The mystery of the distance of galaxies that apparently lacking dark matter

Hosein Haghi

Physics Department, Institute for advanced studies in basic sciences (IASBS)

Abstract: Recently, van Dokkum et al. reported the detection of a couple of ultra-diffuse dwarf (UDG) galaxies lacking dark matter (NGC 1052-DF2 and NGC 1052-DF4). If correct, this could be one of the most important discoveries in Extragalactic Astrophysics and represents a possible challenge to our understanding of the properties of these galaxies and galaxy formation in general. They used surface brightness fluctuation technique yielding a distance of about 20 Mpc implying these galaxies to be dark matter deficient with the larger than expected size, and brighter and larger than expected globular clusters system. But, if the galaxy is a factor of two or three closer to us than the adopted distance of 20 Mpc nearly all the unusual properties vanish. Several authors asked whether these galaxies could be at a smaller distance. Different distance measurement methods that have been applied on these galaxies so far, yielding a distance in the range 10 to 20 Mpc implying it to be “normal” or “dark matter deficient” galaxies. In this talk, first, I will briefly review the current challenge in determining the distance of these galaxies. Then, I will give you a brief report on our recent publication on the IMF-induced intrinsic uncertainties on measuring the ultra-diffuse galaxy distances based on the surface brightness fluctuation technique. 

یکشنبه 22 فروردین 1400، ساعت 19:00

Sunday 11 April 2021 – 19:00 Tehran Time

اتاق سمینار مجازی –Virtual Seminar Room

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Tracing the stellar mass assembly within galaxies over the past 10 Gyr

Moein Mosleh

Biruni Observatory, School of Science, Shiraz University 

Department of Physics, School of Science, Shiraz University

Abstract: One of the important questions in galaxy formation and evolution is to understand how the stellar mass has been assembled within galaxies. The formation of prominent central densities known as bulges is an example of this evolution and the true mechanisms behind this transformation are a matter of debate. In this talk, I will overview the importance of addressing this question and describe the observational methods that we used for tracing the assembly of stellar mass within galaxies. I will also explain how we can benefit from studying the scaling relations such as size-mass relations. In addition, possible connections of the structural evolutions with star-formation activities and environments will be discussed.

یکشنبه 24 اسفند 1399، ساعت 19:00

Sunday 14 March 2021 – 19:00 Tehran Time

اتاق سمینار مجازی –Virtual Seminar Room

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Primordial Black Holes and Gravitational Waves

Antonio Riotto

Department of Theoretical Physics, University of Geneva

 Abstract:  We will discuss the relation between primordial black holes and gravitational waves in the view of the recent Ligo/Virgo and NanoGrav data.

سه شنبه 19 اسفند 1399، ساعت 19:00

Tuesday 9 March 2021 – 19:00 Tehran Time

اتاق سمینار مجازی –Virtual Seminar Room

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

The Hubble-constant crisis

Valerio Marra

Núcleo de Astrofísica e Cosmologia & Departamento de Física, Universidade Federal do Espírito Santo, Brazil

 INAF – Osservatorio Astronomico di Trieste, Italy.

IFPU – Institute for Fundamental Physics of the Universe, Italy.

Abstract:  The determination of the Hubble constant H0 – the present-day expansion rate of the universe – is one of the key goals of cosmology, for it impacts all measurements of cosmological properties. Determinations of H0 from Cosmic Microwave Background observations disagree with local determinations from calibrated supernovas Ia: this tension is now approaching the 5 sigma level. After reviewing the present status of cosmology, I will present recent work on model-independent determinations of H0 and their implications for cosmologies beyond LCDM.

سه شنبه 12 اسفند 1399، ساعت 13:30

Tuesday 2 March 2021 – 13:30 Tehran Time

اتاق سمینار مجازی –Virtual Seminar Room

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Tracing the galaxy evolution and morphologies using the DM and Stellar halo from the cosmological hydrodynamical simulations of TNG

Razieh Emami

Institute for Theory and Computation

Harvard-Smithsonian Center for Astrophysics

Abstract:  One of the fundamental questions about the galaxy evolution is the redshift evolution of the galaxy morphology as traced by DM and stars. Tracking the galaxy morphology enables us to shed light about how the galaxies are evolved with time and also to connect the visible matter, traced by stars, with the invisible matter, DM. In this talk, we will use the next generation (TNG) of cosmological simulations and study the morphology of DM and stars in a sample of Milky Way like galaxies at zero redshift. We will also present how does the galaxy morphology vary with the time. It is intriguing to investigate how does the major mergers affect the galaxy morphology.

یکشنبه 3 اسفند 1399، ساعت 19:00

Sunday 21 February 2021 – 19:00 Tehran Time

اتاق سمینار مجازی –Virtual Seminar Room

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

A conclusive test of the cold dark matter model

Carlos Frenk

Institute for Computational Cosmology, Durham University

Abstract:  The “Lambda cold dark matter” (LCDM) cosmological model is one of the great achievements in Physics of the past thirty years. Theoretical predictions formulated in the 1980s turned out to agree remarkably well with measurements, performed decades later, of the galaxy distribution and the temperature structure of the cosmic microwave background radiation. Yet, these successes do not inform us directly about the nature of the dark matter. Indeed, there are competing (and controversial) claims that the dark matter may have already been discovered, either through the annihilation of cold, or the decay of warm, dark matter particles. In astrophysics the identity of the dark matter manifests itself clearly on subgalactic scales, including the dwarf satellite galaxies of the Milky Way and especially less massive dark matter halos, too small to have made a galaxy.  I will discuss predictions from cosmological simulations assuming cold and warm (in the form of sterile neutrinos) dark matter, including for the properties of the very first halos that ever formed, and show how forthcoming astronomical observations can conclusively distinguish between the two.

