Features RSS

Sagittarius Dwarf Galaxy

Dwarf galaxies are typically very faint, and are therefore hard to find. Given that, what are our chances of finding their distant ancestors, located billions of light-years away? A recent study aims to find out.

Ancient Counterparts

Dwarf galaxies are a hot topic right now, especially as we discover more and more of them nearby. Besides being great places to investigate a variety of astrophysical processes, local group dwarf galaxies are also representative of the most common type of galaxy in the universe. For many of these dwarf galaxies, their low masses and typically old stellar populations suggest that most of their stars were formed early in the universe’s history, and further star formation was suppressed when the universe was reionized at redshifts of z ~ 6–10. If this is true, most dwarf galaxies are essentially fossils: they’ve evolved little since that point.

To test this theory, we’d like to find counterparts to our local group dwarf galaxies at these higher redshifts of z = 6 or 7. But dwarf galaxies, since they don’t exhibit lots of active star formation, have very low surface brightnesses — making them very difficult to detect. What are the chances that current or future telescope sensitivities will allow us to detect these? That’s the question Anna Patej and Abraham Loeb, two theorists at Harvard University, have addressed in a recent study.

Entering a New Regime

SB vs. size

The surface brightness vs. size for 73 local dwarf galaxies scaled back to redshifts of z=6 (top) and z=7 (bottom). So far we’ve been able to observe high-redshift galaxies within the boxed region of the parameter space. JWST will open the shaded region of the parameter space, which includes some of the dwarf galaxies. [Patej & Loeb 2015]

Starting from observational data for 87 Local-Group dwarf galaxies, Patej and Loeb used a stellar population synthesis code to evolve the galaxies backward in time to redshifts of z = 6 and 7. Next, they narrowed this sample to only those dwarfs for which most star formation had already occurred by this time.

Finally, the authors compared the properties of these 73 scaled-back dwarfs to those of high-redshift galaxies that we have already detected with the Hubble and Spitzer Space Telescopes, as well as to the detection limits of the upcoming James Webb Space Telescope (JWST) mission launching in 2018.

Patej and Loeb find that, when scaled back to redshifts of z = 6 or 7, the dwarf galaxies would be too faint to detect with current telescopes — despite being roughly the same size as high-redshift galaxies we’ve already detected. But the capabilities of JWST will push into this regime: according to Patej and Loeb’s calculations, JWST would be able to detect 13 of the 73 galaxies in the sample at a redshift of z = 6, and 9/73 at a redshift of z = 7.

Furthermore, the fraction of detectable galaxies would increase if these ancient dwarfs contained large numbers of Population-III-like, massive, bright stars. But even without such a boost, the hunt for the ancestors of local dwarf galaxies appears to be well within JWST’s capabilities!

Citation

Anna Patej and Abraham Loeb 2015 ApJ 815 L28. doi:10.1088/2041-8205/815/2/L28

M101

Editor’s Note: In these last two weeks of 2015, we’ll be looking at a few selections from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

The Changing Fractions of Type Ia Supernova NUV–Optical Subclasses with Redshift

Published April 2015

 

Main takeaway:

A team of scientists led by Peter Milne (University of Arizona) used ultraviolet observations from the Swift spacecraft to determine that type Ia supernovae, stellar explosions previously thought to all belong in the same class, actually fall into two subgroups: those that are slightly redder in NUV wavelengths and those that are slightly bluer.

Supernovae fraction

Plot of the percentage of supernovae that are NUV-blue (rather than NUV-red), as a function of redshift. NUV-blue supernovae dominate at higher redshifts. [Milne et al. 2015]

Why it’s interesting:

It turns out that the fraction of supernovae in each of these two groups is redshift-dependent. At low redshifts (i.e., nearby), the population of type Ia supernovae is dominated by NUV-red supernovae. At high redshifts (i.e., far away), the population is dominated by NUV-blue supernovae. Since cosmological distances are measured using Type Ia supernovae as standard candles, the fact that we’ve been modeling these supernovae all the same way (rather than treating them as two separate subclasses) means we may have been systematically misinterpreting distances.

What this means for the universe’s expansion:

This seemingly simple discovery carries hefty repercussions — in fact, our estimates of the expansion rate of the universe may be incorrect! The authors believe that if we correct for this error, we’ll find that the universe is not expanding as quickly as we thought.

