The nuclei of galaxies often host small stellar discs with scalelengths of a few tens of parsecs and luminosities up to 107 L. To investigate the formation and properties of nuclear stellar discs ...(NSDs), we look for their presence in a set of N-body simulations studying the dissipationless merging of multiple star clusters in galactic nuclei. A few tens of star clusters with sizes and masses comparable to those of globular clusters observed in the Milky Way are accreted on to a pre-existing nuclear stellar component: either a massive super star cluster or a rapidly rotating, compact disc with a scalelength of a few parsecs, mimicking the variety of observed nuclear structures. Images and kinematic maps of the simulation time-steps are then built and analysed as if they were real and at the distance of the Virgo cluster. We use the Scorza-Bender method to search for the presence of disc structures via photometric decomposition. In one case, the merger remnant has all the observed photometric and kinematic properties of NSDs observed in real galaxies. This shows that current observations are consistent with most of the NSD mass being assembled from the migration and accretion of star clusters into the galactic centre. In the other simulation instead, we detect an elongated structure from the unsharp masked image, that does not develop the photometric or kinematic signature of an NSD. Thus, in the context of searches for a disc structure, the Scorza-Bender method is a robust and necessary tool.
Aims. We study the kinematically distinct components in two early-type galaxies NGC 448 and NGC 4365 aided by integral-field observations with the Multi-Unit Spectroscopic Explorer (MUSE) on the Very ...Large Telescope. The former galaxy has previously been shown to host a counter-rotating stellar disc while the latter harbours a central (apparently) decoupled core that has been suggested to not be physically distinct from the main body and instead stems from the different orbital types in the core and main body due to its triaxial nature. We aim to measure the brightness profiles, kinematics, and stellar population properties of the peculiar kinematic structures in these galaxies and shed light on their true nature and formation mechanism. Methods. We use a kinematic decomposition technique to separate the individual contributions to the spectra of the two distinct kinematic components observed at each spatial position in the field of view. Furthermore, by folding back the outcome of a photometric decomposition we reduce the intrinsic degeneracies in recovering the kinematics and the best-fitting stellar spectral templates. Finally, by extracting the Lick line-strength indices for the individual components and fitting them to single stellar population models we derive their ages, metallicities, and α/Fe overabundances. Results. The two kinematically decoupled stellar components in NGC 448 have similar ages, but different chemical compositions. The distinct kinematic feature in NGC 448 has a nearly exponential surface-brightness light profile, dominates in the innermost ∼10″, is smaller in size, and is very likely an embedded counter-rotating disc as also indicated by its kinematics. It has higher metallicity than the main galaxy stellar body and lower α/Fe overabundance. By contrast, we do not find evidence for true decoupling in the two distinct kinematic components in NGC 4365. This confirms earlier work suggesting that the kinematically distinct core is likely not a separate dynamical structure, but most certainly likely a projection effect stemming from the orbital structure of this galaxy that was previously found to be intrinsically triaxial in shape. Conclusions. Our findings indicate that the kinematically decoupled component in NGC 448 is truly decoupled, has external origin, and was formed through either the acquisition of gas and a subsequent star-formation episode or from the direct accretion of stars from a companion. Conversely, the presence of a kinematically distinct component in NGC 4365 is not associated to a true kinematic decoupling and is instead most likely due to a projection effect stemming from the triaxial nature of this galaxy.
Context. Many disk galaxies host two extended stellar components that rotate in opposite directions. The analysis of the stellar populations of the counter-rotating components provides constraints on ...the environmental and internal processes that drive their formation. Aims. The S0 NGC 1366 in the Fornax cluster is known to host a stellar component that is kinematically decoupled from the main body of the galaxy. Here we successfully separated the two counter-rotating stellar components to independently measure the kinematics and properties of their stellar populations. Methods. We performed a spectroscopic decomposition of the spectrum obtained along the galaxy major axis and separated the relative contribution of the two counter-rotating stellar components and of the ionized-gas component. We measured the line-strength indices of the two counter-rotating stellar components and modeled each of them with single stellar population models that account for the α/Fe overabundance. Results. We found that the counter-rotating stellar component is younger, has nearly the same metallicity, and is less α/Fe enhanced than the corotating component. Unlike most of the counter-rotating galaxies, the ionized gas detected in NGC 1366 is neither associated with the counter-rotating stellar component nor with the main galaxy body. On the contrary, it has a disordered distribution and a disturbed kinematics with multiple velocity components observed along the minor axis of the galaxy. Conclusions. The different properties of the counter-rotating stellar components and the kinematic peculiarities of the ionized gas suggest that NGC 1366 is at an intermediate stage of the acquisition process, building the counter-rotating components with some gas clouds still falling onto the galaxy.
