Feed aggregator

Halpha filaments in Massive Ellipticals: the roles played by Turbulence and type Ia Supernovae

Massive elliptical galaxies are often quiescent in star formation, despite hosting large amounts of -10 million K gas in their ISM , which can cool in a few 100 Myr and fuel star formation. Many of these elliptical galaxies also host massive kpc-scale filaments of cooler atomic (10^4 K) and molecular gas (~10 K) which coexist with the hot ISM .

Two of the outstanding problems related to these systems are – (1) when and how do these cold filaments form? and (2) what keeps the rest of the ISM of the elliptical galaxies hot?

In the first part of this presentation, I will discuss the results of our latest local patch simulations with gravity, turbulence, and radiative cooling physics included. I will present a new condensation criterion to form cold gas from the hot ISM , which takes the effect of turbulence, cooling, and buoyancy into account. (https://arxiv.org/abs/2302.09380)

In the inner few kilo-parsecs (kpc) of the elliptical galaxies, the net heating due to type 1a supernovae (SNIa) is comparable to the net radiative cooling rate and can help maintain the ISM hot. In the second part of the talk, I will present the results from our 1 kpc ISM -patch simulations with individually resolved type SNIa bubbles. I will discuss how these SNIa heat the ISM and drive turbulence in it. Finally, I will compare our results against more commonly implemented smooth heating models in previous theoretical studies. (arxiv: https://arxiv.org/abs/2402.03613)

• Speaker: Rajsekhar Mohapatra (Princeton)
• Friday 10 May 2024, 11:30-12:30
• Venue: Ryle seminar room + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

Add to your calendar or Include in your list

Some questions and answers from protoplanetary disk observations: machine learning, hidden rings, and fake vortices

In this talk, I will discuss some results from different observations of protoplanetary disks. I will first present our work combining disk models with artificial neural networks to model observed SEDs using a statistical approach, and what we learned by applying this to disks in Taurus. I will then discuss our surprising findings when studying the nearby protoplanetary disk MP Mus with ALMA , one of the best young solar analogues which remained relatively unexplored until recently. Finally, I will propose an alternative (and simpler) explanation for some of the disk azimuthal asymmetries that ALMA has revealed, explain how we can use this to gain information about the vertical structure of disks, and provide evidence for at least a number of these asymmetries likely having a geometric origin.

• Speaker: Alvaro Ribas (IOA Cambridge)
• Tuesday 07 May 2024, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Roger Dufresne.

Add to your calendar or Include in your list

Halpha filaments in Massive Ellipticals: the roles played by Turbulence and type Ia Supernovae

Massive elliptical galaxies are often quiescent in star formation, despite hosting large amounts of -10 million K gas in their ISM , which can cool in a few 100 Myr and fuel star formation. Many of these elliptical galaxies also host massive kpc-scale filaments of cooler atomic (10^4 K) and molecular gas (~10 K) which coexist with the hot ISM .

Two of the outstanding problems related to these systems are – (1) when and how do these cold filaments form? and (2) what keeps the rest of the ISM of the elliptical galaxies hot?

In the first part of this presentation, I will discuss the results of our latest local patch simulations with gravity, turbulence, and radiative cooling physics included. I will present a new condition criterion to form cold gas from the hot ISM , which takes the effect of turbulence, cooling, and buoyancy into account. (https://arxiv.org/abs/2302.09380)

In the inner few kilo-parsecs (kpc) of the elliptical galaxies, the net heating due to type 1a supernovae (SNIa) is comparable to the net radiative cooling rate and can help maintain the ISM hot. In the second part of the talk, I will present the results from our 1 kpc ISM -patch simulations with individually resolved type SNIa bubbles. I will discuss how these SNIa heat the ISM and drive turbulence in it. Finally, I will compare our results against more commonly implemented smooth heating models in previous theoretical studies. (arxiv: https://arxiv.org/abs/2402.03613)

• Speaker: Rajsekhar Mohapatra (Princeton)
• Friday 10 May 2024, 11:30-12:30
• Venue: Ryle seminar room + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

