Found 34 talks width keyword star formation
Using the k-means cluster analysis algorithm, we carry out an unsupervised classification of all galaxy spectra in the seventh and final Sloan Digital Sky Survey data release (SDSS/DR7). Except for the shift to rest-frame wavelengths and the normalization to the g-band flux, no manipulation is applied to the original spectra. The algorithm guarantees that galaxies with similar spectra belong to the same class. We find that 99% of the galaxies can be assigned to only 17 major classes, with 11 additional minor classes including the remaining 1%. The classification is not unique since many galaxies appear in between classes; however, our rendering of the algorithm overcomes this weakness with a tool to identify borderline galaxies. Each class is characterized by a template spectrum, which is the average of all the spectra of the galaxies in the class. These low-noise template spectra vary smoothly and continuously along a sequence labeled from 0 to 27, from the reddest class to the bluest class. Our Automatic Spectroscopic K-means-based (ASK) classification separates galaxies in colors, with classes characteristic of the red sequence, the blue cloud, as well as the green valley. When red sequence galaxies and green valley galaxies present emission lines, they are characteristic of active galactic nucleus activity. Blue galaxy classes have emission lines corresponding to star formation regions. We find the expected correlation between spectroscopic class and Hubble type, but this relationship exhibits a high intrinsic scatter. Several potential uses of the ASK classification are identified and sketched, including fast determination of physical properties by interpolation, classes as templates in redshift determinations, and target selection in follow-up works (we find classes of Seyfert galaxies, green valley galaxies, as well as a significant number of outliers). The ASK classification is publicly accessible through various Web sites.
We present the new stellar population synthesis models based on the empirical stellar spectral library MILES, which can be regarded nowadays as standard in the field of stellar population studies. The synthetic SEDs cover the whole optical range at resolution 2.3 Å (FWHM). The unprecedented stellar parameter coverage of MILES allowed us to extend our model predictions from intermediate- to very-old age regimes, and the metallicity coverage from super-solar to [M/H] = -2.3. Observed spectra can be studied by means of full spectrum fitting or line-strengths. For the latter we propose a new Line Index System (LIS) to avoid the intrinsic uncertainties associated with the popular Lick/IDS system and provide more appropriate, uniform, spectral resolution. We present a web-page with a suite of on-line tools to facilitate the handling and transformation of the spectra. Online examples with practical applications to work with stellar spectra for a variety of instrumental setups will be shown. Furthermore we will also show examples of how to compute spectra and colors with varying instrumental setup, redshift and velocity dispersion for a suite of Star Formation Histories.
AbstractThe current databases of empirical star spectra for modelling single-aged stellar populations (SSPs) generally do not chemically characterize their stars completely. Spectral properties of stars and their populations may change considerably if the elemental abundance ratios E/Fe differ from the solar-scaled values. We intend to build up robust integrated spectral energy distribution of SSPs older than 1 Gyr by adopting the MILES database (Medium-resolution Isaac Newton Telescope Library of Empirical Spectra) and taking into account the Mg/Fe ratio of its stars. Magnesium is a proxy of the alpha-capture elements and the alpha/Fe ratio has been widely used as an indicator of the star formation time scale. In this talk, I present how accurate and extensive our compilation and determination of [Mg/Fe] were obtained around MILES to compute state-of-the-art SSP models. Published high resolution measurements were adopted to define a uniform scale of [Mg/Fe] and calibrate our results at medium resolution that were based on the spectral synthesis of two strong Mg features.
AbstractThis talk is divided into two related parts. First, we will call your attention to a basic, but often overlooked worrying fact, and presents ways of dealing with it. The fact is: an enormous number of galaxies in surveys like the SDSS have emission lines which are too weak (low S/N) to be classified by usual schemes (ie, diagnostic diagrams). It turns out that most of these are AGN-like, so ignoring them on the basis of low S/N (which most people do) leaves as much as 2/3 of these emission line galaxies unaccounted for. The solution: We present a number of alternative methods to rescue this numerous population from the classification limbo. We find that about 1/3 of these weak-line galaxies are massive, metal rich star-forming systems, while the remaining 2/3 are more like LINERs. In the second part, we revisit the old idea by Binette et al (1994) that post-AGB stars can account for the emission line properties of some galaxies. A "retired galaxy" model is presented and compared to data in the SDSS. We find that about 1/4 of the galaxies classified as LINERs in the SDSS are consistent with this model, where all ionizing radiation is of stellar origin. More dramatically, nearly 100% of weak-line LINERs are perfectly consistent with being just retired galaxies, with no active nucleus. If these ideas are correct, contrary to current practice, relatively few LINERs should be counted as bona fide AGN.
AbstractWhen we measure the electron density within an H II region using ratios of emission lines we find characteristic values in the range of 100-300 cm-3. But when we make these measurements using the total luminosity in Hα and the overall radial size of an H II region we find average values in the range 3-10. I will first explain how this discrepancy occurs, and then go on to show some measurements of electron densities in the H II regions of M51 (over 2500 regions) and the dwarf galaxy NGC 4449 (over 250 regions) using the second method, by Leonel Gutiérrez and myself. From these measurements we can infer how the electron density varies with the radial size of an individual region, and how it varies as we move from the center of the galaxy disc to the outside. Some interesting simple global relationships are found, which tell us about the interaction of star forming regions with their surroundings and how this interaction varies across the face of a galaxy.
