arXiv subjects

Dimitri A. Gadotti

Publications and source records attributed to Dimitri A. Gadotti.

At least 73 records · Page 4Linked to original sources

arXiv2013

A low-luminosity type-1 QSO sample: I. Overluminous host spheroidals or undermassive black holes?

Recognizing the properties of the host galaxies of quasi-stellar objects (QSOs) is essential to understand the suspected coevolution of central supermassive black holes (BHs) and their host galaxies. We selected a subsample of the Hamburg/ESO survey for bright UV-excess QSOs, containing only the 99 nearest QSOs with redshift z<=0.06, that are close enough to allow detailed structural analysis. From this "low-luminosity type-1 QSO sample", we observed 20 galaxies and performed aperture photometry and bulge-disk-bar-AGN-decomposition with BUDDA on near-infrared J, H, K band images. From the photometric decomposition of these 20 objects and visual inspection of images of another 26, we find that ~50% of the hosts are disk galaxies and most of them (86%) are barred. Stellar masses, calculated from parametric models based on inactive galaxy colors, range from 2x10^9 M_sun to 2x10^11 M_sun. Black hole masses measured from single epoch spectroscopy range from 1x10^6 M_sun to 5x10^8 M_sun. In comparison to higher luminosity QSO samples, LLQSOs tend to have lower stellar and BH masses. Also, in the effective radius vs. mean surface-brightness projection of the fundamental plane, they lie in the transition area between luminous QSOs and "normal" galaxies. This can be seen as further evidence that they can be pictured as a "bridge" between the local Seyfert population and luminous QSOs at higher redshift. Eleven low-luminosity QSOs for which we have reliable morphological decompositions and BH mass estimations lie below the published BH mass vs. bulge luminosity relations for inactive galaxies. This could be partially explained by bulges of active galaxies containing much younger stellar populations than bulges of inactive galaxies. Also, one could suspect that their BHs are undermassive. This might hint at the growth of the host spheroid to precede that of the BH.

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arXiv2013

Unveiling the Structure of Barred Galaxies at 3.6 $μ{\rm m}$ with the Spitzer Survey of Stellar Structure in Galaxies (S$^4$G): I. Disk Breaks

We have performed two-dimensional multicomponent decomposition of 144 local barred spiral galaxies using 3.6 $μ{\rm m}$ images from the Spitzer Survey of Stellar Structure in Galaxies. Our model fit includes up to four components (bulge, disk, bar, and a point source) and, most importantly, takes into account disk breaks. We find that ignoring the disk break and using a single disk scale length in the model fit for Type II (down-bending) disk galaxies can lead to differences of 40% in the disk scale length, 10% in bulge-to-total luminosity ratio (B/T), and 25% in bar-to-total luminosity ratios. We find that for galaxies with B/T $\geq$ 0.1, the break radius to bar radius, $r_{\rm br}/R_{\rm bar}$, varies between 1 and 3, but as a function of B/T the ratio remains roughly constant. This suggests that in bulge-dominated galaxies the disk break is likely related to the outer Lindblad Resonance (OLR) of the bar, and thus moves outwards as the bar grows. For galaxies with small bulges, B/T $<$ 0.1, $r_{\rm br}/R_{\rm bar}$ spans a wide range from 1 to 6. This suggests that the mechanism that produces the break in these galaxies may be different from that in galaxies with more massive bulges. Consistent with previous studies, we conclude that disk breaks in galaxies with small bulges may originate from bar resonances that may be also coupled with the spiral arms, or be related to star formation thresholds.