Citation

Peter A. Milne et al 2015 ApJ 803 20. doi:10.1088/0004-637X/803/1/20

Editor’s Note: In these last two weeks of 2015, we’ll be looking at a few selections from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

An Ancient Extrasolar System with Five Sub-Earth-Size Planets

Published January 2015

 

Main takeaway:

transit light curves

Transit light curves for the five planets orbiting Kepler-444. [Campante et al. 2015]

A team led by Tiago Campante (University of Birmingham, Aarhus University) reported Kepler spacecraft observations of Kepler-444, a system of five transiting exoplanets around a metal-poor, Sun-like star. All five planets are sub-Earth-sized. Furthermore, the system is measured to be over 11 billion years old — making this the oldest known system of terrestrial-size planets.

Why it’s interesting:

While gas-giant planets show a preference for forming around metal-rich stars, smaller planets appear to be less picky. This suggests that Earth-size planets may have been able to form at earlier times in the universe’s history, when metals were scarcer. The determination that Kepler-444 is 11.2 billion years old confirms that terrestrial-size planets have been able to form throughout most of the universe’s 13.8 billion year history.

Awesome technical achievement:

The age of the Kepler-444 system was determined from asteroseismology of the host star. The fact that we can measure oscillations in the interior of this ancient star located 116 light-years away — and use this to determine its age to a precision of 9%! — is a remarkable achievement made possible by 4 years of continuous, high-quality observations of the system.

Citation

T. L. Campante et al 2015 ApJ 799 170. doi:10.1088/0004-637X/799/2/170

VFTS 352

Editor’s Note: In these last two weeks of 2015, we’ll be looking at a few selections from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

Discovery of the Massive Overcontact Binary VFTS 352: Evidence for Enhanced Internal Mixing

Published October 2015

 

Main takeaway:

A team led by Leonardo Almeida (Johns Hopkins University and University of São Paulo, Brazil) discovered the binary star system VFTS 352 in the Large Magellanic Cloud. This pair of O-type stars is an “overcontact binary” — the two stars are orbiting each other so closely that they’re actually touching each other.

Why it’s interesting:

Snapshots of VFTS 352 at a few orbital phases, the system’s light curves, and its radial velocity curves. [Almeida et al. 2015]

Snapshots of VFTS 352 at a few orbital phases, the system’s light curves, and its radial velocity curves. [Almeida et al. 2015]

We know little about the overcontact stage that occurs when two massive stars coalesce — primarily because it’s typically short-lived, so we have few observations of stars in this stage. VFTS 352 is the most massive and earliest spectral type overcontact system known to date. It’s especially interesting because the observations suggest that the strong tidal forces in this system may have caused enhanced internal mixing between the stars’ centers and envelopes. These stars’ interiors may therefore be much more homogenous than is typical.

What to expect:

Ultimately, this pair of stars will likely share one of two fates. In the classical scenario, they’ll expand and eventually merge to produce a single rapidly rotating, massive star. If their internal mixing is large enough, however, they could remain compact rather than expanding. In that case, they would progress to the end of their main-sequence lifetimes without ever merging, potentially evolving to become a black-hole binary system.

Citation

L. A. Almeida et al 2015 ApJ 812 102. doi:10.1088/0004-637X/812/2/102

EGSY8p7

Editor’s Note: In these last two weeks of 2015, we’ll be looking at a few selections from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

Lyα Emission from a Luminous z = 8.68 Galaxy: Implications for Galaxies as Tracers of Cosmic Reionization

Published August 2015

 

Main takeaway:

A team led by Adi Zitrin (Hubble Fellow at California Institute of Technology) detected Lyα emission in the bright galaxy EGSY8p7 using the MOSFIRE spectrograph at Keck Observatory. From this emission line, they calculated that the galaxy has an astonishing redshift of z=8.68.

Why it’s interesting:

This spectroscopic confirmation crowned EGSY8p7 as the record-holder for the farthest-known (and therefore oldest) galaxy. Its redshift shattered the previous record, a galaxy at z=7.73.