The formation of massive stars is not fully understood. The high luminosity and temperature of massive protostars complicate the accretion mechanism at work in intermediate and low mass young stellar objects. Nonetheless, several scenarios exist to explain the formation of massive stars. In this talk, we will focus on the process of triggered star formation on the borders of H II regions. Due to the feedback effects of OB stars, a layer of molecular material is collected during the expansion of the H II region. Instabilities develop in this layer and give birth to new stars. We will present a detailed study of three Galactic H II regions (RCW79, RCW82 and RCW120). Near-infrared integral field observations have been carried out with SINFONI on the VLT. We will see how they reveal the nature of both the ionizing stars and of the YSOs in the collected layer and how they support the scenario of 'triggered star formation'.
In the local universe, galaxies fall into one of two populations: a star-forming blue cloud and a red sequence lacking star formation. At redshift z ~ 1.5, however, the red sequence has yet to develop. Over the past 9 Gyrs some process has quenched star formation in blue galaxies and caused them to evolve onto the red sequence by fading and/or merging of their stellar populations. While such a transformation may be occurring across the full range of masses, the highest rate of evolution occurs in massive starbursts at the luminous end of the blue cloud. These galaxies are the Luminous Compact Blue Galaxies (LCBGs). In this talk I present preliminary results of a comprehensive multiwavelength survey of LCBGs from z ~ 0 to z ~ 3 we will be carrying out over the next 5 years using several space and ground-based observatories, including the GTC.
In this work we have tried to verify what types of bulges are the descendants and the precursors of the bulges with blue colors observed at intermediate z. These may be the result of intense star formation in the central regions of the disks, related to the phenomenon of pseudo-bulges in the local universe or, alternatively, they may result from rejuvenation processes of old and red classical bulges, formed at high z, perhaps through secular evolution produced by internal or external agents. We can identify and distinguish between these processes analyzing the central surface brightness of the galaxies, μ0. For the general bulge population in the local universe, color is strongly correlated with surface density, in the sense that redder bulges are denser. Classical bulges and pseudo-bulges occupy different regions in a color-μ0 diagram. We have studied the redshift evolution of the relation between the colors and the central surface brightness for samples of spiral galaxy bulges selected from HST/ACS GOODS-N survey, and have analysed the ability of color-μ0 diagram to segregate different types of bulges at z ~ 1. The results show that, up to z ~ 0.8, galaxy nuclear and global colors are strongly correlated with the central surface brightness and, therefore, with the central surface density, in the sense that denser bulges are redder. This suggests that these formed the bulk of their stars at earlier epochs than less dense bulges. For z > 0.8, we find an important fraction of galaxies with high central surface brightness and nuclear colors much bluer than the rest of the galaxy, which probably corresponds to episodes of strong nuclear star formation that may result in the growth of the bulges inside the disks. From simple evolution models we can infer that these nuclei with star formation evolve towards the formation/growth of moderate central surface brightness, intermediate color z ~ 0 pseudobulges rather than classical bulges. These models also argue against rejuvenation processes for z ~ 1 dense and old bulges.
AbstractLarge-scale outflows from galaxies are a crucially important yet poorly understood aspect of galaxy evolution. They redistribute gas and metals into the IGM, regulate star formation, affect the galaxy luminosity function and mass-metallicity relation, etc. Unfortunately, their detailed context in galaxy evolution is difficult to understand: locally, they are identified and studied in heterogeneous manners, while we have only recently begun to study them on cosmological scales and then only in known bright, starbursting galaxies. I will discuss increasing evidence that the so-called ultra-strong MgII intervening quasar absorbers select galactic superwinds over a large range of redshift in a manner independent of luminosity. As superwinds cover a small fraction of the sky at any epoch, only with recent huge quasar absorption lines surveys has it been possible to identify significant numbers of outflows in this manner. I will present new results from several of our studies -- including the measurement of the average SFR of their hosts using [O II] emission from SDSS composite spectra, WIYN, Gemini and WHT imaging of the superwind environments, Gemini/GMOS spectroscopy of superwind host galaxies, and VLT/UVES echellegrams of the absorption lines -- with the aim of understanding the nature of the outflows, their host galaxies, environments, and their evolution over cosmic time.
AbstractThe colour distribution of globular cluster (GC) systems in the majority of galaxies is bi/multimodal in optical colours. It is widely accepted that multiple populations differing in metallicity exist implying different mechanisms/epochs of star formation, with small age differences still being allowed due to the large current uncertainties. Recently Yoon, Yi and Lee (2006) challenged this interpretation stating that the metallicity bimodality is an artifact of the horizontal branch (HB) morphologies that can transform a unimodal metallicity distribution in a bimodal (optical) colour distribution. The combination of optical and near-infrared (NIR) colours can in principal break the age/metallicity degeneracy inherent in optical colours alone, allowing age estimates for a large sample of GCs possible at the same time. It has been shown that the colours that best represent the true metallicity distributions are the combination of optical and NIR (eg. Puzia et al. 2002, Cantiello & Blakeslee 2007). Therefore studying GCs in the NIR is crucial to reveal their true metallicity distributions. We are currently building a homogeneous optical/NIR data set of GC systems in a large sample of elliptical and lenticular galaxies. I will present the sample, an attempt to estimate overall ages and metallicities for the GC systems and the optical/NIR colour distributions.
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