A Classical Morphological Analysis of Galaxies in the Spitzer Survey of Stellar Structure in Galaxies (S4G)

2.5 MB·English

by  R. Buta

Accepted for publicationin the Astrophysical JournalSupplementSeries
PreprinttypesetusingLATEXstyleemulateapjv.03/07/07
A CLASSICAL MORPHOLOGICAL ANALYSIS OF GALAXIES IN THE SPITZER SURVEY OF STELLAR
STRUCTURE IN GALAXIES (S4G)
Ronald J. Buta1, Kartik Sheth2, E. Athanassoula3, A. Bosma3, Johan H. Knapen4,5, Eija Laurikainen6,7, Heikki
Salo6, Debra Elmegreen8, Luis C. Ho9,10,11, Dennis Zaritsky12, Helene Courtois13,14, Joannah L. Hinz12,
Juan-Carlos Mun˜oz-Mateos2,15, Taehyun Kim2,15,16, Michael W. Regan17, Dimitri A. Gadotti15, Armando Gil de
Paz18, Jarkko Laine6, Kar´ın Men´endez-Delmestre19, S´ebastien Comero´n6,7, Santiago Erroz Ferrer4,5, Mark
5 Seibert20, Trisha Mizusawa2,21, Benne Holwerda22, Barry F. Madore20
1 Accepted for publication inthe Astrophysical Journal Supplement Series
0
2 ABSTRACT
n The Spitzer Survey of Stellar Structure in Galaxies (S4G) is the largest available database of deep,
a homogeneous middle-infrared (mid-IR) images of galaxies of all types. The survey, which includes
J 2352 nearby galaxies, reveals galaxy morphology only minimally affected by interstellar extinction.
8 This paper presentsan atlas andclassificationsof S4G galaxiesin the Comprehensivede Vaucouleurs
1 revised Hubble-Sandage (CVRHS) system. The CVRHS system follows the precepts of classical de
Vaucouleurs(1959)morphology,modifiedtoincluderecognitionofotherfeaturessuchasinner,outer,

] and nuclear lenses, nuclear rings, bars, and disks, spheroidal galaxies, X patterns and box/peanut
A
structures, OLR subclass outer rings and pseudorings, bar ansae and barlenses, parallel sequence
G late-types, thick disks, and embedded disks in 3D early-type systems. We show that our CVRHS
classifications are internally consistent, and that nearly half of the S4G sample consists of extreme
.
h late-typesystems(mostlybulgeless,purediskgalaxies)intherangeScd-Im. Themostcommonfamily
p
classification for mid-IR types S0/a to Sc is SA while that for types Scd to Sm is SB. The bars in
-
o these two type domains are very different in mid-IR structure and morphology. This paper examines
r the bar, ring, and type classification fractions in the sample, and also includes several montages of
t images highlighting the various kinds of “stellar structures” seen in mid-IR galaxy morphology.
s
a Subject headings: galaxies: structure; galaxies: morphology
[

2
1. INTRODUCTION
v
1Department of Physics & Astronomy, University of Alabama,
4 Galaxy morphology and classification are an essen-
Box870324, Tuscaloosa,AL35487-0324
5 2National Radio Astronomy Observatory / NAASC, 520 tialstep in understanding how galaxiesformandevolve.
4 EdgemontRoad,Charlottesville,VA22903 Morphology is rich in clues to the internal and exter-
0 3Aix Marseille Universite, CNRS, LAM (Laboratoire nal physicalprocesses that have molded galactic shapes.
0 d’Astrophysique de Marseille) UMR 7326, 13388, Marseille, It is, however, non-trivial to determine exactly what a
France
1. 4Departamento de Astrof´ısica, Universidad de La Laguna, given morphology actually implies about the history of
0 38206LaLaguna,Spain a galaxy, because we only see the z 0 end-product of
5 5Instituto deAstrof´ısicadeCanarias,V´ıaL´actea s/n38205 La all of these processes, whether secula≈r in nature or not.
Laguna,Spain
1 6Division of Astronomy, Department of Physical Sciences, Only by examining the collective morphology of galax-
: UniversityofOulu,Oulu,FIN-90014,Finland ies, both nearby and very distant, in conjunction with
v
7Finnish Centre of Astronomy with ESO (FINCA), University physical data (such as luminosities, diameters, inclina-
i
X ofTurku,Vaisalantie20,FI-21500, Piikio,Finland tions, and bulge properties) and numerical simulations
8Vassar College, Deparment of Physics & Astronomy, Pough-
r keepsie,NY12604 of galaxy evolution, can we hope to piece together the
a 9Kavli Institute for Astronomy and Astrophysics, Peking general evolutionary paths of different classes of galax-
University,Beijing100871, China ies.
10DepartmentofAstronomy,PekingUniversity,Beijing100871, The Spitzer Space Telescope (Werner et al. 2004)
China
11The Observatories of the Carnegie Institution for Science, opened a new window on galaxy structure at middle in-
813SantaBarbaraStreet,Pasadena, CA91101, USA frared (mid-IR) wavelengths. With the Infrared Array
12Steward Observatory, University of Arizona, 933 North Camera(IRAC;Fazio etal. 2004),Spitzer providedfour
CherryAvenue, Tucson,Arizona85721
major IR bands for direct imaging: 3.6, 4.5, 5.8, and
13Universit´e Lyon 1, CNRS/IN2P3, Institut de Physique
Nucl´eaire,Lyon,France 8.0µm.23 These bands cover a unique part of the galac-
14Institute for Astronomy, University of Hawaii, 2680 Wood-
lawnDrive,Honolulu,HI26822
15European Southern Observatory, Casilla 19001, Santiago 19, 20The Observatories of the Carnegie Institution for Science,
813SantaBarbaraStreet, Pasadena, CA91101
Chile
16AstronomyProgram,DepartmentofPhysicsandAstronomy, 21Department of Physics and Space Sciences, Florida Institute
ofTechnology,150W.UniversityBoulevard,Melbourne,FL32901
SeoulNationalUniversity,Seoul 151-742,Korea
17Space Telescope Science Institute, 3700 San Martin Drive, 22University of Leiden, Sterrenwacht Leiden, Niels Bohrweg 2,
NL-2333CALeiden,TheNetherlands
Baltimore,MD21218
18Departmento de Astrofisica, Universidad Complutense de 23 Pahre et al. (2004) considered all four IRAC
Madrid,28040Madrid,Spain bands to be mid-IR, while the Infrared Processing and
19University of Rio de Janeiro, Observatorio de Valongo, Analysis Center (IPAC) considers the 3.6 and 4.5µm fil-
LadeiraPedroAntonio,43,CEP20080-090,RiodeJaneiro,Brazil ters as near-IR and the 5.8 and 8.0µm filters as mid-