Why it’s even more interesting than that:

spectroscopic confirmation

Spectroscopic detection of emission in EGSY8p7 with MOSFIRE. The black line is the raw data; the red line shows the best-fit model to the data. [Zitrin et al. 2015]

Based on our understanding of how the universe evolved, the detection of Lyα emission from this galaxy came as a surprise. At EGSY8p7’s redshift of 8.68, the universe was still full of clouds of neutral hydrogen that should have absorbed the galaxy’s Lyα emission long before it reached us. So what does it mean that we do see Lyα emission from EGSY8p7? The reionization of the universe — through which the neutral hydrogen clouds were made transparent — may have been a patchy process. In particular, EGSY8p7 might have emitted an unusual amount of ionizing radiation, creating an early ionized bubble around it that allowed the Lyα emission to escape.

Citation

Adi Zitrin et al 2015 ApJ 810 L12. doi:10.1088/2041-8205/810/1/L12

Coma Cluster

Editor’s Note: In these last two weeks of 2015, we’ll be looking at a few selections from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

Forty-Seven Milky Way-Sized, Extremely Diffuse Galaxies in the Coma Cluster

Published January 2015

 

Main takeaway:

Using the Dragonfly Telephoto Array, a team led by Pieter van Dokkum (Yale University) discovered 47 ultra-diffuse galaxies in the Coma galaxy cluster. These galaxies are very large, with half-light (“effective”) radii of 1.5–4.6 kpc, similar to that of the Milky Way’s disk. But their stellar masses are a factor of 1000 lower than the Milky Way’s, and they’re accordingly much dimmer.

ultra-diffuse galaxies

Plot of the effective radius versus the central surface brightness for the ultra-diffuse Coma cluster galaxies (red markers). These galaxies are similar in size to the Milky Way’s disk (blue), but significantly dimmer. [Van Dokkum et al. 2015]

Why it’s interesting:

These galaxies make up an odd population. Why are their stellar masses so low? The authors posit that these objects may be failed galaxies that lost their gas after having formed their first generation of stars. Adding to the intrigue, the authors find that in order for these galaxies to hold themselves together at their current distance from the cluster core, they must have a whopping dark-matter fraction of 98%.

About the discovery:

These ultra-diffuse galaxies were actually discovered entirely by accident. Van Dokkum and collaborators observed the Coma cluster in a project to measure properties of the intra-cluster light and look for streams and tidal features. Surprisingly, their images revealed these faint, uncataloged galaxies.

Citation

Pieter G. van Dokkum et al 2015 ApJ 798 L45. doi:10.1088/2041-8205/798/2/L45

Spiral arms

Young, forming planets can generate immense spiral structures within their protoplanetary disks. A recent study has shown that observations of these spiral structures may allow astronomers to measure the mass of the planets that create them.

Spirals From Waves

disk simulations

Snapshots of the surface density of a protoplanetary disk in a 2D simulation, 3D simulation, and synthesized scattered-light image. Click for a closer look! [Fung & Dong, 2015]

Recent studies have shown that a single planet, if it is massive enough, can excite multiple density waves within a protoplanetary disk as it orbits. These density waves can then interfere to produce a multiple-armed spiral structure in the disk inside of the planet’s orbit — a structure which can potentially be observed in scattered-light images of the disk.

But what do these arms look like, and what factors determine their structure? In a recently published study, Jeffrey Fung and Ruobing Dong, two researchers at the University of California at Berkeley, have modeled the spiral arms in an effort to answer these questions.

Arms Provide Answers

A useful parameter for describing the structure is the azimuthal separation (φsep) between the primary and secondary spiral arms. If you draw a circle within the disk and measure the angle between the two points where the primary and secondary arms cross it, that’s φsep.

scaling relation

Azimuthal separation of the primary and secondary spiral arms, as a function of the planet-to-star mass ratio q. The different curves represent different disk aspect ratios. [Fung & Dong, 2015]

The authors find that φsep stays roughly constant for different radii, but it’s strongly dependent on the planet’s mass: for larger planets, φsep increases. They discover that φsep scales as a power of the planet mass for companions between Neptune mass and 16 Jupiter masses, orbiting around a solar-mass star. For larger, brown-dwarf-size companions, φsep is a constant 180°.

If this new theory is confirmed, it could have very interesting implications for observations of protoplanetary disks: this would give us the ability to measure the mass of a planet in a disk without ever needing to directly observe the planet itself!

Modeling Observations

Fung and Dong confirm their models by additionally running 3D simulations, which yield very similar outcomes. From these simulation results, they then synthesize scattered-light images similar to what we would expect to be able to observe with telescopes like the VLT, Gemini, or Subaru. The authors demonstrate that from these scattered-light images, they can correctly retrieve the planet’s mass to within 30%.

