اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدLine strengths of early-type galaxies
172
0
0.0
(
0
)
تأليف
Ricardo L.C. Ogando
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
In this paper we present measurements of velocity dispersions and Lick indices for 509 galaxies in the local Universe, based on high signal-to-noise, long slit spectra obtained with the 1.52 m ESO telescope at La Silla. The conversion of our measurements into the Lick/IDS system was carried out following the general prescription of Worthey and Ottaviani 1997. Comparisons of our measurements with those of other authors show, in general, good agreement. We also examine the dependence between these indices (e.g., Hbeta, Mg_2, Fe5270 and NaD) and the central velocity dispersion (sigma), and we find that they are consistent with those previously reported in the literature. Benefiting from the relatively large size of the sample, we are able to investigate the dependence of these relations on morphology and environment, here represented by the local galaxy density. We find that for metallic lines these relations show no significant dependence on environment or morphology, except in the case of NaD, which shows distinct behavior for E and S0. On the other hand, the Hbeta-logsigma shows a significant difference as a function of the local density of galaxies, which we interpret as being caused by the truncation of star formation in high density environments. Comparing our results with those obtained by other authors we find a few discrepancies, adding to the ongoing debate about the nature of these relations. Finally, we report that the scatter of the Mg indices versus sigma relations correlate with Hbeta, suggesting that age may contribute to the scatter. Furthermore, this scatter shows no significant dependence on morphology or environment. Our results are consistent with the current downsizing model, where low mass galaxies have an extended star formation history (abridged).
قيم البحث
اقرأ أيضاً
We present absorption line-strength maps for a sample of 18 Sb-Sd galaxies observed using the integral-field spectrograph SAURON. The SAURON spectral range allows the measurement of the Lick/IDS indices Hbeta, Fe5015 and Mgb, which can be used to est
imate the stellar population parameters. We present here the two-dimensional line-strength maps for each galaxy. From the maps, we learn that late-type spiral galaxies tend to have high Hbeta and low Fe5015 and Mgb values, and that the Hbeta index has often a positive gradient over the field, while the metal indices peak in the central region. We investigate the relations between the central line-strength indices and their correlations with morphological type and central velocity dispersion, and compare the observed behaviour with that for ellipticals, lenticulars and early-type spirals from the SAURON survey. We find that our galaxies lie below the Mg - sigma relation determined for elliptical galaxies and that the indices show a clear trend with morphological type. From the line-strength maps we calculate age, metallicity and abundance ratio maps and discuss the results from a one-SSP approach and from a two-SSP approach. Late-type galaxies are generally younger and more metal poor than ellipticals and have abundance ratios closer to solar values. We also explore a continuous star formation scenario, and try to recover the star formation history using the evolutionary models of Bruzual & Charlot (2003), assuming constant or exponentially declining star formation rate (SFR). We find a correlation between the e-folding time-scale tau of the starburst and the central velocity dispersion: more massive galaxies tend to have shorter tau, suggesting that the star formation happened long ago and has now basically ended, while for smaller objects with larger values of tau it is still active.
[Heavily Abbreviated] In this paper we analyze previously published spectra with high signal-to-noise ratios of E/S0 galaxies in the rich cluster CL1358+62 at z=0.33, and introduce techniques for fitting stellar population models to the data. Here we
focus on the 19 E and S0 galaxies with an homogeneous set of eight blue Lick indices. We explore the galaxy properties using six-parameter stellar population models from the literature, and describe an approach for fitting the models differentially, such that the largest systematic errors are avoided. We find: (1) no differences between the stellar population parameters of Es and S0s, at fixed sigma; (2) the stars in the Es and S0s are uniformly old, consistent with previously published results using M/L ratios; (3) a significant correlation of [Z/H] with sigma, in a manner consistent with the observed B-V colors of the galaxies; (4) no significant correlation of [alpha/Fe] with sigma; and (5) a significant anti-correlation of [alpha/N] with [Z/H], which we interpret as the signature of secondary nitrogen. Neither [alpha/C], nor [alpha/Ca] shows significant variation. While the differences between our conclusions and the current view of stellar populations may point to serious deficiencies, our deduced correlation of mean metallicity with sigma does reproduce the B-V colors of the galaxies, as well as the slope of the local Mg-sigma relation. In matching the inferred population trends with published data on nearby galaxies, the line strength-line width relations match well, save for the narrow iron indices. Taken together, these results reduce early-type galaxies in clusters to a family with one-parameter, velocity dispersion, greatly simplifying scenarios for their formation and evolution.
