A Radio Census of Nuclear Activity in Nearby Galaxies

0.37 MB·English

by  M. E. Filho

Astronomy & Astrophysics manuscript no. 4510 February 5, 2008
(DOI: will be inserted by hand later)
A Radio Census of Nuclear Activity in Nearby Galaxies
Mercedes E. Filho1,2,3, Peter D. Barthel3 and Luis C. Ho4
1 Centro de Astrof´ısica da Universidadedo Porto, Ruadas Estrelas, 4150 – 762 Porto, Portugal
2 Istituto diRadioastronomia, CNR, Via P. Gobetti, 101, 40129 Bologna, Italy
3 Kapteyn Astronomical Institute,P.O. Box 800, 9700 AV Groningen, The Netherlands
6 4 The Observatories of theCarnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101,
0 USA
0
2
Received XX;accepted XX

n
a Abstract. In order to determine the incidence of black hole accretion-driven nuclear activity in nearby galax-
J
ies, as manifested by their radio emission, we have carried out a high-resolution Multi-Element Radio-Linked

4 Interferometer Network (MERLIN) survey of LINERs and composite LINER/Hii galaxies from a complete

magnitude-limitedsampleofbrightnearbygalaxies(Palomarsample)withunknownarcsecond-scaleradioproper-
1
ties. Therearefifteenradiodetections, ofwhich threearenewsubarcsecond-scale radiocoredetections, allbeing
v
candidate AGN. The detected galaxies supplement the already known low-luminosity AGN – low-luminosity
0
Seyferts, LINERs and composite LINER/Hii galaxies – in the Palomar sample. Combining all radio-detected
8
Seyferts, LINERsand composite LINER/Hiigalaxies (LTS sources), weobtain an overall radio detection rate of
0
1 54% (22%ofallbrightnearbygalaxies) andweestimatethatatleast ∼50%(∼20%ofallbrightnearbygalaxies)
0 aretrueAGN.TheradiopowersoftheLTSgalaxiesallowtheconstructionofalocalradioluminosityfunction.By
6 comparing the luminosity function with those of selected moderate-redshift AGN, selected from the 2dF/NVSS
0 survey, we find that LTS sources naturally extend the RLF of powerful AGN down to powers of about 10 times
/ that of Sgr A*.
h
p
- Key words.galaxies: active — galaxies: nuclei — galaxies: luminosity function
o
r
t
s 1. Introduction & Wilson 2003). In the absence of spectral AGN signa-
a
: turessuchasaSeyfertorquasar-typecontinuumorbroad
v The search for low-luminosity active galactic nuclei
emission lines, radio observations can offer an alternative
i
X (LLAGN)innearbygalaxieshasbeenthesubjectofmany method for determining the LLAGN incidence in nearby
optical surveys. Results show that nuclear activity may
r galaxies. Measurements of radio flux, compactness, radio
a be a common phenomenon. The Palomar survey (Ho, spectral index1 and brightness temperatures provide the
Filippenko&Sargent1995,1997a,b)hasbeenveryuseful
necessary diagnostic tools for determining the nature of
in this regard by providing a sensitive magnitude-limited
the radio emission.
(B <12.5mag)sampleofalmost500brightnearbygalax-
T Several important radio surveys have been conducted
ies. About half of the sources are emission-line nuclei,
on the magnitude-limited Palomar bright nearby galaxy
classified as Seyferts, LINERs or composite LINER/Hii
sample (Ho, Filippenko & Sargent 1995, 1997a, b), re-
galaxies,thelastcategorydisplayingbothLINERandHii
vealing a large fraction of radio cores, not only in el-
properties. However, characterizing the powering mecha-
lipticals but also in bulge-dominated spirals. In a re-
nisms of the sources is not straightforward, particularly
cent Very Large Array (VLA) 5 and 1.4GHz, 1′′ resolu-
in low-luminosity sources. Many of these galaxies possess
tion survey of the low-luminosity Seyferts of the Palomar
circum-nuclearstar-formingregionswhichblendwithand
sample (Ho & Ulvestad 2001; HU01 hereafter), it was
mayevendrownoutthepresenceofaweakactivegalactic
found that over 80% of the sources harbour a radio
nucleus (AGN).
core. In a distance-limited sample of low- luminosity
Optimallyitisnecessarytopickspectralregionswhere Seyferts, LINERs and composite LINER/Hii galaxies ob-
the contrast between any hypothetical LLAGN compo- served with the VLA at 15GHz, 0′.′25 resolution (Nagar
nent and circum-nuclear stellar component is maximized.
et al. 2000, 2002; VLA/N00 and VLA/N02 hereafter;
X-rays are very useful in this regard as shown by the
Nagar,Falcke&Wilson2005;VLA/N05hereafter),itwas
hard X-ray studies of LLAGN (Terashima et al. 2000;
Terashima, Ho & Ptak 2000; Ho et al. 2001; Terashima 1 Fν ∝ν−α throughout.
