J/ApJ/773/67 Compact radio sources near the Galactic center (Roy, 2013)
Density of warm ionized gas near the Galactic center: low radio frequency
observations.
Roy S.
<Astrophys. J., 773, 67 (2013)>
=2013ApJ...773...67R 2013ApJ...773...67R
ADC_Keywords: Radio continuum ; Milky Way
Keywords: Galaxy: center; H II regions; ISM: clouds; radio continuum: ISM
Abstract:
We have observed the Galactic center (GC) region at 0.154 and 0.255GHz
with the Giant Metrewave Radio Telescope. A total of 62 compact likely
extragalactic (EG) sources are detected. Their scattering sizes
decrease linearly with increasing angular distance from the GC up to
about 1°. The apparent scattering sizes of the sources are more
than an order of magnitude less than predicted earlier by the NE2001
model of Galactic electron distribution within 359.5°<l<0.5°
and -0.5°<b<0.5° (Hyperstrong Scattering Region) of the
Galaxy. High free-free optical depths (τ) are observed toward most
of the extended non-thermal sources within 0.6° from the GC.
Significant variation of τ indicates that the absorbing medium is
patchy at an angular scale of ∼10' and ne is ∼10/cm3, which matches
the NE2001 model. This model predicts the EG sources to be resolved
out from 1.4GHz interferometric surveys. However, out of 10 EG sources
expected in the region, 8 likely EG are present in the 1.4GHz catalog.
Ionized interfaces of dense molecular clouds to the ambient medium are
most likely responsible for strong scattering and low radio frequency
absorption. However, dense GC clouds traced by CS J=1-0 emission are
found to have a narrow distribution of ∼0.2° across the Galactic
plane. Angular distribution of most EG sources seen through the
so-called Hyperstrong Scattering Region are random in b, and typically
∼7 out of 10 sources will not be seen through the dense molecular
clouds, which explains why most of them are not scatter broadened at
1.4GHz.
Description:
The Galactic center (GC) region was observed with the Giant Metrewave
Radio Telescope (GMRT) on 2003 March 13 at 0.225GHz and on 2005 August
20 at 0.154GHz with pointing centers at RAJ2000=17:46:00 and
DEJ2000=-28:57:00 and at RAJ2000=17:45:40 and DEJ2000=-29:00:28,
respectively. The resolutions of the whole array for a southern source
such as the GC at the above two frequencies are ∼15" and ∼25",
respectively.
Preliminary results of these observations were published in Roy & Rao
(2006JPhCS..54..156R 2006JPhCS..54..156R, 2009ASPC..407..267R 2009ASPC..407..267R). Here, we present a more
detailed analysis from these data sets and examine existing higher
frequency observations in the region.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 64 62 Flux density and source sizes near the
Galactic center (GC)
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See also:
J/AZh/91/503 : Compact radio sources near Galactic center (Pynzar'+, 2014)
J/A+A/535/A38 : Observations of NOAO Bootes field at 153MHz (Intema+, 2011)
J/ApJ/699/186 : VLA observations of the Galactic center (Zhao+, 2009)
J/ApJS/174/481 : 2LC Compact Radio Sources in the GC (Lazio+, 2008)
J/MNRAS/371/38 : X-ray observations of the Galaxy center (Wang+, 2006)
J/AJ/130/586 : Compact radio sources in the galactic plane (White+, 2005)
J/MNRAS/360/1305 : Polarization of extragalactic sources (Roy+, 2005)
J/AJ/128/1646 : The Galactic Center region at 330MHz (Nord+, 2004)
J/A+AS/147/195 : Pulsar spectra of radio emission (Maron+, 2000)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 7 F7.3 deg GLON Galactic longitude
9- 14 F6.3 deg GLAT Galactic latitude
16- 19 I4 mJy S154 [106/1895]? 0.154GHz flux density
21- 24 I4 mJy S255 [94/2170]? 0.255GHz flux density
26- 29 I4 mJy S1400 [51/1623] 1.4GHz flux density
31- 34 F4.1 --- alpha [-3/1.4]? Spectral index (1)
36- 40 I5 --- Sexp [67/10585]? Expected flux density at 154MHz (2)
42- 45 F4.2 --- Sexp/S [0.09/2.9]? Ratio of expected to measured flux
densities at 0.154GHz
47- 51 F5.2 arcsec t154 [0/59.1]? Angular size θ at 0.154GHz
53- 56 F4.1 arcsec t255 [0/44.6]? Angular size θ at 0.255GHz
58- 61 F4.1 arcsec t1400 [0/13]? Angular size θ at 1.4GHz
63- 64 I2 arcsec Size [0/31]? Scattering size at a fiducial
wavelength of 1m (3)
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Note (1): All the sources detected at 0.154 or 0.255GHz were found to have
counterparts at 0.33GHz survey (Nord et al. 2004, J/AJ/128/1646;
in Simbad). We compute spectral indices for all
the 0.33GHz sources based on the detection of counterparts in the
Galactic plane survey of radio sources at 1.4GHz (GPSR; White et al.
2005, J/AJ/130/586, <[WBH2005] GLLL.lll+B.bbb> in Simbad), or in the
survey of compact sources toward the GC (Lazio & Cordes 2008,
J/ApJS/174/481; <2LC LLL.lll+B.bbb> in Simbad).
Spectral indices (α) for these sources have been computed
following the convention of flux density (S)∝να
between 1.4 and 0.33GHz. See section 3.
Note (2): Expected flux density from extrapolation of spectral index between
0.33 and 1.4GHz and flux density at 0.33GHz.
Note (3): Typical error on the size is ∼2.4".
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History:
From electronic version of the journal
(End) Emmanuelle Perret [CDS] 20-Feb-2015