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arXiv2013

Embedded Star Formation in S4G Galaxy Dust Lanes

Star-forming regions that are visible at 3.6 microns and Halpha but not in the u,g,r,i,z bands of the Sloan Digital Sky survey (SDSS), are measured in five nearby spiral galaxies to find extinctions averaging ~3.8 mag and stellar masses averaging ~5x10^4 Msun. These regions are apparently young star complexes embedded in dark filamentary shock fronts connected with spiral arms. The associated cloud masses are ~10^7 Msun. The conditions required to make such complexes are explored, including gravitational instabilities in spiral shocked gas and compression of incident clouds. We find that instabilities are too slow for a complete collapse of the observed spiral filaments, but they could lead to star formation in the denser parts. Compression of incident clouds can produce a faster collapse but has difficulty explaining the semi-regular spacing of some regions along the arms. If gravitational instabilities are involved, then the condensations have the local Jeans mass. Also in this case, the near-simultaneous appearance of equally spaced complexes suggests that the dust lanes, and perhaps the arms too, are relatively young.

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arXiv2013

On the Origin of Lopsidedness in Galaxies as Determined from the Spitzer Survey of Stellar Structure in Galaxies (S4G)

We study the m=1 distortions (lopsidedness) in the stellar components of 167 nearby galaxies that span a wide range of morphologies and luminosities. We confirm the previous findings of 1) a high incidence of lopsidedness in the stellar distributions, 2) increasing lopsidedness as a function of radius out to at least 3.5 exponential scale lengths, and 3) greater lopsidedness, over these radii, for galaxies of later type and lower surface brightness. Additionally, the magnitude of the lopsidedness 1) correlates with the character of the spiral arms (stronger arm patterns occur in galaxies with less lopsidedness), 2) is not correlated with the presence or absence of a bar, or the strength of the bar when one is present, 3) is inversely correlated to the stellar mass fraction, f_*, within one radial scale length, and 4) correlates directly with f_* measured within the radial range over which we measure lopsidedness. We interpret these findings to mean that lopsidedness is a generic feature of galaxies and does not, generally, depend on a rare event, such as a direct accretion of a satellite galaxy onto the disk of the parent galaxy. While lopsidedness may be caused by several phenomena, moderate lopsidedness ( _i + _o)/2 < 0.3) is likely to reflect halo asymmetries to which the disk responds or a gravitationally self-generated mode . We hypothesize that the magnitude of the stellar response depends both on how centrally concentrated the stars are with respect to the dark matter and whether there are enough stars in the region of the lopsidedness that self-gravity is dynamically important.

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arXiv2013

Evidence for Secular Evolution of Disc Structural Parameters in Massive Barred Galaxies

We address the effects of bar-driven secular evolution in discs by comparing their properties in a sample of nearly 700 unbarred and barred (42 +- 3 per cent of the population) massive disc galaxies (M* > 10^10 Msun). We make use of accurate structural parameters derived from i-band bulge/disc/bar decompositions to show that, as a population, barred discs tend to have fainter central surface brightness (Delta mu_0 ~ 0.25 mag), and disc scale lengths that are ~15 per cent larger than those of unbarred galaxies of the same stellar mass. The corresponding distributions of mu_0 and h are statistically inconsistent at the 5.2 sigma and 3.8 sigma levels, respectively. Bars rarely occur in high-surface brightness discs, with less than 5 per cent of the barred population having mu_0 < 19.5 mag arcsec^-2 -- compared to 20 per cent for unbarred galaxies. They tend to reside in moderately blue discs, with a bar fraction that peaks at (g-i)_disc ~ 0.95 mag and mildly declines for both bluer and redder colours. These results demonstrate noticeable structural differences between the discs of barred and unbarred galaxies, which we argue are the result of bar-driven evolution -- in qualitative agreement with longstanding theoretical expectations.

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arXiv2013

Hα Kinematics of S4G spiral galaxies-I. NGC 864

We present a study of the kinematics of the isolated spiral galaxy NGC 864, using Hα Fabry-Perot data obtained with the GHαFaS instrument at the William Herschel Telescope in La Palma, complemented with images at 3.6 μm, in the R band and in Hα filter, and integral field spectroscopic data. The resulting data cubes and velocity maps allow the study of the kinematics of the galaxy, including in-depth investigations of the rotation curve, velocity moment maps, velocity residual maps, gradient maps and position-velocity diagrams. We find asymmetries in the velocity field in the bar zone, caused by non-circular motions, probably in response to the potential of the bar. We also find a flat-profile bar, in agreement with the strong bar, with the grand design spiral pattern, and with the gap between the ends of the bar and the start of the spiral arms. We quantify the rate of massive star formation, which is concentrated in the two spiral arms.

