J/MNRAS/508/3754 Study of nine SPT protocluster candidates (Wang+, 2021)
Overdensities of submillimetre-bright sources around candidate protocluster
cores selected from the South Pole Telescope survey.
Wang G.C.P., Hill R., Chapman S.C., Weiss A., Scott D., Apostolovski Y.,
Aravena M., Archipley M.A., Bethermin M., Canning R.E.A., De Breuck C.,
Dong C., Everett W.B., Gonzalez A., Greve T.R., Hayward C.C., Hezaveh Y.,
Jarugula S., Marrone D.P., Phadke K.A., Reuter C.A., Rotermund K.M.,
Spilker J.S., Vieira J.D.
<Mon. Not. R. Astron. Soc. 508, 3754-3770 (2021)>
=2021MNRAS.508.3754W 2021MNRAS.508.3754W (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, galaxy ; Galaxies, IR ; Photometry ; Infrared ;
Positional data ; Millimetric/submm sources
Keywords: galaxies: abundances - galaxies: clusters: general -
galaxies: high-redshift - submillimetre: galaxies
Abstract:
We present APEX-LABOCA 870-µm observations of the fields
surrounding the nine brightest high-redshift unlensed objects
discovered in the South Pole Telescope's (SPT) 2500 deg2 survey.
Initially seen as point sources by SPT's 1-arcmin beam, the 19-arcsec
resolution of our new data enables us to deblend these objects and
search for submillimetre (submm) sources in the surrounding fields. We
find a total of 98 sources above a threshold of 3.7σ in the
observed area of 1300 arcmin2, where the bright central cores
resolve into multiple components. After applying a radial cut to our
LABOCA sources to achieve uniform sensitivity and angular size across
each of the nine fields, we compute the cumulative and differential
number counts and compare them to estimates of the background, finding
a significant overdensity of δ ≃ 10 at S{870} = 14 mJy. The
large overdensities of bright submm sources surrounding these fields
suggest that they could be candidate protoclusters undergoing massive
star formation events. Photometric and spectroscopic redshifts of the
unlensed central objects range from z= 3 to 7, implying a volume
density of star-forming protoclusters of approximately 0.1 Gpc-3. If
the surrounding submm sources in these fields are at the same
redshifts as the central objects, then the total star formation rates
of these candidate protoclusters reach 10000 M☉ yr-1, making
them much more active at these redshifts than seen so far in either
simulations or observations.
Description:
We report 870 µm observations of the 1.3-1.9 Mpc environment of
nine SPT-selected protocluster candidates using the APEX telescope's
LABOCA which span a redshift range of z = 3-7 and represent a density
of 0.1 sources per Gpc3 (i.e see the section 2.1 Unlensed sources
in the South Pole Telescope millimetre-wave point-source catalogue).
The nine fields observational properties such as positional data,
surface area, the total time exposure and their central flux depth are
reported in the table1.dat, (i.e see the section 2.2 APEX-LABOCA
observations). We also use the Herschel-SPIRE (Griffin et al.
2010A&A...518L...3G 2010A&A...518L...3G) data at 250, 350, and 500 µm primarily
obtained during the initial multiwavelength follow-up campaign. (i.e
see the section 2.3 Herschel-SPIRE observations). Our sample was
observed over six observing runs from 2018 September to 2019 March.
Hereafter, as explained in the section 3.1 LABOCA source extraction,
we proceed to core extractions using the standard Bolometer Array
Analysis Software (BOA; Schuller 2012SPIE.8452E..1TS). We convolve the
LABOCA flux density and noise maps with an 18.6 arcsec Gaussian beam
in order to produce maximum-likelihood signal-to-noise ratio (SNR)
maps for point-source detections. With this method, we identify in
870µm the sources with the highest SNRs (16.8-41.5) in each field
as 'central sources'. Using the significance threshold and source
definition, we identify 98 sources across the nine fields. As
explicited in the section 3.3 Herschel-SPIRE photometry, we extract
from these maps the flux densities of our corresponding sources at
250, 350, and 500 µm. More, these flux densities were previously
dettermined in Spilker et al. (2016ApJ...826..112S 2016ApJ...826..112S), Strandet et al.