Finally, as a proof-of-concept, the authors apply this modeling to an actual system: SAO 206462, a nearly face-on protoplanetary disk with an observed two-armed spiral within it. From the measured azimuthal separation of the two arms, the authors estimate that it contains a planet of about 6 Jupiter masses.

Citation

Jeffrey Fung (馮澤之) and Ruobing Dong (董若冰) 2015 ApJ 815 L21. doi:10.1088/2041-8205/815/2/L21

Leo P

Measurements of metal abundances in galaxies present a conundrum: compared to expectations, there are not nearly enough metals observed within galaxies. New observations of a nearby dwarf galaxy may help us understand where this enriched material went.

Removal Processes

Star formation is responsible for the build-up of metals (elements heavier than helium) in a galaxy. But when we use a galaxy’s star-formation history to estimate the amount of enriched material it should contain, our predictions are inconsistent with measured abundances: large galaxies contain only about 20–25% of the expected metals, and small dwarf galaxies contain as little as 1%!

So what happens to galaxies’ metals after they have been formed? The favored explanation is that metals are removed from galaxies via stellar feedback: stars that explode in violent supernovae can drive high-speed winds, expelling the enriched material from a galaxy. This process should be more efficient in low-mass galaxies due to their smaller gravitational wells, which would explain why low-mass galaxies have especially low metallicities.

But external processes may also contribute to the removal of metals, such as tidal stripping during interactions between galaxies. To determine the role of stellar feedback alone, an ideal test would be to observe an isolated low-mass, star-forming galaxy — i.e., one that is not affected by external processes.

Luckily, such an isolated, low-mass galaxy has recently been discovered just outside of the Local Group: Leo P, a gas-rich dwarf galaxy with a total stellar mass of 5.6 x 105 solar masses.

Isolated Results

Dwarf abundances

Percentage of oxygen lost in Leo P compared to the percentage of metals lost in three other, similar-size dwarfs that are not isolated. If the gas-phase oxygen in Leo P were removed, Leo P’s measurements would be consistent with those of the other dwarfs. [McQuinn et al. 2015]

Led by Kristen McQuinn (University of Minnesota, University of Texas at Austin), a team of researchers has used Hubble observations to reconstruct Leo P’s star formation history. McQuinn and collaborators use this history to determine the dwarf galaxy’s total oxygen production — used as a tracer of its metal production — over its lifetime. They then compare this to the abundance of oxygen currently observed within Leo P.

In non-isolated dwarf-spheroidal galaxies of similar mass to Leo P, 99% of their expected metals are missing. In comparison, the authors find that Leo P is missing 95% of its expected metals. From these results, it seems that expulsion of enriched material by stellar feedback alone can explain most of the missing metals in such galaxies; external factors only remove an additional few percent.

This explanation is further supported by the fact that, of the oxygen remaining in Leo P, 25% is locked up in stars, whereas 75% is found to be in gas form in the galaxy’s interstellar medium. If this 75% were stripped away by external processes, Leo P’s measurements would become consistent with those of the non-isolated dwarf galaxies.

Citation

Kristen B. W. McQuinn et al 2015 ApJ 815 L17. doi:10.1088/2041-8205/815/2/L17

exoplanet atmosphere evaporation

What characteristics must a terrestrial planet exhibit to have the potential to host life? Orbiting within the habitable zone of its host star is certainly a good start, but there’s another important aspect: the planet has to have the right atmosphere. A recent study has determined how host stars can help their planets to lose initial, enormous gaseous envelopes and become more Earth-like.

Collecting An Envelope

When a terrestrial planet forms inside a gaseous protoplanetary disk, it can accumulate a significant envelope of hydrogen gas — causing the planet to bear more similarity to a mini-Neptune than to Earth. Before the planet can become habitable, it must shed this enormous, primordial hydrogen envelope, so that an appropriate secondary atmosphere can form.

So what determines whether a planet can get rid of its protoatmosphere? The dominant process for shedding a hydrogen atmosphere is thermal mass loss: as the planet’s upper atmosphere is heated by X-ray and extreme-ultraviolet (XUV) radiation from the host star, the envelope evaporates.

