxy in the constellation of Aries. It is twice the size of Milky way and is surrounded be several dwarf satellite galaxies including NGC 770. The interactions with the satellite galaxies has caused the emergence of a single elongated tidal arm. In Arp's catalogue, the galaxy is listed under the group, "Spiral galaxy with small high surface brightness companion".
        NGC 772.jpg
          NGC 772 (Arp 78)
          • Arp 85 (M52): The Whirlpool Galaxy is a grand design interacting spiral galaxy in the constellation of Canes Vanatici. It is notably interacting with its companion galaxy M51 b. The galaxy has a Seyfert II nucleus.
            The Whirlpool Galaxy 
            • Arp 133: NGC 541 is a radio galaxy. There is a stellar bridge between NGC 541 and two nearby ellipticals NGC 545 and NGC 547. There is a peculiar fragmant near NGC 541, known as Minkowski's object. It lies in the direction of the radio jet. ​Jacqueline van Gorkom, 2006​ showed that the jet has caused a starburst in Minkowski's object. The current SFR is 0.52 M per year. The image below shows NGC 541 and Minkowski's object in infrared band.
            NGC 541.JPG
              Minkowski's object (marked with green circle) along with NGC 541 (below)

              Merging galaxies

              List of Merging Galaxies
              Arp Number Common Name Merging With Comments 
              55 UGC 4881 NAME Grasshopper B Starforming galaxy 
              90NGC 5929, NGC 5930  - NGC 5929 is starburst and NGC 5930 is AGN
              122 NGC 6040  PGC 56942 Contains a broadline AGN. Severe interaction.
              128UGC 827  UGC 827 NED 02Merging of an elliptical and spiral 
              148  Mayall's Object - A mergeing pair of galaxies with ring shape and tail emerging from it.
              157NGC 520  - Notable intermediate-stage merger
              161  UGC 6665 - Extreme merger
              166 NGC 750, NGC 751 - Merging ellipticals
              167  NGC 2672, NGC 2673- Merging ellipticals 
              177VV 480 -The pair contains an AGN
              192 NGC 3303 LEDA 93104Contains narrow filament 
              194  UGC 6945-  Interacting galaxy triplet 
              195  UGC 4653 VV 243c, VV 243b Interacting galaxy triplet
              198  UGC 6073 [CCA99] UGC 6073bEdge on galaxy merging with a face one galaxy 
              199  NGC 5544, NGC 5545 - Merger of two spirals
              209 NGC 6052 - NGC 6052A and NGC 6052B are in an advanced stage of merger
              211 UGCA 290 MCG+07-26-034 Advanced merger
              237 UGC 5044 MCG+02-24-014  Merging pair
              243  NGC 2623- Late state of merger
              244 Antennae Galaxies- Intense starburst regions
              250  APG 250, NVSS J073556+352304-  Nuclei ver close. A spiral nearby
              299  IC 694, NGC 3690- Both are barred irregular galaxies
              308  NGC 545, NGC 547- Merging ellipticals and radio galaxies
              310  IC 1259-Merging ellipticals just like Arp 308

              Some Notable Merging Galaxies

              Arp 148: Also known as the Mayall's object, Arp 48 is a pair of colliding galaxies in the constellation of Ursa Major. It is resulting in a new ring shaped galaxy with tail emerging from it.

              Arp 148].jpg
                Arp 148

                Arp 308: The Arp 308 is a pair of radio merging galaxies comprising an elliptical galaxy (NGC 547) and a lenticular galaxy (NGC 545) in Cetus. NGC 547 is a prominent radio galaxy while NGC 545 is weak. They both share a common envelope. However, no tidal forces have been detected between the galaxies.

