244 research outputs found
The UV-upturn in brightest cluster galaxies
This paper is part of a series devoted to the investigation of a large sample
of brightest cluster galaxies (BCGs), their properties and the relationships
between these and the properties of the host clusters. In this paper, we
compare the stellar population properties derived from high signal-to-noise,
optical long-slit spectra with the GALEX ultraviolet (UV) colour measurements
for 36 nearby BCGs to understand the diversity in the most rapidly evolving
feature in old stellar systems, the UV-upturn. We investigate: (1) the possible
differences between the UV-upturn of BCGs and those of a control sample of
ordinary ellipticals in the same mass range, as well as possible correlations
between the UV-upturn and other general properties of the galaxies; (2)
possible correlations between the UV-upturn and the properties of the host
clusters; (3) recently proposed scenarios where helium-sedimentation in the
cluster centre can produce an enhanced UV-upturn. We find systematic
differences between the UV-colours of BCGs and ordinary ellipticals, but we do
not find correlations between these colours and the properties of the host
clusters. Furthermore, the observations do not support the predictions made by
the helium-sedimentation model as an enhancer of the UV-upturn.Comment: 12 pages, accepted for publication in MNRA
The stellar metallicity gradients in galaxy discs in a cosmological scenario
Indexación: Web of ScienceContext. The stellar metallicity gradients of disc galaxies provide information on disc assembly, star formation processes, and chemical evolution. They also might store information on dynamical processes that could affect the distribution of chemical elements in the gas phase and the stellar components. Understanding their joint effects within a hierarchical clustering scenario is of paramount importance.
Aims. We studied the stellar metallicity gradients of simulated discs in a cosmological simulation. We explored the dependence of the stellar metallicity gradients on stellar age and on the size and mass of the stellar discs.
Methods. We used a catalogue of galaxies with disc components selected from a cosmological hydrodynamical simulation performed including a physically motivated supernova feedback and chemical evolution. Disc components were defined based on angular momentum and binding energy criteria. The metallicity profiles were estimated for stars with different ages. We confront our numerical findings with results from the Calar Alto Legacy Integral Field Area (CALIFA) Survey.
Results. The simulated stellar discs are found to have metallicity profiles with slopes in global agreement with observations. Low stellar mass galaxies tend to have a larger variety of metallicity slopes. When normalized by the half-mass radius, the stellar metallicity gradients do not show any dependence and the dispersion increases significantly, regardless of the galaxy mass. Galaxies with stellar masses of around 10(10) M-circle dot show steeper negative metallicity gradients. The stellar metallicity gradients correlate with the half-mass radius. However, the correlation signal is not present when they are normalized by the half-mass radius. Stellar discs with positive age gradients are detected to have negative and positive metallicity gradients, depending on the relative importance of recent star formation activity in the central regions.
Conclusions. Our results suggest that inside-out formation is the main process responsible for the metallicity and age profiles. The large dispersions in the metallicity gradients as a function of stellar mass could be ascribed to the effects of dynamical processes such as mergers, interactions and/or migration as well as those regulating the conversion of gas into stars. The fingerprints of the inside-out formation seem better preserved by the stellar metallicity gradients as a function of the half-mass radius.http://www.aanda.org/articles/aa/abs/2016/08/aa28188-16/aa28188-16.htm
New light on the formation and evolution of bars:Trends in the stellar line-strength indices distribution inside the bar region
Aims. Our aim is to study the stellar content of the bar region to constrain its formation and evolution.Methods. Line-strength indices in the bar region of a sample of 6 barred galaxies were employed to derive age and metallicity gradients along the bars using stellar population models.Results. We find clear radial gradients in the line-strength indices for all the galaxies. We find positive gradients within the bar region in the metal indices in four of the six galaxies and opposite trends in the other two. These two galaxies are classified as SAB, and they present exponential bar light profiles. For all the galaxies, we find a positive gradient in the Balmer indices. There is a clear correlation between the position of morphological features inside the bar region with changes in the slope and value of the indices, which indicate changes in the stellar populations, when using stellar population analysis. Therefore, it seems that the bar regions show a gradient in both age and metallicity, changing radially to younger and more meta-rich populations for all the galaxies except for the two with exponential profiles.</p