2 Buta et al.
tic spectrum: the 3.6 and 4.5µm bands largely sample [i.e.,SA(r)a,SB(s)bc,etc.] hasmoremeaningthanitdid
the photospheric light of old stars (Pahre et al. 2004), when the B-band, the waveband in which galaxy clas-
whilethe5.8and8.0µmbandsrevealthedustyinterstel- sification has traditionally been performed (as in, e.g.,
larmedium(Helouetal. 2004). Inallofthesebands,star Sandage 1961, Sandage & Bedke 1994; de Vaucouleurs
formationandtheinterstellarmediumareevidenttovar- 1959; Buta et al. 2007), was the only band used for
ious degrees in the form of either emission lines or ther- such analysis; (3) the high-quality of S4G images with
mal emission from dust heated by massive stars. Most respect to uniformity, depth of exposure, and resolution
importantly, these mid-IR bands show galaxies mostly in the IR, and the detailed information the images pro-
free of the effects of extinction and reddening, revealing vide on both nuclear and outer structure allows us to
previouslyhiddenstructures(e.g.,ringsinedge-ongalax- improve on the morphological types listed in published
ies, or nuclear rings in the dusty central areas of some catalogues like the Third Reference Catalogue of Bright
barred galaxies). Galaxies (RC3, de Vaucouleurs et al. 1991); (4) exam-
The Spitzer Survey of Stellar Structure in Galaxies ining such a high-quality database at the level of detail
(S4G; Sheth et al. 2010) is the largest database of high- needed for classical morphological analysis can draw at-
qualitymid-IRimagesofnearbygalaxiesavailable. Pub- tention to special cases of interest; (5) classical morpho-
liclyreleasedin2013,the S4Gincludes2352galaxiesim- logicaltypes in the mid-IR complement the quantitative
aged in the 3.6µm and 4.5µm bands, selected according analysesthatareamajorpartoftheS4Gproject(Sheth
to redshift, distance, apparent brightness, and galactic et al. 2010);(6) specializedvisual classificationsare still
latitude. Because these filters are predominantly sensi- essential to the automated and crowd-sourcedclassifica-
tive to the light of old stars, they trace the distribution tions that are a common practice in astronomy today,
ofstellarmass. TheprimarygoaloftheS4Gwasto“ob- especially for highredshift studies (e.g., Coe et al. 2006;
tainacompletecensusofthestellarstructuresingalaxies Huertas-Company et al. 2008; Lee et al. 2013); and
in the local volume.” For this purpose, the images have (7) the large number of images that are homogeneous
been used for studies of structures in the faint outskirts in sensitivity, coverage, and spatial resolution avoid the
of galaxiesand of tidal debris (S. Laine et al. 2014;Kim problems that would plague heterogeneous datasets.
etal. 2012),the propertiesofthickdisksseeninedge-on Buta et al. (2010a,hereafter paper I) presenteda pre-
galaxiesoftypesSbtoSdmandprofilebreaks(Comer´on liminarymorphologicalanalysisofnearly200S4Ggalax-
et al. 2011a,b,c; 2012), mid-IR flocculent and grand de- iesfromtheSpitzerarchives,andshowedthattheoldB-
signspiralstructureandstar-formingregions(Elmegreen band classification systems could be effectively applied
et al. 2011, 2014), conversion of 3.6 and 4.5µm light in the mid-IR. This did not mean that there were no
into stellar mass maps (Meidt et al. 2012, 2014; Quera- problems in the actual application of a B-band system
jeta et al. 2014),properties ofstellar mass galacticrings in the mid-IR, only that on the whole the classical sys-
(Comer´on et al. 2014), bar brightness profiles (Kim et temscouldstillbeusedforthemajorityofmid-IRgalaxy
al. 2014), outer disk brightness profiles (Munoz-Mateos types. Eskridge et al. (2002) came to the same conclu-
et al. 2013, J. Laine et al. 2014), quantitative morphol- sion using near-IR H-band (1.65µm) images. H-band
ogy using cosmologically relevant parameters (Holwerda types are compared with our mid-IR classifications in
et al. 2014),mid-IR asymmetries and the mid-IR Tully- Section 3.3.
Fisher relation(Zaritskyetal. 2013,2014),examination In this paper, we present a similar analysis to pa-
of a possible relation between nuclear activity and bar per I of the entire S4G sample. We use the notation
strength(Cisternasetal. 2013),andforthephotometric of the “Comprehensive de Vaucouleurs revised Hubble-
decompositions of bulges, disks, and bars in the mid-IR Sandage” (CVRHS) system (e.g., Buta 2014) to provide
(Salo et al. 2014). Knapen et al. (2014) have also gath- classifications similar to, but more extensive than, those
ered high-quality optical images for about 60% of the provided in the RC3. Much of the background for the
survey galaxies. survey is already described in paper I; only a brief sum-
The S4G also provides an opportunity to examine the mary will be provided here. In addition to an atlas of
mid-IR structure of a large number of galaxies from the images of the 2168 S4G galaxies not included in the pa-
point of view of classical morphological analysis, mean- per I analysis, we highlight specific aspects of mid-IR
ing the classification of galaxies in the well-known Hub- morphology(as wellasinterestingindividualcases). Be-
ble (1926, 1936) system and its later offshoots (Sandage cause the CVRHS has “evolved” since 2010 to include
1961; Sandage & Bedke 1994; de Vaucouleurs 1959). morefeatures (such as ansaebars andbarlenses;Section
This is worth doing for several reasons: (1) the value 4.3) and also uses aspects of the van den Bergh (1976)
of visual classification has increased over the past 20 parallel sequence classification (following developments
yearsowingtotheexplosioninimagingdataavailable(e. summarized by Kormendy & Bender 2012), the present
g., the Sloan Digital Sky Survey) as well as to advances study includes a re-examination of the paper I galaxies.
in numerical simulations and theoretical understanding
of the processes that impact galactic shapes (Kormendy 2. GALAXYSAMPLE
2012; Athanassoula 2012); (2) Because the mid-IR pro- ThesampleselectionfortheS4GisdescribedbySheth
vides the clearest view of galactic stellar mass morphol- etal. (2010). AllgalaxiesintheHyperledadatabase(Pa-
ogy, the symbolism of classical morphological analysis tureletal. 2003)havinganHI radialvelocity (V )<
radio
3000 km s−1 (corresponding to a distance D < 40 Mpc
IR (www.ipac.caltech.edu/outreach/Edu/Regions/irregions.html). for H = 75 km s−1 Mpc−1), a blue light isophotal di-
Hereweusemid-IRforalloftheIRACfilterstodistinguishthem ameteor D > 1′. 0), a blue photographic magnitude m
fromtheground-basednear-IRstudiesofthepastthatwereinthe 25 B
IJHK bands(rangingfrom0.8to2.2µm). <15.5(correctedforinternalextinction),andaGalactic
latitudeofb>30o wereselectedforthesurvey. Theorig-
Classical Morphology of S4G Galaxies 3
inalsamplehad2331galaxies(namedintheon-linetable
accompanying Sheth et al. 2010), but the final sample
has 2352 galaxies owing to Spitzer’s improved efficiency
which allowed 21 galaxies satisfying the original criteria
(except for HI detection) to be added. The additional
galaxies are primarily HI-poor ellipticals and S0s.
Of the selected galaxies, 600 had already been ob-
≈
servedbySpitzerforotherprojects,andthusimageswere
alreadyavailable. Newdatawerecollectedforthe 1750
≈
remainingsamplegalaxies. Thepixelsizeforeachimage
is 0.′′75, achieved using the drizzle technique (Williams
etal. 1996)onoriginalimageshavingapixelsizeof1.′′2.
