Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.arcetri.astro.it/~sperello/ASSP2012_289.pdf Дата изменения: Tue Jul 24 12:15:49 2012 Дата индексирования: Tue Oct 2 03:04:49 2012 Кодировка: Поисковые слова: virgo cluster

Dust Content of Virgo Star-Forming Dwarf Galaxies
M. Grossi, L.K. Hunt, S. Madden, C. Vlahakis, D.J. Bomans, M. Baes, G.J. Bendo, S. Bianchi, A. Boselli, M. Clemens, E. Corbelli, L. Cortese, A. Dariush, J.I. Davies, I. De Looze, S. di Serego Alighieri, D. Fadda, J. Fritz, D.A. Garcia-Appadoo, G. Gavazzi, C. Giovanardi, T.M. Hughes, A.P. Jones, D. Pierini, M. Pohlen, S. Sabatini, M.W.L. Smith, J. Verstappen, E.M. Xilouris, and S. Zibetti

Abstract We investigate the dust properties of a small sample of Virgo cluster dwarf galaxies drawn from the science demonstration phase data set of the Herschel Virgo Cluster Survey (HeViCS). These galaxies have low metallicities (7:8 < 12 C log.O=H/< 8:3) and star formation rates <0.1 M yr 1 . We measure

M. Grossi ( ) ґ ґ CAAUL, Observatorio Astronomico de Lisboa, Universidade de Lisboa, Portugal e-mail: grossi@oal.ul.pt L.K. Hunt S. Bianchi E. Corbelli C. Giovanardi S. di Serego Alighieri INAF-Osservatorio Astrofisico di Arcetri, Italy S. Madden ґ Laboratoire AIM, CEA/DSM- CNRS - Universite Paris Diderot, France C. Vlahakis Universidad de Chile, Santiago, Chile D.J. Bomans Astronomical Institute, Ruhr-University Bochum, Germany M. Baes I. De Looze J. Fritz J. Verstappen G.J. Bendo Astrophysics Group, Imperial College London, Blackett Laboratory, UK A. Boselli Laboratoire d'Astrophysique de Marseille, France M. Clemens INAF-Osservatorio Astronomico di Padova, Italy L. Cortese ESO, Garching, Germany A. Dariush J.I. Davies T.M. Hughes M. Pohlen M.W.L. Smith Department of Physics and Astronomy, Cardiff University, UK D. Fadda NASA Herschel Science Center, California Institute of Technology, USA P. Papaderos et al. (eds.), Dwarf Galaxies: Keys to Galaxy Formation and Evolution, Astrophysics and Space Science Proceedings, DOI 10.1007/978-3-642-22018-0 34, © Springer-Verlag Berlin Heidelberg 2012 289


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the spectral energy distribution (SED) from 100 to 500 m and derive dust temperatures and masses. The SEDs are fitted by a cool component with T < 20 K, implying dust masses around 105 M and dust-to-gas ratios (D) within the range 10 3 ­10 2 .

1 Introduction
Star-forming dwarf galaxies are fragile systems which can be easily affected by the environment where they are evolving through different processes, such as tidal interactions, ram pressure stripping, collisions and mergers. How these processes affect their dust content though is still a matter of debate. Mid/far-infrared (MIR/FIR) and submillimetre (submm) observations have provided information about the different dust components in dwarfs, showing that even metal-poor galaxies may host a significant amount of dust [4, 12]. Since most of these studies have focused mainly on bright and isolated dwarfs [5­7], little is known about the interplay between the environment and the dust content of low-mass and low-metallicity systems. As part of the Herschel Open Time Key Project, the HeViCS survey [2] will map an area of 64 square degrees of the Virgo cluster with PACS [18] and SPIRE [9] to investigate the dust content of the different morphological types within the cluster. The dwarf galaxy population of the Virgo cluster is dominated by dwarf ellipticals, but also contains a non-negligible fraction ( 10%) of late-type dwarfs with signs of current star formation activity [1]. Here we present the FIR properties of three star-forming dwarfs detected in the HeViCS Science Demonstration Phase (SDP) data set.