Early-type galaxies in the Coma cluster: a new piece in the calcium puzzle
We present measurements of the CaII triplet and the Ca4227 Lick-index for a
sample of early-type galaxies in the Coma cluster, deriving, for the first
time, their corresponding relationships with velocity dispersion. Compared with
a similar subsample of ellipticals in the field, Coma galaxies with velocity
dispersions in the range ~ 180-270 km/s exhibit significant differences in the
strengths of the Ca features, suggesting an influence of the environment on the
star formation histories of these galaxies. We argue that the main scenarios
previously proposed to explain the relatively low CaII triplet of galaxies are
not able by themselves to simultaneously reconcile the strengths of the two Ca
indices in both environments.Comment: 6 pages including 2 figures. Accepted for publication in ApJL. For
associated data tables see
http://www.ucm.es/info/Astrof/ellipt/CATRIPLET.htm
Observational constraints to boxy/peanut bulge formation time
Boxy/peanut bulges are considered to be part of the same stellar structure as
bars and both could be linked through the buckling instability. The Milky Way
is our closest example. The goal of this letter is determining if the mass
assembly of the different components leaves an imprint in their stellar
populations allowing to estimate the time of bar formation and its evolution.
To this aim we use integral field spectroscopy to derive the stellar age
distributions, SADs, along the bar and disc of NGC 6032. The analysis shows
clearly different SADs for the different bar areas. There is an underlying old
(>=12 Gyr) stellar population for the whole galaxy. The bulge shows star
formation happening at all times. The inner bar structure shows stars of ages
older than 6 Gyrs with a deficit of younger populations. The outer bar region
presents a SAD similar to that of the disc. To interpret our results, we use a
generic numerical simulation of a barred galaxy. Thus, we constrain, for the
first time, the epoch of bar formation, the buckling instability period and the
posterior growth from disc material. We establish that the bar of NGC 6032 is
old, formed around 10 Gyr ago while the buckling phase possibly happened around
8 Gyr ago. All these results point towards bars being long-lasting even in the
presence of gas.Comment: Accepted for publication in MNRAS Letter
Resolving galaxies in time and space: II: Uncertainties in the spectral synthesis of datacubes
In a companion paper we have presented many products derived from the
application of the spectral synthesis code STARLIGHT to datacubes from the
CALIFA survey, including 2D maps of stellar population properties and 1D
averages in the temporal and spatial dimensions. Here we evaluate the
uncertainties in these products. Uncertainties due to noise and spectral shape
calibration errors and to the synthesis method are investigated by means of a
suite of simulations based on 1638 CALIFA spectra for NGC 2916, with
perturbations amplitudes gauged in terms of the expected errors. A separate
study was conducted to assess uncertainties related to the choice of
evolutionary synthesis models. We compare results obtained with the Bruzual &
Charlot models, a preliminary update of them, and a combination of spectra
derived from the Granada and MILES models. About 100k CALIFA spectra are used
in this comparison.
Noise and shape-related errors at the level expected for CALIFA propagate to
0.10-0.15 dex uncertainties in stellar masses, mean ages and metallicities.
Uncertainties in A_V increase from 0.06 mag in the case of random noise to 0.16
mag for shape errors. Higher order products such as SFHs are more uncertain,
but still relatively stable. Due to the large number statistics of datacubes,
spatial averaging reduces uncertainties while preserving information on the
history and structure of stellar populations. Radial profiles of global
properties, as well as SFHs averaged over different regions are much more
stable than for individual spaxels. Uncertainties related to the choice of base
models are larger than those associated with data and method. Differences in
mean age, mass and metallicity are ~ 0.15 to 0.25 dex, and 0.1 mag in A_V.
Spectral residuals are ~ 1% on average, but with systematic features of up to
4%. The origin of these features is discussed. (Abridged)Comment: A&A, accepte
Malin 1: interacting galaxy pair?