Add to your calendar or Include in your list

Nature, Published online: 03 May 2024; doi:10.1038/d41586-024-01273-4

The first set of results from a pioneering cosmic-mapping project hints that the repulsive force known as dark energy has changed over 11 billion years, which would alter ideas about how the Universe has evolved and what its future will be.

arXiv:2405.01188v1 Announce Type: new Abstract: Patchy reionization generates kinematic Sunyaev-Zeldovich (kSZ) anisotropies in the cosmic microwave background (CMB). Large-scale velocity perturbations along the line of sight modulate the small-scale kSZ power spectrum, leading to a trispectrum (or four-point function) in the CMB that depends on the physics of reionization. We investigate the challenges in detecting this trispectrum and use tools developed for CMB lensing, such as realization-dependent bias subtraction and cross-correlation based estimators, to counter uncertainties in the instrumental noise and assumed CMB power spectrum. We also find that both lensing and extragalactic foregrounds can impart larger trispectrum contributions than the reionization kSZ signal. We present a range of mitigation methods for both of these sources of contamination, validated on microwave-sky simulations. We use ACT DR6 and Planck data to calculate an upper limit on the reionization kSZ trispectrum from a measurement dominated by foregrounds. The upper limit is about 50 times the signal predicted from recent simulations.

Type Ia supernovae: Constraining thermonuclear explosion physics with machine learning

Type Ia supernovae are thermonuclear explosions of white dwarfs in binary systems. They play an important role in many areas of astrophysics, from providing chemical enrichment for galaxies to acting as cosmological distance probes. In spite of this, we still fundamentally do not know how or why some white dwarfs explode as thermonuclear supernovae. Multiple explosion mechanisms have been proposed, but the computational expense associated with developing realistic explosion simulations and the difficulty in observing key diagnostic signatures mean that providing robust constraints on the explosion physics is challenging. In this talk, I will provide a general overview of thermonuclear explosion physics and discuss the main explosion scenarios suggested in the literature. I will present my recent work focused on using machine learning to automatically fit spectral sequences of type Ia supernovae in a much more quantitative and efficient way than existing methods. With automated fitting we can test different explosion scenarios against observations and statistically determine which scenario provides the best overall agreement. As spectroscopic samples of supernovae continue to grow, automated fitting tools will become increasingly important to maximise the physical constraints that can be gained in a quantitative and consistent manner.

• Speaker: Mark Magee (University of Warwick)
• Wednesday 08 May 2024, 13:40-14:05
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Hannah Uebler.

Add to your calendar or Include in your list

Dynamical Field Theories without Detailed Balance

Many condensed matter problems, such as ordering of a classical ferromagnet or fluid-fluid phase separation, are described by dynamical field theories in which a scalar field obeys a noisy gradient flow governed by a quartic effective potential. These theories are called Model A and Model B for the cases of a non-conserved and conserved scalar, respectively. Traditionally, such models are constructed to obey detailed balance, so that the system evolves to the Boltzmann distribution, giving time-reversible fluctuations at stationarity. Reaching the equilibrium state can be nontrivial however: starting from a metastable uniform initial condition, it requires an instanton to nucleates a droplet large enough to then grow spontaneously. In recent years, attention has shifted to systems without detailed balance, whose stationary states are non-Boltzmann and involve continuous entropy production with time-asymmetric fluctuations. (One example is the study of phase separation among self-propelled particles such as swimming bacteria.) To describe such cases, we have recently introduced variants of Models A and B that break detailed balance explicitly. I will outline some of the qualitative and quantitative novelties that arise in the critical phenomena, steady states, and instantons of these new theories.

• Speaker: Michael Cates, University of Cambridge
• Wednesday 15 May 2024, 14:00-15:00
• Venue: MR2.
• Series: Theoretical Physics Colloquium; organiser: Ronak M Soni.

