اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدSpectroscopic Gradients in Early -- Type Galaxies and Implications on Galaxy Formation
87
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
The Coma cluster is the ideal place to study galaxy structure as a function of environmental density in order to constrain theories of galaxy formation and evolution. Here we present the spectroscopy of 35 early type Coma galaxies, which shows that the age spread of early type galaxies in the Coma cluster is large (15 Gyrs). In contrast to the field, the dominant stellar population in all (massive) Coma Es is older than 8 Gyr, while only S0s, which possess extended disks, can be as young as 2 Gyr. The old, most massive Es show a strong light element enhancement, probably due to a rather short star formation time scale and hence to a SNII -- dominated element enrichment. The lower mass S0s are much less enhanced in light elements, indicating a longer star formation time scale. The measured absorption line index gradients support the idea that early type galaxies formed in processes that include both stellar merging and gaseous dissipation.
قيم البحث
اقرأ أيضاً
We study the internal radial gradients of the stellar populations in a sample comprising 522 early-type galaxies (ETGs) from the SAMI (Sydney- AAO Multi-object Integral field spectrograph) Galaxy Survey. We stack the spectra of individual spaxels in
radial bins, and derive basic stellar population properties: total metallicity ([Z/H]), [Mg/Fe], [C/Fe] and age. The radial gradient ($ abla$) and central value of the fits (evaluated at R$_e$/4) are compared against a set of six possible drivers of the trends. We find that velocity dispersion ($sigma$) - or, equivalently gravitational potential - is the dominant driver of the chemical composition gradients. Surface mass density is also correlated with the trends, especially with stellar age. The decrease of $ abla$[Mg/Fe] with increasing $sigma$ is contrasted by a rather shallow dependence of $ abla$[Z/H] with $sigma$ (although this radial gradient is overall rather steep). This result, along with a shallow age slope at the massive end, imposes stringent constraints on the progenitors of the populations that contribute to the formation of the outer envelopes of ETGs. The SAMI sample is split between a field sample and a cluster sample. Only weak environment-related differences are found, most notably a stronger dependence of central total metallicity ([Z/H]$_{e4}$) with $sigma$, along with a marginal trend of $ abla$[Z/H] to steepen in cluster galaxies, a result that is not followed by [Mg/Fe]. The results presented here serve as constraints on numerical models of the formation and evolution of ETGs.
We performed B and R band surface photometry for E/S0 galaxies in a nearby rich cluster ABELL 2199 to investigate their B-R color gradients (d(B-R)/dlogr). Our aims are to study statistical properties of the color gradients and, by comparing them wit
h those in less dense environments, to examine environmental dependence of color gradients in elliptical galaxies. We studied the distribution of the B-R color gradients in the cluster ellipticals and found that the mean value of the color gradients is -0.09 +- 0.04 mag/dex, which can be converted to a metallicity gradient (dlogZ/dlogr) of ~ -0.3 +- 0.1 assuming an old stellar population. We further studied the relations between the B-R color gradients and global properties of the galaxies. Our data suggest that for the galaxies brighter than L*, more luminous and larger galaxies tend to have steeper color gradients. The typical value of the color gradients seems to be consistent with a recent monolithic collapse model and the correlation could also appear if elliptical galaxies formed through the monolithic collapse. On the contrary, it is found based on data from the literature that any such trend is clearly weaker for ellipticals in less dense environments, while the distribution of the color gradients is quite similar to that found in the rich cluster. Based on the results from our data and the published data, we discuss formation process of elliptical galaxy and its environmental dependence.
Using new long-slit spectroscopy obtained with X-Shooter at ESO-VLT, we study, for the first time, radial gradients of optical and Near-Infrared IMF-sensitive features in a representative sample of galaxies at the very high-mass end of the galaxy pop
ulation. The sample consists of seven early-type galaxies (ETGs) at $zsim0.05$, with central velocity dispersion in the range $300<sigma<350$km/s. Using state-of-art stellar population synthesis models, we fit a number of spectral indices, from different chemical species (including TiOs and Na indices), to constrain the IMF slope (i.e. the fraction of low-mass stars), as a function of galactocentric distance, over a radial range out to $sim4$kpc. ETGs in our sample show a significant correlation of IMF slope and surface mass density. The bottom-heavy population (i.e. an excess of low-mass stars in the IMF) is confined to central galaxy regions with surface mass density above $sim 10^{10} M_odot kpc^{-2}$, or, alternatively, within a characteristic radius of $sim2$~kpc. Radial distance, in physical units, and surface mass density, are the best correlators to IMF variations, with respect to other dynamical (e.g. velocity dispersion) and stellar population (e.g. metallicity) properties. Our results for the most massive galaxies suggest that there is no single parameter} that fully explains variations in the stellar IMF, but IMF radial profiles at z$sim$0 rather result from the complex formation and mass accretion history of galaxy inner and outer regions.
We measure radial gradients of the Mg2 index in 15 E-E/S0 and 14 S0 galaxies. Our homogeneous data set covers a large range of internal stellar velocity dispersions (2.0<logsigma<2.5) and Mg2 gradients (dMg2/dlogr/re* up to -0.14mag/dex). We find for
the first time, a noticeable lower boundary in the relation between Mg2 gradient and sigma along the full range of sigma, which may be populated by galaxies predominantly formed by monolithic collapse. At high sigma, galaxies showing flatter gradients could represent objects which suffered either important merging episodes or later gas accretion. These processes contribute to the flattening of the metallicity gradients and their increasing importance could define the distribution of the objects above the boundary expected by the ``classical monolithic process. This result is in marked contrast with previous works which found a correlation between dMg2/dlogr/re* and sigma confined to the low mass galaxies, suggesting that only galaxies below some limiting sigma were formed by collapse whereas the massive ones by mergers. We show observational evidence that a hybrid scenario could arise also among massive galaxies. Finally, we estimated d[Z/H] from Mg2 and Hbeta measurements and single stellar population models. The conclusions remain the same, indicating that the results cannot be ascribed to age effects on Mg2.
Since the near future should see a rapidly expanding set of probes of the halo masses of individual early-type galaxies, we introduce a convenient parameter for characterising the halo masses from both observational and theoretical results: dML, the
logarithmic radial gradient of the mass-to-light ratio. Using halo density profiles from LCDM simulations, we derive predictions for this gradient for various galaxy luminosities and star formation efficiencies $epsilon_{SF}$. As a pilot study, we assemble the available dML data from kinematics in early-type galaxies - representing the first unbiassed study of halo masses in a wide range of early-type galaxy luminosities - and find a correlation between luminosity and dML, such that the brightest galaxies appear the most dark-matter dominated. We find that the gradients in most of the brightest galaxies may fit in well with the LCDM predictions, but that there is also a population of fainter galaxies whose gradients are so low as to imply an unreasonably high star formation efficiency $epsilon_{SF} > 1$. This difficulty is eased if dark haloes are not assumed to have the standard LCDM profiles, but lower central concentrations.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
