A&A 471, 467-474 (2007)
DOI: 10.1051/0004-6361:20065908
D. L. Block1,4 - F. Combes2 - I. Puerari3 - K. C. Freeman4 - A. Stockton5 - G. Canalizo6 - T. H. Jarrett7 - R. Groess1 - G. Worthey8 - R. D. Gehrz9 - C. E. Woodward9 - E. F. Polomski9 - G. G. Fazio10
1 - School of Computational and Applied Mathematics,
University of Witwatersrand, Private Bag 3, WITS 2050, South Africa
2 -
Observatoire de Paris, LERMA, 61 Av. de
l'Observatoire, 75014 Paris, France
3 -
Instituto Nacional de Astrofísica,
Optica y Electrónica,
Calle Luis Enrique Erro 1, 72840 Tonantzintla, Puebla, México
4 -
Mount Stromlo and Siding Spring
Observatories, Research School of Astronomy and Astrophysics, Australian
National University, Australia
5 -
Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive,
Honolulu, Hawaii, USA
6 -
Institute of Geophysics and Planetary Physics and Department of Physics, University of California,
Riverside, CA 92521, USA
7 -
Infrared Processing and Analysis Center,
100-22, CALTECH, 770 South Wilson Ave, Pasadena, CA 91125, USA
8 -
Washington State University, 1245 Webster Hall, Pullman, WA
99163-2814, USA
9 -
Department of Astronomy, University of Minnesota, 116 Church St. SE,
Minneapolis MN 55455, USA
10 -
Harviard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge,
MA 02138, USA
Received 26 June 2006 / Accepted 15 May 2007
Abstract
In an earlier study of the spiral galaxy M 33, we photometrically
identified arcs or outer spiral arms of intermediate
age (0.6-2 Gyr) carbon stars precisely at the commencement of the
HI-warp. Stars in the arcs were unresolved,
but were likely thermally-pulsing asymptotic giant branch carbon stars.
Here we present Keck I spectroscopy of seven intrinsically bright and red
target stars in the outer, northern arc in M 33. The target stars
have estimated visual magnitudes as faint as
.
Absorption bands of CN are seen in all seven spectra
reported here, confirming their carbon star status. In addition, we
present Keck II spectra of a small area 0.5 degree away from the centre of
M 33; the target stars there are also identified as carbon
stars. We also study the non-stellar PAH dust morphology of M 33
secured using IRAC on board the Spitzer Space Telescope.
The Spitzer 8
m image attests to a change of spiral phase at the start of
the HI warp. The Keck spectra confirm that carbon stars may safely be
identified on the basis of their red
colours in the outer, low
metallicity disk of M 33. We propose that
the enhanced number of carbon stars in the outer arms are an indicator
of recent star formation, fueled by gas accretion from the HI-warp
reservoir.
Key words: galaxies: evolution - galaxies: spiral - galaxies: individual: M 33 (NGC598) - galaxies: Local Group - galaxies: formation - galaxies: stellar content
Like the two dominant spiral galaxies in the Local Group
(MW and M 31), M 33 is known to have a prominent warp.
This warp is spectacular in the HI-21cm line:
at many points the line of sight intersects the disk twice. In M 33
the warping commences at a radius of 5 kpc and at a
radius of
10 kpc the rotation axis of neutral hydrogen gas is
inclined by some 40 degrees to the axis of the inner disk,
as revealed by the tilted ring model of Rogstad
et al. (1976). More recently,
Corbelli & Schneider (1997) confirm that at a
radius of 5 kpc (about 3 disk scalelengths), the HI distribution
in M 33 shows a distinct change in inclination.
Warps may be tidally generated when companions are obviously present; but in galaxies such as M 33 which do not have any close companions, warping and gas infall may be inextricably linked (for a review, see Sect. 7 in Binney 1992).
In this paper, we focus our attention on the outer warped disk of M 33, and in particular on its red stellar population. If the HI-warp in M 33 is induced by the infall of gas, then one interesting confirmation would be the presence of a red intermediate age (0.6 Gyr-2 Gyr) population of carbon stars at the locale of the HI-warp. Indeed, C-stars are expected to be associated with recent star formation. Tsalmantza et al. (2006) have found them towards the center in the LMC and associated with spiral arms in M 31, the loci of star formation. In the SMC and in M 33, their sample size was too small to trace their radial distribution, but Rowe et al. (2005) found them also relatively more abundant in the outer parts of M 33.