                NGC 545 and 547.JPG
                  Arp 318
                  Apparent nature bar chart.png
                    Bar chart showing the apparent nature of Arp galaxies in SDSS

                    The Physical Classification

                    With the advancements in the field of spectroscopy, it has become possible to study the spectrum of distant cosmological objects in detail and determine the physical nature of those objects. Spectroscopically, in our work, we divide the galaxies of the Arp catalogue into three main categories:

                    • Starforming Galaxies
                    • Starburst Galaxies
                    • AGNs

                    Starforming galaxies

                    A star forming galaxy is the one that shows prominent Balmer emission lines in its spectrum. For a galaxy to be star forming, it must be rich in gas. SDSS classifies star forming galaxies from the spectrum. Star forming galaxies is set based on whether the galaxy has detectable emission lines that are consistent with star-formation according to the criteria: log10(OIII/Hα) < 0.7 – 1.2(log10(NII/Hα) + 0.4). The table below presents the star forming galaxies in the Arp catalogue along with their NED classification.

                    Star Forming Galaxies In Arp Catalogue
                     Arp Number Common Name NED Classification
                    30 UGC 10832 SBcd
                    34NGC 4615 Scd
                    36UGC 8548  SB
                    42 NGC 5829  SA(s)c
                    45 VV 2b N/A 
                    55 UGC 4881  pair
                    59 NGC 341 SAB(r)bc 
                    62 UGC 6865 S 
                    64 UGC 9503  Sb
                    69 NGC 5579  SABcd
                    95  IC 4461 N/A
                    106 UGC 7277 S  pair
                    242  IC 819, IC 820 S0 pec, SB0/a(s) pec
                    270 IC 3396  IBm pec
                    272 IC 1179  SB(rs)cd
                    280  NGC 3769 SB(r)b
                    285 NGC 2854  SB(s)b
                    303 IC 564  SA(s)cd pec
                    305 NGC 4017  SABbc
                    313 NGC 3994  SA(r)c pec
                    320 NGC 3754 SBb pec 

                    Starburst galaxies

                    A starburst galaxy is a galaxy that has an extremely high star formation rate (SFR) as compared to the long term SFR in that galaxy. The SFR is generally expressed in M per year where M is the mass of Sun (1.99 x 1030 Kg). The SFR of Milky Way is 3M/year. Starburst galaxies can have SFR more than 100 times that of Milky Way. Most, but not all starbursts are the result of galaxy mergers or interaction between galaxies. The galaxies must be home to large quantities of gas to support this extreme SFR. The phenomenon of starburst can be galaxy-wide or be confined to a small region of the galaxies. The spectrum of a Starburst Galaxy looks much like that of an ionized hydrogen region because the light from these galaxies is dominated by giant HII regions ionized by recently formed, massive, hot stars.

                    Arp 45 Sb Spec.gif
                    The Spectrum of Arp 45
                      Arp 45.PNG
                      SDSS Image of Arp 45
                        Arp 33 Sb Spec.gif
                        The Spectrum of Arp 33
                          Arp 33.PNG
                          SDSS Image of Arp 33
                            The Spectrum And Images of Typical Starburst Galaxies

                            Starburst galaxies in Arp catalogue

                            Starburst Galaxies In Arp Catalogu
                             Arp Number Common Name NED Classification
                            5NGC 3664SB(s)m pec
                            6NGC 2537  SB(s)m pec
                            33UGC 8613Integral-sign spiral
                            45 UGC 9178M51 Type
                            49 NGC 5665SAB(rs)c pec
                            60Arp 60 M51 Type
                            63NGC 2944SB(s)c pec
                            69NGC 5579 SABcd
                            72IC 607 SB(rs)bc 
                            73 IC 609SAB(rs)bc pec
                            79 NGC 5665SAB(rs)c pec?
                            82 M 87cD0-1 pec
                            89 UGC 5184SBb
                            90NGC 2418 E
                            91  Tadpole GalaxySB(s)c pec
                            97 NGC 4651SA(rs)c
                            233  NGC 3712SB
                            240  NGC 2782SAB(rs)a pec
                            247  NGC 3310SAB(r)bc pec
                            256 UGC 5720Im pec
                            269 NGC 3738Im
                            296 NGC 14(R)IB(s)m pec
                            305NGC 3239  IB(s)m pec
                            326NGC 3104  IAB(s)m
                            267  UGC 5746 SAB(rs)b? edge-on
                            334  UGC 8498 Sb
                            335 NGC 3509 SA(s)bc pec
                            337 Cigar Galaxy I0 edge-on

