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THE EVOLUTION EXPLORER : GALEX

Denis BURGARELLA1, C. Kevin XU2, Veronique BUAT1 & THE GALEX TEAM 1 Observatoire Astronomique Marseille Provence /LAM traverse du siphon, B.P. 8, 13376 Marseille Cedex 12, France E-mail : denis. [email protected] 2 California Institute of Technology, MC 405-47, 1200 East Califo rnia Ed, PA sadena, CA 91125, USA

This paper presents GALEX, the Galaxy Evolution Explorer P.I. C. Martin) and the planned imaging and spectroscopic surveys that will be carried out by( GALEX in the Far (153 nm) and Near Ultraviolet (251 nm) bands. A first sample of GALEX data was made pub­ lic through the Early Release of October 2003 and the Data Release I including a much larger amount of data will be released in Fall 2004. Some results related to the Young Local Uni­ verse are presented featuring studies of ultraviolet images of local and a spectroscopic

analysis of the ELAIS South I field as observed by GALEX.

Keywords: galaxies : ultraviolet - galaxies : infrared - galaxies extinction - galaxies evolution;

What is GALEX 1 ? GALEX, the Galaxy Evolution Explorer was launched by a PEGASUS rocket on 28 April 2003. GALEX is a 50 - cm telescope devoted to observing the Ultraviolet (UV) in two bands centered at about 153m and 231nm (Martin et al. 2004, Barlow et al. 2004). In Section 2, a sample of Early release Observations data will be shown. In Section 3 a few results from local galaxy imaging will be presented and in Section 4 the analysis of the ELAIS South 1 spectroscopic observations that are part of the ERO. Figure 1 lists the main GALEX surveys with the planned observed areas, sensitivities, spectral and angular resolutions. The estimated number of galaxies and the average distances for each survey are strongly related and selected to address the GALEX science objectives that will be presented in the next Section. One of the main features of this list is the All-sky Imaging Survey (AIS) that will be carried by GALEX over its lifetime. The previous largest UV dataset was observed from the French balloon-borne telescope FOCA that covered about 70 square degrees in the Northern Hemisphere. All the other UV telescopes : the International Ultraviolet Explorer - IUE, ASTR0-1 and ASTR0-2 to quote only a few

333 Figure 1: List of GALEX main surveys in UV. of them were essentially target-oriented as is, of course, the Hubble (HST). Such surveys were already performed at other wavelength ranges (e.g. IRAS) but was left over with only small portions of the sky until Martin's GALEX was accepted by NASA asUV a SMa ll Explorer Mission. France and Korea joined the US team to build GALEX. In addition to these surveys a Guest Investigator Program offers GALEX observation time to the astronomical community. The deadline was on 16 April 2003 and the selected programs should be announced around July 2004. To get used to the data and better know what GALEX characteristics and performances are, an Early Release Observation (ERO) is available since October 2003. The GALEX Science Project is strongly related to galaxies and to their evolution. The m�in objec­ tives is to map the history and evolution of the over 80% of its history. The main questions that we can find on GALEX web site are : What is the history of formation in the Universe What do nearby galaxies look like in light ? When and where did the and elementsUV that? we see today have their origin Schematically, GALEX Science Goals could be divided into i) local ones where we? will try to better understand and calibrate the UV observations of local galaxies and ii) evolutive ones where we will investigate the evolution of galaxiesand their star formationand of some of the main related parameters : dust, morphology, metallicity, etc.

