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Register a new userComposite Bulges -- II. Classical Bulges and Nuclear Discs in Barred Galaxies: The Contrasting Cases of NGC 4608 and NGC 4643
كومبوزيت بلجز -- الجزء الثاني. البلجزات الكلاسيكية والقروض النووية في المجرات المشروطة: الحالات المتناقضة لNGC 4608 وNGC 4643
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Peter Erwin
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نقدم لكم تحليلات مورفولوجية، فوتومترية وكينماتيكية النجومية المفصلة لمناطق المركزية لأجسام باردة نوعها المبكر من الشأن الكبير، والتي تحتويان على باردات كبيرة وقوية مع إضافات داخلية فوتومترية وتشير إلى شكل بوجي/بنت (B/P) للأجسام الشعاعية، والتي تشكل ~ 30٪ من الضوء الشعاعي. داخل أجسامها الشعاعية البوجي/بنت، يحتوي NGC 4608 على بنية مضغوطة، تقريباً مستديرة (نصف نصف الضوء R_e approx. 310 بكسل، Sersic n = 2.2) نحن نعرفها بأنها بنية كلاسيكية، والتي تمثل 12.1٪ من الضوء الكلي، بالإضافة إلى مجموعة نجوم نوكلية (R_e ~ 4 بكسل). وبالمقابل، يحتوي NGC 4643 على قرص نوكلي مع ملف توزيع ضخامة الضوء الظاهري المكسور الأسي (13.2٪ من الضوء)، وبنية صغيرة شعاعية جداً (R_e approx. 35 بكسل، n = 1.6؛ 0.5٪ من الضوء). وتؤيد كينماتيكا النجومية المحددة المحددة هذا الصورة، حيث تتجه بنية كلاسيكية NGC 4608 ببطء وتتمثل في توزيع عالي للسرعة، في حين أن قرص نوكلي NGC 4643 يظهر إنخفاضاً إلى توزيع أقل، تدوير سريع، V-h3 عكس التكافؤ، وh4 مرتفع. يظهر كلا الأجسام على الأقل بعض الأدلة على التكافؤ V-h3 في البار (خارج البنية الكلاسيكية والقرص النوكلي الخاصة بكل منهما)، موافقاً لما يتوقعه النموذج. وتؤدي تقسيمات 2 عنصرية (بنية/قرص) القياسية إلى B/T ~ 0.5-0.7 (وبنية n > 2) لكلا الأجسام. وهذا يفوق الحقيقة لأجسام الشعاعية بمعدلات أربع (NGC 4608) وأكثر من 100 (NGC 4643)، مما يوضح المخاطر المتعلقة بتقسيمات بنية/قرص البسيطة المطبقة على أجسام باردة كبيرة الشأن.
We present detailed morphological, photometric, and stellar-kinematic analyses of the central regions of two massive, early-type barred galaxies with nearly identical large-scale morphologies. Both have large, strong bars with prominent inner photometric excesses that we associate with boxy/peanut-shaped (B/P) bulges; the latter constitute ~ 30% of the galaxy light. Inside its B/P bulge, NGC 4608 has a compact, almost circular structure (half-light radius R_e approx. 310 pc, Sersic n = 2.2) we identify as a classical bulge, amounting to 12.1% of the total light, along with a nuclear star cluster (R_e ~ 4 pc). NGC 4643, in contrast, has a nuclear disc with an unusual broken-exponential surface-brightness profile (13.2% of the light), and a very small spheroidal component (R_e approx. 35 pc, n = 1.6; 0.5% of the light). IFU stellar kinematics support this picture, with NGC 4608s classical bulge slowly rotating and dominated by high velocity dispersion, while NGC 4643s nuclear disc shows a drop to lower dispersion, rapid rotation, V-h3 anticorrelation, and elevated h4. Both galaxies show at least some evidence for V-h3 correlation in the bar (outside the respective classical bulge and nuclear disc), in agreement with model predictions. Standard 2-component (bulge/disc) decompositions yield B/T ~ 0.5-0.7 (and bulge n > 2) for both galaxies. This overestimates the true spheroid components by factors of four (NGC 4608) and over 100 (NGC 4643), illustrating the perils of naive bulge-disc decompositions applied to massive barred galaxies.
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We study nine S0-Sb galaxies with (photometric) bulges consisting of two distinct components. The outer component is a flattened, kinematically cool, disklike structure: a disky pseudobulge. Embedded inside is a rounder, kinematically hot spheroid: a classical bulge. This indicates that pseudobulges and classical bulges are not mutually exclusive: some galaxies have both. The disky pseudobulges almost always have an exponential disk (scale lengths = 125-870 pc, mean $sim 440$ pc) with disk-related subcomponents: nuclear rings, bars, and/or spiral arms. They constitute 11-59% of the galaxy stellar mass (mean PB/T = 0.33), with stellar masses $sim 7 times 10^{9}$-$9 times 10^{10} M_{odot}$. Classical-bulge components have Sersic indices of 0.9-2.2, effective radii of 25-430 pc and stellar masses of $5 times 10^{8}$-$3 times 10^{10} M_{odot}$ (usually < 10% of the galaxys stellar mass; mean B/T = 0.06). The classical bulges show rotation, but are kinematically hotter than the disky pseudobulges. Dynamical modeling of three systems indicates that velocity dispersions are isotropic in the classical bulges and equatorially biased in the disky pseudobulges. In the mass--radius and mass--stellar mass density planes, classical-bulge components follow sequences defined by ellipticals and (larger) classical bulges. Disky pseudobulges also fall on this sequence; they are more compact than similar-mass large-scale disks. Although some classical bulges are quite compact, they are distinct from nuclear star clusters in both size and mass, and coexist with nuclear clusters in at least two galaxies. Since almost all the galaxies in this study are barred, they probably also host boxy/peanut-shaped bulges (vertically thickened inner parts of bars). NGC 3368 shows evidence for such a zone outside its disky pseudobulge, making it a galaxy with all three types of bulge.
The early-type spiral NGC 4698 is known to host a nuclear disc of gas and stars which is rotating perpendicularly with respect to the galaxy main disc. In addition, the bulge and main disc are characterised by a remarkable geometrical decoupling. Indeed they appear elongated orthogonally to each other. In this work the complex structure of the galaxy is investigated by a detailed photometric decomposition of optical and near-infrared images. The intrinsic shape of the bulge was constrained from its apparent ellipticity, its twist angle with respect to the major axis of the main disc, and the inclination of the main disc. The bulge is actually elongated perpendicular to the main disc and it is equally likely to be triaxial or axisymmetric. The central surface brightness, scalelength, inclination, and position angle of the nuclear disc were derived by assuming it is infinitesimally thin and exponential. Its size, orientation, and location do not depend on the observed passband. These findings support a scenario in which the nuclear disc is the end result of the acquisition of external gas by the pre-existing triaxial bulge on the principal plane perpendicular to its shortest axis and perpendicular to the galaxy main disc. The subsequent star formation either occurred homogeneously all over the extension of the nuclear disc or through an inside-out process that ended more than 5 Gyr ago.
(Abridged) We study the incidence, as well as the nature, of composite bulges in a sample of 10 face-on barred galaxies to constrain the formation and evolutionary processes of the central regions of disk galaxies. We analyze the morphological, photometric, and kinematic properties of each bulge. Then, by using a case-by-case analysis we identify composite bulges and classify every component into a classical or pseudobulge. In addition, bar-related boxy/peanut (B/P) structures were also identified and characterised. We find only three galaxies hosting a single-component bulge (two pseudobulges and one classical bulge). We find evidence of composite bulges coming in two main types based on their formation: secular-built and merger- and secular-built. We call secular-built to composite bulges made of entirely by structures associated with secular processes such as pseudo bulges, central disks, or B/P bulges. We find four composite bulges of this kind in our sample. On the other hand, merger- and secular-built bulges are those where structures with different formation paths coexist within the same galaxy, i.e., a classical bulge coexisting with a secular-built structure (pseudobulge, central disk, or B/P). Three bulges of this kind were found in the sample. We remark on the importance of detecting kinematic structures such as sigma-drops to identify composite bulges. A large fraction (80%) of galaxies were found to host sigma-drops or sigma-plateaus in our sample revealing their high incidence in barred galaxies. The high frequency of composite bulges in barred galaxies points towards a complex formation and evolutionary scenario. Moreover, the evidence for coexisting merger- and secular-built bulges reinforce this idea. We discuss how the presence of different bulge types, with different formation histories and timescales, can constrain current models of bulge formation.
We present here a thorough photometric analysis of double-barred galaxies, consisting of i) two-dimensional photometric decompositions including a bulge, inner bar, outer bar, and (truncated) disc; and ii) three-dimensional statistical deprojections to derive the intrinsic shape of bulges, inner bars, and outer bars. This is the first time the combination of both techniques is applied to a sample of double-barred galaxies. It represents a step forward with respect to previous works, which are based on properties of the integrated light through ellipse fitting and unsharp masking. In this first paper of a series of two, we analyse the nature of the dominant bulges within double-barred systems by using several photometric diagnostics, namely Sersic index, Kormendy relation, colours, and the better suited intrinsic flattening. Our results indicate that almost all bulges in our sample are classical, whereas only 2 out of the 17 galaxies under study appear as potential candidates to host secularly-formed disc-like bulges. Such result poses the possibility that having a central hot structure may be a requirement for inner bar formation.
S0 galaxies are known to host classical bulges with a broad range of size and mass, while some such S0s are barred and some not. The origin of the bars has remained as a long-standing problem -- what made bar formation possible in certain S0s? By analysing a large sample of S0s with classical bulges observed by the Spitzer space telescope, we find that most of our barred S0s host comparatively low-mass classical bulges, typically with bulge-to-total ratio ($B/T$) less than $0.5$; whereas S0s with more massive classical bulges than these do not host any bar. Furthermore, we find that amongst the barred S0s, there is a trend for the longer and massive bars to be associated with comparatively bigger and massive classical bulges -- possibly suggesting bar growth being facilitated by these classical bulges. In addition, we find that the bulge effective radius is always less than the bar effective radius --indicating an interesting synergy between the host classical bulge and bars being maintained while bar growth occurred in these S0s.
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