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

Abstract

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

Abbreviations

INTRODUCTION

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.

Background

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:

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.

ABOUT SDSS

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 CLASSIFICATION OF GALAXIES

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.

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.

Fig 1 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.

Fig 2 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.

Fig 3 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.

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".

Fig 4 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.

Fig 5 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.

Fig 6 Minkowski's object (marked with green circle) along with NGC 541 (below)

Merging galaxies

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.

Fig 7 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.

Fig 8 Arp 318

Fig 9 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.

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.

a) The Spectrum of Arp 45

b) SDSS Image of Arp 45

c) The Spectrum of Arp 33

d) SDSS Image of Arp 33

Fig 10 The Spectrum And Images of Typical Starburst Galaxies

Starburst galaxies in Arp catalogue

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.

Fig 11 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.

Method

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(α₁-α₂)

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.

Fig 12 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.

Fig 13 Arp 82

Fig 14 Arp 55

Fig 15 Arp 238

Fig 16 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

CONCLUSIONS

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.

ACKNOWLEDGEMENTS

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.