Quasars
Quasars are superluminous objects powered by active supermassive black holes at the centers of distant galaxies. Although the supermassive black holes are only about the size of our solar system across, as they pull in material from their host galaxy, they can emit hundreds of times more energy than all the stars in that galaxy. The light we observe from quasars originates mostly from material that is superheated as it spirals in towards the central black hole, but also sometimes from winds or jets of high velocity gas.
Quasars are so luminous that they can be observed at vast distances from Earth. As we understand Big Bang cosmology, space is expanding, so that the more distant a quasar is from us, the faster it is moving away from us. That speed, called the recession velocity, stretches out the light waves on their journey through space, causing the wavelength to increase, making the light redder. This “redshift” can be easily translated into a distance measurement of a quasar, but also means that at different redshifts, different parts of a quasar's intrinsic spectrum are measurable in the optical bandpass.
Quasars are variable, normally displaying random, not periodic, brightness changes. Indeed, variability is one of the best ways to discover quasars, since only a small fraction of stars are variable.
Below, we offer sonified videos of three typical quasars, each at a different redshift. Different emission lines begin to appear in spectra towards higher redshift, because intrinsic ultraviolet light is redshifted in the optical range of these spectra as observed on Earth.
Quasar (z = 0.5)
Below is a sonified video of observations of a quasar with a redshift (z) value of approximately 0.5. There are 184 observations taken over about 4 years in the video and it lasts almost 45 seconds, hence each beat of time corresponds to 4 days in real time. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes. During the almost 4 year span, starting in early 2018 and ending in late 2021, the quasar steps up gradually and constantly in brightness, with some shorter peaks and valleys interspersed. All told, its apparent magnitude brightens by about 0.35 magnitudes. Seasonal gaps during each observation year can also be heard, when the quasar wasn’t visible from Earth.
Below is the spectrum for our observed quasar with a z-value of 0.5. This video scans across a plot of brightness measured in flux or intensity of light (y axis) versus wavelength (x axis), moving from blue to red wavelengths from 3600 to 10000 angstroms. Lower pitch represents weaker flux. During the video, the spectrum decreases gradually in flux (pitch) as wavelength increases. To understand the intensity of the redshift that quasars experience, listen for the H-alpha emission line – by far the most powerful in the video. Usually sitting at 6563 Angstroms, for this redshifted quasar the line is seen at around 9800 Angstroms.
This quasar, J3491757.48-103350.04 was targeted for SDSS-IV spectroscopy. The light curve is an optical r-band from the ZTF.
Quasar (z = 1.0)
Below is a sonified video of observations of a quasar with z = 1.0. The 4 year duration of observations, as well as the time base of 4 days, matches the dataset above. In this case, there are 150 observations over the time span. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes. In 2018 the quasar remained relatively constant in brightness, followed by a steep dimming event in 2019 of about 0.2. Next, a seasonal gap in observations can be heard, before a rapid 0.35 magnitude increase in brightness to a level even brighter than the 2018 observations.
Shown next is the spectrum for our observed quasar with a z-value of 1.0. This video scans across a plot of brightness measured in flux or intensity of light (y axis) versus wavelength (x axis), moving from blue to red wavelengths from 3600 to 10000 angstroms. Lower pitch represents weaker flux. During the video, the spectrum decreases gradually in flux (pitch) as wavelength increases. This spectrum is redshifted even further, and the H-alpha line is no longer visible within the observed wavelength range.
This quasar, J3582027.20-011528.12 was targeted for SDSS-IV spectroscopy. The light curve is an optical r-band from the ZTF.
Quasar (z = 2.0)
Below is a sonified video of observations of a quasar with a redshift z-value of 2.0. There are 245 observations of this quasar taken over the same nearly 4 year duration. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes. In 2018 and 2019 the quasar remained relatively constant in brightness, before stepping up about 0.3 magnitudes in brightness and remaining there during 2020 and 2021. Seasonal gaps during each observation year can also be heard, when the quasar wasn’t visible from Earth.
Shown next is the spectrum for our observed quasar with a z-value of 2.0. This video scans across a plot of brightness measured in flux or intensity of light (y axis) versus wavelength (x axis), moving from blue to red wavelengths from 3600 to 10000 angstroms. Lower pitch represents weaker flux. During the video, the spectrum decreases gradually in flux (pitch) as wavelength increases.
This quasar, J3491757.48-103350.04 was targeted for SDSS-IV spectroscopy. The light curve is an optical r-band from the ZTF.
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