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arXiv2012

A unified picture of breaks and truncations in spiral galaxies from SDSS and S^{4}G imaging

The mechanism causing breaks in the radial surface brightness distribution of spiral galaxies is not yet well known. Despite theoretical efforts, there is not a unique explanation for these features and the observational results are not conclusive. In an attempt to address this problem, we have selected a sample of 34 highly inclined spiral galaxies present both in the Sloan Digital Sky Survey and in the Spitzer Survey of Stellar Structure in Galaxies. We have measured the surface brightness profiles in the five Sloan optical bands and in the 3.6$μm$ Spitzer band. We have also calculated the color and stellar surface mass density profiles using the available photometric information, finding two differentiated features: an innermost break radius at distances of $\sim 8 \pm 1$ kpc [$0.77 \pm 0.06$ $R_{25}$] and a second characteristic radius, or truncation radius, close to the outermost optical extent ($\sim 14 \pm 2$ kpc [$1.09 \pm 0.05$ $R_{25}$]) of the galaxy. We propose in this work that the breaks might be a phenomena related to a threshold in the star formation, while truncations are more likely a real drop in the stellar mass density of the disk associated with the maximum angular momentum of the stars.

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arXiv2012

Breaks in thin and thick discs of edge-on galaxies imaged in the Spitzer Survey of Stellar Structure in Galaxies (S4G)

Breaks in the radial luminosity profiles of galaxies have been until now mostly studied averaged over discs. Here we study separately breaks in thin and thick discs in 70 edge-on galaxies using imaging from the Spitzer Survey of Stellar Structure in Galaxies. We built luminosity profiles of the thin and the thick discs parallel to midplanes and we found that thin discs often truncate (77%). Thick discs truncate less often (31%), but when they do, their break radius is comparable with that in the thin disc. This suggests either two different truncation mechanisms - one of dynamical origin affecting both discs simultaneously and another one only affecting the thin disc - or a single mechanism that creates a truncation in one disc or in both depending on some galaxy property. Thin discs apparently antitruncate in around 40% of galaxies. However, in many cases, these antitruncations are an artifact caused by the superposition of a thin disc and a thick disc with the latter having a longer scale length. We estimate the real thin disc antitruncation fraction to be less than 15%. We found that the ratio of the thick and thin stellar disc mass is roughly constant (0.2 120 km/s, but becomes much larger at smaller velocities. We hypothesize that this is due to a combination of a high efficiency of supernova feedback and a slower dynamical evolution in lower-mass galaxies causing stellar thin discs to be younger and less massive than in higher-mass galaxies.

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arXiv2012

Galaxy Bulges and Elliptical Galaxies - Lecture Notes

Our knowledge on the central components of disk galaxies has grown substantially in the past few decades, particularly so in the last. This frantic activity and the complexity of the subject promote confusion in the community. In these notes, I discuss the concept of galactic bulge and its different flavors. I also address fundamental scaling relations and the bulge-elliptical galaxy connection, their central black holes and formation models. In particular, I aim at conveying three important notions: (i): box/peanuts are just the inner parts of bars; (ii): the physical reality of two different families of bulges is evident; and (iii): at the high mass end, at least, classical bulges are not just scaled down ellipticals surrounded by disks.

