Dust in Low Metallicity Blue Compact Dwarf Galaxies

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Dust in Low Metallicity Blue Compact Dwarf
Galaxies
Vassilis Charmandaris, Yanling Wu, Jim
Houck (University of Crete Cornell University)
J. Rosenberg (George Mason Univ.) IRS Instrument
team (Cornell Univ.)
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Outline of Talk

• Some information on dust in the infrared
• Why should one be interested in low-Z galaxies
  and BCDs?
• Studying the BCDs with the Infrared Spectrograph
  (IRS) on Spitzer
• Presence of Polycyclic Aromatic Hydrocarbons
  (PAHs)
• Estimating Elemental Abundances using infrared
  lines
• Spectral Energy Distribution
• Infrared to radio correlation
• Conclusions

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Doing some physics Dust types

• Dust grains range in size from a few hundred Å
  to a few ?m. They are composed mainly of elements
  such as carbon and silicate compounds, and
  various kinds of ices.
• classical dust grains ? 0.1 ?m in size,
  containing ? 10000 atoms
• responsible for the FIR, sub-mm emission
• very small grains ? containing ? 100 atoms
  (less than10nm in size)
• responsible for a rising continuum 10mm
  
• PAHs (Polycyclic Aromatic Hydrocarbons) ?
  benzene rings contain N ? 50 atoms
• trace photodissociation regions.
• Due to the size distribution of grains
  variations in the underlying radiation field the
  dust temperature varies (cold, warm, hot dust)
• Spectra of different dust components are fitted
  by modified Planck curves
• I? ? ?? B?(T)
• Power-law emissivity ?? ? ?n (i.e.
  Dale et al, ApJ, 2001)
• Really hot dust (200-1500K) in a equilibrium is
  observed via a near/mid-IR bump close to tori
  of AGNs.

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PAH Normal Modes

• 10-20 of the total IR luminosity of a galaxy
• Tens - hundreds of C atoms
• Bending, stretching modes ? 3.3,6.2,7.7,8.6,11.2,
  12.7 ?m
• PAH ratios ? ionized or neutral, sizes,
  radiation field, etc.

Leger Puget (1984) Sellgren (1984) Desert, et
al. (1990) Draine Li, (2001) Peeters, et al.
(2004)
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Energy Balance
Helou et al., ApJ, 2001
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Dust in Low Metallicity Environments

• Chicken Egg Problem
• Dust grains act as catalyst for the formation of
  molecules and stars
• Dust is created at the late stages of stellar
  evolution
• If the first grains were created by Pop III
  stars, what were their properties?
• Do we see complex organic molecules (ie PAHs) at
  low metallicities?
• Challenges
• Low abundances in the ISM lead to formation of
  high mass stars.
• High mass (early O type) stars produce more high
  energy UV photons.
• UV photons dissociate / destroy C-H chains
• Lack of metals in the ISM lead to reduced
  efficiency in the gas cooling, hence UV photons
  propagate at longer distances
• Where to look?
• Distant High-z QSOs - Evidence of thermal dust
  emission at z6
• Even at z6 metallicity is elevated
• Galaxies are faint for mid-IR spectroscopy even
  for Spitzer
• Identify nearby low metallicity systems with
  recent bursts of star formation

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Low Metallicity Spitzer Samples

• IRS/GTO Sample
• A total of 61 Blue Compact Dwarfs observed
• Metallicities spanning from 0.02 to 0.6 Solar
• IRS Spectra obtained for 16 of the galaxies
• Additional 16 and 22um images for the whole
  sample
• Rosenberg NDWFS/KISS Sample
• A total of 19 star forming galaxies with MBgt
  -18mag within the NDWFS area
• Spectroscopy available via KISS (AGN excluded)
• Abundances 12log(O/H) 7.8 --gt 9.1 (median
  metallicity 8.05 0.25 Solar)
• IRAC, MIPS and IRS16um data available
• MIPS/GTO Sample (Engelbracht 2005,2008)
• GO Samples (Thuan, Hunt et al)