2 Filho et al.: A Radio Census of Nuclear Activityin Nearby Galaxies
Table 1. TargetMERLIN sources.Col. 1 Source name. Col. 2 and 3: Optical
position from NASA/IPAC Extragalactic Database (NED). Col. 4: Adopted
distance from Ho, Filippenko & Sargent (1997a) with H =75 km s−1 Mpc−1.
0
Col.5:SpectralType fromHo,Filippenko &Sargent(1997a).L =LINERs,S
=Seyferts,andT=compositeLINER/Hiigalaxies.Colonsrefertouncertain
(:) or highly uncertain (::) spectral classification. The number 1.9 refers to
the presence of broad Hα detection and 2 refers to the absence of broad Hα
emission. Col. 6: Hubble type from Ho, Filippenko & Sargent (1997a).
RA(J2000) Dec(J2000) D
Galaxy (hms) (◦′′′) (Mpc) SpectralType HubbleType
(1) (2) (3) (4) (5) (6)
IC356 040746.9 +694845 18.1 T2 SA(s)abpec
IC520 085342.2 +732927 47.0 T2: SAB(rs)ab?
NGC428 011255.6 +005854 14.9 L2/T2: SAB(s)m
NGC488 012146.8 +051525 29.3 T2:: SA(r)b
NGC521 012433.8 +014352 67.0 T2/H: SB(r)bc
NGC718 015313.3 +041144 21.4 L2 SAB(s)a
NGC777 020014.9 +312546 66.5 S2/L2:: E1
NGC841 021117.3 +372950 59.5 L1.9: (R’)SAB(s)ab
NGC1169 030334.7 +462309 33.7 L2 SAB(r)b
NGC1961 054204.8 +692243 53.1 L2 SAB(rs)c
NGC2336 072703.7 +801042 2.9 L2/S2 SAB(r)bc
NGC2681 085332.8 +511850 13.3 L1.9 (R’)SAB(rs)0/a
NGC2768 091137.5 +600215 23.7 L2 E6:
NGC2832 091946.9 +334459 91.6 L2:: E+2:
NGC2841 092202.6 +505835 12.0 L2 SA(r)b:
NGC2985 095021.6 +721644 22.4 T1.9 (R’)SA(rs)ab
NGC3166 101345.6 +032532 22.0 L2 SAB(rs)0/a
NGC3169 101415.0 +032757 19.7 L2 SA(s)apec
NGC3190 101805.8 +214956 22.4 L2 SA(s)apecspin
NGC3226 102327.0 +195354 23.4 L1.9 E2:pec
NGC3301 103655.8 +215255 23.3 L2 (R’)SB(rs)0/a
NGC3368(M96) 104645.7 +114912 8.1 L2 SAB(rs)ab
NGC3414 105116.2 +275830 24.9 L2 S0pec
NGC3433 105203.9 +100854 39.5 L2/T2:: SA(s)c
NGC3507 110325.4 +180812 19.8 L2 SB(s)b
NGC3607 111654.3 +180310 19.9 L2 SA(s)0:
NGC3623(M65) 111855.9 +130532 7.3 L2: SAB(rs)a
NGC3626 112003.8 +182125 26.3 L2: (R)SA(rs)0+
NGC3627(M 66) 112015.0 +125930 6.6 T2/S2 SAB(s)b
NGC3628 112017.0 +133522 7.7 T2 SAbpecspin
NGC3646 112214.7 +201231 56.8 H/T2: SB(s)a
NGC3675 112607.9 +433510 12.8 T2 SA(S)b
NGC3718 113235.3 +530401 17.0 L1.9 SB(s)apec
NGC3780 113922.3 +561614 37.2 L2:: SA(s)c:
NGC3884a 114612.2 +202330 91.6 L1.9 SA(r)0/a;
NGC3898 114915.2 +560504 21.9 T2 SA(s)ab;
NGC3945 115313.6 +604032 22.5 L2 SB(rs)0+
NGC4013 115831.3 +435649 17.0 T2 SAb
NGC4036 120126.9 +615344 24.6 L1.9 S0−
NGC4143 120936.1 +423203 17.0 L1.9 SAB(s)0
NGC4203 121505.0 +331150 9.7 L1.9 SAB0−:
NGC4293 122112.8 +182258 17.0 L2 (R)SB(s)0/a
NGC4321(M100) 122254.9 +154921 16.8 T2 SAB(s)bc
NGC4414 122627.1 +311324 9.7 T2: SA(rs)c?