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arXiv2012

Early type galaxies with tidal debris and their scaling relations in the Spitzer Survey of Stellar Structure in Galaxies (S4G)

Tidal debris around galaxies can yield important clues on their evolution. We have identified tidal debris in 11 early type galaxies (T \leq 0) from a sample of 65 early types drawn from the Spitzer Survey of Stellar Structure in Galaxies (S4G). The tidal debris includes features such as shells, ripples and tidal tails. A variety of techniques, including two-dimensional decomposition of galactic structures, was used to quantify the residual tidal features. The tidal debris contributes ~3 - 10% to the total 3.6 μm luminosity of the host galaxy. Structural parameters of the galaxies were estimated using two-dimensional profile fitting. We investigate the locations of galaxies with tidal debris in the Fundamental Plane and Kormendy relation. We find that galaxies with tidal debris lie within the scatter of early type galaxies without tidal features. Assuming that the tidal debris is indicative of recent gravitational interaction or merger, this suggests that these galaxies have either undergone minor merging events so that the overall structural properties of the galaxies are not significantly altered, or they have undergone a major merging events but already have experienced sufficient relaxation and phase-mixing so that their structural properties become similar to those of the non-interacting early type galaxies.

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arXiv2012

Surprises in Image Decomposition of Edge-on Galaxies: does Sombrero have a (classical) bulge?

The spheroid of the Sombrero galaxy, NGC 4594, is considered a prototype of classical, merger-built bulges. We use a Spitzer, IRAC 3.6 micron image to perform a detailed structural analysis of this galaxy. If one fits to this image only bulge and disc components, the bulge occupies a locus in the mass-size relation close to that of elliptical galaxies. When an outer stellar spheroid is added to improve the fit, the bulge Sersic index drops by a factor of ~ 2, and, if taken at face value, could mean that this bulge is actually a disc-like, pseudo-bulge, or a bar viewed end-on. The bulge effective radius and the bulge-to-total ratio also drop dramatically, putting the bulge in a position closer to that of bulges in the mass-size relation. We discuss implications from these findings, including the locus of the Sombrero bulge in the black hole mass vs. bulge mass relation. With this new bulge mass estimate, current dynamical estimates for the mass of the central black hole in Sombrero are more than 10 times larger than expected, if only the bulge mass is considered. A better agreement is found if the sum of bulge and outer spheroid masses is considered. Furthermore, residual images show the presence of a stellar ring and a stellar, inner ring or disc, with unprecedented clarity. We also show that Sombrero is an outlier in scaling relations of disc galaxies involving the disc, the spheroid and the globular cluster system, but not so when its structural components are considered independently. In this context, the globular cluster system of Sombrero might not be representative of disc galaxies. Finally, we discuss the possibility that Sombrero formed as an elliptical galaxy but accreted a massive disc, which itself has secularly evolved, resulting in a complex and peculiar system.

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arXiv2012

The S4G perspective on circumstellar dust extinction of AGB stars in M100

We examine the effect of circumstellar dust extinction on the near-IR contribution of asymptotic giant branch (AGB) stars in intermediate-age clusters throughout the disk of M100. For our sample of 17 AGB-dominated clusters we extract optical-to-mid-IR SEDs and find that NIR brightness is coupled to the mid-IR dust emission in such a way that a significant reduction of AGB light, of up to 1 mag in K-band, follows from extinction by the dust shell formed during this stage. Since the dust optical depth varies with AGB chemistry (C-rich or O-rich), our results suggest that the contribution of AGB stars to the flux from their host clusters will be closely linked to the metallicity and the progenitor mass of the AGB star, to which dust chemistry and mass-loss rate are sensitive. Our sample of clusters--each the analogue of a ~1 Gyr old post-starburst galaxy--has implications within the context of mass and age estimation via SED modelling at high z: we find that the average ~0.5 mag extinction estimated here may be sufficient to reduce the AGB contribution in (rest-frame) K-band from ~70%, as predicted in the latest generation of synthesis models, to ~35%. Our technique for selecting AGB-dominated clusters in nearby galaxies promises to be effective for discriminating the uncertainties associated with AGB stars in intermediate-age populations that plague age and mass estimation in high-z galaxies.

