astrobites 2018

Find out the latest thinking about our universe.
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Clearing up the Redshifts of Obscured AGNs

Post by bystander » Tue Jul 10, 2018 4:15 pm

Clearing up the Redshifts of Obscured AGNs
Astrobites | 2018 Jul 09
Jamila Pegues wrote:
One of the biggest mysteries of astronomy focuses on, quite fittingly, the absolute biggest astronomical phenomenon we know of: the universe, how it came to be, and where it could possibly be going. To learn more about the evolutionary timeline of the universe, scientists measure the redshifts of far-away objects like black holes and galaxies. In a surreal (and really cool) way, these far-away objects and their redshifts give us snapshots in time of how the universe looked long, long, long ago.

In today’s astrobite, we focus on one particular group of these far-away objects: active galactic nuclei. An active galaxy is a galaxy that has a central core giving off substantial amounts of energy. This core is what’s known as an active galactic nucleus – or AGN for short.

The authors of today’s paper highlighted two ways that scientists measure the redshifts of typical, not-active galaxies. One way is through spectroscopy, which uses the observed spectrum of a galaxy to measure the redshift. Unfortunately this way is pretty costly, and doesn’t work well on a galactic spectrum that is faint at near-infrared and optical wavelengths. The more common way is through photometry, which counts the number of photons, within a given energy band or range, that emit from the galaxy and are collected by a telescope. Given enough different energy bands, scientists can use photometry to construct a low-resolution spectrum of a galaxy and measure the redshift. ...

XZ: Deriving redshifts from X-ray spectra of obscured AGN - Charlotte Simmonds et al
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Spiral Galaxies Show Their Metal

Post by bystander » Tue Jul 10, 2018 4:34 pm

Spiral Galaxies Show Their Metal
Astrobites | 2018 Jul 10
Mia de los Reyes wrote:
To astronomers, the world is composed of hydrogen, helium, and… other stuff. And because astronomers are The Worst At Naming Things™, we call all the other stuff “metals.”

Almost all metals in the universe were produced in stars through nuclear fusion. As stars died, they released these metals into their surrounding environment. These heavy elements were then incorporated into later generations of stars, which eventually exploded as supernovae and released more metals into their surroundings, and so on.

So the abundance of metals in the interstellar medium of a galaxy (the gas-phase metallicity) can be used to study how that galaxy evolved over time! ...

But can we get even more specific? Instead of just considering how metallicities change as a function of radius, can we start to consider how metallicities change around the disk of a galaxy?

Today, we’ll take a look at two papers that consider this question for two different disk galaxies, NGC 1365 and NGC 2997 (Figure 1). ...

The Chemical Evolution Carousel of Spiral Galaxies: Azimuthal Variations of Oxygen Abundance in NGC 1365 - I-Ting Ho et al Azimuthal Variations of Gas-Phase Oxygen Abundance in NGC 2997 - I-Ting Ho et al
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Satellite Galaxies All in a Row—How So?

Post by bystander » Thu Jul 12, 2018 7:44 pm

Satellite Galaxies All in a Row—How So?
Astrobites | 2018 Jul 11
Daniel Berke wrote:
Imagine going for a walk in the summer and coming upon a swarm of bees in flight. But instead of buzzing around in a randomly-distributed cloud, many of the bees are flying in a tightly correlated disk or plane like the rings around Saturn. Not exactly what you’d expect to see, right? (See Figure 1)

Something similar is happening in the universe. (Except with dwarf galaxies instead of bees.) Large galaxies like our Milky Way are surrounded by swarms of smaller satellite galaxies. In simulations of galaxy formation made using the standard model of cosmology, the Lambda Cold Dark Matter model (ΛCDM), these satellite galaxies end up orbiting mostly randomly. That is to say, you generally wouldn’t expect to find multiple satellite galaxies having orbits similar to each other and orbiting in the same direction. However, observations of the Milky Way, the Andromeda Galaxy (M31), and the relatively nearby galaxy Centaurus A have revealed the presence of large, coherent planes of satellite galaxies orbiting around all three. And not only are many of the satellites orbiting in a plane, but in each case the majority of the galaxies in the plane are also orbiting in the same direction. (See Figure 2 for an overview of the various planar structures around the three galaxies.) ...

The Planes of Satellite Galaxies Problem, Suggested Solutions, and Open Questions - Marcel S. Pawlowski
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Multi-messenger observations of a flaring blazar coincident with an IceCube neutrino

Post by bystander » Thu Jul 12, 2018 7:57 pm

Multi-messenger observations of a flaring blazar coincident with an IceCube neutrino
Astrobites | 2018 Jul 12

Interesting article about the multi-messenger observations surrounding neutrino IC-170922A and blazar TXS 0506+056

viewtopic.php?t=38508
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The speeding binary that shouldn’t exist

Post by bystander » Fri Jul 13, 2018 5:37 pm

The speeding binary that shouldn’t exist
Astrobites | 2018 Jul 13
Jennifer Scora wrote:
Hyper-velocity stars (HVS) are so named because they speed through our galaxy really, really fast. So fast, in fact, that they can escape the gravitational pull of our galaxy (about 300 km/s for the Milky Way). This means that either they are flying in from somewhere else altogether, maybe a dwarf galaxy that collided with us, or they were accelerated to such high velocities by a dramatic event within our own galaxy. There are two widely accepted theories about what this event could be, and HVS stars are often divided into two categories based on which origin story they fit best. One theory proposes that when a binary star system gets too close to the supermassive black hole at the center of the Milky Way it is ripped apart by the strong gravitational forces, leaving one star in a close orbit around the black hole and ejecting the other. However, some HVS have been seen travelling from other parts of the galaxy. These can also be explained with binary star systems, but in this case it is the force of the supernova explosion of one star that ejects its companion and accelerates it to high speeds.

To learn more about the supernova theory, the authors picked a star from the Sloan Digital Sky Survey (SDSS) called PB 3877 to be part of a follow-up study of HVS stars. PB 3877 was observed at a speed of 713 ± 140 km/s with respect to the galaxy, putting it well above the requirements for a HVS. They also traced its trajectory backwards and found it did not originate from the black hole at the centre of our galaxy, ruling out the other main theory (see Figure 1). In order to better classify the star and to learn more about the star’s composition and rotation, the authors used the Keck and European Southern Observatory Very Large Telescope (ESO-VLT) to take higher resolution spectra of the star. ...

An extremely fast halo hot subdwarf star in a wide binary system - Péter Németh et al
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Creating a more general deep learning algorithm for galaxies

Post by bystander » Tue Jul 17, 2018 5:09 pm

Creating a more general deep learning algorithm for galaxies
Astrobites | 2018 Jul 16
Avery Schiff wrote:
Between Gaia, LSST, COSMOS, and several other large surveys, astronomers are drowning in far more data than any human could ever hope to handle. As a result machine learning, a branch of computer science that “teaches” a program to rapidly process data, is a very popular topic here at Astrobites. One area of research that previously turned to creative approaches in data analysis–from machine learning to crowdsourcing–is galaxy morphology: what shapes are the hundreds of billions observed galaxies? The authors of today’s paper recently released a massive catalog of galaxy morphologies determined by a deep learning algorithm. In the companion paper discussed here, the authors also explore whether the algorithm can be applied to the data from an entirely different survey. ...

Knowledge Transfer of Deep Learning for Galaxy Morphology from One Survey to Another - H. Domínguez Sánchez et al
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