NGC4435 122740.5 +130444 16.8 T2/H: SB(s)0
NGC4438 122745.6 +130032 16.8 L1.9 SA(s)0/a
NGC4450 122829.5 +170506 16.8 L1.9 SA(s)ab
NGC4457 122859.1 +033414 17.4 L2 (R)SAB(s)0/a
NGC4548(M91) 123526.4 +142947 16.8 L2 SBb(rs)
NGC4589 123725.0 +741131 30.0 L2 E2
NGC4636 124250.0 +024117 17.0 L1.9 E0+
NGC4736(M94) 125053.0 +410714 4.3 L2 (R)SA(r)ab
NGC4750 125007.1 +725230 26.1 L1.9 (R)SA(rs)ab
NGC4772 125329.0 +021002 16.3 L1.9 SA(s)a
NGC4826(M64) 125643.7 +214052 4.1 T2 (R)SA(rs)ab
NGC5077b 131931.6 −123926 40.6 L1.9 E3+
NGC5297 134623.7 +435220 37.8 L2 SAB(s)c:spin
NGC5322 134915.2 +601126 31.6 L2:: E3+
NGC5353 135326.7 +401659 37.8 L2/T2: SA0spin
NGC5363 135607.1 +051520 22.4 L2 IA0?
NGC5371 135539.9 +402743 37.8 L2 SAB(rs)bc
NGC5377 135616.6 +471408 31.0 L2 (R)SB(s)a
NGC5448 140249.7 +491021 32.6 L2 (R)SAB(r)a
NGC5678 143205.8 +575517 35.6 T2 SAB(rs)b
NGC5813 150111.2 +014208 28.5 L2: E1+
NGC5838 150526.2 +020558 28.5 T2:: SA0−
NGC6340 171024.9 +721816 22.0 L2 SA(s)0/a
NGC6501 175603.7 +182223 39.6 L2:: SA0+:
NGC6702 184657.6 +454220 62.8 L2:: E:
NGC6703 184718.8 +453302 35.9 L2:: SA0−
a DoesnotfulfillthemagnitudecriterionforthePalomarsample.
b DoesnotfulfilldeclinationcriterionforthePalomarsample.
Filho et al.: A Radio Census of Nuclear Activityin Nearby Galaxies 3
found that ∼40% of the objects harbour subarcsecond- (68) LINERs or composite sources (plus one Seyfert and
scale compactradiocores.A similar study with the VLA, oneHiinucleus;seeTable1)havingeitherunknownradio
at8.4GHz,2′.′5resolutionofallthecompositeLINER/Hii properties or known radio emission in excess of 2 mJy on
galaxies in the Palomar sample has been presented in arcsecond-scales. Aiming to detect (weak) radio emission
Filho, Barthel & Ho (2000, 2002a), revealing radio cores onsubarcsecondscales,thesourceswereobservedwiththe
in∼25%ofthesamplesources.However,althoughthera- Jodrell Bank MERLIN, at 5GHz, 0′.′1 resolution. These
dio core emission in these sources is consistent with the MERLINobservationswereintendedasafilterforfollow-
presence of a LLAGN, we cannot exclude a stellar ori- up, high-resolution EVN observations, which should in-
gin from the brightness temperature figures (TB∼<105 K; creasethebrightnesstemperaturefigurestothenecessary
Condon 1992) obtained at these resolutions. As con- >105 K (Condon 1992) level and thereby unambiguously
clusive judgement requires Very Long Baseline (VLBI)- determine the nature of the radio emission.In Table 1 we
resolution, multi-wavelength Very Long Baseline Array summarize the properties of the 70 sources observedwith
(VLBA)andEuropeanVeryLongBaselineInterferometer MERLIN.
Network (EVN) observations have been obtained for se-
lectedsubsamplesoflow-luminositySeyferts,LINERsand
composite LINER/Hiigalaxiesthat showedarcsecond-or
subarcsecond-scale radio cores (Falcke et al. 2000; F00 3. Observations and Data Reduction
hereafter;Nagaretal.2002;VLBA/N02hereafter;Nagar,
The 5GHz MERLIN observations were obtained during
Falcke & Wilson 2005; VLBA/N05 hereafter; Ulvestad
2001 October 18–20, with a 15MHz bandwidth. Defford,
& Ho 2001b; Filho, Barthel & Ho 2002b; Filho et al.
Cambridge, Knockin, Darnhall and Tabley telescopes
2004; Anderson, Ulvestad & Ho 2004; AU04 hereafter).
were used, yielding a resolution of about 0′.′1 at 5GHz.
In sources with subarcsecond- or arcsecond-scale radio
Typically four 8 minute scans of the sources were inter-
peak emission above 2.5 mJy, results reveal a 100% de-
tection rate of high-brightness temperature (TB∼>108 K), spersed with 2 minute scans of the respective phase cali-
compact,flatspectrum(α<0.5)radiocores,enforcingthe brators, for a total integration time of about 25 minutes
on each target source. 3C286, the primary flux calibra-
LLAGN scenario for the radio emission (VLBA/N05; see
tor, was observed twice during the observing run. Initial
also Ulvestad & Ho 2001b; Filho, Barthel & Ho 2002b;
calibration, reduction and imaging of the MERLIN data
Filhoetal.2004;AU04).Theirlowradioluminositiessug-
was performed by S. Garrington and A. Richards using
gest we are probing the very faint end of the AGN popu-
the AIPS pipeline at Jodrell Bank.
lation.