In an earlier study (Block et al. 2004),
we presented near-infrared images of M 33 from a deep subsample of
2MASS, in which individual stars were unresolved. The deep 2MASS
images revealed remarkable arcs or spiral arms of red stars in the
outer disk; the northern arc subtends
in azimuth angle
and
5' in width (see Fig. 1). The northern
arc is dominant although a very faint southern counterpart arc, forming
a partial ring, can also be seen. The arcs lie at a radius of 2-3 disk
scale lengths (in V, the disk scale length is 6 arcmin; Ferguson et al.
2006).
Surprisingly, Fourier analysis of the deep 2MASS images
showed that the dominant m=2 peak did not correspond to any inner
spiral arm morphology seen in optical photographs, for which M 33 is
so famous. Rather, the m=2 peak corresponds to the giant outer red arms in the
2MASS images (see Fig. 1).
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Figure 1:
A partial ring of very red stars is seen in this J+H+![]() |
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Block et al. (2004) noted that the very red colour of the arcs
could not be due to extinction by dust grains.
Freedman et al. (1991) derive a mean value for the total colour excess
for the Cepheids in M 33 of
mag, which
includes both foreground (Milky Way) and internal M 33 extinction. The
-band extinction
is approximately one-tenth that in the
optical (Rieke & Lebofsky 1985), so the dust extinction at
is only
0.03 mag. Extinction by dust grains cannot
generate the very red colours found in the arcs in the outer disk of M 33, where the extinction at
would be even smaller.
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Figure 2: The positions of the seven stars listed in Table 1 are indicated by plus signs; the stars lie in the northern swath of carbon bearing stars identified photometrically in an earlier study (Block et al. 2004). Spectra of these stars (see Fig. 3) were secured using the Keck I telescope in Hawaii. Also observed with its twin sister telescope, Keck II, are two stars which lie 0.5 degrees away from the centre of M 33 in an area identified by a white circle. These outlying stars are representative of a very red family identified in near-infrared imaging with the Hale 5 m reflector at Mount Palomar. In this DSS image, North is up and East is to the left. |
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Our methodology differs from conventional ways of photometrically identifying carbon stars using intermediate band filters (V, I, 77, 81) as presented by Rowe et al. (2005) for M 33, using the Canada-France-Hawaii Telescope (CFHT). With such a method, one needs a large enough telescope to resolve individual stars. The (V, I, 77, 81) photometric method was pioneered by Cook et al. (1986); another viable set of filters to identify carbon stars are the Sloan filters. Demers & Battinelli (2005) used the three Sloan filters g, r, i, but again resolution of stars is crucial. Demers & Battinelli (2005) also used the CFHT with the Megacam camera to identify 361 new C-star candidates in M 31.
Some investigators (e.g. Tsalmantza et al. 2006) have focussed their
attention on identifying carbon stars on the basis of the red
tail (see Figs. 1, 4, 6 and 9 in Tsalmantza
et al. 2006). The usefulness of such a methodology is that near-infrared all
sky surveys secured with moderate sized telescopes can potentially be exploited
to identify carbon stars in spiral galaxies well outside our Local Group, where
individual stars are not resolved.
The arcs seen in Fig. 1 are identified on the basis of their red
colours. They lie around 15 arcmin in radius,
between 12 and 18 arcmin, or 2-3 disk scale lengths, whereas the actual
disk truncates at 5 scale lengths (Ferguson et al. 2006).
They are therefore like spiral arms in the outer disk,
as seen also by Rowe et al. (2005).
The latter authors have identified the C and M-type stars in
the AGB population, and traced the carbon star to M star ratio (C/M) as a function of radius.
This ratio is not only an index of age but also of metallicity, increasing in
metal-poor regions. Indeed, a C-star requires that the O-dominated surface
be reversed to C-dominated, due to the dredged-up material from
the star interior. This is easier to do when the surface is metal-poor.
Rowe et al. (2005) show that the C/M ratio increases with radius
and then flattens beyond 20 arcmin (or 5 kpc), indicating a metallicity
gradient covering most of the disk, including the outer spiral arcs.
Our
photometric method uses the fact that the colour of the
northern arc extended to very red colours of
.
It was argued that while very old M
giants of solar abundance can indeed reach
(see e.g. Fig. 2
in Bessell & Brett 1998) and even redder if they are
super-metal-rich (see Frogel & Whitford 1987), stars with
in
low-metallicity regions cannot be M-giants but
rather, very red carbon stars. There is a strong radial metallicity
gradient in M 33 (-0.16 dex/kpc in O/H, over 4-5 kpc, see Beaulieu et al. 2006). Beaulieu et al. have
confirmed this metallicity gradient, already found
in HII regions, B-supergiants or planetary nebulae, by detecting beat Cepheids in
M 33. An excellent discussion of the metallicity gradient in M 33 may be found in Magrini et al. (2007).