(2016ApJ...822...80S 2016ApJ...822...80S) and Reuter et al. (2020ApJ...902...78R 2020ApJ...902...78R, Cat.
J/ApJ/902/78) there, they were derived simply by convolving the raw
maps with the PSF and measuring the values of the peak pixels, while
our approach takes into account the effects of confusion. All flux
extraction results of the nine fields are presented in the tables
tablea1-9.dat.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 49 9 Properties of LABOCA fields
tablea1.dat 92 26 List of LABOCA-detected sources in the field
SPT0303-59 at a photometric redshifts
zphot = 3.33 ± 0.37
tablea2.dat 92 18 List of LABOCA-detected sources in the field
SPT0311-58 at a spectroscopic redshifts
zspec = 6.9011
tablea3.dat 92 16 List of LABOCA-detected sources in the field
SPT0348-62 at a spectroscopic redshifts
zspec = 5.6541
tablea4.dat 92 18 List of LABOCA-detected sources in the field
SPT0457-49 at a spectroscopic redshifts
zspec = 3.9875
tablea5.dat 92 15 List of LABOCA-detected sources in the field
SPT0553-50 at a spectroscopic redshifts
zspec = 5.323
tablea6.dat 92 18 List of LABOCA-detected sources in the field
SPT2018-45 at a photometric redshifts
zphot = 3.18 ± 0.39
tablea7.dat 92 17 List of LABOCA-detected sources in the field
SPT2052-56 at a spectroscopic redshifts
zspec = 4.257
tablea8.dat 92 17 List of LABOCA-detected sources in the field
SPT2335-53 at a spectroscopic redshifts
zspec = 4.7555
tablea9.dat 92 20 List of LABOCA-detected sources in the field
SPT2349-56 at a spectroscopic redshifts
zspec = 4.3020
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See also:
J/ApJ/902/78 : ALMA 3mm new sp. redshifts for SPT galaxies (Reuter+, 2020)
J/ApJ/900/55 : The SPT-SZ catalog at 95, 150, and 220GHz (Everett+, 2020)
J/ApJ/862/96 : Dusty star-forming galaxies with LABOCA 870um obs.
(Lewis+,2018)
J/MNRAS/411/505 : Sub-mm observations in Extended Chandra DFS (Chapin+, 2011)
J/MNRAS/372/1621 : SCUBA Half-Degree Extragalactic Survey. II (Coppin+, 2006)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 --- Field Observed field name
12- 13 I2 h RAh Right ascension (J2000)
15- 16 I2 min RAm Right ascension (J2000)
18- 19 I2 s RAs Right ascension (J2000)
21 A1 --- DE- Sign of declination (J2000)
22- 23 I2 deg DEd Declination (J2000)
25- 26 I2 arcmin DEm Declination (J2000)
28- 29 I2 arcsec DEs Declination (J2000)
31- 32 I2 h Texp Total integration time of all exposures
34- 37 F4.2 mJy Dcent Central depth of the LABOCA field
39- 41 I3 arcmin2 Area Observed field area
43- 49 A7 --- Table Table's name of the field's sources
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Byte-by-byte Description of file: tablea?.dat
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Bytes Format Units Label Explanations
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1 I1 --- Note Note on source extraction (1)
3 A1 --- Name Source name of the core (Source_name)
5 A1 --- n_Name Note on data name (2)
7- 8 I2 h RAh Right ascension (J2000)
10- 11 I2 min RAm Right ascension (J2000)
13- 17 F5.2 s RAs Right ascension (J2000)
19 A1 --- DE- Sign of declination (J2000)
20- 21 I2 deg DEd Declination (J2000)
23- 24 I2 arcmin DEm Declination (J2000)
26- 30 F5.2 arcsec DEs Declination (J2000)
32- 35 F4.1 mJy S870 Flux peak density of the core measured
at 870 µm (S870)
37- 39 F3.1 mJy e_S870 Mean error of S870 (e_S870)
