40,607 research outputs found
Fingerprints of the Hierarchical Building up of the Structure on the Mass-Metallicity Relation
We study the mass-metallicity relation of galactic systems with stellar
masses larger than 10^9 Mo in Lambda-CDM scenarios by using chemical
hydrodynamical simulations. We find that this relation arises naturally as a
consequence of the formation of the structure in a hierarchical scenario. The
hierarchical building up of the structure determines a characteristic stellar
mass at M_c ~10^10.2 Moh^-1 which exhibits approximately solar metallicities
from z ~ 3 to z=0. This characteristic mass separates galactic systems in two
groups with massive ones forming most of their stars and metals at high
redshift. We find evolution in the zero point and slope of the mass-metallicity
relation driven mainly by the low mass systems which exhibit the larger
variations in the chemical properties. Although stellar mass and circular
velocity are directly related, the correlation between circular velocity and
metallicity shows a larger evolution with redshift making this relation more
appropriate to confront models and observations. The dispersion found in both
relations is a function of the stellar mass and reflects the different
dynamical history of evolution of the systems.Comment: 4 pages, 4 figures. Accepted MNRAS Letter
Clues for the origin of the fundamental metallicity relations. I: The hierarchical building up of the structure
We analyse the evolutionary history of galaxies formed in a hierarchical
scenario consistent with the concordance -CDM model focusing on the
study of the relation between their chemical and dynamical properties. Our
simulations consistently describe the formation of the structure and its
chemical enrichment within a cosmological context. Our results indicate that
the luminosity-metallicity (LZR) and the stellar mass-metallicity (MZR)
relations are naturally generated in a hierarchical scenario. Both relations
are found to evolve with redshift. In the case of the MZR, the estimated
evolution is weaker than that deduced from observational works by approximately
0.10 dex. We also determine a characteristic stellar mass, , which segregates the simulated galaxy population
into two distinctive groups and which remains unchanged since , with a
very weak evolution of its metallicity content. The value and role played by
is consistent with the characteristic mass estimated from the SDSS galaxy
survey by Kauffmann et al. (2004). Our findings suggest that systems with
stellar masses smaller than are responsible for the evolution of this
relation at least from . Larger systems are stellar dominated and
have formed more than 50 per cent of their stars at , showing very
weak evolution since this epoch. We also found bimodal metallicity and age
distributions from , which reflects the existence of two different
galaxy populations. Although SN feedback may affect the properties of galaxies
and help to shape the MZR, it is unlikely that it will significantly modify
since, from this stellar mass is found in systems with circular
velocities larger than 100 \kms.Comment: 17 pages, 13 figures. Minor changes to match accepted version.
Accepted October 3 MNRA
Milky Way type galaxies in a LCDM cosmology
We analyse a sample of 52,000 Milky Way (MW) type galaxies drawn from the
publicly available galaxy catalogue of the Millennium Simulation with the aim
of studying statistically the differences and similarities of their properties
in comparison to our Galaxy. Model galaxies are chosen to lie in haloes with
maximum circular velocities in the range 200-250 km/seg and to have
bulge-to-disk ratios similar to that of the Milky Way. We find that model MW
galaxies formed quietly through the accretion of cold gas and small satellite
systems. Only 12 per cent of our model galaxies experienced a major merger
during their lifetime. Most of the stars formed in situ, with only about 15 per
cent of the final mass gathered through accretion. Supernovae and AGN feedback
play an important role in the evolution of these systems. At high redshifts,
when the potential wells of the MW progenitors are shallower, winds driven by
supernovae explosions blow out a large fraction of the gas and metals. As the
systems grow in mass, SN feedback effects decrease and AGN feedback takes over,
playing a more important role in the regulation of the star formation activity
at lower redshifts. Although model Milky Way galaxies have been selected to lie
in a narrow range of maximum circular velocities, they nevertheless exhibit a
significant dispersion in the final stellar masses and metallicities. Our
analysis suggests that this dispersion results from the different accretion
histories of the parent dark matter haloes. Statically, we also find evidences
to support the Milky Way as a typical Sb/Sc galaxy in the same mass range,
providing a suitable benchmark to constrain numerical models of galaxy
formationComment: 10 pages, 7 figures, mne2.cls, MNRAS, replaced with accepted versio
Nucleoside Analogues for Positron Emission Tomography Imaging and to Study Radiation Mediated Generation of Radicals from Azides
Gemcitabine is a potent anticancer cytidine analogue used to treat solid tumors. Its efficacy is diminished by rapid deamination to a toxic uridine derivative by cytidine deaminase. To overcome this limitation and add radioactive nuclei (18F or 68Ga) for PET imaging, I synthesized two 4-NÂ-alkylgemcitabine analogues i) bearing β-keto tosylate moiety for subsequent 18F-fluorination and ii) having SCN-Bn-NOTA chelator to complex 68Ga. The first was synthesized by replacement of tosylamide in 4-NÂ-tosylgemcitabine with 1-amino-10-undecene, followed by elaboration of terminal alkene through dihydroxylation, regioselective tosylation and oxidation. Subsequent fluorination using KF in presence of 18-Crown-6 at 75°C for 1 hr gave 4-NÂ-alkylgemcitabine fluoromethyl ketone. The second was synthesized by analogous replacement of tosylamide with N-Boc-1,3-propanediamine, followed by deprotection with TFA. The reactive terminal amine was condensed with SCN-Bn-NOTA, giving 4-NÂ-alkylgemcitabine-SCN-Bn-NOTA ligand, which efficiently complexed Ga or 68Ga for in vivo PET studies in rats.