The Early Release Observations 2 This description of the Early Data Release is a summary of the more detailed document avalaible on GALEX public web site : http://www.galex.caltech.edu/EROWebSite/Early.release_data_description_partl.htm GALEX Early Release data contains images and catalogs generated by the GALEX data pipeline as of November - December 2003. These data are representative of GALEX data and are intended to provide data in a format similar to those that will be delivered to the Guest Investigators and to the astronomical community, through the first major Data Release (DRl) planned forthe end of 2004. At the beginning of December 2003, GALEX had observed more than 3000 square degrees of sky during more than 2000 orbits. For the all-sky and medium imaging surveys, the early release observations contain less than 1 % of what had been collected, with the single DIS/WSS field one of 10-15 deep fields that had been observed. The ERO fields and exposure times were selected to provide a representative data set for use by prospective inve.stigators.Targets observed at different Galactic latitudes with varying infrared cirrus levels (100 have a wide range of extinction values and background levels.The median cirrus mm) level over the whole sky is about 3.0 MJy/sr. GALEX deep imagingUV and spectroscopy fields are generally selected to have < 1.0 MJy/sr. Currently the All-sky survey is limited to fields with cirrus levels below 3.0 MJy/sr.lioo Two sorts of files areincluded in the ERO release. A first set provides a minimal source extraction catalog and images that would be useful to have a quick look at the data. Moreover, an advanced set contains the full data coming out from the pipeline. This is the information needed to 'test' GALEX performances, get used to the format and make potential Guest Investigators ready to submit good proposals. In Figures 2 & 3, we present some instances of the data that can be found on GALEX Web Site. 1 1 We assume a cosmology with Ho = 70km.s- .Mpc , !1M = 0.3 and !1vAc = 0.7 in this paper.

334 M81 M82 Figure 2: GAL EX Early R>i:lea:1e Observations Color Image of the field containi og and

335 Figure 3 GALRX Early Reie?s0 irna,gez of 1'18 1, the globular

336 3 Local Galaxies : the Stefan's Quintet

A number of UV images of local galaxies have been already analysed and the first analysis will be published in a special issue of ApJ Letters. To cite a few of them : Bianchi et al. (2004) on M51 and MlOl, Popescu et al. (2004) carried out a comparison of UV and FIR fluxes of MlOl, Boissier et al. (2004) performed a radial analysis of the dust attenuation and rate of M83, Hoopes et al. (2004) studied the UV haloes of NGC 253 and M82, Hibbard et al. (2004) studied the tidal tails of the Antennae (NGC 4038/NGC 4039). In this paper, we will have a more detailed look at the paper from Xu et al. (2004) on Stefan's Quintet. The firstUV images of this famous interacting group contain NGC 7317 (E), the binary galaxies NGC 7318a (E) and NGC 7318b (Sbc pee), the Sy2 galaxy 7319 (Sbc pee) and the foreground galaxy NGC 7320 (Sd). Most of the UV emission is located in regions associated to NGC 7319, NGC 7318b and the starburst region in the intragroup medium SQ-A.The total star formationrate (SFR) of Stefan's Quintet is about 6.7 M80z.yr-1 and the main contributors are SQ-A : 1.3 M80z .yr-1 in excellent agreement with the Ha luminosity corrected for dust extinction, 2 M80z.yr-1 for NGC 7319, (68 %) of which coming from the disk of the galaxy and 15 % from the loop-shaped young tail. The UV emission associated with NGC 7318b is emitted from a 80-kpc disk and amounts to 3.4 M80z .yr-1.

4 The ELAIS South Field : Galaxy UV Extinction 1 The ELAIS Sl field wasobserved by GALEX in both its Wide Spectroscopic and Deep Imaging Survey modes (Figure 4). This field was previously observed by the Infrared Space Observatory and Burgarella et al. (2004) made use of the catalogue of multi-wavelength data published by the ELAIS consortium (Rowan-Robinson et al. 2004) to select galaxies common to the two samples. Among the 959 objects with GALEX spectroscopy, 88 are present in the ELAIS catalog and 19 are galaxies with an optical spectroscopic redshift. The distribution of redshifts covers the range 0 < z < 1.6. The selected galaxies have bolometric IR luminosities 10 < Log(Lrn) < 13 (deduced from the 15µm flux) which means that we cover a wide range of galaxies from normal to Ultra Luminous IR Galaxies. The mean UV luminosity (u) for the low-z 0.35) sample. (not corrected for extinction) amounts to Log(>..L 5 0) 9.8(0.6) Lsol (z The UV slope f3 (assuming correlates 1with3 =the GALEX FUV-NUV color if:::; the sample is f>, ex: >.f3) restricted to galaxies below z < 0.1. Taking advantage of the UV and IR data, we estimate the dust attenuation from the IR/UV ratio and compare it to the UV slope (3. We find that it is not possible to uniquely estimate the dust attenuation fromf3 for our sample of galaxies (Burgarella et al. 2004). These galaxies are highly extinguished with a median FUV dust attenuation AFuv 2.7 ± 0.8 in agreement with the median value deduced from a photometric survey from GALEX carried =out by Buat et al. (2004). Once the dust correction applied, the UV- and IR-based SFRs correlate. For the closest galaxy with the best quality spectrum, we see a feature consistent with being produced by a bump near 220nm in the attenuation curve (Figure 5).