                            Active galactic nuclei

                            The centres of most galaxies contain a supermassive black hole. Most of these black holes are quite and invisible, thus being impossible to observe directly. But during the times when material is falling into their massive maws, they blaze with radiation, putting out more light than the rest of the galaxy combined. These bright centers are what is known as Active Galactic Nuclei, and are the strongest proof for the existence of SMBHs. The AGNs have extreme luminosity. This excess non stellar emission has been observed in the radio, microwave, infrared, optical, UV, X-ray and gamma wavelenghts. There are numerous subclasses of AGNs out of which the quasars are the most powerful. There is another subclass known as blazar, whose jets are pointed towards the Earth, and the radiation is enhanced by relativistic beaming.

                            The Arp catalogue is now home to many AGNs. Back in 1966 when Halton Arp completed this catalogue, the fact that galaxy centre contains supermassive black holes was not known. Today, with the advancement in radio astronomy and other spectroscopic techniques, we can identify such objects. The table below presents the list of AGNs in the Arp catalogue as surveyed in SDSS.

                            AGNs In Arp Catalogue As Surveyed By SDSS
                             Arp No. Common Name NED Classification
                            11UGC 717 SBb  -
                            37 M 77(R)SA(rs)b  Sy 2
                            40 IC 4271M51 type  -
                            60LEDA 1762846  M51 type  -
                            83 NGC 3799 SB(s)b pec   -
                            90  NGC 5929 Sab pec Sy 2 
                            92 NGC 7603 SA(rs)b pec  Sy 1.5
                            104 NGC 5216 E0 pec Sy 1
                            105 NGC 3561 SA(r)a pec LINER
                            106  NGC 4211 pair -
                            107UGC 5984 pair  -
                            122 NGC 6040B SA0^+ pecFlat-Spectrum Radio Source 
                            152  M 87 cD0-1 pec LINER
                            153 Centaurus AS0 pec  radio jet
                            154 NGC 1316 SAB0^0(s) pec radio jet
                             169 NGC 7237 SA0^- radio jet
                            177  MCG+04-35-017N/A  -
                            188  TadpoleSB(s)c pec  -
                            195 VV 243c N/A -
                            214  NGC 3718 SB(s)a pec LINER b
                            220[CMS2002] X-3N/AN/A
                            232 NGC 2911 SA0(s) pecFlat-Spectrum Radio Source 
                            242 IC 820 SB0/a(s) pec  -
                            243 NGC 2623 pec -
                            272 NGC 6050 SA(s)c -
                            307 NGC 2874interacting double Radio jet 
                            315 NGC 2831 E0 -
                            318 NGC 833 (R')Sa pec LINER
                            319  NGC 7319 SB(s)bc pecSy 2 
                            320 NGC 3746 SB(r)b -
                            326 UGC 8610 Sa -
                             334UGC 8498  Sb -
                             335NGC 3509  SA(s)bc pec -
                             336 NGC 2685 (R)SB0^+ pec Sy 2
                             337Cigar  I0 edge-on FR I
                              Bar Chart Showing Physical Nature of Galaxies In Arp Catalogue In SDSS

                              CENTRAL VELOCITY DISPERSION

                              In Astrophysics, velocity dispersion is the statistical dispersion of velocities about a mean velocity for a group of objects. A central velocity dispersion refers to the σ of the interior regions of an extended object, such as a galaxy or cluster. Since the Arp catalogue contains hundreds of interacting/merging galaxies, in our work, we aim to study the relation between the central velocity dispersion and the distance between two interacting or merging galaxies.