We present a new technique to quantify the light contribution coming from the faint high-redshift (z ∼ 6) galaxies below the detection threshold of imaging data, set conventionally at S/N = 4.5. We ...illustrate the technique with an application to Hubble Space Telescope Advanced Camera for Surveys images in the F775W and F850LP filters of the Ultra Deep Field parallel field NICP12. The aim of this analysis is to extend by a few magnitudes the faint end of the luminosity function at z ∼ 6. After masking all the detected sources in the field, we apply a fast Fourier transform to obtain the spatial power spectrum of the background signal. The power spectrum permits us to separate the background noise signal, the residuals due to the data reduction of the wide field and the overall signal produced by faint galaxies. The ratio of the signal in the i
775 and z
850 bands is used to estimate the contribution of the faint i-dropout objects. We rely on extensive Monte Carlo simulations to characterize various sources of uncertainty and quantify the number of faint i-dropout galaxies in the field. The analysis allows us to put constraints on the luminosity function at z ∼ 6 down to z
850 = 30 mag, 2.5 mag fainter than with standard techniques on the same data. The data are consistent with a faint-end slope of the luminosity function of α = −1.9. Assuming a specific set of values for the clumping factor, escape fraction and spectral energy distribution, we find that the z ∼ 6 undetected galaxies down to z
850 = 30 mag could have driven cosmic reionization.
Context. A new generation of spectral synthesis models has been developed in recent years, but there is no matching set of template galaxy spectra, in terms of quality and resolution, for testing and ...refining the new models. Aims. Our main goal is to find and calibrate new near-infrared spectral indices along the Hubble sequence of galaxies which will be used to obtain additional constraints to the population analysis based on medium-resolution integrated spectra of galaxies. Methods. Spectra of previously studied and well-understood galaxies with relatively simple stellar populations (e.g., ellipticals or bulge dominated galaxies) are needed to provide a baseline data set for spectral synthesis models. Results. X-shooter spectra spanning the optical and infrared wavelengths (350–2400 nm) of bright nearby elliptical galaxies with a resolving power of R ∼ 4000–5400 were obtained. Heliocentric systemic velocity, velocity dispersion, and Mg, Fe, and Hβ line-strength indices are presented. Conclusions. We present a library of very-high-quality spectra of galaxies covering a large range of age, metallicity, and morphological type. Such a dataset of spectra will be crucial to addressing important questions of the modern investigation concerning galaxy formation and evolution.
ABSTRACT Recent intensive Swift monitoring of the Seyfert 1 galaxy NGC 5548 yielded 282 usable epochs over 125 days across six UV/optical bands and the X-rays. This is the densest extended active ...galactic nucleus (AGN) UV/optical continuum sampling ever obtained, with a mean sampling rate <0.5 day. Approximately daily Hubble Space Telescope UV sampling was also obtained. The UV/optical light curves show strong correlations ( ) and the clearest measurement to date of interband lags. These lags are well-fit by a wavelength dependence, with a normalization that indicates an unexpectedly large disk radius of lt-day at 1367 , assuming a simple face-on model. The U band shows a marginally larger lag than expected from the fit and surrounding bands, which could be due to Balmer continuum emission from the broad-line region as suggested by Korista and Goad. The UV/X-ray correlation is weaker ( ) and less consistent over time. This indicates that while Swift is beginning to measure UV/optical lags in general agreement with accretion disk theory (although the derived size is larger than predicted), the relationship with X-ray variability is less well understood. Combining this accretion disk size estimate with those from quasar microlensing studies suggests that AGN disk sizes scale approximately linearly with central black hole mass over a wide range of masses.
