J/A+A/581/L5 Mid-IR/radio correlation for Ghat galaxies (Garrett+, 2015)
Application of the mid-IR radio correlation to the Ghat sample and the search
for advanced extraterrestrial civilisations.
Garrett M.A.
<Astron. Astrophys., 581, L5-5 (2015)>
=2015A&A...581L...5G 2015A&A...581L...5G
ADC_Keywords: Galaxies, radio ; Galaxies, IR
Keywords: astrobiology - galaxies: general - galaxies: star formation -
radio continuum: galaxies - infrared: general
Abstract:
Wright et al. (2014ApJ...792...26W 2014ApJ...792...26W) have embarked on a search for
advanced Karadashev Type III civilisations via the compilation of a
sample of sources with extreme mid-IR emission and colours. The aim is
to furnish a list of candidate galaxies that might harbour an advanced
Kardashev Type III civilisation; in this scenario, the mid-IR emission
is then primarily associated with waste heat energy by-products. I
apply the mid-IR radio correlation to this Glimpsing Heat from Alien
Technology (Ghat) sample, a catalogue of 93 candidate galaxies compiled
by Griffith et al., 2015ApJS..217...25G 2015ApJS..217...25G. I demonstrate that the mid-IR
and radio luminosities are correlated for the sample, determining a
k-corrected value of q22=1.35±0.42 . By comparison, a similar
measurement for 124 galaxies drawn from the First Look Survey (FLS)
has q22=0.87±0.27. The statistically significant difference of the
mean value of q22 for these two samples, taken together with their
more comparable far-IR properties, suggests that the Ghat sample shows
excessive emission in the mid-IR. The fact that the Ghat sample largely
follows the mid-IR radio correlation strongly suggests that the vast
majority of these sources are associated with galaxies in which
natural astrophysical processes are dominant. This simple application
of the mid-IR radio correlation can substantially reduce the number of
false positives in the Ghat catalogue since galaxies occupied by advanced
Kardashev Type III civilisations would be expected to exhibit very
high values of q. I identify nine outliers in the sample with q22>2
of which at least three have properties that are relatively well
explained via standard astrophysical interpretations e.g. dust
emission associated with nascent star formation and/or nuclear
activity from a heavily obscured AGN. The other outliers have not been
studied in any great detail, and are deserving of further observation.
I also note that the comparison of resolved mid-IR and radio images of
galaxies on sub-galactic (kpc) scales can also be useful in
identifying and recognising artificial mid-IR emission from less
advanced intermediate Type II/III civilisations. Nevertheless, from
the bulk properties of the Ghat sample, I conclude that Kardashev Type
III civilisations are either very rare or do not exist in the local
Universe.
Description:
Cross-match of the Ghat sample of 93 sources (see Griffith et al.,
2015ApJS..217...25G 2015ApJS..217...25G, Table 9) with the NRAO/VLA Sky Survey (NVSS)
1.4GHz (20cm) radio catalogue (Condon et al., 1998, Cat. VIII/65).
The study was restricted to the 92 sources that fall within the NVSS
survey area and have measured redshifts.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 86 92 The Ghat sample studied in this paper, including
values (or upper limits) of q22
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See also:
VIII/65 : 1.4GHz NRAO VLA Sky Survey (NVSS) (Condon+ 1998)
II/328 : AllWISE Data Release (Cutri+ 2013)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 2 I2 h RAh Right ascension (J2000)
4- 5 I2 min RAm Right ascension (J2000)
7- 11 F5.2 s RAs Right ascension (J2000)
13 A1 --- DE- Declination sign (J2000)
14- 15 I2 deg DEd Declination (J2000)
17- 18 I2 arcmin DEm Declination (J2000)
20- 23 F4.1 arcsec DEs Declination (J2000)
25 A1 --- l_S20cm [<] Limit flag on S20cm
26- 30 F5.1 mJy S20cm NVSS flux density at 20cm (1.4GHz)
32- 36 F5.2 mag W4 WISE W4 magnitude
38 A1 --- l_q22 [>] Limit flag on q22
39- 43 F5.3 --- q22 [0.1/3] q22=log[S(22µm)/S(20cm)] (1)
45- 52 F8.6 --- z [0/0.15] Redshift
54- 57 F4.2 --- gamma [0/1] Fraction of starlight re-emitted in the
mid-IR as waste heat products (γ)
59- 61 A3 --- Type SIMBAD object type
63- 86 A24 --- Name Source Name
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Note (1): S(22um) is the source flux density measured by WISE band 4,
and S(20cm) is the source flux density measured by NVSS (1.4GHz)
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 05-Nov-2015