Blueberry galaxy


Blueberry galaxies are dwarf starburst galaxies that have very high ionization rates and some of the lowest stellar masses and metallicities.
They are smaller counterparts of Green Pea galaxies, but
  • are more compact being less than th the size of the Milky Way,
  • are less distant, existing in low-density environments that are within the local universe, and
  • have lower luminosities. BBs form one of the youngest classes of star-forming galaxies with median ages ~70 Myr. Two BBs are among the most metal-poor galaxies known within the local universe.
BBs were first named in the scientific literature by Yang et al. as GPs that were at a distance of redshift z=0.05 or less, although similar galaxies had originally been named BBs on the Galaxy Zoo website. While Yang et al. identified a sample of 40 BBs, a much larger sample was acquired using data from the LAMOST DR9 survey. Liu et al. found 270 BBs, as well as GPs and "Purple Grapes". Researchers undertook a systematic study of the star formation rates, metallicities and environments of the compact galaxies that have different colours because of the different positions of emission lines in the photometric bands.
BBs at redshift z < 0.05 have the λ5007 emission line within the g band, which makes their colors blue; purple grapes are those at z > 0.36 with λ5007 within the i band and the UV continuum redshifted to the g band ; GPs are in the redshift range of 0.112 < z < 0.36 and have the λ5007 line within the r band.

Comparison to high-redshift galaxies

BBs, or "peas of various colours", have recently been studied as analogs for high-redshift galaxies that have been observed by the James Webb Space Telescope . Three examples of this are:
  • "JADES: Probing interstellar medium conditions at z ~ 5.5–9.5 with ultra-deep JWST/NIRSpec spectroscopy" examines 27 galaxies at high redshifts that were observed with JWST/NIRSpec. The study measured various emission lines from the ultra-deep JADES survey so as to gauge the ratios between the chemicals identified and categorise them into groupings, or 'spaces'. BBs are identified as local analogs to these JADES galaxies, along with GPs, as they have "extreme properties" such as metallicity, low mass and very high ionization. They find that "galaxies in this sample occupy regions of line-ratio space that are offset from those inhabited by typical galaxies at z ~ 0 or z ~ 2, although generally aligned with more extreme low-redshift populations such as 'blueberry' and 'green pea' dwarf starbursts".
  • In the study "Evolution of the Mass–Metallicity Relation from Redshift z ≈ 8 to the Local Universe", BBs and GPs are compared to a sample of 11 galaxies from JWST that are at redshift z ≈ 8. Using NIRCam and NIRSpec spectroscopy, metallicities and stellar masses are measured and then compared to extremely low metallicity analogs such as BBs. The study finds that the z ≈ 8 sample are generally distinct from extreme emission line galaxies or GPs but are similar in strong emission line ratios and metallicities to BBs. The study finds that BBs and GPs have metallicities similar to the z ≈ 8 galaxies, but "Despite this similarity, at a fixed stellar mass, the z ≈ 8 galaxies have systematically lower metallicities compared to BBs."
  • The study "NGDEEP: The Star Formation and Ionization Properties of Galaxies at 1.7 < z < 3.4" uses the JWST FGS-NIRISS from the Next Generation Deep Extragalactic Exploratory Public Survey to investigate a sample of 178 star-forming galaxies at redshifts 1.7 < z < 3.4. Using slitless spectroscopy, the authors select galaxies with ionizing oxygen and hydrogen emission features. BBs and GPs are used extensively as comparison galaxies at redshft z ~ 0 as they have strong emission lines "which may have properties more similar to those of high-redshift galaxies". As well as 36 BBs and 43 GPs, galaxies from SDSS and 45 from the Cosmic Origins Spectrograph Legacy Spectroscopic Survey with similar redshifts are used all of which have ionized oxygen measurements. Among their results, they find that:
  • * Galaxies with a redshift z ~ 2–3 have comparable or lower ionized oxygen ratios "to that measured for extreme galaxies at z ~ 0 at the fixed stellar mass, SFR, sSFR, and Hα and Hβ EW, indicating these extreme galaxies at z ~ 0 have similar to higher ionization parameters and similar to lower metallicity" and
  • * The authors' NGDEEP sample spans a wide range of O32 and stellar masses, "which helps bridge the gap between the local and the z > 5 universe. We find an evolutionary trend in O32 and EW from z ~ 0 to z ~ 5, where higher redshift galaxies show increased O32 and EW, with possibly higher O32 at fixed EW."