The point spread function for the 3.6 µm images has a
mean full width at half maximum of 1.′′66 (1.′′72 for the
4.5µm filter; IRAC Instrument Handbook), which limits
the accuracy of the classifications of some of the smaller
Fig. 1.0001.— UGC12893; filter: 3.6µm; mean (Phase1,2)
or more distant galaxies in the sample. The processing CVRHS type: dSA(l)0o / Sph; north up, east left; field: 2′.73
of all S4G images followed a pipeline with a number of 2.′10; surfacebrightness range: 18.0-26.5 magarcsec−2. (Figure×1
steps (P1-P4) outlined by Sheth et al. (2010). is published in its entirety in the electronic edition of the Astro-
physical Journal Supplement Series. A portion is shown here for
The use of 21-cm radial velocities to select S4G sam-
guidanceregardingitsformandcontent.)
ple galaxies introduced a bias against inclusion of gas-
poor early-type galaxies, for which an HI radial velocity cially forlow luminosity, low surfacebrightness galaxies.
would not be available. The bias is being rectified in a Also as in paper I, only the 3.6µm images were used
supplementary survey (Sheth et al. 2013) of 465 early- forthevisualclassificationspresentedinthispaper. The
type galaxies that satisfy the same selection criteria as reasonis that these images tend to have a greaterdepth
the original survey but using an optical radial velocity ofexposure(and thus greatersensitivity to stellarmass)
for the distance limit. With these galaxies, the full S4G than the 4.5µm dataset. With azimuthal averaging of
will include 2817 galaxies. A comparable morphological the luminosity distribution, Sheth et al. (2010) showed
analysis of these additional galaxies will be provided in that these images can detect surface mass densities as
a later study. low as 1M⊙ pc−2.
The images used for our morphological analysis are Altho≈ugh 3.6µm is an excellent wavelength for seeing
the Pipeline 1 (P1)images. These are the final, “science the distribution of stellar mass in galaxies, it is not per-
ready” mosaics where individual sub-images have been fect, and indeed no IR band perfectly traces such mass.
matched with regard to background levels and drizzled The main drawbacks of the 3.6µm filter are contamina-
to get the final pixel scale. S4G images are generally tionby“hotdust”andthe inclusionofa3.3µmemission
much more sensitive to low light levels than are typi- featuredue toapolycyclicaromatichydrocarbonassoci-
cal ground-based near-IR images, owing to the greatly ated with star-forming regions (e.g., Meidt et al. 2012).
reduced and much more stable background levels that As shown by Kendall et al. (2008), hot dust emission
space observations have compared to ground-based im- at 3.6µm can be removed using an IRAC 8.0µm image
ages. if available. However, most S4G galaxies do not have
Our approachto galaxy classification from the P1 im- an 8.0µm image. Meidt et al. (2012, 2014) and Quera-
ages is the same as was used in paper I. The final P1 jeta et al. (2014) use [3.6] [4.5] colors and a technique
images were background-subtracted and then converted known as “independent co−mponent analysis” to locate
into units of mag arcsec−2 using a common (Vega) zero and remove young contaminants and derive stellar mass
point of 17.6935. This type of “classification-ready”im- maps. Our morphological analysis is based on the origi-
agehasthe advantagethatallofthe galaxiescanbedis- nal 3.6µm images and not on the corrected stellar mass
playedin a homogeneous way;with the Vega zero point, maps. The main reasonfor this is that galaxy classifica-
therange11.5-26.5magarcsec−2 coveredthefullrange tion has traditionally been based on the distribution of
of surface brightnesses for the sample. The same faint luminosity and not the distribution of mass.
limit was used for most of the galaxies, but the bright
limit was adjusted for individual objects. 3. S4GMORPHOLOGY
Our final list has 2412 galaxies (NGC 4038 and 4039 3.1. Classification System
are counted as one), 60 more than the extended S4G
3.1.1. The VRHS classification
sample. All ofthe additionalgalaxiesare companionsor
in the same area as an S4G sample galaxy. As in paper The CVRHS system is a modified version of the de
I,weinclude the full setofclassification-readyimagesas Vaucouleurs(1959)revisedHubble-Sandage(VRHS)sys-
an atlas (Figure 1.0001). Each image was displayed on tem that is described in the de Vaucouleurs Atlas of
a 24-bit monitor within an area that is recorded in the Galaxies (dVA, Buta et al. 2007). More detail on the
captiontotheimage. Many,butnotall,oftheadditional application of the system, and extensive illustrations of
galaxies are covered in the atlas images. different CVRHS morphological features, is provided in
Smaller or more distant galaxiesin the sample are not the complementary reviews of Buta (2012; IAC Winter
well-resolved in the IRAC image. Even so, S4G im- schoollecturesonmorphologyandsecularevolution)and
ages are of far higher depth than could ever have been Buta (2013; phenomenology of galaxy morphology and
achieved from the ground at near-IR wavelengths, espe- classification). Table1providesasummaryofthemean-
4 Buta et al.
ing of the notations of CVRHS morphology used in this dimension, while outer rings and pseudorings are about
paper. twicethebarlengthindiameter. Intheabsenceofabar,
The hallmark of VRHS classification is continuity of the distinction may be ambiguous unless more than one
structure along three morphological dimensions (in the ring is present. In some cases, it may not be possible to
formofaclassificationvolume): thestage,whichrefersto resolve the ambiguity from visual inspection alone.
the E-S0-S-I position along a modified Hubble sequence The VRHS provides information on inclination
(the VRHS sequence); the family, referring to the pres- through the “spindle” (sp) notation. A spindle is a
enceorabsenceofabar;andthevariety, referringtothe highly-inclineddiskgalaxy. Forexample,theVRHSclas-
presence or absence of an inner ring. In addition, there sificationforNGC 4565isSb sp. Itis tooinclined toget
is a fourth dimension known as the outer ring classifica- a full classification with family and variety, but stage is
tion,referringtothe presenceofalargeringinthe outer still distinguishable. The sp after the stage points to its
disk. Becausestagecorrelateswithseveralbasicphysical near edge-on orientation.
properties of galaxies (e.g., average surface brightness, Peculiarities are recognized using “Pec” as the classi-
color, HI mass-to-blue light ratio), it is considered the fication, or “pec” after a regular classification. For ex-
fundamental dimension of the system. ample, NGC 4038-9is a well-known merger system with
The positioning of galaxies in stage depends on spe- distorted components and tidal tails. The object does
cific morphological characteristics: elliptical galaxies notfitintoanyVRHS“cell”andsoisclassifiedas“Pec.”
are defined by a smoothly declining brightness gradi- If instead, “pec” follows a classification,it implies some-
ent and little or no evidence for a disk component; thing unusual about the object, often uncharacteristic
S0 galaxies are armless disk galaxies and form a se- asymmetry or odd shape.
