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| 2. |
- Atek, Hakim, et al.
(författare)
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On the detectability of Ly-alpha emission in star forming galaxies. The role of dust
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 488:491
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- Context: Lyman-alpha (Lyα) radiation is now widely used to investigate the galaxy formation and evolution in the high redshift universe. However, without a rigorous understanding of the processes regulating the Lyα escape fraction, physical interpretations of high-z observations remain questionable. Aims: We examine six nearby star forming galaxies to disentangle the role of the dust from other parameters such as gas kinematics, geometry, and ISM morphology in the obscuration of Lyα. Thereby, we attempt to understand the Lyα escape physics and infer the implications for high-redshift studies. Methods: We use HST/ACS imaging to produce continuum-subtracted Lyα maps, and ground-based observations (ESO/NTT and NOT) to map the Hα emission and the extinction E(B-V) in the gas phase derived from the Balmer decrement Hα/Hβ. Results: When large outflows are present, the Lyα emission does not appear to be correlated with the dust content, confirming the role of the Hi kinematics in the escape of Lyα photons. In the case of a dense, static Hi covering, we observe a damped absorption with a negative correlation between Lyα and E(B-V). We found that the Lyα escape fraction does not exceed 10% in all our galaxies and is mostly about 3% or below. Finally, because of the radiative transfer complexity of the Lyα line, star formation rate based on Lyα luminosity is underestimated with respect to that derived from UV luminosity. Simple reddening correction does not reconcile SFR(Lyα) with the total star formation rate. Conclusions: The dust is not necessarily the main Lyα escape regulatory factor. ISM kinematics and geometry may play a more significant role. The failure of simple dust correction to recover the intrinsic Lyα/Hα ratio or the total star formation rate should prompt us to be more cautious when interpreting high-z observations and related properties, such as SFRs based on Lyα alone. To this end, we propose a more realistic calibration for SFR(Lyα), which accounts for dust attenuation and resonant scattering effects via the Lyα escape fraction.
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| 3. |
- Gehren, Thomas, et al.
(författare)
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Na, Mg and Al abundances as a population discriminant for nearby metal-poor stars
- 2006
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; :451, s. 1065-1079
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- Aims. Parameters for 55 nearby metal-poor stars are determined using high-resolution spectroscopy. Together with similar data takenfrom a recent analysis, they are used to show trends of their Galactic evolution with stellar [Fe/H] or [Mg/H] abundances. Theseparation of abundance ratios between disk and halo stars is used as a basic criterion for population membership.Methods. After careful selection of a clean subsample free of suspected or known binaries and peculiar stars, abundances of Mg, Na and Al are based on NLTE kinetic equilibrium calculations applied to spectrum synthesis methods.Results. The relation between [Na/Mg] and [Fe/H] is a continuous enrichment through all three Galactic populations spanning a rangeof values between a metal-poor plateau at [Na/Mg] = −0.7 and solar values. [Al/Mg] displays a step-like difference between starsof the Galactic halo with [Al/Mg] ∼ −0.45 and the two disk populations with [Al/Mg] ∼ +0.10. [Al/Mg] ratios, together with the[Mg/Fe] ratios, asymmetric drift velocities V, and stellar evolutionary ages, make possible the individual discrimination between stars of the thick disk and the halo. At present, this evidence is limited by the small number of stars, and by the theoretical and empirical uncertainties of stellar age determinations, but it achieves a high significance.Conclusions. While the stellar sample is not complete with respect to space volume, the resulting abundances indicate the necessityto revise current models of chemical evolution to allow for an adequate production of Al in early stellar generations.
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| 4. |
- Korn, Andreas J., et al.
(författare)
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The sensitivity of Lick indices to abundance variations
- 2005
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 438, s. 685-704
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- We present results of model atmosphere/line formation calculations which quantitatively test how the 21 classical and four higher-order Balmer-line Lick/IDS indices (Worthey et al. 1994, ApJS, 94, 687; Worthey & Ottaviani 1997, ApJS, 111, 377) depend on individual elemental abundances (of carbon, nitrogen, oxygen, magnesium, iron, calcium, sodium, silicon, chromium, titanium) and overall metallicity in various stellar evolutionary stages and at various metallicities. At low metallicities the effects of an overall enhancement of α-elements are also investigated. The general results obtained by Tripicco & Bell (1995, ApJ, 110, 3035) at solar metallicity are confirmed, while details do differ. Tables are given detailing to which element every index reacts significantly, as a function of evolutionary stage and composition.This work validates a number of assumptions implicitly made in the stellar population models of Thomas et al. (2003a, MNRAS, 339, 897), which utilized the results of Tripicco & Bell (1995) to include the effects of element abundance ratios variations. In particular, these computations confirm that fractional changes to index strengths computed at solar metallicity (and solar age) can be applied over a wide range of abundances and ages, also to model old stellar populations with non-solar abundance ratios. The use of metallicity-dependent response functions not only leads to a higher degree of self-consistency in the stellar population models, but is even required for the proper modelling of the Balmer-line indices. We find that the latter become increasingly sensitive to element abundances with increasing metallicity and decreasing wavelength. While Hβ still responds only moderately to abundance ratio variations, the higher-order Balmer-line indices Hγ and Hδ display very strong dependencies at high metallicities. As shown in Thomas et al. (2004, MNRAS, 351, L19), this result allows to remove systematic effects in age determinations based on different Balmer-line indices.
