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The SCUBA Local Universe Galaxy Survey
Dust along the Hubble sequence Catherine Vlahakis
University of Bonn

Collaborators: Steve Eales (Cardiff University) Loretta Dunne (University of Nottingham)

Overview
What is SLUGS? Results from the survey Recent work using SLUGS results
the FIR-radio relation

Introduction: SLUGS
SLUGS a submm survey of galaxies in the local Universe at 850m and 450m Survey of ~200 nearby galaxies: 104 60m-selected ("IRS")
(Dunne et al. 2000, Dunne & Eales 2001)

81 optically-selected ("OS")
drawn from right along the Hubble sequence (Vlahakis, Dunne & Eales 2005)

"OS" sample: of the Hubble

investigate sequence,

the properties of dust along the whole length in particular the cool 20 K dust

OS SLUGS results: submm morphology
Detected 52 galaxies in the OS sample, 17 of which also detected at 450m Several common features in the submm morphology of spirals:
many exhibit two peaks of 850-m emission, seemingly coincident with spiral arms others are core dominated and exhibit a single central peak of submm emission many have a combination of these features or have irregular morphologies in a number of cases the submm emission clearly follows a prominent dust lane

OS SLUGS results: submm morphology
NGC 7047

NGC 99

NGC 7442

UGC 12519

OS SLUGS results: submm morphology

NGC 3689

NGC 6131

NGC 803

OS SLUGS results: submm morphology

NGC 3987

NGC 7722

Any correlation between Hubble type and submm morphology?

OS SLUGS results: two-component SED
NGC 7722

T=(54,20) n=1224

OS SLUGS results: two-component SED
Tw = 56 K Tc = 18 K n = 2684 NGC 6190

OS SLUGS results: two-component SED
We find that the 60-, 100- (IRAS), 450- and 850-m (SCUBA) data are well fitted by a two-component dust model with dust emissivity index =2 The temperatures of the warm component range from 28 to 59 K The cold component temperatures range from 17 to 24 K Ratio of mass of cold dust to mass of warm dust (Nc/Nw) much higher for our OS galaxies than for IRAS-selected galaxies can reach values of ~1000

OS SLUGS results: single-component SED
Distribution of values

Mean dust emissivity index =1.1 Significantly lower than the IRAS selected sample Rather than a physical difference in the emissivity behaviour of the grains () we believe that it is due to a difference in the ratios of cold to warm dust

SLUGS results: Colour-colour plot

Filled optically-selected Open IRAS-selected

Population of galaxies containing a large proportion of cold dust - unrepresented in the IRAS sample

FIR-radio relation
Relation between non-thermal radio and FIR emission from galaxies One of strongest correlations in astronomy Tight correlation over 5 decades of luminosity Cause of relationship still unclear "Standard" explanation: both FIR and radio emission caused by high-mass stars
FIR emission

dust heating massive stars supernovae

relativistic electrons & Synchrotron radiation

FIR-radio relation
Optically-selected SLUGS submm measurements allow us to test a basic prediction of the standard theory Standard model prediction: FIR-radio correlation will be tighter than the submm-radio
Reason:

Regions with large no. of OB stars : · ISRF more intense · dust hotter than in general ISM · gives rise to 60m emission 850m emission: · traces colder dust heated by ISRF · includes component from older stellar populations

SLUGS FIR-radio relation

Likely AGN

Circles optically-selected Crosses IRAS-selected

Tight correlation between FIR and radio for OS sample

SLUGS submm-radio relation

Circles optically-selected Crosses IRAS-selected

Much larger scatter of submm-radio relation for OS sample Exactly the behaviour we would expect if standard model is correct

High-z Universe: -redshift relation
Carilli & Yun method: Use redshift-sensitive nature of submm-radio flux density ratio as redshift estimator

850 1.4

S850 log S1.4

Based on assumption that the FIR-radio relation is the same at low-z and high-z We use fitted SEDs for 17 OS SLUGS galaxies to Predict how depends on redshift, for "normal" low-z galaxies
Compare with deep SCUBA sources with spectroscopic redshifts (Chapman et al. 2005) Use this to assess reliability of CY method

-redshift relation
· Source could be IRS-like and
high-z or OS-like and low-z · Temp affects position on z diagram · Difficult to get reliable estimates of redshift in this way

Dashed lines OS SLUGS predictions Solid lines IRS SLUGS

Deep SCUBA sources: · No correlation, but... · Brighter in radio (or fainter in FIR) than the predictions for our local SLUGS samples · A number of possible explanations

Plotted points: deep SCUBA sources with spectroscopic redshifts (Chapman et al. 2005)

-redshift relation: possible explanations
i. Correlation between and luminosity at given z
Unlikely No evidence for sufficient correlations out to high redshift Unlikely: Not strong x-ray sources Radio morphologies not typical of AGN Optical spectra often starburst not AGN Unlikely Test this using subsample with robustly determined redshifts (Aretxaga et al. 2007) and find no difference

ii. Chapman sources: FIR and radio emission comes from AGN

iii. Redshifts of Chapman sources are unreliable

iv. Relation between FIR and radio different at high and low z FIR
Very different conditions compared to today surprising if relation were the same We feel is the most likely explanation

Summary
60-, 100- (IRAS), 450- and 850-m (SCUBA) fluxes are well fitted by a two-component dust model with dust emissivity index =2 Ratio of mass of cold dust to mass of warm dust much higher for our OS galaxies than for IRAS-selected galaxies FIR-radio correlation for OS SLUGS much stronger than submmradio, evidence that massive star formation is cause of FIR-radio relation Much more scatter in z relation for "normal" galaxies than for bright IRAS galaxies For CY method to be reliable as redshift estimator for deep submm sources, first need measurement of dust temp z relation: deep submm galaxies brighter sources of radio emission than predicted from properties of local galaxies possible explanation is evolution of FIR-radio relation