quence (S0− S0o S0+) of increasing structure rang- de Vaucouleurs (1963) modified the VRHS to include
→ →
ing from subtle inflections in the brightness distribution underline notation for stage, family, and (inner) variety,
(type S0−) to prominent rings (type S0+); spirals form where in a combined classification symbol such as Sab,
a sequence (S0/a-Sa-Sab-Sb-Sbc-Sc-Scd-Sd-Sdm-Sm) of Sbc, Scd, Sdm, SAB, and (rs), one symbol is underlined
decreasing bulge-to-total luminosity ratio, increasingly to imply that it is “closest to actual type.” For exam-
open spiral arms, an increasing degree of star forma- ple, a classification like SA(s)ab would indicate a stage
tion,andincreasingasymmetry;andMagellanicirregular Sab galaxy that is more Sb than Sa, SB(s)cd would be
galaxies (Im) are the endpoint characterized by signifi- a stage Scd galaxy that is more Sc than Sd, etc. For
cant asymmetries, often a high degree of scattered star family,anSAB galaxyshowsonly atraceofabar(often
formation, and a significant range in luminosity. merelyanoval),whileanSABgalaxyismorebarredthan
ThefamilyclassificationrangesfromSAfornonbarred nonbarred but not as strongly barred as an SB galaxy.
galaxies to SB for barred galaxies, with an intermediate Similarly, for variety an (rs) galaxy shows a well-defined
category of SAB to account for “weakly-barred” galax- inner ring only slightly broken by spiral structure, while
ies, or galaxies intermediate in apparent bar strength an rs galaxy shows only a trace of an inner ring.
between SA and SB. Both relative bar length and bar Although very useful [and applied in the Catalogue of
contrast play a role in family classification. Although Southern Ringed Galaxies (CSRG, Buta 1995) and the
bars can be recognized in edge-on spiral galaxies (Sec- dVA], for practical reasons underline notation was not
tion 4.3), reliable family classification is still something used in any of the reference catalogues (RC1, de Vau-
that can be done only for low inclination galaxies. High couleurs&deVaucouleurs1964;RC2,deVaucouleurset
inclinationcanconsiderablyforeshortenabar,orconfuse al. 1976;andRC3). deVaucouleurs(1963)alsousedun-
inner structure. derline notation sparingly: in his survey of 1500 bright
The variety classification ranges from (r) for a closed galaxies, including 1263 spirals and S0s, underline no-
inner ringto (s)for anopenspiral,with anintermediate tation for stages was used for only 2% of the galaxies,
category (rs) to account for partial inner rings having a while underline notationfor family and varietywas used
spiral character (called “inner pseudorings”). The outer for only 9-10% of the galaxies. In VRHS classifications,
ringclassificationrangesfrom(R)foraclosedouterring the bulk of classifications will be in the main categories,
to(R′)foranouterpseudoringmadeofvariablepitchan- while underline categories will generally be underrepre-
gle outer spiral arms. Because outer and inner rings are sented.
similar aspects ofgalaxymorphology,we willhenceforth
refer to the conventional variety as the “inner variety”
and the outer ring classification as the “outer variety” 3.1.2. Comprehensive VRHS Classification
(Section 3.3).
The four parts of a VRHS spiral galaxy classification WhattheCVRHSaddstotheoriginalVRHSsystemis
in order are: recognitionof details whose significance to galaxy struc-
(outer variety)–family–(inner variety)–stage. ture and evolution has only recently been appreciated.
(R′) SB (r) ab For example, Kormendy (1979) showed that lenses, disk
For example, the VRHS RC3 classification of NGC morphologicalfeaturescharacterizedbyashallowbright-
1433is(R′)SB(r)ab. Ifthereisnoouterringorpseudor- ness gradient interior to a sharp edge, are prominent in
ing, or if the inner variety cannot be determined, these barred galaxies and could be intimately connected with
can be dropped from a classification. the evolution of bars. He argued that lenses were often
The distinction between inner and outer varieties is misclassified as rings in RC2, and noted that there was
not always clearcut. Inner and outer rings and pseudor- a lens analogue of each type of ring in the VRHS. He
ings are easily distinguished in barred galaxies because suggestedusingthe symbol(l) forinnerlenses(analogue
the bar usually fills an inner ring or pseudoring in one ofinnerrings)and(L)forouterlenses(analogueofouter
Classical Morphology of S4G Galaxies 5
TABLE 1
Explanation ofCVRHS Symbolsa
Symbol Description
1 2
General Terms
ETG Anearly-typegalaxy,collectivelyreferringtoagalaxyintherangeof
types EtoSa
ITG Anintermediate-type galaxy,takentobeintherangeSabtoSbc
LTG Alate-typegalaxy,collectivelyreferringtoagalaxyintherangeof
types SctoIm
ETS Anearly-typespiral,takentobeintherangeS0/atoSa
ITS Anintermediate-type spiral,takentobeintherangeSabtoSbc
LTS Alate-typespiral,taken tobeintherangeSctoScd
XLTS Anextremelate-typespiral,takentobeintherangeSdtoSm
classicalbulge Agalaxybulgethatlikelyformedfromearlymergersofsmallergalaxies
(Kormendy&Kennicutt2004; Athanassoula2005)
pseudobulge Agalaxybulgemadeofdiskmaterialthathassecularlycollectedinto
thecentralregionsofabarredgalaxy(Kormendy2012)
PDG Apurediskgalaxy, agalaxylackingaclassicalbulgeandoftenalso
lackingapseudobulge
Stage
stage Thecharacteristicofgalaxymorphologythatrecognizesdevelopment of
structure,thewidespreaddistributionofstarformation,andthe
relativeimportanceofabulgecomponent alongasequencethat
correlates wellwithbasiccharacteristics suchasintegratedcolor,
averagesurfacebrightness,andHImass-to-blueluminosityratio
Elliptical Galaxies
Egalaxy Agalaxyhavingasmoothlydecliningbrightnessdistributionwithlittle
ornoevidenceofadiskcomponentandnoinflections(suchaslenses)
intheluminositydistribution(examples: NGC1052,3193, 4472)
En Anellipticalgalaxyofvisualflatteningn=10(1 b/a), whereb/aisthe
visualisophotalaxisratio(Hubble1926) −
E+n A“late”ellipticalofvisualflatteningn,atransitionstagetotheS0class
(deVaucouleurs1959); showslighttraces ofdifferentiatedstructure,
usuallysubtleevidenceoflenses(example: NGC5846) orafaint
outer envelope; alsohasbeenusedasa“home”forMorgancDgalaxies
inRC3
E/E+ AnEgalaxythatinourPhase1,2analysisaveragesbetween EandE+
(example: NGC3226)
E(d)n Adiskyellipticalgalaxyofvisualflattening n(Kormendy&Bender
1996); asubclassificationofellipticalshavingpointyouter
isophotes thatisvisuallydetectable onlyforthemostobvious or
rings).24 Thesignificanceoflensestogalaxymorphology respectively. Thesearealsousedwithunderlinenotation
wasfurtherestablishedwiththeNear-InfraredS0Survey to emphasize the ring or lens aspect.