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The Blue Compact Dwarfs (BCDs)

• The BCDs are
• Blue colors typical of O,B,A stars
• Compact dimensions less than 1kpc
• Low Luminosity MB gt -18mag
• Dominated by a recent burst of star formation
• Have low metallicity (lt 0.3 Solar)
• The are interesting to study
• They are found nearby (Second Buyrakan Survey)
• Hierarchical galaxy formation scenario proposes
  that bigger structures are built from smaller
  galaxies. Building-block galaxies are too faint
  and small to be studied at high-redshift.
• Review Kunth Ostlin 2000

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IRS spectroscopy of low metallicity galaxies
Wu et al. 2006
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Mid-IR spectra as a function of metallicity

• Engelbracht et al. 2008

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Decompositions of PAHs

• (SINGS - Smith et al. 2007)

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Fractional PAH power
(SINGS - Smith et al. 2007)
6.2µm, 12.6µm gt 10 7.7µm gt 30 11.2µm, 17µm
gt 15
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PAHs vs Metallicity in BCDs
Wu et al. 2006
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PAHs vs Neon line ratio
Wu et al. 2006
Starbursts
NeII 21.56eV NeIII 40.96eV
see Brandl et al. 2006
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Metallicity and presence of PAH
Rosenberg et al. 2007
Engelbracht et al. 2005
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Star Formation Rates of BCDs

• The metallicity-corrected SFRs agree well with
  those estimated from radio continuum, though none
  might reflect the true SFRs.
• Both SFRs from IR and radio are underestimated,
  but with a similar factor (Bell 2003)

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Star Formation Rates as a function of metallicity
Rosenberg et al. 2007

• Substantial scatter depending on the method
  used. (Boquien -Lisenfeld)
• Attention in high-redshift systems

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Ne/H (IR) correlates with S/H (IR)
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Neon and Sulfur abundances - IR vs Optical
Wu et al. (2008)

• Overall consistent results between optical and
  IR studies
• Slightly elevated Ne values as estimated in the
  IR

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Ne/S (IR) vs O/H (Optical)

• Discrepancy in Ne/S estimates - already been
  seen in PNe (Bernard-Salas 2002)
• Possibly due to optical Ionization Correction
  Factors - Temperature dependence in optical
  (TeOIII)

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(Crude) SED template fitting
Rosenberg et al. 2007

• The SEDs of low-metallicity dwarfs are warmer

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(Crude) SED template fitting (2)
Rosenberg et al. 2007
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Average SEDs
Engelbracht et al. 2008
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Radio to far-IR correlation in BCDs

• The low luminosity dwarf galaxies appear to have
  a similar slope as compared to that of normal
  galaxies.
• ltqFIRgt2.3 0.2 (Condon et al. 1992)
• ltqFIRgt2.37 0.26 (this sample)

Wu et al. (2008b)
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Mid-IR to Radio Correlation - q24

• Mid-IR emission also traces star formation
  activity, and shows more variation than FIR.
• Mid-IR luminosities are becoming available for
  low luminosity systems from deep Spitzer surveys.
• ltq24gt1.25 0.41
• lt q24gt0.94 0.23
• (Appleton et al. 2004)

Wu et al. (2008b)
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q24 as a function of metallicity

• No clear correlation is seen between the q24 and
  the galaxy metallicities.
• q24 appears to decrease with metallicity in
  metal-poor sources
• SBS0335-052E is an outlier.

Wu et al. (2008b)
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Conclusions

• Decreasing metallicity (and the presence of an
  AGN), suppress PAH emission and decrease their
  fractional contribution to the total IR
  luminosity.
• No PAH emission has been detected in systems
  with 12 log(O/H) lt 8.0 (small number
  statistics)
• SED template fitting appears consistent with this
  finding.
• Infrared to Radio correlation holds in BCDs
• Possible decrease in mid-IR to radio with
  metallicity
• Warmer dust at lower metallicities
• Unclear why SBS0335-052E and IZw18 are so
  different.

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