Orbits of Ultra-Faint MW Satellites

Gaia Proper Motions and Orbits of the Ultra-Faint Milky Way Satellites

The second data release from the Gaia mission (DR2) provides a comprehensive and unprecedented picture of the motions of astronomical sources in the plane of the sky, extending from the solar neighborhood to the outer reaches of the Milky Way. I present proper motion measurements based on Gaia DR2 for 17 ultra-faint dwarf galaxies within 100 kpc of the Milky Way. I compile the spectroscopically-confirmed member stars in each dwarf bright enough for Gaia astrometry from the literature, producing member samples ranging from 2 stars in Triangulum II to 68 stars in Bootes I. From the spectroscopic member catalogs I estimate the proper motion of each system. I find good agreement with the proper motions derived by the Gaia collaboration for Bootes I and Leo I. The tangential velocities for 14 of the 17 dwarfs are determined to better than 50 km/s, more than doubling the sample of such measurements for Milky Way satellite galaxies. The orbital pericenters are well-constrained, with a median value of 38 kpc. Only one satellite, Tucana III, is on an orbit passing within 15 kpc of the Galactic center, suggesting that the remaining ultra-faint dwarfs are unlikely to have experienced severe tidal stripping. As a group, the ultra-faint dwarfs are on high-velocity, eccentric, retrograde trajectories, with nearly all of them having space motions exceeding 370 km/s. In the default low-mass Milky Way potential I assume, eight out of the 17 galaxies lack well-defined apocenters and appear likely to be on their first infall, indicating that the Milky Way mass may be larger than previously estimated or that many of the ultra-faint dwarfs are associated with the Magellanic Clouds. The median eccentricity of the ultra-faint dwarf orbits is 0.79, similar to the values seen in numerical simulations, but distinct from the rounder orbits of the more luminous dwarf spheroidals.

 

Lithopanspermia by interstellar objects

Planet seeding and lithopanspermia through gas-assisted capture of interstellar objects

Planet formation begins with collisional-growth of small planetesimals accumulating into larger-ones. Such growth occurs while planetesimals are embedded in a gaseous protoplanetary-disk. However, small-planetesimals experience collisions and gas-drag that lead to their destruction on short-timescales, not allowing, or requiring fine-tuned conditions for the efficient growth of ~meter-size planetesimals. Here we show that small (up-to 0.1-10 km-size) unbound interstellar-objects passing through a gaseous protoplanetary-disk can be efficiently captured to become embedded in the disk. ‘Seeding’ of such planetesimals then catalyze further planetary-growth into planetary embryos, and potentially alleviate the main-challenges with the meter-size growth-“barrier”. Moreover, planetesimal-capture provides a far-more efficient route for lithopanspermia than previously thought, and ∼104 interstellar objects such as the recently detected 1I/2017-U1 (‘Oumuamua) could have been captured, and become part of the young Solar System.

 

GAIA Data Release 2

European satellite reveals motions of more than 1 billion stars and shape of the Milky Way

By Daniel Clery |

“It’s like waiting for Christmas,” said Vasily Belokurov, an astronomer at the University of Cambridge in the United Kingdom last week. Today, the gifts arrived: the exact positions, motions, brightnesses, and colors of 1.3 billion stars in and around the Milky Way, as tracked by the European Space Agency’s (ESA’s) €750 million Gaia satellite, which after launch in 2013 began measuring the positions of stars and, over time, how they move. On 25 April, ESA made Gaia’s second data set—based on 22 months of observations—publicly available, which should enable a precise 3D map of large portions of the galaxy and the way it moves. “Nothing comes close to what Gaia will release,” Belokurov says.

One might think that the galaxy is completely mapped. But large parts of it are obscured by gas and dust, and it is hard to discern structure from the vantage of the solar system. Gaia is not only expected to clarify the spiral structures of the galaxy today, but because the satellite traces how stars move, astronomers can wind the clock backward and see how the galaxy evolved over the past 13 billion years—a field known as galactic archaeology. With Gaia’s color and brightness information, astronomers can classify the stars by composition and identify the stellar nurseries where different types were born, to understand how chemical elements were forged and distributed.

