- The Formation and Evolution of Lenticular (S0) Galaxies
Lenticular (S0) galaxies form a morphological transition class between ellipticals and early-type spirals in the Hubble (1936) classification system. When comparing properties, it is found that the bulges of lenticulars are very similar to elliptical galaxies, while their disks have similarities to the disks of early type spiral galaxies, except that they lack conspicuous spiral arms. Our understanding of the formation and evolution of lenticular galaxies, in terms of the individual physical processes involved, is still unclear, inspite of extensive efforts both by observational and theoretical means.
We made detailed multiband investigation of surface photometry of the morphology of representative samples of lenticulars (S0), and their integrated magnitudes and colors along with a careful comparison with results obtained by other authors. We obtained color profiles and color gradients, and studied the presence and spatial distribution of dust using extinction maps and color maps obtained from the optical and near-IR images. The colors f the galaxies in the sample become bluer outwards, in keeping with the trend observed in ellipticals. A comparison of the mean color gradients with corresponding values from the literature for ellipticals and bulges of early-type spirals shows that the (B-V) color gradients for lenticulars are more negative than the gradients for ellipticals, while the (B-R) gradient for lenticulars is less negative than the corresponding gradient for bulges of early-type spirals. The steeper color gradients could imply that the metallicity gradients are stronger in lenticulars and/or that there is more recent star formation in the outer parts of lenticulars as compared to those of ellipticals.
A detailed study of the morphology of lenticular galaxies in near-infrared band, with possible separation of bulge and disk components is carried out by our group. We find that lenticular galaxies show markedly different correlations between their bulge effective radius (re) and disk scale length (rd) as a function of their total luminosity (MT). For faint lenticular galaxies (MT > -24.5), re and rd are positively correlated, in line with predictions of secular formation processes that likely formed the pseudo bulges of late-type disk galaxies. Such a formation scenario is also consistent with the predictions of numerical simulations of lenticular galaxy formation. Bright lenticular galaxies with MT < -24.5, on the other hand, do not exhibit this correlation, indicating a different formation mechanism. Further, we have shown that several correlations such as the Kormendy relation, Photometric Plane, etc. support the hypothesis that bright and faint lenticulars (in cluster or field environments) are fundamentally different, with different formation histories. These trends seem to hold irrespective of galaxy environment, although more luminous lenticulars are largely missing from our cluster sample. The relative fraction of lenticular galaxies is, of course, very different in clusters and in the field.
- Barway, Sudhanshu, Mayya, Y. D., Kembhavi, Ajit K., Pandey, S. K., 2005, Astronomical Journal, 129, 630
- Barway, Sudhanshu, Kembhavi, Ajit, Wadadekar, Yogesh, Ravikumar, C. D., Mayya, Y. D. , 2007, Astrophysical Journal, 661, L37
- Barway, Sudhanshu, Wadadekar, Yogesh, Kembhavi, Ajit K. Mayya, Y. D., 2009, MNRAS, 394, 1991
- SDSS View of Formation of Lenticular (S0) Galaxies
- Bar fraction in lenticular galaxies: dependence on luminosity and environment
A study of bars in lenticular galaxies based on a sample of 371 galaxies from the SDSS-DR 7 and 2MASS in optical and near-infrared bands, respectively, reveled 15% of the lenticular galaxies are barred in our sample. The barred galaxy fraction shows a luminosity dependence, with faint lenticular galaxies (M_K > -24.5, total absolute magnitude in K band) having a larger fraction of bars than bright lenticular galaxies (M_K < -24.5). A similar trend is seen when M_r = -21.5, the total absolute magnitude in SDSS r band is used to divide the sample into faint and bright lenticular galaxies. We find that faint galaxies in clusters show a higher bar fraction than their counterparts in the field. This suggests that the formation of bars in lenticular galaxies not only depends on the total luminosity of galaxy but also on the environment of the host galaxy (Barway et al. 2010 ).