(NIRS0S), a K -band (2.2µm) survey of 206 early-type TheCVRHSincludesrecognitionofimportantnuclear
s
galaxies, including 160 S0-S0/a galaxies (Laurikainen et features,such as nuclear rings,pseudorings,lenses, ring-
al. 2011, 2013). lenses,bars,anddisks(Buta&Combes1996). Anuclear
Animportantquestionishowlensesdifferfrombulges. ring (nr) is a small ring, often defined by star-forming
Inthecaseofinnerlenses,thereisnoambiguitybetween regions, typically found in the centers of barred galax-
thesefeaturesbecausethebartendstofillthelensinone ies and on average 1 kpc in diameter (Comer´on et al.
≈
dimension (Kormendy 1979). However, there is another 2010). A nuclear lens (nl) is the lens analogue of a nu-
type oflens, calleda “barlens”(Laurikainenet al. 2011) clear ring. A nuclear bar (nb) is a small bar often found
that canbe mistakenfor a classicalbulge (Athanassoula within a nuclear ring or lens, or which may be present
etal. 2014). ThesearediscussedfurtherinSection4.3.1. independent of these features. A nuclear disk (nd) is
Therecognitionoflensesbringsattentiontootherfea- typically a distinct, highly-flattened feature seen most
tures known as ring-lenses, where the apparently sharp easily in edge-on S0 galaxies. Nuclear ring-lenses (nrl),
edgeofalensisslightlyenhancedtoappearasalowcon- nuclear pseudorings (nr′), and nuclear spirals (ns) are
trastring. In CVRHS classification,we use the notation alsoknown. Similar to inner andouter rings,inCVRHS
(rl) for an inner ring-lens and (RL) for an outer ring- classificationthepresenceorabsenceofanuclearringor
lens, with pseudoring-lensequivalents of (r′l) and (R′L), related feature will be referred to as the “nuclear vari-
ety.”
24 Kormendy (2012) prefers the use of (lens) in place of (l) for The fact that there are three ring types (R,r,nr) and
innerlenses,toavoidpossibleconfusionwith(1). Herewecontinue three types of lenses (L,l,nl), with ring-lenses (RL, rl,
touse(l)and(L)tobeconsistentwiththedVAandtheCSRG.
6 Buta et al.
TABLE 1
Explanation of CVRHS Symbols(cont.)a
Symbol Description
1 2
mostfavorablyorientedcases(example: NGC3377)
E(b)n Aboxyellipticalgalaxyofvisualflatteningn(Kormendy&Bender1996);
asubclassificationofellipticalshavingboxyouterisophotes thatis
visuallydetectable onlyforthemostobvious cases
E(b,nd) AboxyEgalaxywithaninnerdisk(example: NGC4370)
S0 Galaxies
S0galaxy Adisk-shapedgalaxylackingstrongorobviousspiralstructure;at
minimum,atwo-component systemwithabulgeandadisk
S0− S0o S0+ astagesequenceofS0galaxiesbasedonincreasing
→ → development ofstructure,suchasbars,lenses,andrings
E+/S0− AnETGthatinourPhase1,2analysisaverages betweenE+ and
S0− (example: NGC4649)
S0− AnearlyS0showingclearevidenceforadisk(envelope) butlittle
structure;allfeaturesaresubtle(examples: NGC4442,5507)
S0−/o AnS0galaxythatinourPhase1,2analysisaverages between S0−
andS0o
S0o AnintermediatestageS0showingclearlensesortracesof
rings/pseudorings(examples: NGC1411, 1533)
S0o[d] Oneexampleofanotationusedforanapparentlyearly-typegalaxyin
thecatalogue havinglittleornoapparentbulge;other examples
include[c],[cd],[m]. ThesearerelatedtothevandenBergh
(1976)parallel-sequenceHubbleclassificationsystem,although
thatsystemisnotfullybuiltintothecatalogue (example:
NGC693)
S0o/+ AnS0galaxythatinourPhase1,2analysisaverages between S0o
andS0+
S0+ AlateS0stageshowingstrongringsandbars,andinsomecasestrace
spiralstructure(examples: NGC1291,1326,4138)
S0+[c] AlateS0withanSc-likecentralconcentration (examples: NGC4344,
4451)
Spiral and Irregular Galaxies
Spiralgalaxy Agalaxywhereaspiralpatternisamajorpartofthemorphology
S0/a-Sa-Sb-Sc-Sd-Sm Astagesequence forspirals(withintermediatetypes Sab,Sbc,Scd,and
Sdm)basedonHubble’sthreecriteria: relativeprominenceofthe
bulge,thedegreeofopenness ofthearms,andthedegreeof
resolutionofthearmsintostarclustersorveryluminousstars.
Irregulargalaxy Acomplexsystemcharacterized byanirregulardistributionofstar
formation;thisirregulardistributioncan,however,beembeddedina
moreregularbackground
S0/a AnS0/agalaxythatisclosertoS0+ thantoSa(examples: NGC522,
2681,3626)
nrl) as intermediate categories, all with a similar rela- der (2012). In a cluster environment, parallel sequence
tionship to bar extent, could imply a close connection classificationis a moreaccurateview of galaxymorphol-
between rings and lenses in a dynamical or even evolu- ogy than the VRHS sequence, and provides a natural
tionary sense. Kormendy (1979) originally argued that home for what Kormendy & Bender (2012) refer to as
inner lenses could be the result of secular dissolution of “spheroidalgalaxies.”Nevertheless,this does not negate
aprimarybar. Comero´n(2013)hasmostrecentlyexam- the value of CVRHS morphology because CVRHS clas-
ined the ring-lens issue and concluded that once a star- sificationis,forthe mostpart,purelymorphologicaland
forming ring exhausts or is stripped of its gas, it may notbasedona“whiffoftheory”(Sandage2005). [Anex-
dissolve into a ring-lens in as little as 200 Myr. ceptionisthe OLRouter ring/pseudoringmorphological
Other characteristics considered part of CVRHS mor- subclasses (Buta & Crocker 1991), which were theoreti-
phologyincludeX-patterns,boxy/diskystructures,outer cally predicted by Schwarz (1981).]
Lindblad resonance (OLR) ring morphologies, warps, Paper I described the application of the CVRHS to
spheroidalgalaxies,andotherfeaturesdescribedinmore 200 S4G galaxies, where it was shown that, in spite of
detail in the next Sections. CVRHS morphology also the significant differences between modern mid-IR digi-
considers alternative points of view, such as the paral- talimagesandtheopticalphotographicblue-lightplates
lelsequenceclassificationofvandenBergh(1976)where that were the historical basis for the originalVRHS sys-
S0 galaxiesare stripped spirals on a sequence parallelto tem, many galaxiesshow the same essentialmorphologi-
regular spirals. Three recent studies have provided con- calfeaturesinthemid-IRasintheB-band,allowingthe
siderable supportfor this idea: Laurikainenet al. (2011, effectiveapplicationoftheVRHSsystemtoS4Gimages.