Malin 1 is a unique, extraordinarily large low surface brightness galaxy. The
structure and the origins of the galaxy are poorly understood. The reason for
such a situation is an absence of detailed observational data, especially, of
high-resolution kinematics. In this Letter we study the stellar kinematics of
the inner part (r < 15 kpc) of Malin 1. We present spectroscopic arguments in
favour of a small galaxy - Malin 1B - being a companion probably interacting
with the main galaxy - Malin 1. This object is clearly seen in many published
images of Malin 1 but is not mentioned in any astronomical databases. Malin 1B
is located at the projected distance of 14 kpc from the Malin 1's nucleus and
has small - 6516 km/s - relative velocity, which we determined for the
first time. We suggest that ongoing interaction with Malin 1B can explain main
morphological features of the Malin 1's central region - two-armed spiral
structure, a bar, and an external one-armed spiral pattern. We also
investigated the large scale environment of Malin 1 and postulate that the
galaxy SDSS J123708.91+142253.2 might be responsible for the formation of
extended low-surface brightness envelope by means of head-on collision with
Malin 1 (in the framework of collision scenario proposed by Mapelli et al.
2008). To test the collisional origins of Malin 1 global structure, more
observational data and new numerical models are needed.Comment: 5 pages, 4 figures, accepted for publication in MNRA
Spotting the differences between active and non-active twin galaxies on kpc-scales. A pilot study
We present a pilot study aimed to identify large-scale galaxy properties that
could play a role in activating a quiescent nucleus. To do so, we compare the
properties of two isolated nearby active galaxies and their non-active twins
selected from the Calar Alto Legacy Integral Field Area (CALIFA) survey. This
pilot sample includes two barred and two unbarred galaxies. We characterise the
stellar and ionised gas kinematics and also their stellar content. We obtain
simple kinematic models by fitting the full stellar and ionised gas velocity
fields and just the approaching/receding sides. We find that the analysed
active galaxies present lopsided disks and higher values of the global stellar
angular momentum () than their non-active twins. This could be
indicating that the stellar disks of the AGN gained angular momentum from the
inflowing gas that triggered the nuclear activity. The inflow of gas could have
been produced by a twisted disk instability in the case of the unbarred AGN,
and by the bar in the case of the barred AGN. In addition, we find that the
central regions of the studied active galaxies show older stellar populations
than their non-active twins. The next step is to statistically explore these
galaxy properties in a larger sample of twin galaxies.Comment: 24 pages, 24 figures. Accepted by MNRA
Dissecting galactic bulges in space and time - I. The importance of early formation scenarios versus secular evolution
The details of bulge formation via collapse, mergers, secular processes or their interplay remain unresolved. To start answering this question and quantify the importance of distinct mechanisms, we mapped a sample of three galactic bulges using data from the integral field spectrograph WiFeS on the ANU's 2.3-m telescope in Siding Spring Observatory. Its high-resolution gratings (R ∼ 7000) allow us to present a detailed kinematic and stellar population analysis of their inner structures with classical and novel techniques. The comparison of those techniques calls for the necessity of inversion algorithms in order to understand complex substructures and separate populations. We use line-strength indices to derive single stellar population equivalent ages and metallicities. Additionally, we use full spectral fitting methods, here the code STECKMAP, to extract their star formation histories. The high quality of our data allows us to study the 2D distribution of different stellar populations (i.e. young, intermediate and old). We can identify their dominant populations based on these age-discriminated 2D light and mass contribution. In all galactic bulges studied, at least 50 per cent of the stellar mass already existed 12 Gyr ago, more than currently predicted by simulations. A younger component (age between ∼1 and ∼8 Gyr) is also prominent and its present day distribution seems to be affected much more strongly by morphological structures, especially bars, than the older one. This in-depth analysis of the three bulges supports the notion of increasing complexity in their evolution, likely to be found in numerous bulge structures if studied at this level of detail, which cannot be achieved by mergers alone and require a non-negligible contribution of secular evolution
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