    Further studies

In "X-ray observations of Blueberry galaxies" BBs are studied using the XMM-Newton space telescope. These observations are the first to use x-rays and deliver surprising results. Of the 7 BBs studied, only 2 were detected as having significant x-ray emissions, while the remaining 5 are considered under-luminous. One theory as to why this might be is that because the stellar population of BBs is very young and "hasn't yet evolved enough to produce binary systems with a normal star paired with a neutron star or black hole, which shine brightly in X-ray."
A massive BB named SHOC 579 has been studied using the SDSS MaNGA survey by Paswan et al.. Using data from MaNGA and a variety of sources such as GALEX and Spitzer, a BB next to an older disk-like structure is investigated. Both objects are at redshift z=~ 0.0472. Their conclusions find that the BB is:
  • is the most massive and metal-rich one for which we have direct observational evidence of an old stellar population,
  • the age of the stellar population to be ~5 Gyr and ~7 Gyr for the blueberry component and the stellar disk, respectively,
  • GPs and BBs generally and their extreme emission-line properties are likely due to recent strong starburst events, potentially triggered by an external gas accretion process,
  • the presence of old stars imply that mechanisms that allow the escape of ionizing photons in these local objects may be different from those at play during the epoch of reionization.
In "FAST H I 21 cm Study of Blueberry Galaxies" 28 BBs are studied using the Five-hundred-meter Aperture Spherical Telescope. The sample of BBs are observed over a 3 year period using FAST to measure the H I, or neutral hydrogen, using the 21 cm spectral line. By finding out the H I levels, the depletion rate of any H I 'reservoirs' can be deduced. Generally, the lower the stellar mass, the higher the amount of H I is present i.e. has not yet been used up in star formation. Two of the 28 are found to have these reservoirs and overall, only 7% of the 28 have an H I detection, which are lower values than those of main sequence galaxies.
The study "Blueberry galaxies up to 200 Mpc and their optical and infrared properties" analyses 48 BBs. Using data from the HECATE catalog, photometry from Pan-STARRS, SDSS and ALLWISE, and spectroscopy from MPA-JHU, 40 previously known BBs and 8 unknowns were identified. 14 of the 48 were from the less-studied southern hemisphere. They conclude that BBs are the most intensely starforming sources among dwarf galaxies in the local universe. They are less massive, more blue in visible light and redder in the infrared. BBs "have higher specific starformation rates, equivalent widths, lower metallicities, and the most strongly ionized interstellar medium compared to typical SFGs and GPs."
In "H I imaging of a Blueberry galaxy suggests a merger origin" a BB is observed with the Giant Metrewave Radio Telescope. H I is detected in the BB J1509+3731, which is at redshift z = 0.03259, The H I is found to have a depletion time of 0.2 Gyr which indicates a high star formation rate than comparable standard blue compact galaxies. Combining the radio observations with images from the DESI Legacy Survey, it is shown that there is an H I offset outside the optical boundaries as seen on the DESI image. They conclude that "such an offset could be a sign of a merger event which can also trigger a starburst" and that, combined with other studies, this highlights "the role of dwarf galaxy mergers in the leakage of ionizing photons, and thus their role in cosmic reionization ".
"Radio Continuum Emission from Local Analogs of High-z Faint LAEs: Blueberry Galaxies" studies the radio continuum from BBs using the GMRT. The authors find that the star formation rate of BBs is suppressed by a factor of ~3 when compared to optical emissions lines and infer that this might be due to:
  • the young ages of these galaxies or,
  • the escape of cosmic ray electrons via diffusion or galactic scale outflows.
From the BBs sample in Yang et al., they select a subset of 10 of the brightest galaxies. Having captured the flux densities from 9 of these, they were combined with images from DECaLS showing how far the radio observations extended from the visible galaxies. After assuming that the stellar mass was built up from starburst activity, they calculate the time from the onset of this burst to within a range of 30 Myr to 370 Myr with a median of ~70 Myr. The emission of synchrotron radiation is found to be small and that it is plausible that the star formation in BBs has not been going on long enough for this to become a dominant force. They conclude: "In our study, we are not able to distinguish between the different scenarios that lead to the decrement in the radio continuum emission."
A sample of Extreme Emission Line Galaxies was found by a survey detailed in "J-PLUS: Uncovering a large population of extreme emitters in the local Universe". Using the Javalambre Photometric Local Universe Survey 466 EELGs at redshift z < 0.06 with Doubly ionized oxygen| EW over 300 Å and an r-band magnitude below 20 were identified, of which 411 were previously unknown. Using additional data from WISE, Gaia and SDSS, a much fainter sample of galaxies was found than had previously been observed. Among the galaxy sample were BBs which resulted in a ten-fold increase in the density of BBs known. In the diagram on the right, it is shown that the "sample of EELGs covers the region of the Yang et al. blueberry galaxies, as expected, but it also covers the same color space as extremely metal-poor galaxies both from observations and models". Among the results, it was found that the EELGs had:
  • low stellar masses
  • very young ages
  • moderately low metallicities
  • exceptionally high EW, all of which are typical of low-mass galaxies with high star formation activity.
The authors' sample was "much more efficient than searches made using broadband surveys, such as the blueberry galaxies or the green peas ". They conclude that the EELGs they identified share characteristics that make them similar to galaxies at high redshift and might help "shed light onto the properties of the galaxies forming in the very early Universe."
While many sources have identified GPs/BBs as analogs of early galaxies, several studies have identified the presence of an older stellar population within some examples, something that could not have been possible in the early universe.