Gaia isn’t only about the Milky Way. For solar system scientists, the new data set will contain data on 14,000 asteroids. That’s a small fraction of the roughly 750,000 known minor bodies, but Gaia provides orbit information 100 times more accurate than before, says University of Cambridge astronomer Gerry Gilmore, who heads the U.K. branch of Gaia’s data processing consortium. That should help astronomers identify families of asteroids and trace how they relate to each other, shedding light on the solar system’s past and how planets formed from smaller bodies.

For cosmologists, the data set will improve distance measures to stars of known brightness such as Cepheid variables, crucial stepping stones that allow a “distance ladder” to be built out to other galaxies—so that the expansion rate of the universe, also known as the Hubble constant, can be calculated. And exoplanet hunters expect that Gaia will eventually see thousands of stars shifting from side to side because of the gravitational tugs of Jupiter-size planets in distant orbits, but these won’t emerge until the satellite’s precision improves in later data releases. “No one in the world knows what we’ll find,” says David Hogg of New York University in New York City.

The Gaia team released an initial catalog in 2016 and, although it contained more than a billion stars, it only provided motions for 2 million of them. It was a “sampler to get people used to handling Gaia-type data,” Gilmore says. The 2016 release showed that the Milky Way was larger in size than previously thought. The first paper exploiting the data appeared on the arXiv preprint server on the same day. Ever since, Gilmore says, there’s been an average of one paper per day.

This time, astronomers are even more geared up with algorithms that can crunch the tabular data. Belokurov says he and his group have about 50 ideas to pursue, including an assessment of the distribution of mass across the Milky Way and the Large Magellanic Cloud (LMC), a nearby satellite galaxy. Astronomers have long estimated the LMC’s mass at about a billion times that of the sun, but recently studies have suggested it may be heftier. With Gaia data, they may be able to see Milky Way objects that are perturbed by the LMC, which would be a sign of its more massive gravitational influence. “There’s going to be a complete science explosion,” Belokurov says. “I’m planning on not sleeping for a week or two.”

Hogg is also ready for some heavy-duty Gaia hacking. For the release, he invited colleagues from around the world to gather in New York City to work on analyzing the data. He plans to start by drawing up plots that were not possible previously, to look for new trends. Graphing color versus brightness for white dwarf stars, for example, could illuminate how these stellar remnants change as they cool off and eventually become black stellar cinders. Following Gaia’s first release, “almost every plot led to a paper,” he says.

The 450-strong Gaia consortium is already at work on a third data release, planned for 2020. “There are very clear areas we can improve,” says ESA’s Timo Prusti, Gaia project scientist, at the European Space Research and Technology Centre in Noordwijk, the Netherlands. For example, the team wants to return to the very brightest stars, which saturate the detector, in a special short-exposure observing mode. The team also wants to improve on ways to deal with stray light getting onto the detector, a problem which only emerged after launch.

Gaia is also unusual because the scientists who work on the mission are not given a period of exclusive access to the data, a common practice in astronomy. Although Gaia consortium members get to intimately know the way the data are collected and processed, they cannot use the data to do science until after the release, just like everyone else. “It’s brave and very admirable,” Belokurov says.

Gilmore says his team members have been laying bets on how many papers will hit the preprint servers on Day One. Belokurov says: “It’s like going to a festival—the festival of Gaia.”