Unambiguouslydeterminingthephysicalnatureofthe
nearby galaxy radio cores is more than of mere phe-
nomenologicalinterest.Ifthey trulycontainanaccretion- Table 2. Map parameters of the MERLIN-detected
powerednucleus,then they obviouslyneedto be included sources. Col. 1: Source name. Col. 2: Spectral class from
intheAGNpopulation.Theirnon-trivialnumbersimpact Ho, Filippenko & Sargent (1997a). Col. 3: Restoring
onseveralastrophysicalproblemsrangingfromthecosmo- beam. Col. 4: Position angle of the beam. Col. 5: rms
logical evolution of the AGN luminosity function to their noise level of the image.
contribution to the X-ray background.
The present paper deals with high-resolution radio- Spectral Beamsize P.A. rms
Galaxy Class (mas2) (deg) (mJy/beam)
imaging of LLAGN, carried out with the Jodrell Bank
(1) (2) (3) (4) (5)
Multi-Element Radio-Linked Interferometer Network
N2768 L2 130 × 68 −7.07 0.10
(MERLIN), completing the radio census of nuclear ac-
N3169 L2 178 × 56 −45.27 0.09
tivity in the Palomar galaxy survey of Ho, Filippenko & N3226 L1.9 153 × 48 −51.23 0.07
Sargent (1997a). N3414 L2 153 × 49 −49.53 0.06
N3718 L1.9 161 × 130 +54.66 0.06
2. Source Selection N3884a L1.9 86 × 58 −79.10 0.05
N4143 L1.9 71 × 41 +50.37 0.06
ThetargetsourcesweretakenfromthePalomarsurveyof N4203 L1.9 126 × 47 −28.85 0.06
bright nearby galaxies (Ho, Filippenko & Sargent1997a). N4293 L2 159 × 55 −18.22 0.06
The Palomar sample contains a total of 417 emission-line N4321 T2 174 × 55 −15.41 0.06
N4450 L1.9 177 × 57 −14.73 0.06
objects (486 galaxies in total), of which 206 are stellar-
powered Hii nuclei. Among the remaining sources, 52 are N5077a L1.9 145 × 47 +12.02 0.08
N5297a L2 71 × 42 +29.53 0.06
classifiedaslow-luminositySeyfertgalaxies,94asLINERs
N5353 L2 73 × 44 +23.66 0.06
and 65 as composite LINER/Hii galaxies, many of which N5363 L2/T2: 100 × 46 +24.83 0.11
may be AGN-powered. After the extensive radio studies
a Newsubarcsecond-scaleradiodetection.
describedabovewhichtargetedalllow-luminositySeyfert
andpartoftheLINERsandcompositeLINER/Hiigalax-
iesinthePalomarsample,wehaveselectedtheremaining
4 Filho et al.: A Radio Census of Nuclear Activityin Nearby Galaxies
Table 3.The5GHzradioparametersoftheMERLIN-detectedsources.
Col. 1: Source name. Col. 2: 5GHz peak radio flux density. Col. 3 and
4: 5GHz radio position. Col. 5: Integrated 5GHz flux density. Col. 6:
Brightness temperature.
Fpeak RA(J2000) Dec(J2000) Fint TB
Galaxy (mJy) (h m s ) (◦ ′′′ ) (mJy) (× 104 K)
(1) (2) (3) (4) (5) (6)
NGC2768 7.06 09 11 37.414 +60 02 14.86 7.22 3.9
NGC3169 2.23 10 14 15.014 +03 27 57.26 2.96 1.1
NGC3226 3.50 10 23 27.008 +19 53 54.67 3.88 2.3
NGC3414 1.05 10 51 16.208 +27 58 30.27 1.13 0.7
NGC3718 1.65 11 32 34.854 +53 04 04.47 2.07 0.4
NGC3884 1.74 11 46 12.183 +20 23 29.93 1.75 1.7
NGC4143 0.97 12 09 36.065 +42 32 03.00 1.00 1.6
NGC4203 4.30 12 15 05.052 +33 11 50.35 4.86 3.6
NGC4293 0.54 12 21 12.797 +18 22 57.41 1.81 0.3
NGC4321 0.50 12 22 54.938 +15 49 20.87 0.52 0.3
NGC4450 1.21 12 28 29.590 +17 05 06.00 1.23 0.6
NGC5077A 65.86 13 19 31.726 −12 39 27.88 67.37 47.3
NGC5077B 85.03 13 19 31.603 −12 39 29.11 86.66 61.1
NGC5297 0.91 13 46 23.772 +43 52 18.88 0.95 1.5
NGC5353 16.21 13 53 26.695 +40 16 58.89 17.42 24.7
NGC5363A 10.93 13 56 07.215 +05 15 17.15 11.92 11.6
NGC5363B 10.95 13 56 07.213 +05 15 17.26 11.28 11.7
4. Radio Properties to be ’compact’ if the FIRST flux density is >50% the
NVSS value, otherwise the source is said to be resolved.