A Critical Dependence

In a recent study led by Colin Johnstone (University of Vienna), a team of scientists has developed models of this evaporation process for hydrogen planetary atmospheres. In particular, Johnstone and collaborators examine how the host star’s initial rotation rate — which strongly impacts the star’s level of XUV activity — affects the degree to which the planet’s hydrogen atmosphere is evaporated, and the rate at which the evaporation occurs.

The authors’ findings can be illustrated with the example of an Earth-mass planet located in the habitable zone of a solar-mass star. In this case, the authors find four interesting regimes (shown in the plot to the right):

  • Atmosphere evolution

    Evolution of the hydrogen protoatmosphere of an Earth-mass planet in the habitable zone of a solar-mass star. The four lettered cases describe different initial atmospheric masses. The three curves for each case describe the stellar rotation rate: slow (red), average (green), or fast (blue). [Johnstone et al. 2015]

    Case A
    (I    nitial atmospheric mass of 10-4 Earth masses)
    Entire atmosphere evaporates quickly, regardless of the rotation speed of the host star.
  • Case B
    (I    nitial atmospheric mass of 10-3 Earth masses)
    Entire atmosphere evaporates, but the timescale is much shorter if the stellar host is fast-rotating as opposed to slow-rotating.
  • Case C
    (Initial atmospheric mass of 10-2 Earth masses)
    If the stellar host is fast-rotating, entire atmosphere evaporates on a short timescale. If the host is slow-rotating, very little of the atmosphere evaporates.
  • Case D
    (Initial atmospheric mass of 10-1 Earth masses)
    Very little of the atmosphere evaporates, regardless of the rotation speed of the host star.

These results demonstrate that the initial rotation rate of a host star not only determines whether a planet will lose its protoatmosphere, but also how long this process will take. Thus, the evolution of host stars’ rotation rates is an important component in our understanding of how planets might evolve to become habitable.

Citation

C. P. Johnstone et al 2015 ApJ 815 L12. doi:10.1088/2041-8205/815/1/L12

Q2237+0305

What happens when light from a distant quasar — powered by a supermassive black hole — is bent not only by a foreground galaxy, but also by individual stars within that galaxy’s nucleus? The neighborhood of the central black hole can be magnified, and we get a close look at the inner regions of its accretion disk!

What is Microlensing?

Our view of Q2237+0305 is heavily affected by a process called gravitational lensing. As evidenced by the four copies of the quasar in the image above, Q2237+0305 undergoes macrolensing, wherein the gravity of a massive foreground galaxy pulls on the light of a background object, distorting the image into arcs or multiple copies.

But Q2237+0305 also undergoes an effect called microlensing. Due to the fortuitous alignment of Q2237+0305 with the nucleus of the foreground galaxy lensing it, stars within the foreground galaxy pass in front of the quasar images. As a star passes, its own gravitational pull also affects the light of the image, causing the image to brighten and/or magnify.

How can we tell the difference between intrinsic brightening of Q2237+0305 and brightening due to microlensing? Brightening that occurs in all four images of the quasar is intrinsic. But if the brightening occurs in only one image, it must be caused by microlensing of that image. The timescale of this effect, which depends on how quickly the foreground galaxy moves relative to the background quasar, is on the order of a few hundred days for Q2237+0305.

Resolving Structure

The light curve of a microlensed image can reveal information about the structure of the distant object. For this reason, a team of scientists led by Evencio Mediavilla (Institute of Astrophysics of the Canaries, University of La Laguna) has studied the light curves of three independent microlensing events of Q2237+0305 images.

double-peaked light curve

Average light curve of the three microlensing events near the peak brightness. The double-peaked structure may be due to light from the innermost region of the quasar’s accretion disk. [Mediavilla et al. 2015]

Mediavilla and collaborators find a two-peaked structure in the light curves. Modeling the data as a standard thin disk that has been lensed, the team shows that the light curve features are consistent with fine structure relatable to the region near the quasar’s central supermassive black hole. The authors find that the diameter of the fine structure is ~6 Schwarzschild radii, leading them to believe that this structure actually represents the innermost region of the accretion disk.

If the team’s models are correct, this represents the first direct measurement of the size of the innermost region of a quasar’s accretion disk. The authors encourage further monitoring of Q2237+0305: as stars within the dense foreground nucleus continue to pass in front of the quasar, many more microlensing events should be observable, allowing further analysis.

Citation

E. Mediavilla et al 2015 ApJ 814 L26. doi:10.1088/2041-8205/814/2/L26

1 … 89 90 91 92 93 … 97