Gaia Data Release 2. Summary of the contents and survey properties
Gaia Data Release 2: The astrometric solution
Gaia Data Release 2: processing of the photometric data
Gaia Data Release 2: Photometric content and validation
Gaia Radial Velocity Spectrometer
Gaia Data Release 2: The catalogue of radial velocity standard stars
Gaia Data Release 2: Processing the spectroscopic data
Gaia Data Release 2: Properties and validation of the radial velocities
Gaia Data Release 2: Summary of the variability processing & analysis results
Gaia Data Release 2: first stellar parameters from Apsis
Gaia Data Release 2: Catalogue validation
Gaia Data Release 2: using Gaia parallaxes
Gaia Data Release 2: The Celestial reference frame (Gaia-CRF2)
Gaia Data Release 2: Observational Hertzsprung-Russell diagrams
Gaia Data Release 2: Observations of solar system objects
Gaia Data Release 2: Mapping the Milky Way disc kinematics
Gaia Data Release 2: Kinematics of globular clusters and dwarf galaxies around the Milky Way
Gaia Data Release 2: Variable stars in the colour-absolute magnitude diagram
Gaia Data Release 2. Calibration and mitigation of electronic offset effects in the data
Estimating distances from parallaxes IV: Distances to 1.33 billion stars in Gaia Data Release 2
Radial Distribution of Stellar Motions in Gaia DR2
Revisiting hypervelocity stars after Gaia DR2
Wrinkles in the Gaia data unveil a dynamically young and perturbed Milky Way disk
A hypervelocity star with a Magellanic origin
Milky Way Cepheid Standards for Measuring Cosmic Distances and Application to Gaia DR2: Implications for the Hubble Constant
Gaia DR2 Gravitational Lens Systems I: New lensed quasar candidates around known quasars
Evidence for unresolved exoplanet-hosting binaries in Gaia DR2
Three Hypervelocity White Dwarfs in Gaia DR2: Evidence for Dynamically Driven Double-Degenerate Double-Detonation Type Ia Supernovae
STREAMFINDER I: A New Algorithm for detecting Stellar Streams
Ghostly Tributaries to the Milky Way: Charting the Halo’s Stellar Streams with the Gaia DR2 catalogue
One large blob and many streams frosting the nearby stellar halo in Gaia DR2
Evidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions
Gaia DR2 Distances and Peculiar Velocities for Galactic Black Hole Transients
Revised Radii of Kepler Stars and Planets using Gaia Data Release 2
Gaia Data Release 2: Rotational modulation in late-type dwarfs
Off the beaten path: Gaia reveals GD-1 stars outside of the main stream
LISA verification binaries with updated distances from Gaia Data Release 2
Gaia Data Release 2: The Short Timescale Variability Processing and Analysis
Gaia DR2 Proper Motions of Dwarf Galaxies within 420 kpc: Orbits, Milky Way Mass, Tidal Influences, Planar Alignments, and Group Infall
A Late-type L Dwarf at 11 pc Hiding in the Galactic Plane Characterized Using Gaia DR2
Isochrone fitting in the Gaia era. II. Distances, ages and masses from UniDAM using Gaia DR2 data
With and without spectroscopy: Gaia DR2 proper motions of seven Ultra-Faint Dwarf Galaxies
Gaia Data Release 2: The first Gaia catalog of Long Period Variable candidates
Gaia Data Release 2: Specific characterisation and validation of all-sky Cepheids and RR Lyrae stars
The properties of the known QSOs astrometric solutions in Gaia DR2
A Gaia DR2 search for dwarf galaxies towards Fermi-LAT sources: implications for annihilating dark matter
In disguise or out of reach: first clues about in-situ and accreted stars in the stellar halo of the Milky Way from Gaia DR2
An independent confirmation of the future flyby of Gliese 710 to the solar system using Gaia DR2
Confirmation of the zero-point offset in Gaia Data Release 2 parallaxes using asteroseismology and APOGEE spectroscopy in the Kepler field
An Ultraviolet-Optical Color-Metallicity relation for Red Clump Stars using GALEX and Gaia
On the Gaia DR2 distances for Galactic Luminous Blue Variables
Unidentified quasars among stationary objects from Gaia DR2
Evidence for a systematic offset of -80 micro-arcseconds in the Gaia DR2 parallaxes
The Galactic Disc in Action Space as seen by Gaia DR2
The sdA problem – III. Hundreds of new extremely low-mass white dwarfs and their precursors from Gaia astrometry
First Gaia Dynamics of the Andromeda System: DR2 Proper Motions, Orbits, and Rotation of M31 and M33
Validating TGAS wide binaries with Gaia DR2 Radial Velocities and Parallaxes
Gaia and the Galactic Center Origin of Hypervelocity Stars
Discovery of the most ultra-luminous QSO using Gaia, SkyMapper and WISE
Anisotropy of the Milky Way’s stellar halo using K giants from LAMOST and Gaia

 

Radio Background Below 100 MHz

The Radio Background Below 100 MHz

The recent detection of the “cosmic dawn” redshifted 21 cm signal at 78 MHz by the EDGES experiment differs significantly from theoretical predictions. In particular, the absorption trough is roughly a factor of two stronger than the most optimistic theoretical models. The early interpretations of the origin of this discrepancy fall into two categories. The first is that there is increased cooling of the gas due to interactions with dark matter, while the second is that the background radiation field includes a contribution from a component in addition to the cosmic microwave background. In this paper we examine the feasibility of the second idea using new data from the first station of the Long Wavelength Array. The data span 40 to 80 MHz and provide important constraints on the present-day background in a frequency range where there are few surveys with absolute temperature calibration suitable for measuring the strength of the radio monopole. We find support for a strong, diffuse radio background that was suggested by the ARCARDE 2 results in the 3 to 10 GHz range. We find that this background is well modeled by a power law with a spectral index of -2.58±0.05 and a temperature at the rest frame 21 cm frequency of 603+102-92 mK.