2013), Cappellari et al. (2011), and Kormendy & Ben- There is no need to invent a new classification system
Classical Morphology of S4G Galaxies 7
TABLE 1
Explanation of CVRHS Symbols(cont.)a
Symbol Description
1 2
S0/a Atransitionstageshowingclearbutsubtletightly-wrappedspiral
arms;structureisgenerallysmoothbuttracestarformationisseen
innearbyexamples(examples: NGC1350,1452,4394,4454, 4984,5701,
6340,7098)
S0/a AnS0/agalaxythatisclosertoSathantoS0+ (examples: NGC3185,3900)
Sa Anearly-typespiral,usuallydefinedbyrelativelysmooth,tightly-wrappedspiral
armsandasignificantbulge;standardinterpretationmaybeviolatedina
clusterenvironmentorinpresenceofabar(examples: NGC1433, 1512,3031,
3788,4260,4450, 4548,4800,7513)
Sab AnSabgalaxythatisclosertoSathantoSb(example: NGC2985)
Sab Anintermediate-typespiralgalaxysimilartoSabutwithamoreknotty
structure(examples: NGC210,1097,3992, 4995;IC1993)
Sab AnSabgalaxythatisclosertoSbthantoSa(examples: NGC3177,4902)
Sb Anintermediate-typespiralhavingrelativelymoreopen,knottyarmsanda
smallerbulgethanSaorSabgalaxies(examples: NGC908,3433,3689,
3705,4237,7479)
Sbc AnSbcgalaxythatisclosertoSbthantoSc(examples: NGC3344,3512;
IC769)
Sbc Typically,anintermediate-typegalaxyhavingwell-developed,open,knotty
spiralarmslikeanScgalaxybutwithamoresignificantbulge(examples:
NGC1365,3184,3198,3338, 3726)
Sbc AnSbcgalaxythatisclosertoScthantoSb(examples: NGC2715,4303)
Sc Alate-typespiralhavingwell-developed,open,andknottyspiralarmswitha
smallbutsignificantbulge(examples: NGC1042,1084,3486,3810, 3893,
4411B, 5457,5970,7448;IC1953)
Scd AnScdgalaxythatisclosertoScthantoSd(examples: NGC1073,5033, 5468)
Scd SimilartoanScbutwithlittleornobulge;typicallyasmallcentral object
(nuclearstarclusterorpseudobulge) maybeseen;armscanbemoreopenthan
foranSc;thesearegenerallypurediskgalaxies(PDGs)(examples: NGC1255,
1559,3346,5334, 5595,7741)
Scd AnScdgalaxythatisclosertoSdthantoSc(examples: NGC255,1253,3359,
4411A,5597,5668)
Sd Anextremelate-typespiral,similartoScdbutwithlessapparentcentral
concentration thananScd;amongthemostcommonPDGs;asymmetryisoften
presentbutlessextremethaninSdmorSm,Imtypes (examples: NGC3003,
4294,4731,5068, 5669)
Sdm AnSdmgalaxythatisclosertoSdthantoSm(examples: NGC247,3556,7151,
7497)
Sdm Anextremelate-typespiralshowingconsiderableasymmetry,usuallywithone
armlongerandbetter definedthantheother;considerablestarformationalso
characterizes thesePDGs(examples: NGC300,3906,4395, 4630,7154)
Sdm AnSdmgalaxythatisclosertoSmthantoSd(example: NGC2552)
Sm Amagellanicspiral,usuallycharacterized byasinglespiralarmemergingfrom
abarorcentral regionandlittleornobulgeorcentral concentration (de
Vaucouleurs&Freeman1972)(examples: NGC55,5474, 7091)
to accomodate mid-IR galaxy morphology; instead, it canbe difficult because of competing factors. For exam-
should be sufficient to build on what we already have ple, the contrast of the spiral structure in mid-IR light
from studies of blue-light images. This does not mean can be very low comparedto the brightness of the back-
thatthe “essentialfeatures”aredefinedinthe sameway ground disk light. This can make it difficult to fit some
inthetwowavebands. Forexample,thedegreeofresolu- galaxiesintotheCVRHSsystem,especiallyifagalaxyis
tion into star-forming regions was one of Hubble’s orig- small or distant enough to be poorly resolved. Another
inal criteria for classifying spiral galaxies into Sa-Sb-Sc issue is the greater sensitivity of the IRAC bands to old
bins. In the B-band, this resolution is determined by stellar population bulges [also known as classical bulges
the distributionof aggregatesofyoung blue supergiants. (Kormendy&Kennicutt2004,Athanassoula2005],while
In the mid-IR, however, these stars are not prominent. at the same time being less sensitive to spiral structure.
Instead, we see the thermal emissionfrom the local dust The galaxies that show the most drastic differences
heated by these stars. Paper I also noted that when between mid-IR and B-band morphology are usually
CVRHS mid-IR stages were compared to RC3 stages, those having considerable internal dust extinction, such
galaxies of types Sc and later or S0+ and earlier were as edge-on spirals, starburst galaxies, and major or mi-
often classified the same as in RC3, while intermediate nor merger systems. Extinction in the mid-IR bands is
stagessuchasS0/atoSbcwereclassifiedaboutonestage lessthan 5%ofthat inthe B-band,and generallyallows
earlier. fairlygoodpenetrationintothickplanardustlayers. S4G
Even if it is possible to use an existing classification imagesdoallowustoimproveourinterpretationofsome
system effectively in the 3.6µm and 4.5µm IRAC bands, edge-ongalaxies,butevensoclassificationisstilldifficult
theclassificationofgalaxiesbasedonS4Gmid-IRimages for highly inclined galaxies.
8 Buta et al.
TABLE 1
Explanation of CVRHS Symbols(cont.)a
Symbol Description
1 2
S/Im AmagellanicgalaxythatisclosertoImthantoSm(example: NGC4353)
Im Amagellanicirregulargalaxy,characterized byanirregularshapesometimes
withinasmoothbackgroundofstarlight;oftenshowconsiderablestarformation
andawiderangeofluminosities(examples: NGC4214,4242,4449, 4605)
Im(cc) Aclumpcluster,usuallyreferringtoanirregulargalaxywithalargenumberof
scatteredstarformingregions. Thetermwasoriginallyappliedtohigh
redshiftclumpygalaxies(e.g.,Elmegreenetal. 2009)(examples: IC1826,
2040;UGC1945)
I0 Atypeofgalaxyseenmainlyinbluelightimageswhereahighlyirregulardust
distributionisseenwithinanS0orE-likebackground;at3.6µm,I0
galaxiesarerelativelynormal-lookingETGs(examples: NGC2968,3077, 5195,
5253,5363)
Dwarf and Spheroidal Galaxies
dE A“dwarfelliptical”galaxy, definedtohavea“smoothintensitydistribution
overthefaceand... lowsurfacebrightness”(Sandage &Binggeli1984;
Non-VirgoexampleinS4Gcatalogue: NGC59)
dS0 A“dwarfS0”galaxy,aclassofdwarfswhichresembledEgalaxiesexcept for
“achangeofslopeintheradialgradientofthelightdistribution”
showing“eitherdirectevidenceofadisk,or... evidenceoftwo
components” (Sandage&Binggeli1984)
dE,N AnucleateddwarfEgalaxy(Sandage &Binggeli1984)
dS0,N AnucleateddwarfS0galaxy(Sandage &Binggeli1984)
dIm Adwarfirregulargalaxy,typicallyanImgalaxyhavinganabsoluteB-band
magnitudeMo & 17;lowsurfacebrightnessandoftenresolved
inS4GimagesB −
Sph A“spheroidal”galaxy,atypeofgalaxyhavingtheappearance ofanEorS0
galaxy,butthephotometriccharacteristics ofmuchlatertype,lower
luminositygalaxies(likeSm,Imtypes); thedEanddS0galaxiesinthe
VirgoClusterareallofthisbasictype(Kormendy&Bender 2012).