Of the 70 objects observed with MERLIN, fifteen objects
Furthermore,weassumethatanyradiovariabilityissmall
weredetectedabovethe5σ(∼0.5mJybeam−1)threshold.
compared to the total NVSS flux. We also present a brief
Three of these are new radio core detections on subarc-
discussion on detected background sources (Fig. 5).
secondscales.Withtheexceptionofthecompositegalaxy
NGC4321, all of the detected sources are spectroscopi-
NGC2768: This source has a NVSS (14.5 mJy) and
callyclassifiedasLINERs.Alldetectedsourcesexceptfor
a FIRST (12.3 mJy) radio detection, denoting that the
NGC4293andNGC4321showcompact,unresolvedcores
sourceis compactonarcsecondscales.NGC2768alsohas
nearthephasecenter,coincidentwiththeopticalnucleus.
anEVN(7mJy;P.Barthel,privatecommunication2005)
In Table 2 we summarize the radio parameters for the
andaVLBA/N05detectionof7.3mJyComparisonofour
MERLIN-detected sources.The rms noise levelwas mea-
MERLIN measurement with that of VLA/N05 yields an
suredinasource-freeregionusingtheAIPStaskIMSTAT.
α∼0.0.
MERLIN radio maps of the detected sources (excluding
NGC4321)areshowninFig.1–4.The mapforNGC4321 NGC3169: There is a NVSS (87.0 mJy) and FIRST
canbe seeninFig.5,alongwiththe detectedbackground (16.4 mJy) radio detection; the flux density values show
sources.Contour levels are rms×(−3, 3, 6, 12,24,48, 96, resolution effects. VLA/N00 (see also VLA/05) has de-
192). tected a 6.8 mJy source and FOO (see also VLBA/N05)
Table 3 includes the model fits to the detected radio detect 6.2 mJy; the source is compact on milliarcsecond
components. The AIPS task IMFIT was used to fit bi- scales. Using the VLA/N00 and F00 (see also VLA/N05
dimensional Gaussians to the brightness peaks in each and VLBA/N05) values implies a flat spectral index
radio component. The brightness temperature has been for this radio source (α ∼0.0). However, comparison of
calculated using the formula (Weedman 1986): thesevalueswithourMERLINmeasurementsuggeststhe
source could be radio variable.
TB = 4.9×107(cid:16)mFJνy(cid:17)(cid:16)θmajomraxsθ2minor(cid:17)−1(cid:0)5GνHz(cid:1)−2, NGC3226: There is no NVSS value for this source,
mostlikelybecauseitisspatiallyconfusedwithNGC3227.
where Fν is the peak flux density of the sources and However, there is a 4.5 mJy FIRST detection. VLA/N00
θ and θ refer to the major and minor axis of (seealsoVLA/N05)andF00(seealsoVLBA/N05)detect
major minor
the Gaussian beam (Table 2). a 5.4 and 3.5 mJy radio source, respectively, showing the
Below we give a source description, quoting values sourcetobecompactonmilliarcsecondscales.Fromthese
from surveys such as the NVSS, FIRST, VLA/N05 and lastvaluesthesourceappearstohaveaflat/invertedspec-
VLBA/N05 (see Table 4 for details). Sources are said trum (α>∼−0.4). The F00 measurement is consistent with
Filho et al.: A Radio Census of Nuclear Activityin Nearby Galaxies 5
Table 4. Bibliographical radio data for measurement shows that the source must be radio vari-
the MERLIN-detected sources. Col. 1: able.
Survey. Col. 2: Observing frequency. Col. NGC4203: This source has been extensively observed
3: Resolution. Col. 4: Instrument. Col. 5: at radio frequencies. There is a NVSS (6.1 mJy), FIRST
Reference. (6.9 mJy), VLA/N00 and VLA/N02 (9.5 and 9.0 mJy;
see also VLA/N05), F00 (8.9 mJy; see also VLBA/N05),
ν Res. HU01 (11.2 mJy) and multifrequency AUH04 radio de-
Survey (GHz) (mas) Instr. Ref.
tections. The flux density values, including the MERLIN
(1) (2) (3) (4) (5)
detection, show that the source is compact and has a flat
NVSS 1.4 45000 VLA 1 spectrum (α∼0.1).
FIRST 1.4 5000 VLA 2
NGC4293: This source has a NVSS (18.5 mJy) and
VLA/N00 15 150 VLA 3
a FIRST (13.9 mJy) radio detection, denoting that it is
VLA/N02 15 150 VLA 4
compact on arcsecond scales. The present MERLIN and
VLA/N05 15 150 VLA 5
VLA/N02 (1.4 mJy; see also VLA/N05) measurements
VLBA/N02 5 2 VLBA 4
VLBA/N05 5 2 VLBA 5 show that most of the source has been resolved and pos-
F00 5 2 VLBA 6 sesses a flat spectrum (α<∼0.2). The weak amorphous na-
HU01 5.0 1000 VLA 7 ture of the MERLIN emission, associated with the flat
AUH04 1.7 10 VLBA 8 spectrum, suggests the radio emission to be mainly ther-
2.3 6 mal.