- Luminosity-dependent star formation history of S0 galaxies: evidence from GALEX-SDSS-2MASS-WISE colours
We combine UV/optical/near-IR/mid-IR data on a sample of nearly 240 S0 galaxies to examine various star formation related processes in them. We split the sample into bright and faint S0 galaxies based on their K-band luminosity. Comparing the far-ultraviolet (FUV)-near-ultraviolet (NUV) versus NUV-K colour-colour diagram with a simple stellar population model shows that ellipticals and bright S0 galaxies are dominated by a stellar population of age >109 yr while faint S0 galaxies may contain stars as young as 108 yr, providing evidence for relatively recent star formation activity. The strength of the 4000 Å break is also systematically higher in brighter S0 galaxies, again indicating the presence of an old stellar population. Their mid-IR colours indicate that bright S0 colours are like those of ellipticals while faint S0 colours are more like spirals. All these observations are consistent with a scenario in which low-luminosity S0 galaxies likely formed by the stripping of gas from the discs of late-type spiral galaxies, which in turn formed their pseudo-bulges through secular evolution processes, possibly involving multiple episodes of star formation. On the other hand, more luminous S0 galaxies likely formed the bulk of their stars at early epochs, similar to the star formation in elliptical galaxies, and are characterized by an old coeval stellar population and classical bulges.
- Barway, Sudhanshu, Wadadekar, Yogesh, Vaghmare, Kaustubh, Kembhavi, Ajit K., 2013, MNRAS, 432, 430
- Barway, Sudhanshu, Wadadekar, Yogesh, Vaghmare, Kaustubh, Kembhavi, Ajit K., 2013, IAUS, 292, 153
- SuperMassive Black Holes in Nearby Galaxies
The existence of massive Black Holes (hereafter BHs) at the center of nearby inactive galaxies as well as in the nuclei of active galaxies and quasars is well established. Observations based on high resolution HST data and reverberation mapping are now available which allow measurement of the masses of BHs using different techniques has shown that the measured BH mass (MBH) is tightly with correlated physical properties of host galaxy such as bulge luminosity (MB), bulge mass (Mbulge) and the velocity dispersion (σ). It is believed that these massive BHs play an important role in the formation and evolution of galaxies, and the growth of BHs and bulges must be linked to the same physical processes which results in BH masses that are related to the properties of host galaxies. We (Barway & Kembhavi, 2007, Astrophysical Journal, 662, L67) obtain a new fundamental plane for supermassive black holes at the centers of elliptical galaxies, involving measured central black hole mass and photometric parameters (mean bulge surface brightness (SBe) and effective radius (Re) of bulge) which define the light distribution. The galaxies are tightly distributed around this mass fundamental plane, with improvement in the rms residual over those obtained from the MBH-σ and MBH-MB relations. The mass FP provides a convenient way for estimating BH mass from photometric data alone and implies a strong multidimensional link between the central massive black hole formation and global photometric properties of elliptical galaxies and provides an improved estimate of black hole mass from galaxy data.
- Stellar Activity Cycles in Chromospherically Active Stars (PhD Thesis)
At some level all stars are probably spotted, however, enhanced solar like activity, seems to be inherent properties of all late type stars regardless of their evolutionary stages. Solar like magnetic activity such as spots, highly energetic flares, facular networks chromospheric plagues, emission from transition region and corona are found to be present in cool late-type stars. Stellar magnetic activities with far extreme activity level than their solar counterparts have been found in several groups of stars and these groups are broadly known as chromospherically active stars. The chromospherically active stars display appreciable changes in their light curves within very short period of time typically covering a couple of rotation periods. These short term variations are usually attributed to the secular evolution of starspot area and/or redistribution of the starspots on the stellar surface as reported in previous studies. The light curves of these stars obtained during any two seasons do not agree in shape, amplitude, and phase of light minimum and maximum and mean light level. Chromospherically active stars include single and binary stars as well as pre- and post-mainsequence stars having rapid rotation and deep convective layers. At present at least ten different groups of chromospherically active stars such as RS CVn binaries, BY Dra, W Uma, T Tauri etc. have been classified according to their properties of being a member of single or binary system, evolutionary stage and rotation period. As far as strength of activity among these active stars is concerned RS CVn may be considered as the group leader. With the availability of small and efficient telescopes equipped with instruments like photoelectric photometer and small format CCD’s, simplicity of photometric observations and its reduction procedure, the conventional photometric monitoring has become a highly potential tool for the study of chromospherically active stars. The objective of PhD thesis was to study the short term as well as long term photometric light variation of chromospherically active stars in the framework of starspot model and look for the stellar activity cycles akin to solar 11-year sunspot cycle using variety of photometric activity tracers.