Sph,N Anucleatedspheroidalgalaxy(Binggelietal. 1985; Kormendyetal. 2009)
BCD Astar-forming,bluecompact dwarfgalaxy;at3.6µm,canappearasadEor
dS0galaxy(example: NGC1705)
Family
family Thecharacteristicofgalaxymorphologythatrecognizes theapparentstrength
ofabarorothertypeofnonaxisymmetricstructure,suchasanoval
SA AnonbarredspiralorS0galaxy(examples: NGC488,628,1411,4698; IC1993,
5267)
SAB Agalaxyshowingatraceofabar,usuallyintheformofabroadovalora
verylowcontrastregularbar(examples: NGC4203,4899)
SAB Abarredgalaxyofintermediateapparentbarstrength(examples: NGC4535,
5236,7743)
3.2. Application to the Full S4G Sample the agreement is better, with 1370 (57%) of the sample
The classification of the full S4G sample was carried classified identically and 659 (27%) differing by only 1
±
step (e.g., as in Sab vs. Sb), 167 (7%) differing by 2
out by R. Buta in three phases: Phase 1, the initial ex- ±
steps (as in Sbc vs. Scd), and 83 (3%) differing by more
aminationofthefulldatasetasdatawerebeingcollected;
than 2 steps. Some of the cases of large disagreement
Phase 2, a re-examinationof the full dataset made more
±
are galaxies which appear to be of an early type, yet
than a year after data collection ended and without any
havelittleornobulge. Thesehaveclassificationssuchas
reference to the Phase 1 results; and a partial (10%)
“SB(s)0/a[d],” where the “[d]” is meant to highlight the
Phase 3 made 6 months after Phase 2 (also without ref-
apparent lack of a bulge [alluding to the van den Bergh
erence to the previous phases)to better assessthe inter-
(1976) parallel sequence idea]. This inconsistency could
nalconsistencyofthe typesderivedbyaveraging thefull
Phase 1 and Phase 2 catalogues (Section 3.3).25 bereal,butcouldalsopartlybearesolutioneffectinthe
sample.
Table 2 provides the classifications from Phases 1 and
For 1709 galaxies where it was possible to reliably as-
2. (The Phase 3 classifications are listed in the Ap-
sign family classifications, the Phase 1 and 2 classifica-
pendix.) A visual comparison shows generally good
tions were identical for 1160 (68%) of the sample, and
agreement between them, with differences appearing to
differed by one classification interval (e.g., SB vs SAB
bemostlyrandom. Ofthe2412galaxies,thePhase1and
for 264 (15%), two intervals (e.g, SAB vs. SB) for 264
2 classifications are identical for 382 galaxies, or 16% of
(15%),3intervals(e.g.,SABvs. SBfor6(0.4%),andthe
the sample. Restricting to the stage classification alone,
full range (e.g., SB vs. SA) for 15 (0.9%) of the sample.
25 Wethanktherefereeforsuggestingthisapproach. The classification and recognition of inner and outer
ring and lens features can show variation, with some
Classical Morphology of S4G Galaxies 9
TABLE 1
Explanation of CVRHS Symbols(cont.)a
Symbol Description
1 2
SAB Abarthatisclearandwell-definedbutweaker-lookingthanatypicalSB
galaxybar(examples: NGC4639,4818,5566,5701)
SB Abarredgalaxywithaconspicuous bar,usuallystrongandobvious
(examples: NGC1300,1365,1452,7513)
SABa,SBa Abarredgalaxywherethebarisdefinedbybrightnessenhancements (“ansae”;
Sandage1961;Danby1965)atitsends;theseenhancements mayberound
spots,shortlinearfeatures,arcs,orstar-formingclumps(Buta2013)
[examples: NGC2787,5375,7098(seeFigures12&13forothers)]
SABx,SBx AgalaxyshowingaprominentXorbox/peanut structureinitsinnerregions;
oftenseeninedge-ongalaxies,thebox/peanut/X shapeisbelievedto
beamanifestationofverticalresonantorbitsinabarpotential, thusit
indicatesthepresenceofabar(Athanassoula2005). AnXmayalsobeseen
intheclearbarsofnon-edgegalaxies(Butaetal. 2007;Erwin&Debattista
2013)[examples: NGC2654(Figure14;nearlyedge-on); NGC5377
(intermediateinclination)]
SABxa,SBxa AgalaxyshowingbothanXandapairofansae;theXisusually
three-dimensionalwhiletheansaeareflat. [example: NGC4216(Figures12
&13)]
Standard Inner Varieties
innervariety Thecharacteristicofgalaxymorphologythatrecognizes thepresenceorabsence
ofaninnerring
(r) Aninnerring,aclosedcircularorovalfeatureenvelopingeithertheendsof
abarifpresentorthecentralbulge(examples: NGC1433, 3351,3486,
4245,5566)
(rs) Awell-definedinnerring,butmadeoftightly-wrappedspiralstructure
(examples: NGC613,1398,3368,3705)
(rs) Aninner“pseudoring”, apartialinnerringmadeofspiralarms(examples:
NGC779,1232, 3346,4548)
(rs) Aweakinnerpseudoring,usuallyfairlyopenandcharacterizedbyapitch
angleonlyalittledifferentfromthatoftheouter spiralarms
(examples: NGC1365,3513,4501,4548, 5383)
(s) Apures-shapedspiralwherethearmsbreakfromtheendsofabarorfrom
thebulgewithoutformingapseudoring(examples: NGC1300,7721)
Special Inner Varieties
(rr) Agalaxyhavingtwoinnerrings(example: NGC4698)
(x1r) Aring-likefeaturethatoutlinesabar,possiblyrelatedtothex1 family
ofbarorbitsdiscussedbyRegan&Teuben(2004)(example: NGC6012)
(r,s) Avarietywheretheinsideofaninnerringincludesaspiralpatternunrelated
tothemainouterspiralarms;prototype inthiscatalogue isNGC5364
(rs,rs) Agalaxyhavingtwoinnerpseudorings,usuallyofverydifferentsizes
(examples: NGC289,4689)
classifications in one phase not being noted in the other al. (2011) we defined an index F which ranges from 0.0
phase. In 14 cases in the catalogue, the same feature for SA galaxies to 1.0 for SB galaxies. We then set F
has been classified as an inner ring or pseudoring in one = 0.25, 0.50, and 0.75 for SAB, SAB, and SAB galax-
phase, and as an outer ring or pseudoring in the other. ies, respectively. Similar codings were used for inner
AsnotedinSection3.1,suchambiguitiescanoccurespe- ring/pseudoring/spiral and outer ring/pseudoring clas-
ciallyfornonbarredorveryweakly-barredgalaxies(Buta sifications. In Tables 3 and 4, these are referred to as
1995). We adopt either the Phase 1 or Phase 2 classifi- the inner variety IV and outer variety OV, respectively.
cation for such cases, after a re-inspection of the image. Although the parentheses in VRHS classifications nor-
mallyincludeonlyasingleouterorinnerfeature[e.g.,as
3.3. Comparison of Classifications in(R)SB(rs)ab],CVRHSclassificationscanincludemul-
tiple inner and outer rings, lenses, or nuclear features.