Clofarabine is a highly effective chemotherapeutic adenosine analogue used for treatment of acute lymphoblastic leukemia. Clofarabine undergoes rate limiting phosphorylation from its 5\u27-monophosphate to 5\u27-diphosphate by purine monophosphate kinase, and possible dephosphorylation of its respective 5\u27-monophosphate by 5\u27-nucleotidases. Synthesis of clofarabine diphosphate prodrugs, and potentially their 18F-radiolabeled analogues, were undertaken to overcome these limitations. Successful synthesis of model adenosine diphosphate prodrug, by coupling adenosine monophosphate with bis(benzoyloxybenzyl) phosphoramidite in presence of 5-phenyl-1-HÂ-tetrazole activator was achieved.
The aminyl radical generated from azide moiety in 3\u27-azido-3\u27-deoxythymidine (3\u27-AZT) or 5-azidomethyl-2\u27-deoxyuridine (AmdU), upon addition of radiation-produced electrons, is thought to be the source of their radiosensitizing effects. Herein, I report synthesis of azido-modified purine and pyrimidine analogues for EPR study of formation of reactive aminyl radical in guanine, adenine and cytidine bases. The EPR studies of electron addition to 2-azidoguanosine (i.e. 2-azidoinsoine), protected 4-azidocytidine and 4-tetrazolocytidine analogues clearly establish that the position of the azide in base moiety dictates reactivity. The azide directly attached to nucleobases at ortho/para position to ring nitrogens produce stable RN3•- that does not rapidly convert to aminyl radical, except in the excited state. Hence, these did not display much radiosensitizing effects in in vivo biological studies in MDA-MB-231, MCF7 and U87 cell lines
On the mass assembly of low-mass galaxies in hydrodynamical simulations of structure formation
Cosmological hydrodynamical simulations are studied in order to analyse
generic trends for the stellar, baryonic and halo mass assembly of low-mass
galaxies (M_* < 3 x 10^10 M_sun) as a function of their present halo mass, in
the context of the Lambda-CDM scenario and common subgrid physics schemes. We
obtain that smaller galaxies exhibit higher specific star formation rates and
higher gas fractions. Although these trends are in rough agreement with
observations, the absolute values of these quantities tend to be lower than
observed ones since z~2. The simulated galaxy stellar mass fraction increases
with halo mass, consistently with semi-empirical inferences. However, the
predicted correlation between them shows negligible variations up to high z,
while these inferences seem to indicate some evolution. The hot gas mass in z=0
halos is higher than the central galaxy mass by a factor of ~1-1.5 and this
factor increases up to ~5-7 at z~2 for the smallest galaxies. The stellar,
baryonic and halo evolutionary tracks of simulated galaxies show that smaller
galaxies tend to delay their baryonic and stellar mass assembly with respect to
the halo one. The Supernova feedback treatment included in this model plays a
key role on this behaviour albeit the trend is still weaker than the one
inferred from observations. At z>2, the overall properties of simulated
galaxies are not in large disagreement with those derived from observations.Comment: 19 pages, 12 figures. Accepted for publication in MNRAS: 6th August
2013. First submitted: 7th July 201
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