The outer regions of M 33 are relatively metal-poor, and solar abundance is reached
only in the very central domain of M 33.
In this paper, we examine the robustness of our
photometric technique as a stepping stone to exploring
the outer disks of more distant spiral galaxies,
wherein individual carbon stars may be
present, but unresolved. To this end, we need follow-up
spectroscopy. Here we discuss exploratory Keck I and Keck II
spectroscopic observations of a few of the
reddest and intrinsically brightest stars in the northern arc. All
seven of our target candidates have very red
colours and
should be confirmed to be TP-AGB carbon stars if our methodology is correct.
As a prelude to conducting the Keck spectroscopy, we imaged a
section of the northern plume of M 33
with the Hale 5 m reflector at Mount Palomar. We used the
array near-infrared camera WIRC (Wilson et al. 2003),
to resolve individual stars.
The field of view is
with 0.25 arcsec pixels.
The seeing FWHM is 0.8'' in J, and 0.7'' in
the
band. The telescope was centered at
01h34m28.1s, +30d54m00s (J2000).
The total
integration time was
9 min,
reaching a limiting surface brightness at
of 23.2 mag per pixel2, or 20.2 mag per arcsec2
(
pixels gives 1 square arsec).
The point source photometry S/N=10 limits are 19.0, 18.0, 16.9 mag in
,
respectively.
Table 1 lists the magnitudes and colours of 7 candidate targets in the outer northern
arc seen in Block et al. (2004). Their positions are indicated by plus
signs (+) in Fig. 2. In deriving the absolute magnitudes in Table 1,
we assume a distance modulus to M 33 of 24.64m, corresponding to a linear distance of 840 kpc (Freedman
et al. 1991). Noting that
mag
(Freedman et al. 1991), and that the
-band extinction is approximately
one-tenth that in the optical (Rieke & Lebofsky 1985), we use a
dust extinction correction for the
apparent magnitudes of 0.03 mag.
Table 1: Magnitudes and colors of our target stars in the northern red arc of M 33.
Spectra of these carbon star candidates were obtained on 2004 Aug. 17 UT with the Low Resolution Imaging Spectrograph (LRIS; Oke et al.
1995) attached to the Keck I telescope. The aperture mask slitlets had widths and lengths
corresponding to 1
2 and 11
5, respectively. The 600 groove mm-1 grating gave a resolution of
8 Å per projected slit
width and a spectral range from 6825 to 9415 Å for slitlets near
the centre of the mask. The stars were dithered along the slitlets
between the 2 exposures, each 600 s long. The spectrum from each
slitlet was extracted and reduced independently, the wavelength
calibration being derived from airglow lines. An approximate flux
calibration was produced by observing the spectrophotometric standard
star G191B2B (Massey et al. 1988) in one of the slitlets near the
centre of the mask and transfering the system response function found
for this slitlet to the same wavelength intervals for the other
slitlets. This approach neglects any differential vignetting or other
field-dependent variations in response, but these are expected to be
small for LRIS in any case.
Also secured with the Keck II telescope were spectroscopic observations of an area 0.5 degree away from the centre
of M 33 (circled in Fig. 2). Near-infrared imaging with the
Hale 5 m reflector at Palomar had revealed stars with very red
colours
in this outer low metallicity region. The data were obtained on 2004 July 18 UT using the
Echellette Spectrograph and Imager (ESI; Sheinis et al. 2002) on the
Keck II telescope. We used the echellete mode which allows a wavelength
coverage from 3900 to 11 060 Å and yields a dispersion of
56 km s-1 with the 1'' slit. The spectra were obtained at
parallactic angle near transit, and the total exposure was 1200 s
for each object.
The spectra were reduced using a combination of J. Prochaska's IDL reduction package, ESIRedux, and the echelle tasks in IRAF. The spectra were flux calibrated using spectrophotometric standards from Massey et al. (1988) also observed with the slit at the parallactic angle.
Observations of M 33 were also made using all four bands of the Infrared Array Camera (IRAC) mounted on the Spitzer Space Telescope (Werner et al. 2004; Gehrz et al. 2007) on 2005 January 21 as part of a Spitzer Guaranteed Time Observing Program (Program ID 5) conducted by Spitzer Science Working Group member R. D. Gehrz.
The IRAC instrument (Fazio et al. 2004) is composed of four detectors
that operate at 3.6 m (channel 1), 4.5
m (channel 2), 5.8
m (channel 3) and 8.0
m (channel 4).