Add to your calendar or Include in your list

Interferometric measurements of the 21-cm signal with SKA

The Cosmic Dawn marks the first star formations and preceded the Epoch-of-Reionization, when the Universe underwent a fundamental transformation propelled by the radiation from these first stars and galaxies. Interferometric 21-cm experiments aim to probe redshifted neutral hydrogen signals from these periods, constraining the conditions of the early Universe. The SKA -LOW instrument of the Square Kilometre Array telescope is envisaged to be the largest and most sensitive radio telescope at m and cm wavelengths. In this talk we present a full SKA pipeline that consist of forward modelling and data analysis that were also tested in the SKA Science Data Challenge 3a: Epoch of Reionisation (SKA SDC3a) to process the novel data products expected from the SKA . The forward modelling enables simulation of the astrophysical signals from the Epoch of Reionization and chosen systematic effects of the SKA -LOW. In the analysis part we implement predictive foreground and Bayesian Gaussian Process Regression models alongside a foreground avoidance strategy to isolate the 21-cm signal from that of the astrophysical radio frequency (RF) foregrounds. Together these will determine whether a successful 21-cm detection is possible with the envisaged SKA .

• Speaker: Yuchen Liu and Oscar O'Hara (Cavendish Astrophysics)
• Tuesday 07 May 2024, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Charles Walker.

Add to your calendar or Include in your list

arXiv:2311.05364v2 Announce Type: replace Abstract: The redshifted 21 cm signal from neutral hydrogen is a direct probe of the physics of the early universe and has been an important science driver of many present and upcoming radio interferometers. In this study we use a single night of observations with the New Extension in Nan\c{c}ay Upgrading LOFAR (NenuFAR) to place upper limits on the 21 cm power spectrum from cosmic dawn at a redshift of $z$ = 20.3. NenuFAR is a new low-frequency radio interferometer, operating in the 10-85 MHz frequency range, currently under construction at the Nan\c{c}ay Radio Observatory in France. It is a phased array instrument with a very dense uv coverage at short baselines, making it one of the most sensitive instruments for 21 cm cosmology analyses at these frequencies. Our analysis adopts the foreground subtraction approach, in which sky sources are modeled and subtracted through calibration and residual foregrounds are subsequently removed using Gaussian process regression. The final power spectra are constructed from the gridded residual data cubes in the uv plane. Signal injection tests are performed at each step of the analysis pipeline, the relevant pipeline settings are optimized to ensure minimal signal loss, and any signal suppression is accounted for through a bias correction on our final upper limits. We obtain a best 2$\sigma$ upper limit of $2.4\times 10^7$ $\text{mK}^{2}$ at $z$ = 20.3 and $k$ = 0.041 $h\,\text{cMpc}^{-1}$. We see a strong excess power in the data, making our upper limits two orders of magnitude higher than the thermal noise limit. We investigate the origin and nature of this excess power and discuss further improvements to the analysis pipeline that can potentially mitigate it and consequently allow us to reach thermal noise sensitivity when multiple nights of observations are processed in the future.

arXiv:2404.19504v1 Announce Type: new Abstract: We carried out very deep VLT/SPHERE imaging polarimetry of the nearby system Eps Eri based on 38.5 hours of integration time with a 600 - 900 nm broadband filter to search for polarized scattered light from a planet or from circumstellar dust using AO, coronagraphy, high precision differential polarimetry, and angular differential imaging. We have improved several data reduction and post-processing techniques and also developed new ones to further increase the sensitivity of SPHERE/ZIMPOL. The data provide unprecedented contrast limits, but no significant detection of a point source or an extended signal from circumstellar dust. For each observing epoch, we obtained a point source contrast for the polarized intensity between $2\cdot 10^{-8}$ and $4\cdot 10^{-8}$ at the expected separation of the planet Eps Eri b of 1'' near quadrature phase. The polarimetric contrast limits are about six to 50 times better than the intensity limits because polarimetric imaging is much more efficient in speckle suppression. Combining the entire 14-month data set to the search for a planet moving on a Keplerian orbit with the K-Stacker software further improves the contrast limits by a factor of about two, to about $8 \cdot 10^{-9}$ at 1''. This would allow the detection of a planet with a radius of about 2.5 Jupiter radii. The surface brightness contrast limits achieved for the polarized intensity from an extended scattering region are about 15 mag arcsec$^{-2}$ at 1'', or up to 3 mag arcsec$^{-2}$ deeper than previous limits. For Eps Eri, these limits exclude the presence of a narrow dust ring and they constrain the dust properties. This study shows that the polarimetric contrast limits for reflecting planets with SPHERE/ZIMPOL can be improved to a level $