                              The first task is to calculate the angular distance between the centers of interacting galaxies. For that, we first find the right ascension (RA) α and declination (dec) δ from SDSS. The RA lies in the range [0,2π] and dec lies between [-π/2, +π/2]. Once we know RA and dec, the angular distance θ (in arcseconds) can be found using the relation:

                              cos θ = sin(δ1) sin(δ2) + cos(δ1) cos(δ2) cos(α12)

                              δ1, δ2, α1, α2 represent the dec and RA of the two galaxies. The angular seperation can be then converted into degrees. After this, we find the redshifts z1 and z2 of the two galaxies from NED. Using Hubble's law, we then find the distance to those galaxies. First we find the velocity of recession using v = cz, c being the speed of light in Km/s. Then we plug this value in Hubble's law,

                              v = Hd

                              where v is the velocity of recession in Km/s, H is Hubble's constant and d is the distance to the galaxy in megaparsec. The value of Hubble's constant is taken as given in NED (73 km/s/Mpc). Once the distances d1 and d2 are known, the seperation between galaxies in Mpc can be determined using the formula

                              d = d1 - d2

                              If σ1 and σ2 are the central velocity dispersion, the mean velocity dispersion is calculated as σ = (σ1 + σ2)/2.

                              In the table below, we have compiled the list of interacting galaxies whose central velcoity dispersion is given in SDSS.

                              Central Velocity Dispersion And Distance Between Galaxies
                              Arp No.  z1 z2Inter-nuclear distance (in Mpc) σ (in Km/s)
                              11  0.037 0.059 3.4162.59 
                              30  0.0266 0.0281 6 88.78
                               45 0.029213 0.029253 0.169 33.37
                               47 0.0423 0.0411 4.72 140.25
                               64 0.031342 0.031393 0.21 91.80
                               71 0.0333 0.03118 8.73 133.05
                               72 0.010974 0.010998 0.100 71.36
                               73 0.030769 0.030401 1.50 95.12
                               82 0.01366 0.01373 0.313 70.45
                               90 0.008312 0.008727 1.701 128.59
                               95 0.030708 0.030705 0.0234 109.18
                               97 0.023366 0.023353 0.063161.96 
                               105 0.029356 0.029367 0.055 197.06
                               106 0.022012 0.022302 1.191 145.07
                               107 0.034868 0.033176 6.96 165.59
                               115 0.030918 0.031395 2.4 126.06
                              124 0.013020 0.012715 1.25 90.12
                              125  0028593 0.028033 2.30 140.17
                               142 0.023313 0.022702 2.51 235.53
                               148 0.035193 0.034524 2.75 97.22
                               169 0.026282 0.026245 0.15 197.92
                               171 0.026675 0.027376 2.88 203.12
                               172 0.033687 0.033066 2.552 255.15
                               173 0.029590 0.029441 0.613 145.39
                               174 0.020869 0.021088 0.90 197.55
                               201 0.063467 0.063154 1.28 226.42
                              238 0.030788 0.031065 1.13 155.41
                              242  0.022059 0.022039 0.083 169.81
                               247 0.018012 0.018079 0.27 75.47
                               255 0.040858 0.040689 0.69 120.41
                               272 0.036852 0.037116 1.08 68.29
                               285 0.009243 0.008799 1.82 106.33
                               290 0.012168 0.012162 0.04 144.36
                              297  0.032099 0.032229 0.539 145.94
                              303 0.019980 0.020019 0.164 113.82
                               305 0.011469 0.011521 0.228 61.22
                               323 0.026508 0.026548 0.17 245.34
                              Classified Graph_1.png
                                Velocity Dispersion Relation vs Distance Between Galaxies

                                TIDAL DWARF GALAXIES

                                During galaxy interactions, the tidal forces expel large amounts of gas from the galaxies. Sometimes, this gas may recollapse and form new stellar systems which are large enough to be considered as a dwarf galaxy. Due to their nature of origin, they are known as Tidal Dwarf Galaxies (TDGs). These galaxies have the following characteristics:

                                • They do not have old stars and are producing their first ones.
                                • They have much higher metallicity as they are formed from recycled material.
                                • They are devoid of the dark matter.

                                Since the major proportion of galaxies in the Arp catalogue are interacting or merging, it provides an excellent source to hunt for TDGs. In our study, we have catalogued a few TDG candidates, as presented in the list below.

                                • Arp 32
                                • Arp 33
                                • Arp 34
                                • Arp 45
                                • Arp 47
                                • Arp 55
                                • Arp 57
                                • Arp 59
                                • Arp 82
                                • Arp 104
                                • Arp 105
                                • Arp 111
                                • Arp 125
                                • Arp 142
                                • Arp 143
                                • Arp 149
                                • Arp 160
                                • Arp 178
                                • Arp 191
                                • Arp 197
                                • Arp 218
                                • Arp 238
                                • Arp 242
                                • Arp 255
                                • Arp 272

                                Below are the images of a few galaxies that may be host to tidal dwarf galaxies.

                                Arp 82.JPG
                                  Arp 82
                                  Arp 55_1.JPG
                                    Arp 55
                                    Arp 238.JPG
                                      Arp 238
                                      Arp 272.JPG
                                        Arp 272

                                        There were a few galaxies that show much more complex features and might be home to TDGs. They are:

                                        • Arp 195
                                        • Arp 206
                                        • Arp 208
                                        • Arp 239
                                        • Arp 250
                                        • Arp 260
                                        • Arp 270
                                        • Arp 277
                                        • Arp 301
                                        • Arp 305


                                        In our project, we re-examined the peculiar galaxies in the Arp catalogue and classified them on the Apparent and Physical basis. We found that a large proportion of the Arp galaxies are interacting galaxies while the merging and single galaxies are comparitively lesser in proportion. We catalogued the galaxies as single, interacting and merging, by looking up at the SDSS. In the terms of physical classification, we catagorized the galaxies as starforming, starbursts and AGNs. There are many interesting galaxies that we found in terms of AGN activity, a few of them containing multiple AGNs. We also investigated the relation between the central velocity dispersion and internuclear distance of interacting and merging galaxies. The result was a scattered graph with no defined relation. This may be due to the noisy spectrum and the presence of rotational component in the velocity dispersion. In the end, we looked up at the Arp interacting and merging galaxies and found some promising TDG candidates. The TDGs are important to understand the evolution of the dwarf galaxies. Further spectroscopic studies on the list of TDG candidates will provide more insights to the nature and properties of these galaxies.


                                        The whole project uses the data of SDSS and NED, without these websites the project would
                                        not be fulfilled. I would like to thank Indian Academy of Science (IASc-INSA-NASI) for selecting me for the Summer Research Fellowship programme and giving me a great opportunity to work in Indian Institute of Astrophysics, one of the prestigious institutes of the country.

                                        I owe my sincere gratitude to Dr. Mousumi Das, my project guide who was constantly supporting me throughout my project by making me understand the basics and by clearing my doubts now and then. This work under mam has inspired me pursue research in the field of galaxies.

                                        I also thank Dr. Saini from Guru Nanak Dev University for giving me first hand experience in research in theoretical plasma physics and for recommending me to the Academy's Summer Research Program.


                                        • Halton Arp, 1966, Atlas of Peculiar Galaxies, The Astrophysical Journal Supplement Series, vol. 14, pp. 1

                                        • Steve Croft, Wil van Breugel, Wim de Vries, Mike Dopita, Chris Martin, Raffaella Morganti, Susan Neff, Tom Oosterloo, David Schiminovich, S. A. Stanford, Jacqueline van Gorkom, 2006, Minkowski’s Object: A Starburst Triggered by a Radio Jet, Revisited, The Astrophysical Journal, vol. 647, no. 2, pp. 1040-1055


                                        • Fig 4: Liverpool Telescope
                                        • Fig 5: Image: NASA and ESA
                                        • Fig 7: Hubble Space Telescope
                                        Written, reviewed, revised, proofed and published with