4.1 GALaxy UV Attenuation Models

Light do not undergo the same extinction when it is emitted by young stars (i.e. age < 10 Myrs) as compared to older stars (age > 10 Myrs). Young stars probably undergo more dust extinction because they are still concentrated and embedded in the remaining of the molecular cloud. Old stars probably destroyed their cocoon and move away. In our models, we assume an attenuation law for young stars which is identical. to Calzetti et al. (2000) attenuation law while the Milky Way ou LMC attenuation law is assumed for old stars. Dust associated to old stars is either mixed with the stars i.e. optical depth proportional to the starburst optical depth (multiplied by 5 & 1, divided by 2, 5 & 10) or concentrated in clumps (5, 10, 15, 20 clumps) with optical depths proportional to young star optical depth (/5 up /80). We show in Figure 6 how the GALEX colors (FUV-NUV) evolves. When a MW extinction law is assumed for old stars, the Log (FIR/UV) ratio (which is directly related to the dust attenuation) could be totally decoupled from the FUV-NUV color. A same trend is observed for the UV slope /3 (assuming It seems that the degeneracy could prevent us from determining the dust attenuation from the f>. ex: >.f3). UV slope for Luminous IR galaxies (LIRGs) and Ultra-LIRGs (see also Fig. 5).

337 Figure GALEX Early m the ELAIS South Fidd (top) the 2-D 4: Release Obs·�rvationg Color I age of l and imag•c of the same' field. The fireworks--like due t.u spectroscopic each suh--exposure, <:bangingaspect the of the latter is tlH' fact that the grism Ls rot1tted for direction nf the disp<'!·sio1i.

338 -14.70

� >< -14.80 :s

-14.90

- -15.00

-15.10 3.15 3.20 3.25 3.30 3.35 3.40 log(wave)

Figure 5: The highest-quality spectra is shown with its fits (/>. This object seems to exhibit a trough ex >.il). consistent with being due to a bump around 220 nm 3.34) simHar to the Milky Way Extinction law. (log10(>.)= The regular fit (heavy line) gives f3 = -1.58 ±0.04. If we estimate f3 without the trough by exluding pixels in the range 1900A - 2500A, i.e. 3.28 3.39 (thin line), we obtain f3 -1.26 at 8 from the previous value. � log10(>.) � = rI

339 3.0 -sb+oldlMC"'5.l -s;b�old(LMC) 0"O°' • � sb+old(LMC/5) 0" --. -.. •i::b+old(LI.,C/10) 2.5 -Cf sb+old/2 0" Q0 sb+old/3 0" - -CF sb+old/S • • •Cf 0° ---- ..CF sb +sb+old/10 5 clumps(LMC/5) O" 0" > 2.0 clumps(LMC/10) " -... "so + 5 O" z ,,_ +- 5 dumps(LMC/20) 0" -C.alzSB- sb 1.5 - --� sb + 20 dumps(LMC/80) 0° !!:. - sb + 20 c!umps(LMG20) 0" t � II.OI 1.0 .2 0.5

SBSFR + LMC: o.o r· expo 5 Gyrs

·0.5 -0.2 0.0 0.2 0.4 0.6 0.8 LO FUV·NUV 3.0 SFR : expo S Gyrs 2.5

2.0 > " z !t. 1.5 ..

� 1.0 II.1:11

.2 0.5

o.o

-0. S -0.2 0.0 0.2 0.4 0.6 0.8 1.0 FUV·NUV

Figure 6: We show the resulting models in a Log(Fdu,,/FNuv) vs. FUV-NUV for several configurations : Calzetti

law for young stars (age < 10 Myrs) plus LMC (top) and MW (bottom) for older stars. We assume, here, a exponential SFR with a decaying rate of 5 Gyrs. Other laws e.g. Charlot & Fall (2000), Meurer et al. (1999) are also shown for comparison.

340 References

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