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]. H alpha 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]
Group	Description	Examples
1	Morphologically normal galaxies with nuclear starbursts. In all cases a companion is detected well within 1 Mpc projected separation, and V < 500 km/sec.	To 1004-296 UM 0499 Mrk 0710
2	A 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
3	Magellanic irregular, or dwarf irregular. Appears to be near other galaxies, and could be interacting.	To 1400-41 UM 0523 NGC 1487
4	Small and compact. At least one massive galaxy is seen in the neighbourhood.	To 1147-283 UM 0462 Mrk 1318
5	Small and compact. Nearby galaxies are not normal, being Magellanic irregulars or unclassified.	To 1924-416 UM 0160 II Zw 040
6	Compact 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]
Class	Host Morphology Class	Interaction Class
0	amorphous/unresolved	isolated
1	spiral	companion
2	bar and/or ring	companion is disturbed or at same redshift
3	peculiar or distorted	bridge, tail or jet

**Table C. Interaction Classes (IAC) for Single Galaxies** [25]
IAC	Description	Example
1	symmetric	Arp 027
2	slightly asymmetric; diffuse extensions	Arp 026
3	asymmetric, extended arm	Arp 222
4	distorted; out of shape	Arp 224
5	strongly distorted	Arp 220
6	aftermath; severely distorted	Arp 157

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

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

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

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

**Table H. Nuclear Emission-Line Type (NET)** [77, 89, 133, 134, 135]
NET	Class	Description	Example
1	Seyfert 1	widths of the Balmer emission lines are much broader (several 1,000 km/sec) than the ``narrow'' forbidden line widths	NGC 4151
1.5	Seyfert 1.5 Seyfert 1.8 Seyfert 1.9	easily apparent, narrow H profile superposed on broad wings narrow component of H stronger than Sy 1.5 weak, but definite, broad H broad H cannot be detected by mere visual inspection	Mrk 0006 NGC 2622
2	Seyfert 2	Balmer lines and forbidden lines have similar widths, typically 500 - 1000 km s^-1 5007 / H > 3.0 6583 / H > 0.5 6300 / H > 0.1	NGC 1068
2.5	marginal Seyfert	5007 / H ~ 3.0 6583 / H ~ 0.5 6300 / H > 0.1	NGC 4388 NGC 7436
3	LINER (Seyfert 3)	5007 / H < 3.0 6583 / H > 0.5 6300 / H > 0.1	NGC 3312 NGC 7393
4	HII region	5007 / H > 3.0 6583 / H < 0.4 6300 / H < 0.1	NGC 4765 NGC 7253
5	weak H		NGC 3920 NGC 5410
6	no emission	EW H < 10	NGC 1143
7	no emission	noisy or not observed when no nucleus is seen	NGC 0942 NGC 1347

**Table I. Byurakan Nuclear Types** [3, 57]
**(N galaxies)**
Type	Description	Example
1	No central condensation	NGC 4088
2	Weak central condensation	NGC 5850
3	Strong central condensation but not stellar	NGC 4442
4	Stellar nucleus blending into nebulous background	NGC 1300
5	Strongly stellar nucleus	NGC 3992