یکشنبه 26 بهمن 1399، ساعت 19:00

Sunday 14 February 2021 – 19:00 Tehran Time

اتاق سمینار مجازی –Virtual Seminar Room

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Characterizing the non-linear evolution of dark energy models

Farbod Hassani

Institute of theoretical astrophysics of the University of Oslo

Abstract: Understanding the reason behind the observed accelerating expansion of the Universe is one of the most notable puzzles in modern cosmology, and conceivably in fundamental physics. In the upcoming years, near future surveys will probe structure formation with unprecedented precision and will put firm constraints on the cosmological parameters, including those that describe properties of dark energy. In light of this, during past years I mainly focused on precision cosmology by characterizing the non-linear evolution of cosmological components using N-body simulations. In this talk, first I will introduce gevolution and the idea of developing EFT-evolution N-body code, in which we consistently study the dark matter and dark energy structures up to non-linear scales. Then I will discuss the consequences of clustering dark energy on cosmological observables.

 یکشنبه 19 بهمن 1399، ساعت 19:00

Sunday 7 February 2021 – 19:00 Tehran Time

اتاق سمینار مجازی –Virtual Seminar Room

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Application of Neural Networks and Machine Learning in CMB data analysis

Farida Farsian

Alma Mater Studiorum – Università di Bologna

Abstract:  In the next decade, Primordial Gravitation Waves detection as a CMB B-mode polarization source will play an important role in leading the CMB experiments and constraining inflationary models. To reach this detection, CMB foregrounds seem to be the most challenging problem.

On the other hand, the application of Neural Networks and Machine Learning (ML) in general, as computational tools, expands exponentially in the scientific fields. With their unique computational power, these methods can become a very helpful implement in solving the computational challenges in Cosmology and specifically in the CMB field.

In this talk, at first, I will quickly review the ML methods and then introduce new applications of NNs in the CMB data analysis that we recently developed. We will see that the implemented methods gain advantages in terms of accuracy and efficiency compared to traditional methods.

یکشنبه 12 بهمن 1399، ساعت 19:00

Sunday 31 January 2021 – 19:00 Tehran Time

اتاق سمینار مجازی –Virtual Seminar Room

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Non-parametric modeling of data

Ahmad Mehrabi

Department of Physics, Bu-Ali Sina University

Abstract: Given a data set, one can assume a model with some free parameters and then use a parameter inference method to find the best values of parameters as well as their uncertainties.

The maximum likelihood estimator and Bayesian inference provide two important methods to infer free parameters. Fixing all parameters of a model might result in losing some features or predicting biased values in those parts in which the data set is poor. On the other hand, one might develop some non-parametric modeling to reconstruct all possible curves that are consistent with the data. In this talk, I will review some non-parametric modeling of data and then apply them to Hubble data to reconstructing the Hubble diagram. Finally, I will compare these methods and give some discussions. 

یکشنبه 5 بهمن 1399، ساعت 15:00

Sunday 24 January 2021 – 15:00 Tehran Time

اتاق سمینار مجازی –Virtual Seminar Room

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

The MOSDEF Survey: Environmental dependence of the gas-phase metallicity of galaxies at 1.4≤ z ≤2.6

Nima Chartab

Department of Physics and Astronomy, University of California at Riverside

Abstract:  Using the near-IR spectroscopy of the MOSFIRE Deep Evolution Field (MOSDEF) survey, we investigate the role of the local environment in the gas-phase metallicity of galaxies. The local environment measurements are derived from accurate and uniformly calculated photometric redshifts with well-calibrated probability distributions. Based on rest-frame optical emission lines, [NII]λ6584 and Hα, we measure gas-phase oxygen abundance of 167 galaxies at 1.37≤z≤1.7 and 303 galaxies at 2.09≤z≤2.61, located in diverse environments. We find that at z∼1.5, the average metallicity of galaxies in overdensities with M∗∼10^ 9.8 M⊙,10^10.2 M⊙ and 10^10.8 M⊙ is higher relative to their field counterparts by 0.094±0.051, 0.068±0.028 and 0.052±0.043 dex, respectively.

 However, this metallicity enhancement does not exist at higher redshift, z∼2.3, where, compared to the field galaxies, we find 0.056±0.043, 0.056±0.028 and 0.096±0.034 dex lower metallicity for galaxies in overdense environments with M∗∼10^9.8M⊙,10^10.2 M⊙ and 10^10.7 M⊙, respectively. Our results suggest that, at 1.37≤ z ≤2.61, the variation of mass-metallicity relation with the local environment is small (<0.1dex), and reverses at z∼2. Our results support the hypothesis that, at the early stages of cluster formation, owing to efficient gas cooling, galaxies residing in overdensities host a higher fraction of pristine gas with prominent primordial gas accretion, which lowers their gas-phase metallicity compared to their coeval field galaxies. However, as the Universe evolves to lower redshifts (z≲2), shock-heated gas in overdensities cannot cool down efficiently, and galaxies become metal-rich rapidly due to the suppression of pristine gas inflow and re-accretion of metal-enriched outflows in overdensities.

یکشنبه 21 دی 1399، ساعت 19:00

Sunday 10 January 2021 – 19:00 Tehran Time

اتاق سمینار مجازی –Virtual Seminar Room

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest