Dragonfly 44: A potential Dark Matter Galaxy

Title: A High Stellar Velocity Dispersion and ~100 Globular Clusters for the Ultra Diffuse Galaxy Dragonfly 44

PublicationApJ, v828, Number 1, arXiv: 1606.06291

The title of this paper sounds like some standard astrophysics analyses; but, dig a little deeper and you’ll find – what I think – is an incredibly interesting, surprising and unexpected observation.

The Coma Cluster: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Last year, using the WM Keck Observatory and the Gemini North Telescope in Manuakea, Hawaii, the Dragonfly Telephoto Array observed the Coma cluster (a large cluster of galaxies in the constellation Coma – I’ve included a Hubble Image to the left). The team identified a population of large, very low surface brightness (ie: not a lot of stars), spheroidal galaxies around an Ultra Diffuse Galaxy (UDG) called Dragonfly 44 (shown below). They determined that Dragonfly 44 has so few stars that gravity could not hold it together – so some other matter had to be involved – namely DARK MATTER (my favorite kind of unknown matter).

 

The ultra-diffuse galaxy Dragonfly 44. The galaxy consists almost entirely of dark matter. It is surrounded by faint, compact sources. Image credit: Pieter van Dokkum / Roberto Abraham / Gemini Observatory / SDSS / AURA.
The ultra-diffuse galaxy Dragonfly 44. The galaxy consists almost entirely of dark matter. It is surrounded by faint, compact sources. Image credit: Pieter van Dokkum / Roberto Abraham / Gemini Observatory / SDSS / AURA

The team used the DEIMOS instrument installed on Keck II to measure the velocities of stars for 33.5 hours over a period of six nights so they could determine the galaxy’s mass. Observations of Dragonfly 44’s rotational speed suggest that it has a mass of about one trillion solar masses, about the same as the Milky Way. However, the galaxy emits only 1% of the light emitted by the Milky Way. In other words, the Milky Way has more than a hundred times more stars than Dragonfly 44. I’ve also included the Mass-to-Light ratio plot vs. the dynamical mass. This illustrates how unique Dragonfly 44 is compared to other dark matter dominated galaxies like dwarf spheroidal galaxies.

 

 

MLratio
Relation between dynamical mass-to-light ratio and dynamical mass. Open symbols are dispersion-dominated objects from Zaritsky, Gonzalez, & Zabludoff (2006) and Wolf et al. (2010). The UDGs VCC 1287 (Beasley et al. 2016) and Dragonfly 44 fall outside of the band defined by the other galaxies, having a very high M/L ratio for their mass.

What is particularly exciting is that we don’t understand how galaxies like this form.

Their research indicates that these UDGs could be failed galaxies, with the sizes, dark matter content, and globular cluster systems of much more luminous objects. But we’ll need to discover more to fully understand them.

 

 

 

 

 

 

 

 

Further reading (works by the same authors)
Forty-Seven Milky Way-Sized, Extremely Diffuse Galaxies in the Coma Cluster,arXiv: 1410.8141
Spectroscopic Confirmation of the Existence of Large, Diffuse Galaxies in the Coma Cluster: arXiv: 1504.03320

Searching for Magnetic Monopoles with MoEDAL

Article: Search for magnetic monopoles with the MoEDAL prototype trapping detector in 8 TeV proton-proton collisions at the LHC
Authors: The ATLAS Collaboration
Reference:  arXiv:1604.06645v4 [hep-ex]

Somewhere in a tiny corner of the massive LHC cavern, nestled next to the veteran LHCb detector, a new experiment is coming to life.

The Monopole & Exotics Detector at the LHC, nicknamed the MoEDAL experiment, recently published its first ever results on the search for magnetic monopoles and other highly ionizing new particles. The data collected for this result is from the 2012 run of the LHC, when the MoEDAL detector was still a prototype. But it’s still enough to achieve the best limit to date on the magnetic monopole mass.

Figure 1: Breaking a magnet.

Magnetic monopoles are a very appealing idea. From basic electromagnetism, we expect to swap electric and magnetic fields under duality without changing Maxwell’s equations. Furthermore, Dirac showed that a magnetic monopole is not inconsistent with quantum electrodynamics (although they do not appear natually.) The only problem is that in the history of scientific experimentation, we’ve never actually seen one. We know that if we break a magnet in half, we will get two new magnetics, each with its own North and South pole (see Figure 1).

This is proving to be a thorn in the side of many physicists. Finding a magnetic monopole would be great from a theoretical standpoint. Many Grand Unified Theories predict monopoles as a natural byproduct of symmetry breaking in the early universe. In fact, the theory of cosmological inflation so confidently predicts a monopole that its absence is known as the “monopole problem”. There have been occasional blips of evidence for monopoles in the past (such as a single event in a detector), but nothing has been reproducible to date.

Enter MoEDAL (Figure 2). It is the seventh addition to the LHC family, having been approved in 2010. If the monopole is a fundamental particle, it will be produced in proton-proton collisions. It is also expected to be very massive and long-lived. MoEDAL is designed to search for such a particle with a three-subdetector system.

Figure 2: The MoEDAL detector.
Figure 2: The MoEDAL detector.

The Nuclear Track Detector is composed of plastics that are damaged when a charged particle passes through them. The size and shape of the damage can then be observed with an optical microscope. Next is the TimePix Radiation Monitor system, a pixel detector which absorbs charge deposits induced by ionizing radiation. The newest addition is the Trapping Detector system, which is simply a large aluminum volume that will trap a monopole with its large nuclear magnetic moment.

The collaboration collected data using these distinct technologies in 2012, and studied the resulting materials and signals. The ultimate limit in the paper excludes spin-0 and spin-1/2 monopoles with masses between 100 GeV and 3500 GeV, and a magnetic charge > 0.5gD (the Dirac magnetic charge). See Figures 3 and 4 for the exclusion curves. It’s worth noting that this upper limit is larger than any fundamental particle we know of to date. So this is a pretty stringent result.

Figure 3: Cross-section upper limits at 95% confidence level for DY spin-1/2 monopole production as a function of mass, with different charge models.
Figure 3: Cross-section upper limits at 95% confidence level for DY spin-1/2 monopole production as
a function of mass, with different charge models.
Figure 4: Cross-section upper limits at 95% confidence level for DY spin-1/2 monopole production as a function of charge, with different mass models.
Figure 4: Cross-section upper limits at 95% confidence level for DY spin-1/2 monopole production as
a function of charge, with different mass models.

 

As for moving forward, we’ve only talked about monopoles here, but the physics programme for MoEDAL is vast. Since the detector technology is fairly broad-based, it is possible to find anything from SUSY to Universal Extra Dimensions to doubly charged particles. Furthermore, this paper is only published on LHC data from September to December of 2012, which is not a whole lot. In fact, we’ve collected over 25x that much data in this year’s run alone (although this detector was not in use this year.) More data means better statistics and more extensive limits, so this is definitely a measurement that will be greatly improved in future runs. A new version of the detector was installed in 2015, and we can expect to see new results within the next few years.

 

Further Reading:

  1. CERN press release 
  2. The MoEDAL collaboration website 
  3. “The Phyiscs Programme of the MoEDAL experiment at the LHC”. arXiv.1405.7662v4 [hep-ph]
  4. “Introduction to Magnetic Monopoles”. arxiv.1204.30771 [hep-th]
  5. Condensed matter physics has recently made strides in the study of a different sort of monopole; see “Observation of Magnetic Monopoles in Spin Ice”, arxiv.0908.3568 [cond-mat.dis-nn]

 

The CMB sheds light on galaxy clusters: Observing the kSZ signal with ACT and BOSS

Article: Detection of the pairwise kinematic Sunyaev-Zel’dovich effect with BOSS DR11 and the Atacama Cosmology Telescope
Authors: F. De Bernardis, S. Aiola, E. M. Vavagiakis, M. D. Niemack, N. Battaglia, and the ACT Collaboration
Reference: arXiv:1607.02139

Editor’s note: this post is written by one of the students involved in the published result.

Like X-rays shining through your body can inform you about your health, the cosmic microwave background (CMB) shining through galaxy clusters can tell us about the universe we live in. When light from the CMB is distorted by the high energy electrons present in galaxy clusters, it’s called the Sunyaev-Zel’dovich effect. A new 4.1σ measurement of the kinematic Sunyaev-Zel’dovich (kSZ) signal has been made from the most recent Atacama Cosmology Telescope (ACT) cosmic microwave background (CMB) maps and galaxy data from the Baryon Oscillation Spectroscopic Survey (BOSS). With steps forward like this one, the kinematic Sunyaev-Zel’dovich signal could become a probe of cosmology, astrophysics and particle physics alike.

The Kinematic Sunyaev-Zel’dovich Effect

It rolls right off the tongue, but what exactly is the kinematic Sunyaev-Zel’dovich signal? Galaxy clusters distort the cosmic microwave background before it reaches Earth, so we can learn about these clusters by looking at these CMB distortions. In our X-ray metaphor, the map of the CMB is the image of the X-ray of your arm, and the galaxy clusters are the bones. Galaxy clusters are the largest gravitationally bound structures we can observe, so they serve as important tools to learn more about our universe. In its essence, the Sunyaev-Zel’dovich effect is inverse-Compton scattering of cosmic microwave background photons off of the gas in these galaxy clusters, whereby the photons gain a “kick” in energy by interacting with the high energy electrons present in the clusters.

The Sunyaev-Zel’dovich effect can be divided up into two categories: thermal and kinematic. The thermal Sunyaev-Zel’dovich (tSZ) effect is the spectral distortion of the cosmic microwave background in a characteristic manner due to the photons gaining, on average, energy from the hot (~107 – 108 K) gas of the galaxy clusters. The kinematic (or kinetic) Sunyaev-Zel’dovich (kSZ) effect is a second-order effect—about a factor of 10 smaller than the tSZ effect—that is caused by the motion of galaxy clusters with respect to the cosmic microwave background rest frame. If the CMB photons pass through galaxy clusters that are moving, they are Doppler shifted due to the cluster’s peculiar velocity (the velocity that cannot be explained by Hubble’s law, which states that objects recede from us at a speed proportional to their distance). The kinematic Sunyaev-Zel’dovich effect is the only known way to directly measure the peculiar velocities of objects at cosmological distances, and is thus a valuable source of information for cosmology. It allows us to probe megaparsec and gigaparsec scales – that’s around 30,000 times the diameter of the Milky Way!

A schematic of the Sunyaev-Zel’dovich effect resulting in higher energy (or blue shifted) photons of the cosmic microwave background (CMB) when viewed through the hot gas present in galaxy clusters. Source: UChicago Astronomy.

 

Measuring the kSZ Effect

To make the measurement of the kinematic Sunyaev-Zel’dovich signal, the Atacama Cosmology Telescope (ACT) collaboration used a combination of cosmic microwave background maps from two years of observations by ACT. The CMB map used for the analysis overlapped with ~68000 galaxy sources from the Large Scale Structure (LSS) DR11 catalog of the Baryon Oscillation Spectroscopic Survey (BOSS). The catalog lists the coordinate positions of galaxies along with some of their properties. The most luminous of these galaxies were assumed to be located at the centers of galaxy clusters, so temperature signals from the CMB map were taken at the coordinates of these galaxy sources in order to extract the Sunyaev-Zel’dovich signal.