All four detector arrays
are
pixels in size with mean pixel scales of 1.221, 1.213, 1.222,
and 1.220
respectively. The IRAC filter band centers are at 3.548, 4.492, 5.661 and 7.87
m and the adopted zero magnitude fluxes
are 280.9 Jy (channel 1), 179.7 Jy (channel 2), 115.0 Jy (channel 3) and 64.1 Jy (channel 4) as described in the IRAC Data Handbook V3.0 (see Gehrz et al. 2007).
The M 33 mapping sequence consisted of 438 frames per channel, including a 3 point 1/2 pixel dither for each map position. The integration time was 12 s per frame.
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Figure 3:
Spectra of the seven stars seen in Fig. 2,
secured using the Keck I telescope.
All C stars candidates are very red, with
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The raw Spitzer data were processed and flux calibrated with
version 11.0.2 of the Spitzer Science Center (SSC) pipeline. Post-BCD
processing was carried out using an artifact mitigation algorithm
developed by Carey (2005) and
the 2005 September 30 Linux version of the
SSC MOPEX software (Makovoz & Khan 2005). The artifact mitigation
algorithm alleviates the effects of muxbleed, column pulldown/pullup,
electronic banding, and bias variations between the images. Three
additional corrections were implemented with MOPEX:
Background Matching, Outlier Detection, and Mosaicing.
Background matching was performed by minimizing the pixel differences in
overlapping areas with respect to a constant offset computed by the
program. Cosmic rays and other outliers were detected and eliminated
with the Outlier Detection module.
In the final step the images were
reinterpolated to a pixel scale of approximately 1.224'' pixel-1 and
mosaiced to create a final image spanning approximately 1.2
.
Even for nearby M 33, identification of carbon stars by spectroscopic means
necessitates the use of the largest of groundbased telescopes.
The first carbon star to be
spectroscopically confirmed in M 33 dates back to the work of Mould &
Aaronson (1986; Fig. 7) who used the Hale 5 m reflector at
Palomar. The
individual V magnitudes of each target star in Table 1 are estimated, from their
colours, to be fainter
than 22m, with one star (star 3 in Fig. 3)
as faint as
.
The spectra sample only the bright end of the
-band luminosity function
for carbon stars, which ranges from
6 to -9.
Much longer integration times would be needed for intrinsically fainter
carbon stars.
Carbon stars show a plethora of molecular spectral features, including the C2 Swan bands and the CN bands. The presence of the hugely dominant CN bands shortward of 7000 Å and 8000 Å and longward of 9000 Å are indicated by tick marks in Fig. 3 and unambiguously reveal the C-star status for each of these seven stars.
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Figure 4:
Spectra with the Keck II telescope, of
two stars 0.5 degree (![]() ![]() ![]() |
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Figure 5:
The topmost panel shows a dust emissivity 8 ![]() |
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The carbon star arcs are clearly seen in
images, but the
global ISM dust morphology can also be
effectively traced at 8
m. The reason is as follows:
as carbon stars pulsate, their atmospheres become extended
and matter may reach a temperature low enough for some
elements to condense into very small particles ("star dust'').
An outflow develops as a result of
radiation pressure and the star is progressively
surrounded by an expanding circumstellar shell of dust and gas
(Le Bertre et al. 2003). Emission from the shells
of both carbon stars and also dust from O-rich shells is detected at
8
m (Buchanan et al. 2006). Buchanan and co-workers note that "most of the C-rich stars have spectra that are
dominated by warm dust'' and these authors show that stars such as MSXLMC587 (O-rich) and MSXLMC1488 (C-rich)
both show emission in the IRAC 8
m window (see their Fig. 2, for example). The trend is
for the flux to climb from longer (
38
m) to the shorter
wavelength of 8
m. As an interesting aside, one does not need high ultraviolet excitation to produce
infrared emission features:
stars as cool as 3000 K can generate
PAH features, for example, as discussed by Li and Draine (2002).
Carbon stars are one of the most
important contributors to the replenishment of dust in the
interstellar medium. Our IRAC image of M 33 at wavelength 8 m offers
unprecendented insight into the global ISM dust morphology.
Deprojection of our IRAC images of M 33 were facilitated by adopting log
R25 = 0.23(RC3)
where R is the ratio of the major to minor axis, and a position angle of 23 degrees
(Deul & van der Hulst 1987; Regan & Vogel 1994).
In order to remove the small contribution of starlight
(at the end of the Rayleigh-Jeans tail)
from the 8
m image, we subtract a scaled
version of the IRAC 3.6
m map.