Much of the far-UV emission from elliptical galaxies is thought to arise from extreme horizontal branch stars and related objects. Only about 10% of the stellar population needs to evolve through this phase even in galaxies with the strongest UV uptu
rn. However it is not yet clear if this population represents the extreme low-metallicity or high-metallicty tail of the distribution, or rather arises from the overall population through some metallicity-insensitive mechanism that causes increased mass loss in a small fraction of RGB stars. We investigate the utility of far-UV line strengths for deciding between these possiblities. Complications include the fact that the line strengths reflect both the temperature distribution and the metallicity distribution of the stars, that there may be abundance anomalies introduced on the RGB, and that metals are likely to be redistributed by gravitational settling and radiative diffusion in the atmospheres of hot high-gravity stars. Line-strength measurements from Astro-2 HUT spectra are considered in this context.
We use the integral-field spectrograph SAURON to measure the stellar line-of-sight velocity distribution and absorption line strengths out to four effective radii (Re) in the early-type galaxies NGC 3379 and NGC 821. With our newly developed observin
g technique we can now probe these faint regions in galaxies that were previously not accessible with traditional long-slit spectroscopy. We make optimal use of the large field-of-view and high throughput of the spectrograph: by adding the signal of all ~1400 lenslets into one spectrum, we obtain sufficient signal-to-noise in a few hours of observing time to reliably measure the absorption line kinematics and line strengths out to large radius. We find that the line strength gradients previously observed within 1 Re remain constant out to at least 4 Re, which puts constraints on the merger histories of these galaxies. The stellar halo populations are old and metal-poor. By constructing orbit-based Schwarzschild dynamical models we find that dark matter is necessary to explain the observed kinematics in NGC 3379 and NGC 821, with 30 - 50 per cent of the total matter being dark within 4 Re. The radial anisotropy in our best-fit halo models is less than in our models without halo, due to differences in orbital structure. The halo also has an effect on the Mgb - Vesc relation: its slope is steeper when a dark matter halo is added to the model.
The star formation rates (SFRs) in weak emission line (WEL) galaxies in a volume-limited ($0.02 < z < 0.05$) sample of blue early-type galaxies (ETGs) identified from SDSS, are constrained here using 1.4 GHz radio continuum emission. The direct detec
tion of 1.4 GHz radio continuum emission is made in 8 WEL galaxies and a median stacking is performed on 57 WEL galaxies using VLA FIRST images. The median stacked 1.4 GHz flux density and luminosity are estimated as 79 $pm$ 19 $mu$Jy and 0.20 $pm$ 0.05 $times$ 10$^{21}$ W Hz$^{-1}$ respectively. The radio far-infrared correlation in 4 WEL galaxies suggests that the radio continuum emission from WEL galaxies is most likely due to star formation activities. The median SFR for WEL galaxies is estimated as 0.23 $pm$ 0.06 M$_{odot}$yr$^{-1}$, which is much less compared to SFRs ($0.5 - 50$ M$_{odot}$yr$^{-1}$) in purely star forming blue ETGs. The SFRs in blue ETGs are found to be correlated with their stellar velocity dispersions ($sigma$) and decreasing gradually beyond $sigma$ of $sim 100$ km s$^{-1}$. This effect is most likely linked with the growth of black hole and suppression of star formation via AGN feedback. The color differences between SF and WEL sub-types of blue ETGs appear to be driven to large extent by the level of current star formation activities. In a likely scenario of an evolutionary sequence between sub-types, the observed color distribution in blue ETGs can be explained best in terms of fast evolution through AGN feedback.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