While the smallness of the kSZ signal with respect to the tSZ signal and the noise level in current CMB maps poses an analysis challenge, there exist several approaches to extracting the kSZ signal. To make their measurement, the ACT collaboration employed a pairwise statistic. “Pairwise” refers to the momentum between pairs of galaxy clusters, and “statistic” indicates that a large sample is used to rule out the influence of unwanted effects.

Here’s the approach: nearby galaxy clusters move towards each other on average, due to gravity. We can’t easily measure the three-dimensional momentum of clusters, but the average pairwise momentum can be estimated by using the line of sight component of the momentum, along with other information such as redshift and angular separations between clusters. The line of sight momentum is directly proportional to the measured kSZ signal: the microwave temperature fluctuation which is measured from the CMB map. We want to know if we’re measuring the kSZ signal when we look in the direction of galaxy clusters in the CMB map. Using the observed CMB temperature to find the line of sight momenta of galaxy clusters, we can estimate the mean pairwise momentum as a function of cluster separation distance, and check to see if we find that nearby galaxies are indeed falling towards each other. If so, we know that we’re observing the kSZ effect in action in the CMB map.

For the measurement quoted in their paper, the ACT collaboration finds the average pairwise momentum as a function of galaxy cluster separation, and explores a variety of error determinations and sources of systematic error. The most conservative errors based on simulations give signal-to-noise estimates that vary between 3.6 and 4.1.

The mean pairwise momentum estimator and best fit model for a selection of 20000 objects from the DR11 Large Scale Structure catalog, plotted as a function of comoving separation. The dashed line is the linear model, and the solid line is the model prediction including nonlinear redshift space corrections. The best fit provides a 4.1σ evidence of the kSZ signal in the ACTPol-ACT CMB map. Source: arXiv:1607.02139.
The mean pairwise momentum estimator and best fit model for a selection of 20000 objects from the DR11 Large Scale Structure catalog, plotted as a function of comoving separation. The dashed line is the linear model, and the solid line is the model prediction including nonlinear redshift space corrections. The best fit provides a 4.1σ evidence of the kSZ signal in the ACTPol-ACT CMB map. Source: arXiv:1607.02139.

The ACT and BOSS results are an improvement on the 2012 ACT detection, and are comparable with results from the South Pole Telescope (SPT) collaboration that use galaxies from the Dark Energy Survey. The ACT and BOSS measurement represents a step forward towards improved extraction of kSZ signals from CMB maps. Future surveys such as Advanced ACTPol, SPT-3G, the Simons Observatory, and next-generation CMB experiments will be able to apply the methods discussed here to improved CMB maps in order to achieve strong detections of the kSZ effect. With new data that will enable better measurements of galaxy cluster peculiar velocities, the pairwise kSZ signal will become a powerful probe of our universe in the years to come.

Implications and Future Experiments

One interesting consequence for particle physics will be more stringent constraints on the sum of the neutrino masses from the pairwise kinematic Sunyaev-Zel’dovich effect. Upper bounds on the neutrino mass sum from cosmological measurements of large scale structure and the CMB have the potential to determine the neutrino mass hierarchy, one of the next major unknowns of the Standard Model to be resolved, if the mass hierarchy is indeed a “normal hierarchy” with ν3 being the heaviest mass state. If the upper bound of the neutrino mass sum is measured to be less than 0.1 eV, the inverted hierarchy scenario would be ruled out, due to there being a lower limit on the mass sum of ~0.095 eV for an inverted hierarchy and ~0.056 eV for a normal hierarchy.

Forecasts for kSZ measurements in combination with input from Planck predict possible constraints on the neutrino mass sum with a precision of 0.29 eV, 0.22 eV and 0.096 eV for Stage II (ACTPol + BOSS), Stage III (Advanced ACTPol + BOSS) and Stage IV (next generation CMB experiment + DESI) surveys respectively, with the possibility of much improved constraints with optimal conditions. As cosmic microwave background maps are improved and Sunyaev-Zel’dovich analysis methods are developed, we have a lot to look forward to.

 

Background reading: