Found 185 talks archived in Galaxies
Microvariations probe the physics and internal structure of quasars. Unpredictability and small flux variations make this phenomenon elusive and difficult to detect. Variance based probes such as the C and F tests, or a combination of both, are popular methods to compare the light-curves of the quasar and a comparison star. Recently, detection claims in some studies depend on the agreement of the results of the C and F tests, or of two instances of the F-test, in rejecting the non-variation null hypothesis. However, the C-test is a non-reliable statistical procedure, the F-test is not robust, and the combination of tests with concurrent results is anything but a straightforward methodology. A priori Power Analysis calculations and post hoc analysis of Monte-Carlo simulations show excellent agreement for the Analysis of Variance test to detect microvariations, as well as the limitations of the F-test. Additionally, combined tests yield correlated probabilities that make the assessment of statistical significance unworkable. However, it is possible to include data from several field stars to enhance the power in a single F - test or ANOVA nested designs, increasing the reliability of the statistical analysis. These would be the preferred methodology when several comparison stars are available. These results show the importance of using adequate methodologies, and avoid inappropriate procedures that can jeopardize microvariability detections. Power analysis and Monte-Carlo simulations are useful tools for research planning, as they can reveal the robustness and reliability of different research approaches.
The ultra-deep multiwavelength HST Frontier Fields coverage of the Abell Cluster 2744 is used to derive the stellar population properties of its intra-cluster light (ICL). The restframe colors of the ICL of this intermediate redshift (z=0.3064) massive cluster are bluer (g-r=0.68 ±0.04; i-J=0.56±0.01) than those found in the stellar populations of its main galaxy members (g-r=0.83±0.01; i-J=0.75±0.01). Based on these colors, we derive the following mean metallicity Z=0.018±0.007 for the ICL. The ICL age is 6±3 Gyr younger than the average age of the most massive galaxies of the cluster. The fraction of stellar mass in the ICL component comprises at least 6% of the total stellar mass of the galaxy cluster. Our data is consistent with a scenario where the bulk of the ICL of Abell 2744 has been formed relatively recently (z<1). The stellar population properties of the ICL suggest that this diffuse component is mainly the result of the disruption of infalling galaxies with similar characteristics in mass (M*~ 3x10^10 Msolar) and metallicity than our own Milky Way. The amount of ICL mass in the central part of the cluster (<400 kpc) is equivalent to the disruption of 4-6 Milky Way-type galaxies.
The active galactic nuclei is conformed by a number of classes. Optically they are defined using diagnostics based on optical emission lines. At X-rays they are classified by the power of the AGN continuum and the shape of the X-ray spectra. Therefore, optical and X-ray classes are independent classifications. However, optical and X-ray classes show many discrepancies not fully understood yet. Some AGN at X-rays do not show any AGN signature at optical wavelengths (called optical elusive). Classical obscured AGN are ’sometimes’ not obscured at X-rays.
We have studied the ‘synapses’ between them using artificial neural networks (Gonzalez-Martin+14). To do so, we used flux-calibrated X-ray spectra of a sample of 90 emission line nuclei (ELN) observed with XMM-Newton. It includes starbursts (SB), transition objects (T2), LINERs (L1.8 and L2), and Seyferts (S1, S1.8, and S2).
The ELN can be classified into six classes, based on the shape of their X-ray spectra. These classes are associated with most of the optical classes. The key parameters to explain them at X-rays are three. The first parameter is an AGN-like component, which is present in all of them (even non-AGN at optical wavelengths!). The second one is obscuration, which almost certainly drives the Type-1/Type-2 dichotomy, but may also explain why L1.8 are more similar to S1s while L2/T2 are more similar to S1.8s. The third component is star-forming activity happening at the host galaxy and contributing at X-rays. The AGN strength, relative to the host-galaxy component, determines the average X-ray spectrum for these classes as follows: S1 -> S1.8 -> L1.8/S2 -> L2/T2/ -> SB.
There is increasing speculation that quasars are intimately linked to the evolution of their host galaxies. Not only are they triggered as galaxies build up mass through gas accretion, but they also have the potential to drive massive outflows that can directly affect galaxy evolution by heating the gas and expelling it from galaxy bulges. However, there remain considerable uncertainties about how, when and where quasars are triggered as galaxies evolve, and the true energetic significance for the quasar-induced outflows and their acceleration mechanism have yet to be established. In this talk I will present new Gemini, VLT, Spitzer and Herschel results on samples of luminous AGN in the local Universe which provide key information on the triggering mechanisms for quasars and physics of their outflows.
The Magellanic Clouds are the closest star forming galaxies, and their star formation histories can be derived in great details from color-magnitude diagrams reaching the oldest main sequence turnoffs. In the last several years, we have been conducting a wide research program on the Magellanic Clouds, including both photometry and spectroscopy, and have analysed the star formation history across both the Large and the Small Magellanic Clouds. This has revealed the nature of the stellar population gradients of these galaxies, as well as signatures that can possibly be related to their interaction history, among them and with the Milky Way.