arXiv:2312.08095v2 Announce Type: replace Abstract: The early Universe, spanning 400,000 to 400 million years after the Big Bang ($z\approx1100-11$), has been left largely unexplored as the light from luminous objects is too faint to be observed directly. While new experiments are pushing the redshift limit of direct observations, measurements in the low-frequency radio band promise to probe early star and black hole formation via observations of the hydrogen 21-cm line. In this work we explore synergies between 21-cm data from the HERA and SARAS 3 experiments and observations of the unresolved radio and X-ray backgrounds using multi-wavelength Bayesian analysis. We use the combined data set to constrain properties of Population II and Population III stars as well as early X-ray and radio sources. The joint fit reveals a 68 percentile disfavouring of Population III star formation efficiencies $\gtrsim5.7\%$. We also show how the 21-cm and the X-ray background data synergistically constrain opposite ends of the X-ray efficiency prior distribution to produce a peak in the 1D posterior of the X-ray luminosity per star formation rate. We find (at 68\% confidence) that early galaxies were likely 0.3 to 318 times as X-ray efficient as present-day starburst galaxies. We also show that the functional posteriors from our joint fit rule out global 21-cm signals deeper than $\lesssim-203\ \mathrm{mK}$ and power spectrum amplitudes at $k=0.34\ h\mathrm{Mpc^{-1}}$ greater than $\Delta_{21}^2 \gtrsim 946\ \mathrm{mK}^2$ with $3\sigma$ confidence.

arXiv:2405.00504v1 Announce Type: new Abstract: We investigate the X-ray properties of a large sample of 71 broad line and narrow line AGN at 2

arXiv:2405.00113v1 Announce Type: new Abstract: We present extensive observations of the Type II supernova (SN II) 2023ufx which is likely the most metal-poor SN II observed to-date. It exploded in the outskirts of a low-metallicity ($Z_{\rm host} \sim 0.1~Z_\odot$) dwarf ($M_g = -13.23\pm0.15$~mag; $r_e\sim 1$~kpc) galaxy. The explosion is luminous, peaking at $M_g\approx -18.5~$mag, and shows rapid evolution. The $r$-band (pseudo-bolometric) light curve has a shock-cooling phase lasting 20 (17) days followed by a 19 (23)-day plateau. The entire optically-thick phase lasts only $\approx 55~$days following explosion, indicating that the red supergiant progenitor had a thinned H envelope prior to explosion. The early spectra obtained during the shock-cooling phase show no evidence for narrow emission features and limit the pre-explosion mass-loss rate to $\dot{M} \lesssim 10^{-3}~\rm M_\odot$/yr. The photospheric-phase spectra are devoid of prominent metal absorption features, indicating a progenitor metallicity of $\lesssim 0.1~Z_\odot$. The semi-nebular ($\sim 60-130~$d) spectra reveal weak Fe II, but other metal species typically observed at these phases (Ti II, Sc II, Ba II) are conspicuously absent. The late-phase optical and near-infrared spectra also reveal broad ($\approx 10^4~\rm{km}~\rm s^{-1}$) double-peaked H$\alpha$, P$\beta$, and P$\gamma$ emission profiles suggestive of a fast outflow launched during the explosion. Outflows are typically attributed to rapidly-rotating progenitors which also prefer metal-poor environments. This is only the second SN II with $\lesssim 0.1~Z_\odot$ and both exhibit peculiar evolution, suggesting a sizable fraction of metal-poor SNe II have distinct properties compared to nearby metal-enriched SNe II. These observations lay the groundwork for modeling the metal-poor SNe II expected in the early Universe.