Context. Galaxies accrete material from the environment through acquisition and merging events. These processes contribute to galaxy assembly and leave their fingerprints on the galactic morphology, ...internal kinematics of gas and stars, and stellar populations. Aims. We study the nearby S0 galaxy IC 5181 to address the origin of the ionized gas component that orbits the galaxy on polar orbits. Methods. We measure the surface brightness distribution of the stars and ionized gas of IC 5181 from broadband and narrow-band imaging. The structural parameters of the galaxy are obtained with a photometric decomposition assuming a Sérsic and exponential profile for the bulge and disk, respectively. We measure the ionized-gas and stellar kinematics and the line strengths of the Lick indices of the stellar component along both the major and minor axis. The age, metallicity, and α/Fe enhancement of the stellar populations are derived using single stellar population models with variable element abundance ratios. The ionized-gas metallicity is obtained from the equivalent width of the emission lines. Results. The galaxy IC 5181 is a morphologically undisturbed S0 galaxy with a classical bulge made by old stars with super solar metallicity and overabundance. Stellar age and metallicity decrease in the disk region. The galaxy hosts a geometrically and kinematically decoupled component of ionized gas. It is elongated along the galaxy minor axis and in orthogonal rotation with respect to the galaxy disk. Conclusions. We interpret the kinematical decoupling as suggesting that there is a component of gas, which is not related to the stars and having an external origin. The gas was accreted by IC 5181 on polar orbits from the surrounding environment.
Aims.
We characterised the properties of the bar hosted in lenticular galaxy NGC 4277, which is located behind the Virgo cluster.
Methods.
We measured the bar length and strength from the surface ...photometry obtained from the broad-band imaging of the Sloan Digital Sky Survey and we derived the bar pattern speed from the stellar kinematics obtained from the integral-field spectroscopy performed with the Multi Unit Spectroscopic Explorer at the Very Large Telescope. We also estimated the co-rotation radius from the circular velocity, which we constrained by correcting the stellar streaming motions for asymmetric drift, and we finally derived the bar rotation rate.
Results.
We found that NGC 4277 hosts a short (
R
bar
= 3.2
−0.6
+0.9
kpc), weak (
S
bar
= 0.21 ± 0.02), and slow (ℛ = 1.8
−0.3
+0.5
) bar and its pattern speed (Ω
bar
= 24.7 ± 3.4 km s
−1
kpc
−1
) is amongst the best-constrained ones ever obtained with the Tremaine–Weinberg (TW) method with relative statistical errors of ∼0.2.
Conclusions.
NGC 4277 is the first clear-cut case of a galaxy hosting a slow stellar bar (ℛ > 1.4 at more than a 1
σ
confidence level) measured with the model-independent TW method. A possible interaction with the neighbour galaxy NGC 4273 could have triggered the formation of such a slow bar and/or the bar could be slowed down due to the dynamical friction with a significant amount of dark matter within the bar region.
We present the ionized gas kinematics of the SB0 galaxy NGC 4435 from spectra obtained with the Space Telescope Imaging Spectrograph. This galaxy has been selected on the basis of its ground-based ...spectroscopy, for displaying a position—velocity diagram consistent with the presence of a circumnuclear Keplerian disc rotating around a supermassive black hole (SMBH). We obtained the Hα and N ii λ6583 kinematics in the galaxy nucleus along the major axis and two parallel offset positions. We built a dynamical model of the gaseous disc taking into account the whole bi-dimensional velocity field and the instrumental setup. For the mass of the central SMBH, we found an upper limit of 7.5 × 106 M⊙ at the 3σ level. This indicates that the mass of the SMBH of NGC 4435 is lower than the one expected from the M•—σc (5 × 107 M⊙) and near-infrared M•−Lbulge (4 × 107 M⊙) relationships.
Context.
A new generation of spectral synthesis models has been developed in recent years, but there is no matching set of template galaxy spectra, in terms of quality and resolution, for testing and ...refining the new models.
Aims.
Our main goal is to find and calibrate new near-infrared spectral indices along the Hubble sequence of galaxies which will be used to obtain additional constraints to the population analysis based on medium-resolution integrated spectra of galaxies.
Methods.
Spectra of previously studied and well-understood galaxies with relatively simple stellar populations (e.g., ellipticals or bulge dominated galaxies) are needed to provide a baseline data set for spectral synthesis models.
Results.
X-shooter spectra spanning the optical and infrared wavelengths (350–2400 nm) of bright nearby elliptical galaxies with a resolving power of
R
∼ 4000–5400 were obtained. Heliocentric systemic velocity, velocity dispersion, and Mg, Fe, and H
β
line-strength indices are presented.
Conclusions.
We present a library of very-high-quality spectra of galaxies covering a large range of age, metallicity, and morphological type. Such a dataset of spectra will be crucial to addressing important questions of the modern investigation concerning galaxy formation and evolution.
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