- Features in Early-type Galaxies
In 1983, Malin and Carter published “A catalog of elliptical galaxies with shells”, a list of 137 elliptical galaxies in the southern sky, causing a minor revolution in the study of galaxies and their stellar populations. Since then, the possibility has arisen that such structural features are ubiquitous in ellipticals though these galaxies were (and to a large extent, still are) usually thought to belong to a homogeneous class. Theoreticians have since shown that these shells (and many other kinds of features found later on) could be linked to the recent merger history of these galaxies. This was coupled with the discovery of minor axis rotation and kinematically de-coupled cores, as well as other photometric features underlying the smooth light, all of which hold clues to the history of formation of early-type galaxies (E + S0). Furthermore, as observations unfolded at other wavelengths, it was clear that these apparently smooth clumps of old stars show remarkable features in other wavelength regimes: hot gas in x-rays, dust features and ionized gas in carefully-examined optical features, molecular gas in radio observations.
In collaboration with Somak Raychaudhury (University of Birmingham), Prof. A.K. Kembhavi (IUCAA, Pune) and Prof. S.K. Pandey (Pt. Ravishankar Shukla University, Raipur) observed a sample of early-type galaxies chosen for carrying out observations in optical, infrared as well as in X-rays bands, in J,H and Ks band from 3.5m New Technology Telescope (NTT), La Silla, Chile. Image processing techniques unveiled various structures and substructures in the sample galaxies, e.g., shells, dust lanes and patches, etc., especially a spiral structure in the core of the galaxy NGC 1553 and shells in the inner and outer regions. This is the first time such features have been unveiled from NIR images of early-type galaxies. These observations will be the first of its kind for a well defined large sample, and will lead to a detailed understanding of the nature, origin and evolution of structures, substructures, shells and ISM in early-type galaxies. Please watch this space for more information on Shell galaxies from SDSS.
- The Central Regions of Lenticular Galaxies- A HST WFPC-2 Imaging Survey
The central regions of galaxies hold important clues to the physical processes that govern galaxy evolution. In the last decade, several studies focussing on early-type galaxies have shown a dichotomy in the nuclear structures, which can be traced to two possible formation mechanisms. While the low luminosity spheroids have steep cusps suggestive of recent gas accretion and star formation, the most luminous galaxies show flat cores in their centers. The core formation has been attributed to the scouring action of binary black holes during gas-poor galaxy mergers. Spiral galaxies, on the other hand, almost always have nuclei or star clusters in the centers, suggesting gas-rich, dissipative processes are important in their formation. With Swara Ravindranath, I have undertaken a study of lenticular galaxies, the global properties of which straddle the regime between ellipticals and spiral bulges. The initial results show that the lenticulars along with the spiral bulges, and ellipticals form a continuous sequence of cusp slopes from the lowest luminosity (MV > -15 to MV ~ -21). The correlation between the cusp slope and luminosity, and the frequency of compact stellar nuclei, are suggestive of a dissipative formation history involving gas-rich mergers, gas accretion to the central region, and subsequent star formation. The most luminous ellipticals, and BCGs at MV < -21 depart from this sequence and exhibit very flat cores, favouring a formation involving gas-poor mergers and binary blackhole effects. It appears that the observed dichotomy in the properties of spheroids that were reported in the literature previously, may have been the results of viewing only a part of the parameter space occupied by galaxies with MV < -19. Taking all bulges, over a large range in luminosities, clearly shows a dominant, continuous sequence of gas-rich mergers, with core galaxies clearly falling off this sequence at high luminosities.