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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