41 A1 --- l_S870deb Upper limit flag on S870deb (3)
43- 46 F4.1 mJy S870deb Deboosted flux peak density of the core
measured at 870 µm (Sdeb870) (4)
48- 50 F3.1 mJy E_S870deb ?=- Mean upper errorbar of S870deb (E_S870deb)
52- 54 F3.1 mJy e_S870deb ?=- Mean lower errorbar of S870deb (e_S870deb)
56 A1 --- l_S500 Upper limit flag on S500
58- 61 F4.1 mJy S500 ?=- Flux peak density of the core measured
at 500 µm (S500)
63- 66 F4.1 mJy e_S500 ?=- Mean error of S500 (e_S500)
68 A1 --- l_S350 Upper limit flag on S350
70- 74 F5.1 mJy S350 ?=- Flux peak density of the core measured
at 350 µm (S350)
76- 79 F4.1 mJy e_S350 ?=- Mean error of S350 (e_S350)
81 A1 --- l_S250 Upper limit flag on S250
83- 87 F5.1 mJy S250 ?=- Flux peak density of the core measured
at 250 µm (S250)
89- 92 F4.1 mJy e_S250 ?=- Mean error of S250 (e_S250)
--------------------------------------------------------------------------------
Note (1): We group the sources into four parts as follows:
1 = Sources fall within our radial cutoff 240 arcsec
2 = Sources within our radial cutoff 240 arcsec and deboosted to the
lower limit of the background prior which is zero
3 = Sources fall outside our radial cutoff 240 arcsec
4 = Sources found with a lower signal to noise ratio cut of 3σ,
detected at 3σ
Note (2): Central (1) and Northern (2) cores where Herschel-SPIRE data are
confused : Since our Herschel-SPIRE maps are significantly confused
by the cosmic infrared background (i.e. sources near to the line of
sight at redshifts other than that of the SPT source), we need to use
an appropriate filter to measure the flux densities of our sources
at 250, 350, and 500 µm. Convolving these maps with the
point-spread function (PSF) is sufficient for isolated point-source
detection of well-characterized data, but in this case, an optimal
filter must take into account the background source number counts.
We choose to filter all of our SPIRE maps using the matched-filter
technique described in Chapin et al.
(2011MNRAS.411..505C 2011MNRAS.411..505C, Cat. J/MNRAS/411/505) more details in the
section 3.3 Herschel-SPIRE photometry.
Note (3): Deboosted flux densities, 98 per cent confidence upper limits are
quoted when deboosted to the lower limits of the prior.
Note (4): Since our source-extraction technique involves an SNR cut, there
is a systematic boosting of lower flux objects due to Eddington-type
bias. Even though the objects are normally distributed under our
assumption of Gaussian statistics, the underlying population
distribution of luminous objects means that intrinsically, there are
many more faint objects in our fields below our detection threshold
than bright objects above.
We discard the ones below the threshold and on average, the flux
density of an object near the threshold will be overestimated.
Our raw flux densities need to be 'deboosted' to correct for this
effect. We use the method described in Coppin et al.
(2005MNRAS.357.1022C 2005MNRAS.357.1022C; 2006MNRAS.372.1621C 2006MNRAS.372.1621C, Cat. J/MNRAS/372/1621)
in order to obtain the posterior distribution for the actual flux
density. We take the posterior distribution peak to be the deboosted
flux density of each source, with the uncertainties determined by
calculating 68 per cent confidence intervals.
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History:
From electronic version of the journal
(End) Luc Trabelsi [CDS] 19-Aug-2024