 

Dipole Anisotropy of Cosmic-Dawn

Dipole Anisotropy as an Essential Qualifier for the Monopole Component of the Cosmic-Dawn Spectral Signature

While the importance of detecting the Global spectral signatures of the red-shifted 21-cm line of atomic hydrogen from the very early epochs cannot be overstated, the associated challenges are not limited to isolating the weak signal of interest from the orders of magnitude brighter foregrounds, and extend equally to reliably establishing the origin of the ‘apparent’ global signal to the very early epochs. This letter, aimed to tackle the latter, proposes a critical test that the measurements of the monopole component of the spectrum of interest should necessarily pass. The criterion is based on a unique correspondence between the intrinsic monopole spectrum and the differential spectrum as an imprint of dipole anisotropy resulting from motion of observer with respect to the rest frame of our source (such as that of our Solar system, interpreted from the dipole anisotropy in CMBR). More importantly, the spectral manifestation of the dipole anisotropy gets ‘amplified’ by a significant factor, depending on the monopole spectral slopes, rendering it feasible to measure. We describe details of such a test, and illustrate its application with the help of simulations. Such dipole qualifier for the monopole spectrum is expected to pave way for in situ validation of spectral signatures from early epochs, important to presently reported and future detections of EoR signal.

 

Angular momentum-mass relation

The angular momentum-mass relation: a fundamental law from dwarf irregulars to massive spirals

Specific stellar angular momentum – stellar mass relation for a sample of 92 nearby disc galaxies. The specific angular momentum is defined as the angular momentum per mass.

In a ΛCDM Universe, the specific stellar angular momentum (j*) and stellar mass (M*) of a galaxy are correlated as a consequence of the scaling existing for dark matter haloes (jh∝Mh2/3). The shape of this law is crucial to test galaxy formation models, which are currently discrepant especially at the lowest masses, allowing to constrain fundamental parameters, e.g. the retained fraction of angular momentum. In this study, we accurately determine the empirical j*-M* relation (Fall relation) for 92 nearby spiral galaxies (from S0 to Irr) selected from the Spitzer Photometry and Accurate Rotation Curves (SPARC) sample in the unprecedented mass range 7 ≲ log M*/M ≲ 11.5. We significantly improve all previous estimates of the Fall relation by determining j* profiles homogeneously for all galaxies, using extended HI rotation curves, and selecting only galaxies for which a robust j* could be measured (converged j*(<R) radial profile). We find the relation to be well described by a single, unbroken power-law j*∝M*α over the entire mass range, with α=0.55±0.02 and orthogonal intrinsic scatter of 0.17±0.01 dex. We finally discuss some implications for galaxy formation models of this fundamental scaling law and, in particular, the fact that it excludes models in which discs of all masses retain the same fraction of the halo angular momentum.

Denne ubrudte relation for skivegalakser over over et masseområde svarende til en faktor 100000 kan sandsynligvis forklare den tætte relation, som man finder mellem den radiale acceleration fra baryoner (stjerner) og den totale radiale acceleration fra baryoner og den mørke halo.

 

TESS’s search for Earthlike exoplanets

NASA’s new satellite brings the search for Earthlike exoplanets closer to home

By Daniel Clery | 

Thanks to NASA’s pioneering Kepler probe, we know our galaxy is teeming with exoplanets. Now, a new generation of exoplanet hunters is set to home in on rocky worlds closer to home.

Over 9 years in space, Kepler has found more than 2600 confirmed exoplanets, implying hundreds of billions in the Milky Way. The new efforts sacrifice sheer numbers and target Earth-size planets whose composition, atmosphere, and climate—factors in whether they might be hospitable to life—could be studied. Leading the charge is the Transiting Exoplanet Survey Satellite (TESS), a NASA mission due for launch on 16 April.

