IMAGER: Expected results from UV dust observations of M101
M. E. Danowski1, T. A. Cook1, K. D. Gordon2, S. Chakrabarti1
1Department of Astronomy and Center for Space Physics, Boston University; 2 Space Telescope Science Institute
Background
The Interstellar Medium Absorption Gradient Experiment Rocket (IMAGER)
will probe the correlation between ultraviolet dust extinction, and the
metallicity and radiation environment in M101. Evidence from studies of
starburst galaxies (Englebracht et al. 2008) indicate that active, high-mass star
formation modifies the UV dust extinction curve, demonstrated by the lack of
a 2175 Å bump.
With photometry from IMAGER, we measure the apparent strength of the
2175Å bump, the UV continuum, and the far-UV rise− the spectral features
which probe the nature of dust formation and processing in these areas near
massive star formation. With four 400Å-wide bandpasses, IMAGER is sensitive
to these UV extinction features and will utilize M101 as a laboratory for
studying dust in HII regions. M101 is nearly face-on, has large angular extent,
contains many well studied HII regions, and has a steep metallicity gradient,
log(O/H)+12 = 7.4 8.8 (Kennicutt et al. 2003).
RUV1
RUV2
RUV3
RUV4
Our data, when combined with infrared data from Spitzer, the DIRTY radiative
transfer model, and models of stellar evolution, will allow us to examine the
changes seen in the UV extinction curve and the IR emission features as a
function of metallicity and radiation field hardness. This study will directly
impact our understanding of the nature of dust and our ability to accurately
account for the effects of dust on observations at all redshifts.
Experiment
Dichroics
Detector
Secondary
Mirror
Results
RUV4 RUV3 RUV2
Electronics
Tertiary
OAP
Fold Mirror
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Mirror
Figure 3 (Top Left): Color-color plot of expected IMAGER data from idealized dichroics. The
lines are for different combinations of PEGASE stellar evolutionary synthesis (Fioc & RoccaVolmerange 1998), and DIRTY (radiative transfer) models. The large angle between
trajectories for these models combined with the age dependency ensures that any point can be
unambigulously assigned to a given dust model. Figure 4 (Top Right) A similar color-color
plot, but for GALEX & u-band data. The smaller trajectory angle makes it difficult to clearly
determine the type of dust. Figure 5 (Bottom Left): Color-color plot from expected IMAGER
data with system response. Figure 6 (Bottom Right) Simulated IMAGER Observations with
superimposed Hα-selected circular apertures.
RUV1
Star Tracker
Fold Mirror
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Field of View 30 arcminutes, fits M101 with pointing and 0alignment
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Å
Dichroic
Bandpasses
Four from three dichroic beamsplitters (see Figure 2)
One on 2175 Å bump, one on FUV rise
Two on UV continuum
Detector
Photon-counting microchannel plate z-stack detector
wedge-and-strip with four anodes from Photek
Photocathode
CsTe, QE ~10-15% across observed wavelengths
Electronics
Refurbished previously flown sounding rocket electronics
Resolution
9 arcseconds including pointing & alignment error, and detector
resolution
Figure 1 (Top): Simulated IMAGER Observations with nominal flight/observing time
t=389s, 6 arcsecond resolution, and expected dichroic and detector response. Figure 2
(Bottom) Measured extinction curves (Gordon et al. 2003). Normalized IMAGER
response curves to illustrate sensitivities of IMAGER bands to dust features.
Analysis
All IMAGER data will be calibrated for sensitivity, bandpass, and
resolution at Boston University. Using this information, we reduce raw
data and perform aperture photometry on the HII regions in the four
IMAGER ultraviolet bands. HII regions are visually identified by
correlating the UV images with Hα (van Zee et al. 1998). We measure the
flux within a circular aperture of 12 arc seconds at each HII region. We
also calculate and correct for the sky background.
To examine the strength of the 2175Å feature and the FUV rise, we
generate color-color plots and compare these to curves calculated from
models of stellar evolution, taking into account our system response and
radiative transfer effects. We then compare the strength of these features
at a given galactic location and compare to the local galactic properties.
Local dust content and aromatic feature strength are measured from
Spitzer, radiation field hardness is measured from line ratios, and the
metallicity is also available from measured emission lines. We will
compare galactic environment (metallicity and radiation hardness) on a
pixel-to-pixel scale with the observations to determine the relationship of
the dust properties to the environment.
Observations with the Spitzer Space Telescope have revealed that the strength
of the aromatic features (PAH features) correlates better with radiation field
hardness, a tracer of processing due to massive star formation, than it does
with metallicity, a tracer unaffected until a threshold of radiation hardness.
We will examine if this correlation extends to the UV– Are the UV extinction
features best correlated with the aromatic features and radiation hardness, or
are they better correlated with the metallicity?
If, as we expect, the 2175Å feature traces the aromatic/PAH features, we would
anticipate the central region of M101 (radius of a few arcminutes) to all have
the same strength 2175Å bump; past this ‘threshold radius’ the strength of the
bump should decrease rapidly. However, if the strength of this feature is
better correlated with metallicity, which strongly varies from the galactic
center to the outer edges, then we would expect the strongest 2175Å features
to lie in the center of the galaxy, at the nucleus, and then decline outwards.
Some dust grain models suggest that the 2175Å bump and the aromatic
features are carried by the same material (Li & Draine 2002).
Observations from IMAGER will provide an excellent probe of this
hypothesis.
References
Fioc, M., & Rocca-Volmerange, B. 1997, A&A, 326, 950
Gordon, K.D., Clayton, G.C., Misselt, K.A., Landolt, A.U., & Wolff, M.J. 2003, ApJ,
594, 279
Gordon, K.D., Engelbracht, C.W., Rieke, G.H., Misselt, K.A, Smith, J.-D.T.,
Kennicutt, Jr., R.C. 2008, 682, 336
Kennicutt, Jr., R.C., Bresolin, F., & Garnett, D.R. 2003, ApJ, 591, 80
Li, A. & Draine, B.T. 2002, ApJ, 576, 762
van Zee, L., Salzer, J.J., Haynes, M.P., O’Donoghue, A.A., & Balonek, T.J. 1998,
AJ, 116, 2805
Acknowledgments
IMAGER is a NASA sounding rocket experiment set to launch in 2011. This
work is supported by NASA grant NNX09AE23G.