In this Section, we use our independent classification
ThiscomplicatescombiningPhase1and2classifications
phases to examine the internal agreement of CVRHS
for some of the galaxies, which had to be treated on an
stage, family, and variety classifications of S4G galaxies.
individual basis. For most of the galaxies, the assign-
The stages and families are also compared with other
ment of a T, F, IV, and OV index was straightforward.
sources.
Nevertheless, we strongly emphasize that none of these
For comparing classifications, and also for combining
numbers is a measured quantity; they are merely conve-
the Phase 1 and 2 catalogues, the letter classifications
nient codings for a given classification symbol. Table 4
were coded with numbers (Table 3). For stages, the
shows how the Phase 1 and 2 < T >, < F >, < IV >,
standard numerical T index, which ranges from T = 5
− and < OV > values convert into final classifications for
for E galaxies to T = +10 for Im galaxies, was used as
most of the sample.
in RC3. For family classifications, following Baillard et
10 Buta et al.
TABLE 1
Explanation of CVRHS Symbols(cont.)a
Symbol Description
1 2
(s,rs) Agalaxyhavingastrongs-shapedspiralinthepresenceofaninnerpseudoring
(example: NGC986)
(l) Aninnerlens,atypeoffeature,oftenseeninS0galaxies,havingashallow
brightnessgradientinteriortoasharpedge(Kormendy1979)(examples:
NGC1291,1411, 4269,5602)
(rl) Aninnerring-lens,recognizedasalowcontrastinnerring;types
(rl)and(rl)recognizedifferentdegreesofcontrastenhancement
(examples: NGC2859,4250)
(rs,rl) Agalaxyhavinganinnerpseudoringandaninnerring-lens,thelatterthe
smaller(example: NGC5055)
(r′l) Aninnerpseudoring-lens,wheretheinnerringshowsazimuthalcontrast
differences likespiralarms(examples: NGC210,1415,3147)
(ls) Aninnerlenswithasubtleembeddedspiralpattern(example: NGC3675)
(p) A“plume”,usuallyseenasasecondaryspiralarcpositionedjustoffthe
leadingsidesofabrightinnerring(Buta1984); muchrarerthaninnerrings
orlenses(example: NGC1433)
(bl) A“barlens”,afeaturerecognizedbyLaurikainenetal. (2011, 2013,2014)
andAthanassoulaetal. (2014)astheinnerpartofanearly-typebar
[examples: NGC1433, 2787,3351(seeFigure11)]
Nuclear Varieties
nuclearvariety Thenuclearvarietyclassificationreferstorings,lenses,bars,andotherfeatures
thatareoftenfoundinthecenters ofbarredgalaxies,butwhichmayalso
befoundinnonbarredgalaxies
(nr) Anuclear(orcircumnuclear)ring,asmallstar-formingfeaturefoundin
thecenters ofbarredgalaxies butalsosometimesseeninnonbarredgalaxies
(example: NGC1097; seeFigure20forothers)
(nr′) Anuclearpseudoring,wheretheringappearsformedbyawrappedspiral
pattern; inoptical images,thischaracter maysometimesbeanartifactof
innerdustlanesinbars(examples: NGC1068, 1090)
(ns) Anuclearspiral,atypeoffeaturethatmayalsobeanartifactofdustin
optical bands(example: NGC1022)
(nl) Anuclearlens;anexcellent exampleintheS4Gcatalogue isNGC4250
(nrl) Anuclearring-lens(examples: NGC210,1300,1433,5566)
(nb) Anuclear(orsecondary)bar,afeaturefoundinthecenters ofbarredgalaxiesbut
whichmayalsobefoundinSAgalaxies(examples: NGC1291,1433,4725)
(nba) Anuclearansae-type bar
(nd) Anucleardisk,usuallyseeninwell-resolvededge-ondiskgalaxies
(examples: NGC24,678,1532,3079,3628, 4111,5907)
(np) a“nuclearpattern,”anuclearstructureofuncertainnature
(tb) atriaxialbulge,anelongated centralcomponent whosemajoraxisismisaligned
withthegalaxymajoraxis
(psb) apseudobulge,anelongatedbulgecomponent whosemajoraxisisapproximately
alignedwiththediskmajoraxis(example: NGC4536)
For some galaxies, the classification has two parts, as andF forPhase1ateachPhase2type. Thedifferences,
in “S0− sp / E(d)7. This refers to an edge-on S0 show- ∆T = T T and ∆F = F F , can be used to judge
2 1 2 1
− −
ing a thin disk embedded within a disky (pointy-ended) howconsistentthe Phase1and2classificationsarewith
E-likethickdisk. (AnexamplehereisNGC 1032,shown respect to the refined divisions of CVRHS morphology.
in Figure 1.0193.) In such a case, only the first part de- The internal consistency of our Phase 1 and 2 stage
terminesthe T-index,whichinthis exampleis 3. Note andfamilyclassificationscanbe quantifiedusingthenu-
−
thatnormalellipticalsareveryrarelymoreflattenedthan merical codings in Table 3 to calculate σ (T) = σ(∆T)
12
E4andgenuineEgalaxiesarenotnecessarilyfoundmore = Σ(T2−T1)2 and σ (F) = σ(∆F) = Σ(F2−F1)2 for
flattenedthanE6(vandenBergh2009). The E(d)nota- q N−1 12 q N−1
tion is from Kormendy & Bender (1996), and was orig- all galaxies in the sample having stage and family clas-
inally proposed for genuine elliptical galaxies. Two-part sifications in both phases. From these, we estimate the
classifications recognizing a thick disk are only given for standard deviation of a single T classification as σ1(T)
highly-inclined galaxies. σ (T) = σ(T) = σ(∆T)/√2 and of a single F classi-
2
≈
fication as σ (F) σ (F) = σ(F) = σ(∆F)/√2. In a
3.3.1. Stage and Family Phase 1,2 Comparisons 1 ≈ 2
similar manner, when the Phase 1 and 2 classifications
Figure2showscomparisonsbetweenthemeannumer- are averaged, we estimate the standard deviation of the
ical type and family indices for the Phase 1 and 2 clas- mean types and families as σ(< T >) = σ(∆T)/2 and
sifications for the full sample (top panels) and for the σ(<F >)=σ(∆F)/2, respectively.
much smaller paper I subsample (middle panels). Filled The results of this analysis are compiled in Table 5,
circlesshowthemeanT andF valuesforPhase2ateach which gives these standarddeviations in units of 1 stage
Phase 1 type, while the filled triangles show the mean T intervalforT (∆T =1.0)and1familyintervalforF (∆F