D.A. Garcia-Appadoo ESO, Santiago, Chile G. Gavazzi Universita' di Milano-Bicocca, Italy A.P. Jones Institut d'Astrophysique Spatiale (IAS), France D. Pierini Max-Planck-Institut fuer extraterrestrische Physik, Germany S. Sabatini INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, Italy E.M. Xilouris Institute of Astronomy and Astrophysics, National Observatory of Athens, Greece S. Zibetti Max-Planck-Institut fuer Astronomie, Germany


Dust Content of Virgo Star-Forming Dwarf Galaxies

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250 m m

Fig. 1 SDSS and Herschel images of VCC 562, VCC 1179, VCC 1356 at 250 m

2 Data Set
The SDP data set of the HeViCS survey probes a 4i 4i region around the center of the cluster. More details on the observations can be found in [2]. The data were reduced using the Level I procedures described in [19]. The angular resolution for PACS in fast scan parallel mode is 700 1200.7 and 1100.6 1500.7, at 100, and 160 m, respectively. For SPIRE, the PSF FWHM is 1800.1, 2500.2, and 3600.9 at 250, 350, and 500 m respectively. The SDP field contains 28 late-type low-luminosity galaxies classified in the Virgo cluster catalog [1] as Sm (7), Im (12), blue compact dwarf (BCD) (6) and dIrr (3). Three BCDs have been detected with both PACS and SPIRE: VCC 562, VCC 1179, VCC 1356 and here we present their FIR properties. Figure 1 shows optical SDSS and SPIRE (250 m) data for the three BCDs in our sample.

3 Dust Masses and Dust-to-Gas Mass Ratios
We fitted the PACSCSPIRE SEDs with a single modified Planck function and an emissivity law k / with D 2 to derive the temperature of the dust (Fig. 2; left panel). We assessed the dust masses using the 250 m flux densities, and the emissivity k D 4.67 cm2 g 1 at 250 m [15]. The resulting dust masses are few times 105 M , and the dust temperatures are around 20 K. However, Fig. 2 shows that the 500 m fluxes of VCC 1179 and VCC 1356 tend to be underestimated by the single-temperature fits. This difference could either be due to an additional cold ( 10 K) dust component [5­7], to an enhanced abundance of small grains [16] or to the different optical properties of the amorphous dust grains [17]. A more


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VCC562 F (mJy)

VCC1179

VCC1356
log(Mdust /Mgas)

­1 ­2 ­3 ­4 ­5 ­6
James et ol. 2002 Hunt et ol. 2005 Walter et ol. 2007 HeViCS BCDs

100

10 100 100 (mm) 100

6.5 7.0 7.5 8.0 8.5 9.0 9.5 12 + log (O / H)

Fig. 2 Left: SED fitting of the three dwarfs under study. Right: D vs oxygen abundance; HeViCS dwarfs are shown as filled circles and they are compared to data and models from the literature

detailed discussion on the excess at 500 m can be found in [10]. We determined the gas-to-dust mass ratios (D) using HI masses available from the ALFALFA catalog [8]. We assumed the total gas mass was given by the atomic component only, including a correction of 1.36 for helium. Figure 2 (right panel) shows D for the Virgo BCDs (filled circles) as a function of nebular oxygen abundance. Also illustrated are data from the literature [13, 14, 20]. Selected model predictions for the D vs metallicity relation are also shown [3, 11]. Our estimates of D for the three dwarfs, based on the single-temperature fit to the data, are roughly consistent with the predictions of dust formation models from [3].

4 Conclusions
We presented the FIR properties of three BCDs in Virgo detected in the HeViCS survey. The data indicate a cool dust component with T < 20 K and dust masses around 105 M . The completion of the full area of the survey will enable us to study a larger sample of star-forming dwarf galaxies, and to place more stringent constraints on the dust content of these systems in a dense cluster environment.

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