5.0 2 NGC4321, M100: There are several radio frequency
8.4 1.6 measurements for this source (see source discussion in
15.4 1.2
Filho, Barthel & Ho 2000). NVSS has detected 85.9 mJy
22.2 0.8
and FIRST a 37.4 mJy source. The position of the
43.2 0.4
MERLIN radio source is coincident with the optical nu-
References–(1)Condonetal.1998;(2)Becker,White,
cleus of the galaxy as given by NED and with compo-
& Helfand 1995; (3) Nagar et al. 2000; (4) Nagar et al.
2002; (5) Nagar, Falcke & Wilson 2005 and references nent A3 in Collison et al. (1994), the strongest compo-
therein;(6) Falcke et al. 2000; (7) Ho & Ulvestad 2001;
nent in their multiple radio structure map. The radio
(8)Anderson,Ulvestad&Ho2004.
measurements clearly show the source has been resolved.
Comparison of the Collison et al. (1994) values and our
MERLIN detection, shows that the source is not variable
our MERLIN value; the source does not appear variable at5GHzandappearstohaveafairlysteepradiospectrum
at 5GHz. (α< 1.6).
NGC3414:TheNVSS(4.4mJy)andFIRST(4.0mJy) NGC4450: Comparison of the NVSS (9.4 mJy),
flux densities show that this source is compact on arc- FIRST (6.7 mJy), HU01 (6.5 mJy), VLA/N02 (2.7 mJy;
second scales. Our MERLIN combined with VLA/N05 seealsoVLA/N05),multifrequencyAUH04andMERLIN
(2.4 mJy) yields an α∼0.0. measurementsshow thatthe sourceis compactonarcsec-
NGC3718: There is a NVSS (14.9 mJy) and FIRST ondscalesbuthasbeenpartiallyresolvedonsubarcsecond
(10.6mJy)radiodetection,whosefluxdensityvaluesshow scales. The AUH04 multifrequency data yield a spectral
the source to be compact on arcsecond scales. There is index of α<∼0.3.
also a VLA/N02 and VLBA/N02 (10.8 and 5.3 mJy de- NGC4589: NVSS has detected a 36.8 mJy radio
tection,respectively(seealsoVLA/N05andVLBA/N05). source, VLA/N05 data yield 11.9 mJy and VLBA/N05
Comparison of these last values with our MERLIN mea- data yield 11.5 mJy; the source is compact on subparsec
surementshowsthatthesourcemaysufferfromresolution scales. Our MERLIN data suffer from severe phase varia-
effects and/or can be radio variable (α>∼−0.6). tions.
NGC3884: This source has a NVSS (14.0 mJy) and NGC5077: The NVSS has detecteda 156.7mJy radio
a FIRST (8.9 mJy) radio detection, showing some res- source. Our MERLIN map shows two very strong radio
olution effects. Taking into consideration the resolution sources (A and B) in the nuclear region of this galaxy.
mismatch, comparison of the FIRST and our MERLIN The NVSS flux density appears to be the sum of the
detection shows that the source may be resolved and/or MERLIN flux of these two sources,denoting that there is
radio variable (α <1.3). Because it does not fulfill the little extended flux on arcsecond scales. While the APM
magnitude criterionfor the Palomarsurvey,this source is and NED identify these radio sources with the nucleus of
not included in the final sample. NGC5077, it is difficult to constrain the origin of the ra-
NGC4143: There is a NVSS (9.9 mJy) and FIRST dioemissionwithoutspectralinformation.Becauseitdoes
(5.0 mJy) radio detection, showing some signs of res- notfulfillthedeclinationcriterionforthePalomarsurvey,
olution. This source was also observed in VLBA/N02 this source is not included in the final sample.
(8.7 mJy; see also VLBA/N05) and twice in the
VLA/N00andVLA/N02(3.3mJyand10.0mJy;seealso
VLA/N05).ComparisonofthesevalueswithourMERLIN
6 Filho et al.: A Radio Census of Nuclear Activityin Nearby Galaxies
60 02 15.4
03 27 57.8
15.2
57.7
DECLINATION (J2000) 1154..08 CLINATION (J2000) 5555577777.....65432
14.6 DE 57.1
57.0
56.9
14.4
56.8
09 11 37.48 37.46 37.44 37.42 37.40 37.38 37.36 37.34 10 14 15.06 15.05 15.04 15.03 15.02 15.01 15.00 14.99 14.98 14.97
RIGHT ASCENSION (J2000) a) RIGHT ASCENSION (J2000) b)
19 53 55.2 27 58 30.7
55.1
30.6
55.0
DECLINATION (J2000) 5555544444.....98765 DECLINATION (J2000) 33330000....5432
30.1
54.4
54.3 30.0
54.2 29.9
10 23 27.05 27.04 27.03 27.02 27.01 27.00 26.99 26.98 26.97 26.96 10 51 16.24 16.23 16.22 16.21 16.20 16.19 16.18 16.17
RIGHT ASCENSION (J2000) c) RIGHT ASCENSION (J2000) d)
Fig.1. Radio emission contours of the MERLIN-detected sources. Contour levels are rms× (−3, 3, 6, 12, 24, 48, 96,
192),(see Table 2). The size of the restoringbeamis givenin eachimage at the bottom lefthand corner(see Table 2).
a) NGC2768, b) NGC3169, c) NGC3226 and d) NGC3414.
