Last updated: July-7-99

Byurakan Nuclear Types, Bright Nuclei Galaxies, and Seyfert Galaxies

Kalloglyan and Tovmassian [57] have introduced a classification of galaxies judged solely on the degree of central concentration of galaxy images (Table I); examples of this system are also to be found in [56, 126, 136, 153, 154, 155, 186, 187, 188].

Keel and Weedman [78] provided a survey of 448 so-called `bright-nuclei' (BN) galaxies of Type 4 and 5 (Table I), drawn from the Byurakan master list [3] of nuclear types for 711 galaxies. They draw attention to 10 galaxies which have nuclei morphologically resembling Seyfert galaxies, were originally defined by their optical appearance [168] but now have a strict spectroscopic classification first suggested in a simpler form [77], and then refined and expanded upon by [133, 134, 135] and outlined in Table H. More sophisticated diagnostic diagrams involving various optical emission-line ratios can be found in [197] and [10], which quantitatively discriminate between HII regions, starburst galaxies (see [12] for far infrared and optical models), HII galaxies, narrow-line galaxies, LINER (low ionization nuclear emission region) galaxies and Seyfert 2 galaxies. In this respect it is worth noting that the N galaxies [120] now satisfy Seyfert's original morphological criteria as a separate class. Compilations of Seyfert galaxies and related objects can be found in [58, 89, 204, 205]. Early-type galaxies with emission lines are brought together in [13]. Halpha emission-line strength surveys of 200 normal galaxies [74] and 26 Virgo galaxies [75].

Several surveys of Seyfert nuclei have discussed the morphology of the host galaxy [1, 7, 25, 172, 206]; other studies [45, 81, 82, 83, 132] have investigated the relation of Seyfert activity to their environment: Dahari [25] defines Interaction Classes for galaxies, in the context of nuclear activity, based on environmental criteria, given in Tables C, D, E, F; MacKenty [97] offers a simple two-way classification scheme (Table B) involving both the host morphology and the environment (in the form of a statement concerning interaction); [83] lists 113 galaxies in 15 Seyfert and 9 non-Seyfert groups and assigns Activity Classes (see Table G) to these galaxies.

Further environmental groupings have been assigned [20] for violently star-forming galaxies, as given in Table A.

Table A. Environmental Groupings for Violently Star-Forming Galaxies [20]
GroupDescriptionExamples
1Morphologically normal galaxies with nuclear starbursts.
In all cases a companion is detected well within 1 Mpc projected separation, and DeltaV < 500 km/sec.
To 1004-296
UM 0499
Mrk 0710
2A burst of star formation appears to be located in a small companion, or corresponds to an HII region in the galaxy.To 1116-325
UM 0501
Mrk 1315
3Magellanic irregular, or dwarf irregular.
Appears to be near other galaxies, and could be interacting.
To 1400-41
UM 0523
NGC 1487
4Small and compact.
At least one massive galaxy is seen in the neighbourhood.
To 1147-283
UM 0462
Mrk 1318
5Small and compact.
Nearby galaxies are not normal, being Magellanic irregulars or unclassified.
To 1924-416
UM 0160
II Zw 040
6Compact morphology and small size.
Isolated (no companions within 500 km/sec or 1 Mpc projected separation).
To 1148-2020
UM 0439
NGC 5253

Table B. Classification Sequence for Seyfert Galaxies [97]
ClassHost Morphology ClassInteraction Class
0amorphous/unresolvedisolated
1spiralcompanion
2bar and/or ringcompanion is disturbed or at same redshift
3peculiar or distortedbridge, tail or jet

Table C. Interaction Classes (IAC) for Single Galaxies [25]
IACDescriptionExample
1symmetricArp 027
2slightly asymmetric; diffuse extensionsArp 026
3asymmetric, extended armArp 222
4distorted; out of shapeArp 224
5strongly distortedArp 220
6aftermath; severely distortedArp 157

Table D. Interaction Classes (IAC) for Double Galaxies [25]
IACDescriptionExample
3large separation, and no apparent contactArp 305
4large separation, but components are connected OR
small separation, but no contact
Arp 314
Arp 271
5small separation, and evidence of contactArp 283
6galaxies overlapArp 166

Table E. Interaction Classes (IAC) for Galaxies with Companions [25]
IACDescriptionExample
2large separation, no contactArp 023
3large separation, but signs of connectionArp 304
4small separation, but no contact   OR
small separation, and signs of contact
Arp 112
Arp 085
5companion overlaps parent galaxyArp 309

Table F. Interaction Classes (IAC) for Galaxies with Much Smaller Companions [25]
IACDescriptionExample
1large separation, no contact. . .
2large separation, but connectedArp 024
3small separation, but no contactArp 290
4small separation, connected    OR
companion overlaps primary galaxy
Arp 082
Arp 239
5companion overlaps parent galaxyArp 309

Table G. Spectroscopic Criteria for Nuclear Activity Classes [83]
ClassSpectrum
1Seyfert 1 or Seyfert 2
2Starburst nucleus with [OIII]lambda 5007 > Hbeta
3Halpha present; [OIII]lambda 5007 < Hbeta
4Halpha + [NII] present, but weak
5Halpha or [NII] weak
6absorbtion lines only

Table H. Nuclear Emission-Line Type (NET) [77, 89, 133, 134, 135]
NETClassDescriptionExample
1Seyfert 1widths of the Balmer emission lines are much broader (several 1,000 km/sec) than the ``narrow'' forbidden line widthsNGC 4151
1.5Seyfert 1.5


Seyfert 1.8

Seyfert 1.9
easily apparent, narrow Hbeta profile superposed on broad wings
narrow component of Hbeta stronger than Sy 1.5 weak, but definite, broad Hbeta
broad Hbeta cannot be detected by mere visual inspection
Mrk 0006

NGC 2622
2Seyfert 2Balmer lines and forbidden lines have similar widths, typically 500 - 1000 km s-1
lambda5007 / Hbeta > 3.0
lambda6583 / Halpha > 0.5
lambda6300 / Halpha > 0.1
NGC 1068
2.5marginal Seyfertlambda5007 / Hbeta ~ 3.0
lambda6583 / Halpha ~ 0.5
lambda6300 / Halpha > 0.1
NGC 4388
NGC 7436
3LINER
(Seyfert 3)
lambda5007 / Hbeta < 3.0
lambda6583 / Halpha > 0.5
lambda6300 / Halpha > 0.1
NGC 3312
NGC 7393
4HII regionlambda5007 / Hbeta > 3.0
lambda6583 / Halpha < 0.4
lambda6300 / Halpha < 0.1
NGC 4765
NGC 7253
5weak Halpha NGC 3920
NGC 5410
6no emissionEW Halpha < 10NGC 1143
7no emissionnoisy or not observed when no nucleus is seenNGC 0942
NGC 1347

Table I. Byurakan Nuclear Types [3, 57]
(N galaxies)
TypeDescriptionExample
1No central condensationNGC 4088
2Weak central condensationNGC 5850
3Strong central condensation but not stellarNGC 4442
4Stellar nucleus blending into nebulous backgroundNGC 1300
5Strongly stellar nucleusNGC 3992


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