2024-08-06 12:40:46
To calculate the distance to the galaxy, we need to measure the speed of the movement – and thus the reddening of the light. In principle, this is easy to do if we can measure the spectrum of the galaxy – the dependence of the light on the wavelength. It is then sufficient to identify a few spectral lines and, knowing their true wavelengths, we can determine the shift.
However, measuring the spectrum requires a lot of time and observational resources, so it can only be done for a small number of galaxies. It is much easier to measure the galactic light in different broad sections of the spectrum, which are called photometric bands. But how do you relate such rough data to the actual distance of the galaxy?
The most accurate calibration of this connection is now presented. For the analysis, the researchers selected 230 thousand galaxies that fell into the observation field of two relatively new surveys. The first overview – of the Dark Energy Spectroscopic Instrument. Dark Energy Spectroscopic Instrument, DESI), which is intended to measure the spectrum of almost 50 million galaxies. The second is “KiDS-VIKING”, with which many galaxies are observed photometrically.
After identifying galaxies that match each other between the two data sets, the researchers correlated their positions in color space with the spectroscopically measured redshift.
Color space is a set of parameters that indicate the values of several colors of a galaxy, and color in astronomy is called the difference between the brightness of an object in two adjacent photometric bands. KiDS-VIKING uses nine bands, resulting in eight colors.
Differences in the colors of different galaxies include both their redshift due to the expansion of the Universe and physical differences due to their size, mass, direction of rotation, and star-forming history.
However, redshift appears to be the dominant effect: the typical deviation between redshift estimated from galaxy colors and that estimated from the spectrum does not exceed 0.04. The relation of this quantity to the value of distance or light travel time depends on the redshift at which we calculate it.
In the nearest environment, a redshift error of 0.04 corresponds to half a billion years of light travel, and at z=1 (roughly half the age of the Universe) 150 million years. On the one hand, such errors seem considerable. On the other hand, these data can still be used for rough calculations.
The color space for this survey covers more than half of what the Euclid Telescope, which is starting to operate, and the Vera Rubin Telescope, due to launch in the coming years, will be based on. Both will take photometry of hundreds of millions, perhaps even billions, of galaxies, with the goal of elucidating the details of the growth of the Universe’s largest structures.
By being able to practically automatically determine the distance to these galaxies with relative reliability, astronomers will be able to analyze their evolution much more easily and reliably.
Research results published by MNRAS.
2024-08-06 12:40:46