The cometary delivery of prebiotic feedstock molecules to the early-Earth and rocky exoplanets

The delivery of prebiotic feedstocks molecules, such as hydrogen cyanide (HCN), during cometary impacts may have significantly influenced prebiotic chemistry on the early Earth, motivated by the discovery of a rich diversity of CHN - and CHS -bearing molecules on solar system comets. Numerical experiments have demonstrated that HCN survival during cometary impacts is however only possible in oblique impacts at very low velocities. In this talk I will discuss the effects of stellar mass, and planetary architecture on minimum cometary impact velocities onto rocky exoplanets. Using both an analytical model and numerical N-body simulations, we show the lowest impact velocities occur for low-mass planets in tightly-packed planetary systems around high-mass (i.e., Solar-mass) stars, enabling the intact delivery of prebiotic feedstock molecules. I will finish by discussing a specific origins scenario, proposed to achieve favourable conditions for subsequent prebiotic chemistry, which invokes the arrival of a secondary impactor in the same location. We consider the atmospheric fragmentation of cometary impactors, and use the lunar crater record to quantitatively evaluate the likelihood of these `double impact’ scenarios on the early-Earth. These scenarios are found to be extremely unlikely settings for the initial stages of prebiotic chemistry, unless there was a particularly high impact rate on the early-Earth, and global environmental conditions conducive to successful cometary delivery.

• Speaker: Richard Anslow (University of Cambridge)
• Wednesday 08 May 2024, 13:15-13:40
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Hannah Uebler.

Add to your calendar or Include in your list

Results of beamline testing at the MROI

TBC

• Speaker: Dr. John Young (Cavendish Astrophysics)
• Tuesday 26 November 2024, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Charles Walker.

Add to your calendar or Include in your list

Title to be confirmed

Abstract not available

• Speaker: Siyi Xu (NOIRlab, USA)
• Thursday 13 June 2024, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: eb694.

Add to your calendar or Include in your list

Title to be confirmed

Abstract not available

• Speaker: Siyi Xu (NOIRlab, USA)
• Thursday 13 June 2024, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: eb694.

Add to your calendar or Include in your list

Insights into cosmological simulations from modified initial conditions

I will discuss the GMGalaxies programme, which is pursuing a new ‘hybrid’ approach to cosmological galaxy formation simulations combining the best of cosmological zooms and idealised approaches of the past. By customising (‘genetically modifying’) our initial conditions, we can construct controlled tests of structure formation within a fully cosmological environment. This approach has allowed us to obtain new and unique insights into ultra-faint dwarf galaxy formation, AGN -driven galaxy quenching, large scale structure formation and — in soon-to-be-released ultra-high-resolution simulations — the Milky Way fossil record seen by Gaia. In this talk, I will summarise some of these results but focus especially on recent insights into dwarf galaxy formation.

• Speaker: Andrew Pontzen (UCL)
• Thursday 09 May 2024, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: eb694.

Add to your calendar or Include in your list

Black hole accretion in the TDAMM Era

Most of the power from an Active Galactic Nucleus is released close to the black hole, and thus studying accretion at event horizon scales—at the intersection of inflow and outflow—is essential for understanding how much matter accretes and grows the black hole vs. how much matter is ejected, thus effecting the black hole’s large-scale environments. In the past decade, we have had a breakthrough in how we probe the inner accretion flow, through the discovery of X-ray Reverberation Mapping, where X-rays produced close to the black hole reverberate off inflowing gas. By measuring reverberation time delays, we can quantify the effects of strongly curved space time and measure black hole spin, which is key for understanding how efficiently energy can be tapped from the accretion process. In this talk, I will give an overview of this field, and will show how extending these spectral-timing techniques to extreme, transient (and possibly multi-messenger) accretion events like Tidal Disruption Events and Quasi Periodic Eruptions can help us understand the growth and impact of black holes in galactic centers.

• Speaker: Erin Kara (MIT)
• Thursday 06 June 2024, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: eb694.

Add to your calendar or Include in your list

Title to be confirmed

Abstract not available

• Speaker: Anshu Gupta (Curtin)
• Friday 05 July 2024, 11:30-12:30
• Venue: Ryle seminar room + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

Add to your calendar or Include in your list