Filho et al.: A Radio Census of Nuclear Activityin Nearby Galaxies 7
53 04 05.0
20 23 30.3
04.8
DECLINATION (J2000) 0044..64 NATION (J2000) 33320009....2109
04.2 DECLI 29.8
04.0 29.7
29.6
11 32 34.92 34.90 34.88 34.86 34.84 34.82 34.80 34.78 11 46 12.22 12.21 12.20 12.19 12.18 12.17 12.16 12.15
RIGHT ASCENSION (J2000) a) RIGHT ASCENSION (J2000) b)
33 11 50.8
50.7
42 32 03.3
50.6
03.2
CLINATION (J2000) 000332...109 DECLINATION (J2000) 55550000....5432
E
D
50.1
02.8
50.0
02.7
49.9
02.6
12 09 36.10 36.09 36.08 36.07 36.06 36.05 36.04 36.03 12 15 05.09 05.08 05.07 05.06 05.05 05.04 05.03 05.02 05.01
RIGHT ASCENSION (J2000) c) RIGHT ASCENSION (J2000) d)
Fig.2. As in Figure 1. a) NGC3718, b) NGC3884,c) NGC4143 and d) NGC4203.
8 Filho et al.: A Radio Census of Nuclear Activityin Nearby Galaxies
17 05 06.5
06.4
06.3
18 22 57.8 N (J2000) 0066..21
ON (J2000) 555777...765 DECLINATIO 0065..09
ATI 57.4 05.8
CLIN 57.3 05.7
DE 57.2
57.1 05.6
57.0 05.5
12 21 12.84 12.82 12.80 12.78 12.76 12.74 12 28 29.63 29.62 29.61 29.60 29.59 29.58 29.57 29.56 29.55
RIGHT ASCENSION (J2000) a) RIGHT ASCENSION (J2000) b)
43 52 19.2
-12 39 27.6 19.1
27.8 B
CLINATION (J2000) 22228888....0246 DECLINATION (J2000) 111988...098
DE 28.8
18.7
29.0
A
29.2 18.6
29.4
13 19 31.74 31.72 31.70 31.68 31.66 31.64 31.62 31.60 31.58 13 46 23.80 23.79 23.78 23.77 23.76 23.75 23.74
RIGHT ASCENSION (J2000) c) RIGHT ASCENSION (J2000) d)
Fig.3. As in Figure 1. a) NGC4293, b) NGC4450,c) NGC5077 and d) NGC5297.
Filho et al.: A Radio Census of Nuclear Activityin Nearby Galaxies 9
40 16 59.3
59.2
59.1
N (J2000) 59.0
O
ATI 58.9
N
CLI 58.8
E
D
58.7
58.6
58.5
13 53 26.73 26.72 26.71 26.70 26.69 26.68 26.67 26.66
RIGHT ASCENSION (J2000) a)
05 15 17.6
17.5
17.4
DECLINATION (J2000) 111777...321 A B
17.0
16.9
16.8
16.7
13 56 07.24 07.23 07.22 07.21 07.20 07.19
RIGHT ASCENSION (J2000) b)
Fig.4. As in Figure 1. a) NGC5353 and b) NGC5363.
10 Filho et al.: A Radio Census of Nuclear Activityin Nearby Galaxies
NGC5297: NVSS has detected a 23.0 mJy radio detected LINERs. This result is consistent with the 42%
source.FIRST has failed to detect this source most likely detectionrateofLINERs inthe VLA/N00andVLA/N02
due to resolution. The MERLIN observations have re- (see also VLA/N05) distance-limited sample, if we con-
solved out a large part of the radio emission. siderthat64%ofourMERLINLINERtargetsarefarther
NGC5353: This source has a NVSS (39.0 mJy) and than 20 Mpc.