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| 5. |
- Korn, Andreas, et al.
(författare)
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Sulphur abundances in metal-poor stars
- 2005
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 443, s. 1029-1032
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Tidskriftsartikel (populärvet., debatt m.m.)
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| 6. |
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| 7. |
- Neichel, B., et al.
(författare)
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IMAGES. II. A surprisingly low fraction of undisturbed rotating spiral disks at z ~ 0.6 The morpho-kinematical relation 6 Gyr ago
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 484:159
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- We present a first combined analysis of the morphological and dynamical properties for the intermediate-mass Galaxy Evolution Sequence (IMAGES) sample. It is a representative sample of 52 z˜0.6 galaxies with M_stell from 1.5 to 15 × 1010~Mȯ that possesses 3D resolved kinematics and HST deep imaging in at least two broad band filters. We aim at evaluating the evolution of rotating spirals robustly since z˜ 0.6, as well as at testing the different schemes for classifying galaxies morphologically. We used all the information provided by multi-band images, color maps, and 2D light fitting to assign a morphological class to each object. We divided our sample into spiral disks, peculiar objects, compact objects, and mergers. Using our morphological classification scheme, 4/5 of the identified spirals are rotating disks, and more than 4/5 of identified peculiar galaxies show complex kinematics, while automatic classification methods such as concentration-asymmetry and GINI-M20 severely overestimate the fraction of relaxed disk galaxies. Using this methodology, we find that the fraction of undisturbed rotating spirals has increased by a factor ~2 during the past 6 Gyr, a much higher fraction than was found previously based on morphologies alone. These rotating spiral disks are forming stars very rapidly, even doubling their stellar masses over the past 6 Gyr, while most of their stars were formed a few Gyr earlier, which reveals a large gas supply. Because they are the likely progenitors of local spirals, we can conjecture how their properties are evolving. Their disks show some evidence of inside-out growth, and the gas supply/accretion is not random since the disk needs to be stable in order to match the local disk properties.
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| 8. |
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| 9. |
- Puech, M., et al.
(författare)
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IMAGES. III. The evolution of the near-infrared Tully-Fisher relation over the last 6 Gyr
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 484:173
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- Using the multi-integral field spectrograph GIRAFFE at VLT, we have derived the K-band Tully-Fisher relation (TFR) at z ˜ 0.6 for a representative sample of 65 galaxies with emission lines (W_0(OII)≥15 Å). We confirm that the scatter in the z ˜ 0.6 TFR is caused by galaxies with anomalous kinematics, and find a positive and strong correlation between the complexity of the kinematics and the scatter that they contribute to the TFR. Considering only relaxed-rotating disks, the scatter, and possibly also the slope, of the TFR, do not appear to evolve with redshift. We detect an evolution of the K-band TFR zero point between z ˜ 0.6 and z=0, which, if interpreted as an evolution of the K-band luminosity of rotating disks, would imply that a brightening of 0.66 ± 0.14 mag occurs between z ˜ 0.6 and z=0. Any disagreement with the results of Flores et al. (2006, A&A, 455, 107) are attributed to both an improvement of the local TFR and the more detailed accurate measurement of the rotation velocities in the distant sample. Most of the uncertainty can be explained by the relatively coarse spatial-resolution of the kinematical data. Because most rotating disks at z˜0.6 are unlikely to experience further merging events, one may assume that their rotational velocity, which is taken as a proxy of the total mass, does not evolve dramatically. If true, our result implies that rotating disks observed at z˜0.6 are rapidly transforming their gas into stars, to be able to double their stellar masses and be observed on the TFR at z=0. The rotating disks observed are indeed emission-line galaxies that are either starbursts or LIRGs, which implies that they are forming stars at a high rate. Thus, a significant fraction of the rotating disks are forming the bulk of their stars within 6 to 8 Gyr, in good agreement with former studies of the evolution of the mass-metallicity relationship.
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| 10. |
- Karlsson, Torgny, et al.
(författare)
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Chemical abundance patterns - fingerprints of nucleosynthesis in the first stars
- 2001
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Ingår i: Astronomy & Astrophysics. - 0004-6361. ; 379:2, s. 461-481
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- The interstellar medium of low-metallicity systems undergoing star formation will show chemical abundance inhomogeneities due to supernova events enriching the medium on a local scale. If the star formation time-scale is shorter than the time-scale of mixing of the interstellar matter, the inhomogeneities are reflected in the surface abundances of low-mass stars and thereby detailed information on the nucleosynthesis in the first generations of supernovae is preserved. Characteristic patterns and substructures are therefore expected to be found, apart from the large scatter behaviour, in the distributions of stars when displayed in diagrams relating different element abundance ratios. These patterns emerge from specific variations with progenitor stellar mass of the supernova yields and it is demonstrated that the patterns are insensitive to the initial mass function (IMF) even though the relative density of stars within the patterns may vary. An analytical theory of the formation of patterns is presented and it is shown that from a statistical point of view the abundance ratios can trace the different nucleosynthesis sites even when mixing of the interstellar medium occurs. Using these results, it should be possible to empirically determine supernova yields from the information on relative abundance ratios of a large, homogeneous sample of extremely metal-poor Galactic halo stars.
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