The brainchild of researchers at the Massachusetts Institute of Technology (MIT) in Cambridge, the $337 million TESS project aims to identify at least 50 rocky exoplanets—Earth-size or bigger—close enough for their atmospheres to be scrutinized by the much larger James Webb Space Telescope (JWST), due for launch in 2020. “Where do we point Webb?” TESS Principal Investigator George Ricker asked rhetorically at the American Astronomical Society annual meeting at National Harbor in Maryland in January. “This is the finder scope.”

Like Kepler, TESS finds planets by staring at stars and looking for a dip in brightness as a planet passes in front, blocking some of the star’s light in a so-called transit. But whereas Kepler kept a fixed view, watching just 0.25% of the sky out to a distance of 3000 lightyears, TESS will maneuver to observe 85% of it, out to about 300 light-years.

The spacecraft carries four telescopes that together will survey a strip of sky extending from the solar system’s pole to its equator, known as the ecliptic. The scopes will watch a strip for 27 days, then shift sideways and repeat the process. After observing 13 such strips over a year, covering almost an entire hemisphere of sky, TESS will flip over and survey the other hemisphere.

Over 2 years, TESS should measure the brightness of some 2 million stars, says project scientist Stephen Rinehart of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “If there is one planet per star [as Kepler predicts], we will see many. It’ll be a firehose of data.”

TESS’s primary targets are red dwarf stars, the most common stars in our neighborhood. Red dwarfs weigh less than half as much as the sun, so they do not burn brightly, offering several advantages to exoplanet hunters. A planet passing in front of a small, dim red dwarf blocks more of its light, yielding a stronger transit signal. Moreover, planets can whip around red dwarfs in orbits closer than Mercury’s, and still have hospitable climates. More orbits mean more opportunities for transit detections. TESS researchers are targeting speedsters that would circle the star at least twice during a 27-day TESS watch. Spotting two transits is key because it tells astronomers the length of the planet’s orbit. Other features of the transit—its duration, how much light is blocked, and how quickly the brightness dips—provide additional details such as the planet’s diameter.

Transits don’t reveal a planet’s mass, however, which is vital to determining its density—a clue to whether it is made of iron, rock, or ice. For this, TESS is relying on follow-up studies by ground-based telescopes, which can watch for tiny periodic Doppler shifts in the frequency of a star’s light caused by an orbiting planet tugging on it. The shift is a clue to the planet’s mass. Of the 5000 transitlike signals that the TESS team expects to detect, the clearest will be chosen for ground-based follow-up, says MIT’s Sara Seager, the mission’s deputy science director. The aim is to identify and weigh 50 planets to serve up to the JWST.

Although detecting planets around red dwarfs is easier, life may be less likely to arise there. Red dwarfs are erratic, prone to blasts of lethal radiation, and because the planets are so close, “they feel the effects of the star,” says astronomer Elisa Quintana of NASA Goddard. Close-in planets are also likely to be “tidally locked,” with one side always facing the star in an eternal scorching day while the other side freezes in an endless night. “Can they be habitable?” Quintana asks. “The debate goes back more than 10 years.”

Later this year, the European Space Agency will launch another eye on exoplanets: the Characterising Exoplanets Satellite. Rather than searching for new worlds, it will take a second, much longer look at transits of known planets to pin down their sizes more precisely. In combination with mass measurements from the ground, that should provide a better fix on planets’ densities.

Also debuting in the next few months is a ground-based search in Chile: SPECULOOS, the search for habitable planets eclipsing ultracool stars. The project’s four 1-meter telescopes have near-infrared sensors to detect transits of the very dimmest, coolest stars; a similar array in the Canary Islands will survey the northern sky. These stars are too faint for TESS’s small telescopes to see, but they could give the JWST valuable targets, says Michaël Gillon of the University of Liège in Belgium, which is leading the project.

SPECULOOS may be especially sensitive to small planets, because even small bodies will block noticeable amounts of light from the dim target stars. “TESS will find many more planets, but in the temperate—and potentially habitable—Earth-size regime, SPECULOOS’s detection potential should be significantly better,” Gillon says. “The next years are going to be very exciting!”