This paper discusses how cosmic gas accretion controls star formation, and summarizes the physical properties expected for the cosmic gas accreted by galaxies. The paper also collects observational evidence for gas accretion sustaining star formation. It reviews evidence inferred from neutral and ionized hydrogen, as well as from stars. A number of properties characterizing large samples of star-forming galaxies can be explained by metal-poor gas accretion, in particular, the relationship between stellar mass, metallicity, and star formation rate (the so-called fundamental metallicity relationship). They are put forward and analyzed. Theory predicts gas accretion to be particularly important at high redshift, so indications based on distant objects are reviewed, including the global star formation history of the universe, and the gas around galaxies as inferred from absorption features in the spectra of background sources.
Different components of galaxies are the result of internal and environmental processes during their lifetimes. Disentangling these processes is an important issue for understanding how galaxies form and evolve. In this context isolated galaxies provide a fruitful sample for exploring galaxies which have evolved mainly by internal processes (minimal merger/accretion/tidal effects). I will present the structural analysis performed as part of the AMIGA (Analysis of the interstellar Medium of Isolated GAlaxies; http://www.amiga.iaa.es) project. The analysis of the stellar mass-size relation of our spiral galaxies reveals a larger size for disks in low-density environments, as well as a dependence of disk size on the number of satellites. A 2D bulge/disk/bar decomposition of SDSS i-band images was performed in order to identify the pseudobulges in our sample. We derived (g-i) bulge colors and find a large fraction of pseudobulges in the red sequence of early-type galaxies. The bluer pseudobulges in our sample tend to be located in those galaxies more affected by tidal interactions. The properties of the majority of bulges in isolated galaxies suggest that pseudobulges formed most of their mass at an early epoch, and that specific environmental events may rejuvenate pseudobulges.
One of the most widely researched topics in Extragalactic Astrophysics
in the last decades is how early-type galaxies have formed their stars
and assembled. In this context, we now have unequivocal observational
evidences about the existence of a numerous population of massive
galaxies which not only had assembled a considerable amount of stars
(~10 11 M_sun) by z~2, but were already evolving passively by that
time. These galaxies, the likely progenitors of nearby ellipticals,
are also quite compact in comparison with local galaxies of the same
mass. These result are mainly based on measurements designed to obtain
stellar masses and sizes, and our estimations of these parameters are
now quite robust. Now we need a more secure determination of how
exactly they formed and assembled their stellar mass in just 2-3 Gyr
(z>2). In other words, how was their Star Formation History and which
are the properties (age, metallicity, dust content) of their stellar
populations? And how could they end up with such high masses and small
sizes? In this talk, we will present our results about the SFH (mainly
ages and duty cycles) of massive galaxies at z=1-3 based on the
deepest spectro-photometric data ever taken. These data were gathered
by the Survey for High-z Absorption Red and Dead Sources (SHARDS), a
ESO/GTC Large Program aimed at obtaining R~50 optical spectra of
distant galaxies. This resolution is especially suited to measure
absorption indices such as D(4000), Mg_UV, the Balmer break,etc.. for
galaxies up to z~3 (merging our SHARDS data with HST/WFC3 grism
observations) or emission-line fluxes for faint targets up to
z~6. These measurements represent a big step forward for the robust
determination of the stellar population properties, providing a much
more certain characterization of the stellar content of distant
galaxies than the typical broad-band studies. Our results uniquely
allow to study the stellar content of red and dead galaxies at z~2 and
identify progenitors at higher redshifts, as well as helping to
constrain the models of galaxy formation.
Most massive galaxies have supermassive black holes at their centres, and the masses of the black holes correlate with properties of the host-galaxy bulge component. These empirical scaling relations are important for distinguishing between various theoretical models of galaxy evolution, and they furthermore form the basis for all black-hole mass measurements at large distances. Observations have shown that the mass of the black hole is typically 0.1 per cent of the mass of the stellar bulge of the galaxy. Our spectroscopic survey with the Hobby-Eberly Telescope of 1000 nearby galaxies revealed several compact lenticular galaxies with extremely high velocity dispersions. The first example is NGC1277, which is a small, Re=1kpc, compact, lenticular galaxy with a mass of 1.2×10^11 solar masses. From the stellar kinematics we determined that the mass of the central black hole is 10^10 solar masses, more than 10 per cent of its bulge mass. I will present HST images and IFU spectroscopy of a dozen more compact galaxies that all appear to host extremely big black holes and have Salpeter-like IMFs. These local systems, with distances less than 100 Mpc, could be the passively evolved descendents of the quiescent compact nugget galaxies found at z~2 and the >10e9 Msun quasars that are found at z>6.
Dwarf galaxies are a complex population. They comprise objects with young and old stellar populations, slow and fast rotation, as well as single- and multi-component structure. These characteristics show correlations with environmental density - we thus believe that dwarf galaxies hold a fossil record of how environment affected galaxy evolution. In this talk I will review and discuss recent progress on our understanding of dwarf galaxies in clusters, both from the observational and the modelling side. In particular, I will attempt to reconcile the proposed formation mechanisms of early-type dwarf galaxies - the most abundant population in clusters - with the continuous environmental influence predicted by cosmological simulations.