FIRST (38.3 mJy) radio detection, denoting that the We can also compare the MERLIN radio detec-
source is compact on arcsecond scales. There is also a tions for the subset of sources (LINERs and com-
VLBA/N05 (21.6 mJy) and VLA/N05 (18.7 mJy) detec- posite galaxies) overlapping with the galaxy samples
tion.TheMERLINandVLA/N05measurementsyieldan VLA/N00, VLA/N02 (see also VLA/N05), F00 (see also
α∼0.0. VLBA/05), HU01, and Filho et al. (2004). NGC2678,
NGC5363: The FIRST (141.8 mJy) and NVSS NGC2841, NGC3169, NGC3190, NGC3226, NGC3414,
(160.3 mJy) flux densities show that this source is com- NGC3607, NGC3627, NGC3628, NGC3718, NGC3780,
pact on arcsecond scales. There is also a VLA/N05 and NGC3945, NGC4143, NGC4203, NGC4293, NGC4450,
VLBA/N05 measurement of 40.7 and 39.6 mJy, respec- NGC4548, NGC4589, NGC4636, NGC4736, NGC4772,
tively.TheMERLINobservationshave,however,resolved NGC5233, NGC5353, NGC5363, NGC5377, NGC5813
part of the radio emission into two components – A and and NGC5838, have been detected with the VLA
B, with a total flux density of ∼25 mJy. Consultation of and/or VLBA (VLA/N00; VLA/N02; VLA/N05; F00;
NEDandtheAPMprovidesnootheropticalidentification VLBA/N05). NGC777 (HU01) and NGC5838 (Filho et
for these sources except sources belonging to the nuclear al. 2004) have been detected with the VLA. Several
region of NGC5363. It is possible that this source has a sources (NGC3190, NGC3607, NGC3627, NGC3780,
double nucleus. NGC4548, NGC4736 and NGC4772) went undetected
with MERLIN due to mispointing relative to the doc-
Background Sources
umented radio core. One (NGC4589) perhaps two
NGC3627: As given by NED, we may have detected
(NGC5322; see also Feretti et al. 1984) sources suffer
thesupernovaSN1997bsat(α,δ) =(112014.2,+12
J2000 from severe phase erros. As for NGC2841, NGC3628,
5820)withinlessthan20′′ oftheradiopositionmeasured
NGC3945,NGC4636,NGC5377,NGC5813(VLA/N002;
in the map of this galaxy.
VLA/N02; VLA/N05), NGC777 (HU01) and NGC5838
NGC4321: To the North, at the NED position
(Filho et al. 2004; VLA/N05) which have peak flux den-
(α,δ) = (12 22 54.9, +15 49 25), there is emission
J2000 sities below 2 mJy, it is likely our MERLIN observa-
coincidentwithinlessthan20′′withtheHiiregion(source
tions either resolved out the emission or simply do not
158), as documented in Hodge & Kennicutt (1983).
haveenoughsignal-to-noiseratio.WenotethattheLovell
NGC4826:Wedonotdetectthemultiplesourcestruc-
Telescope (Jodrell Bank), the most sensitive telescope of
tureasfoundbyTurner&Ho(1994)becausethe individ-
the MERLIN array, was offline during our observations.
ual sources either fall below our detection limit or fall
Of the remaining sources we have detected NGC2768,
outside the mapped region. We do, however, detect a ra-
NGC3169, NGC3226, NGC3718, NGC4143, NGC4203,
diosourceatthe NEDposition(α,δ) =(125643.66,
J2000 NGC4293,NGC4450,NGC5353,andNGC5363(Table2
+21 41 03), which we identify, within less than 20′′, with
and 3).
a RSN (source 6) in Turner & Ho (1994). Comparison
of our and the Turner & Ho (1994) fluxes suggest some
5. Local Radio Luminosity Function
variability.
Giventhe detection limitofthe survey–5σ ∼0.5mJy InordertoconstructarepresentativeAGNradioluminos-
beam−1 – the overall detection rate for the MERLIN ityfunction(RLF)ofthelocalUniverse,wehaveusedthe
observations is 27%. We have excluded from this cal- emission-line(excluding Hii) sourcesinthe Palomarsam-
culation the two sources which do not fulfill the mag- ple.Allofthesesourceshavenowbeenobservedat2.′′5res-
nitude/declination criteria, the Hii and Seyfert galaxies olution or better, when we include the present MERLIN
(Table 1), eight sources with incorrect pointing relative observations. We shall refer to the 196 LINERs, compos-
to the NED/radio position and two sources with severe ite sources and Seyferts which satisfy both the magni-
phase errors (see below). We can also estimate the frac- tude (B <12.5 mag) and declination criteria (δ >0◦) of
T
tionofMERLIN-detectedsources–58%–thataregenuine the Palomar survey as the ‘LTS sources’for brevity (LTS
AGN,duetothepresenceofbroadHαemission(Type1.9 meaningLINER-Transition-Seyfert);thesesourcesconsti-
sources; see also Section 5). Only 19% of the MERLIN- tute the present sample.
detectedsourcesareType2,whichexhibitnobroademis- Ideally, we would like to have a homogeneous set of
sion lines. It is possible that Type 2 sources are a mixed radioobservations.Butlackingsucha survey,wehaveas-
case, whereby only a small fraction of these are genuine sembled in Table 5 a list of radio measurements used to
AGN. However, various lines of evidence to be discussed derivetheRLFfortheLTSsamplesources.Individualob-
below suggest that a significant fraction of these sources jects takenfromotherradiosurveysarealsolisted. Radio
do indeed harbour a LLAGN. We will return to this issue observations at different frequencies have been converted
in Section 5. The detection rate is 25% for the MERLIN- to 5GHz assuming a spectral index of 0.7 and also cor-