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arXiv2011

Bars rejuvenating bulges? Evidence from stellar population analysis

We obtained stellar ages and metallicities via spectrum fitting for a sample of 575 bulges with spectra available from the Sloan Digital Sky Survey. The structural properties of the galaxies have been studied in detail in Gadotti (2009b) and the sample contains 251 bulges in galaxies with bars. Using the whole sample, where galaxy stellar mass distributions for barred and unbarred galaxies are similar, we find that bulges in barred and unbarred galaxies occupy similar loci in the age vs. metallicity plane. However, the distribution of bulge ages in barred galaxies shows an excess of populations younger than ~ 4 Gyr, when compared to bulges in unbarred galaxies. Kolmogorov-Smirnov statistics confirm that the age distributions are different with a significance of 99.94%. If we select sub-samples for which the bulge stellar mass distributions are similar for barred and unbarred galaxies, this excess vanishes for galaxies with bulge mass log M < 10.1 M_Sun while for more massive galaxies we find a bimodal bulge age distribution for barred galaxies only, corresponding to two normal distributions with mean ages of 10.4 and 4.7 Gyr. We also find twice as much AGN among barred galaxies, as compared to unbarred galaxies, for low-mass bulges. By combining a large sample of high quality data with sophisticated image and spectral analysis, we are able to find evidence that the presence of bars affect the mean stellar ages of bulges. This lends strong support to models in which bars trigger star formation activity in the centers of galaxies.

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arXiv2011

Reconstructing the stellar mass distributions of galaxies using S4G IRAC 3.6 and 4.5 micron images: I. Correcting for contamination by PAH, hot dust, and intermediate age stars

With the aim of constructing accurate 2D maps of the stellar mass distribution in nearby galaxies from S4G 3.6 and 4.5 micron images, we report on the separation of the light from old stars from the emission contributed by contaminants (e.g. hot dust and the 3.3 micron PAH feature). Results for a small sample of six disk galaxies (NGC 1566, NGC 2976, NGC 3031, NGC 3184, NGC 4321, and NGC 5194) with a range of morphological properties, dust contents and star formation histories are presented to demonstrate our approach. We use an Independent Component Analysis (ICA) technique designed to separate statistically independent source distributions, maximizing the distinction in the [3.6]-[4.5] colors of the sources. The technique also removes emission from intermediate-age evolved red objects with a low mass-to-light ratio, such as asymptotic giant branch (AGB) and red supergiant (RSG) stars, revealing maps of the underlying old distribution of light with [3.6]-[4.5] colors consistent with the colors of K and M giants. Contaminants are identified via comparison to the non-stellar emission imaged at 8 microns, which is dominated by the broad PAH feature. Using the measured 3.6/8 micron ratio to select the individual contaminants, we find that hot dust and PAH together contribute between ~5-15% to the integrated light at 3.6 microns, while light from regions dominated by intermediate-age stars accounts for only 1-5%. Locally, however, the contribution from either contaminant can reach much higher levels; dust contributes on average 22% to the emission in star-forming regions throughout the sample, while intermediate age-stars contribute upwards of 50% in localized knots. The removal of these contaminants with ICA leaves maps of the old stellar disk that retain a high degree of structural information and are ideally suited for tracing the stellar mass, as will be the focus in a companion paper.

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arXiv2011

Thick disks of edge-on galaxies seen through the Spitzer Survey of Stellar Structure in Galaxies (S4G): Lair of missing baryons?