 

The Excited Spin State of `Oumuamua

The Excited Spin State of 1I/2017 U1 `Oumuamua

We show that `Oumuamua’s excited spin could be in a high energy Long Axis Mode (LAM) state, which implies that its shape could be far from the highly elongated shape found in previous studies. CLEAN and ANOVA algorithms are used to analyze `Oumuamua’s lightcurve using 818 observations over 29.3 days. Two fundamental periodicities are found at frequencies (2.77±0.11) and (6.42±0.18) cycles/day, corresponding to (8.67±0.34) h and (3.74±0.11) h, respectively. The phased data show that the lightcurve does not repeat in a simple manner, but approximately shows a double minimum at 2.77 cycles/day and a single minimum at 6.42 cycles/day. This is characteristic of an excited spin state. `Oumuamua could be spinning in either the long (LAM) or short (SAM) axis mode. For both, the long axis precesses around the total angular momentum vector (TAMV) with an average period of (8.67±0.34) h. For the three LAMs we have found, the possible rotation periods around the long axis are 6.58, 13.15, or 54.48 h, with 54.48 h being the most likely. `Oumuamua may also be nutating with respective periods of half of these values. We have also found two possible SAM states where `Oumuamua oscillates around the long axis with possible periods at 13.15 and 54.48 h, the latter as the most likely. In this case any nutation will occur with the same periods. Determination of the spin state, the amplitude of the nutation, the direction of the TAMV, and the average total spin period may be possible with a direct model fit to the lightcurve. We find that ‘Oumuamua is “cigar-shaped”, if close to its lowest rotational energy, and an extremely “oblate spheroid” if close to its highest energy state for its total angular momentum.

 

The Atmosphere of Neoproterozoic Earth

Exploring the Atmosphere of Neoproterozoic Earth: The Effect of O2 on Haze Formation and Composition

Previous studies of haze formation in the atmosphere of the Early Earth have focused on N2/CO2/CH4 atmospheres. Here, we experimentally investigate the effect of O2 on the formation and composition of aerosols to improve our understanding of haze formation on the Neoproterozoic Earth. We obtained in situ size, particle density, and composition measurements of aerosol particles produced from N2/CO2/CH4/O2 gas mixtures subjected to FUV radiation (115-400 nm) for a range of initial CO2/CH4/O2 mixing ratios (O2 ranging from 2 ppm to 0.2%). At the lowest O2 concentration (2 ppm), the addition increased particle production for all but one gas mixture. At higher oxygen concentrations (20 ppm and greater) particles are still produced, but the addition of O2 decreases the production rate. Both the particle size and number density decrease with increasing O2, indicating that O2 affects particle nucleation and growth. The particle density increases with increasing O2. The addition of CO2 and O2 not only increases the amount of oxygen in the aerosol, but it also increases the degree of nitrogen incorporation. In particular, the addition of O2 results in the formation of nitrate bearing molecules. The fact that the presence of oxygen bearing molecules increases the efficiency of nitrogen fixation has implications for the role of haze as a source of molecules required for the origin and evolution of life. The composition changes also likely affect the absorption and scattering behavior of these particles but optical properties measurements are required to fully understand the implications for the effect on the planetary radiative energy balance and climate.

 

Heating of the intergalactic medium

Heating of the intergalactic medium by the cosmic microwave background during cosmic dawn

The intergalactic medium is expected to be at its coldest point before the formation of the first stars in the universe. Motivated by recent results from the EDGES experiment, we revisit the standard calculation of the kinetic temperature of the neutral gas through this period. When the first ultraviolet (UV) sources turn on, photons redshift into the Lyman lines of neutral hydrogen and repeatedly scatter within the Lyman-α line. They heat the gas via atomic recoils, and, through the Wouthuysen-Field effect, set the spin temperature of the 21-cm hyperfine (spin-flip) line of atomic hydrogen in competition with the resonant cosmic microwave background (CMB) photons. We show that the Lyman-α photons also mediate energy transfer between the CMB photons and the thermal motions of the hydrogen atoms. In the absence of X-ray heating, this new mechanism is the major correction to the temperature of the adiabatically cooling gas (∼10% at z=17), and is several times the size of the heating rate found in previous calculations. We also find that the effect is more dramatic in non-standard scenarios that either enhance the radio background above the CMB or invoke new physics to cool the gas in order to explain the EDGES results. The coupling with the radio background can reduce the depth of the 21-cm absorption feature by almost a factor of two relative to the case with no sources of heating, and prevent the feature from developing a flattened bottom. As an inevitable consequence of the UV background that generates the absorption feature, this heating should be accounted for in any theoretical prediction.