Most, if not all, disk galaxies have a thin (classical) disk and a thick disk. In most models thick disks are thought to be a necessary consequence of the disk formation and/or evolution of the galaxy. We present the results of a study of the thick disk properties in a sample of carefully selected edge-on galaxies with types ranging from T=3 to T=8. We fitted one-dimensional luminosity profiles with physically motivated functions - the solutions of two stellar and one gaseous isothermal coupled disks in equilibrium - which are likely to yield more accurate results than other functions used in previous studies. The images used for the fits come from the Spitzer Survey of Stellar Structure in Galaxies (S4G). We found that thick disks are on average more massive than previously reported, mostly due to the selected fitting function. Typically, the thin and the thick disk have similar masses. We also found that thick disks do not flare significantly within the observed range in galactocentric radii and that the ratio of thick to thin disk scaleheights is higher for galaxies of earlier types. Our results tend to favor an in situ origin for most of the stars in the thick disk. In addition the thick disk may contain a significant amount of stars coming from satellites accreted after the initial build-up of the galaxy and an extra fraction of stars coming from the secular heating of the thin disk by its own overdensities. Assigning thick disk light to the thin disk component may lead to an underestimate of the overall stellar mass in galaxies, because of different mass to light ratios in the two disk components. On the basis of our new results, we estimate that disk stellar masses are between 10% and 50% higher than previously thought and we suggest that thick disks are a reservoir of "local missing baryons".

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arXiv2011

The unusual vertical mass distribution of NGC 4013 seen through the Spitzer Survey of Stellar Structure in Galaxies (S4G)

NGC 4013 is a nearby Sb edge-on galaxy known for its "prodigious" HI warp and its "giant" tidal stream. Previous work on this unusual object shows that it cannot be fitted satisfactorily by a canonical thin+thick disk structure. We have produced a new decomposition of NGC 4013, considering three stellar flattened components (thin+thick disk plus an extra and more extended component) and one gaseous disk. All four components are considered to be gravitationally coupled and isothermal. To do so, we have used the 3.6micron images from the Spitzer Survey of Stellar Structure in Galaxies (S4G). We find evidence for NGC4013 indeed having a thin and a thick disk and an extra flattened component. This smooth and extended component (scaleheight z_EC kpc) could be interpreted as a thick disk or as a squashed ellipsoidal halo and contains ~20% of the total mass of all three stellar components. We argue it is unlikely to be related to the ongoing merger or due to the off-plane stars from a warp in the other two disk components. Instead, we favor a scenario in which the thick disk and the extended component were formed in a two-stage process, in which an initially thick disk has been dynamically heated by a merger soon enough in the galaxy history to have a new thick disk formed within it.

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arXiv2011

Secular Evolution and Structural Properties of Stellar Bars in Galaxies

I present results from the modeling of stellar bars in nearly 300 barred galaxies in the local universe through parametric multi-component multi-band image fitting. The surface brightness radial profile of bars is described using a Sersic function, and parameters such as bar effective radius, ellipticity, boxiness, length and mass, and bar-to-total luminosity and mass ratios, are determined, which is unprecedented for a sample of this size. The properties of bars in galaxies with classical bulges and pseudo-bulges are compared. For a fixed bar-to-total mass ratio, pseudo-bulges are on average significantly less massive than classical bulges, indicating that, if pseudo-bulges are formed through bars, further processes are necessary to build a classical bulge. I find a correlation between bar ellipticity and boxiness, and define a new parameter as the product of these two quantities. I also find correlations between this product and normalised bar size, between the sizes of bars and bulges, and between normalised bar size and bulge-to-total ratio. Bars with different ellipticities follow parallel lines in the latter two correlations. These correlations can arise if, starting off with different normalised sizes and ellipticities, bars grow longer and stronger with dynamical age, as a result of angular momentum exchange from the inner to the outer parts of galaxies, consistent with previous theoretical predictions. A plausible consequence is that bar pattern speeds should become lower with bar dynamical age, and towards galaxies with more prominent bulges.

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arXiv2011

The Rotation of Bulges

I briefly review early and recent progress concerning the dynamical properties of bulges. I also show results from a study on the kinematics of bulges and disks in edge-on galaxies, where the bulge rotation curve is obtained with little contamination from the disk. I show that a proper structural analysis is crucial to understand such measurements. The results suggest that exponential bulges have a higher degree of rotational support than bulges with higher Sersic indexes, giving support to the concept that photometric bulges include different categories of stellar systems.

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