As in Block et al. (2006), we generate a "non-stellar'' pure dust
map from
8
m-
3.6
m, where
(a constant) assumes the
value of 0.25. This subtraction removes the contribution from stellar
photospheres and leaves only the emission from dust grains, which trace
the ISM morphology. The
major uncertainties entering into the precise value of
are, firstly, the extended
aperture correction,
and secondly, the exact mid-IR colour of the stellar populations which one
is modeling by
scaling the 3.6
m emission. The second correction
introduces uncertainties of order 0.05 mag or perhaps larger (Ashby, private communication).
A slightly smaller value of
(0.232 instead of 0.25) has been suggested by Helou et al. (2004),
but our use of a slightly higher photospheric contribution (at a level of 2 percent) is well within the
correction uncertainties; the choice
of
is only accurate at the 10-15 percent (and not one or two percent) level.
A complimentary method could, of course, be to focus our attention on
photospheric emission, and not ISM dust emission. At the shorter IRAC
wavelength of 3.6 m, carbon star photospheres would
be detected, but so would the photospheres of O-rich stars. A full
presentation and analysis of all IRAC images of M 33 will be presented
elsewhere (Gehrz et al., in preparation). In this paper, we confine our
attention to the dust grain morphology, as we did for the Andromeda
Spiral in Block et al. (2006).
The Fourier method has been extensively discussed in a number of papers (e.g.,
Considère & Athanassoula 1982;
Puerari & Dottori 1992, among others). In the Fourier method, an image is
decomposed into a basis of logarithmic spirals
of the form
.
The Fourier coefficients A(p,m) can be written as
The inverse Fourier transform can be written as
Firstly, the Fourier m=2 mode in the 8 m image also does not
correspond to the inner spiral arm morphology, but rather shows a local
peak at a radius of 14', just longward of 2 scalelengths. This is
precisely where the outer red arcs were detected by Block et al. (2004).
Another intriguing fact is that there is a distinct change of phase in the ISM dust morphology at 3 disk scale-lengths (18'), which is precisely the region of the outer boundaries of the "arcs'' or outer spiral arms, where Corbelli & Schneider (1997) find the prominent HI-warp.
We believe it fully consistent to propose that the very red, and relatively metal-poor stars have recently been formed by gas infall which is inextricably tied to its strong HI-warp. It seems highly plausible that fresh low-metallicity gas is being fed to the host galaxy M 33. The outer disk from which mass is accreted is inclined to the inner disk of M 33, and has a different angular momentum, as revealed by the tilted disk model.
Our conclusions are compatible with the work of Rowe et al. (2005) who published a spatial map of the C/M-star ratio, that reveals conspicuous maxima at exactly the positions of what we call "arcs'', or outer spiral arms. They interpret the radial distribution of the C/M ratio uniquely as a metallicity indicator, and conclude that there is a strong metallicity gradient up to 5 kpc (20 arcmin) and then the gradient flattens out. Their interpretation is in terms of the viscous disk model, where the outer shear of the rotation curve produces mixing in the outer parts, able to wash out abundance gradients.
The viscous disk model certainly applies to M 33 to some extent, and is certainly a needed ingredient of the external gas accretion model. The flattening of the metallicity gradient could be linked to the accretion of low-metallicity gas. The viscous accretion disk model has been frequently invoked to build exponential galaxy disks, either with viscous torques to exchange angular momentum, or with gravitational torques due to spiral arms or bars. In the latter case the equivalent viscosity is called "gravitational viscosity'' (e.g. Lin & Pringle 1987). If the viscous and star formation time scales are of the same order, then the resulting stellar disk has an exponential distribution independent of the disk rotation law and of the assumed viscosity prescription. The expected metallicity gradient is also exponential, as computed by Tsujimoto et al. (1995). As for M 33, it is highly possible that the "viscous disk'' model applies until a radius of 5 kpc (20 arcmin), i.e. just outside the arcs, and is the mechanism with which the accreted gas coming from the warped component is consumed into stars and disk formation. The optical disk is exponential up to this radius.
We have confirmed, through Keck spectroscopy of individual stars,
the presence of an enhanced abundance of carbon stars in the
outer spiral arms in M 33, in particular in the two "arcs'' identified
previously from their red colour (Block et al. 2004).
Our conclusions are in very good agreement with the recent work of
Rowe et al. (2005), who have produced a spatial map of the
C/M-star ratio in M 33 showing the conspicuous outer arms.
We also see the outer spiral arms in the Fourier m=2 component of the Spitzer
8 m dust emission image. These arms
correspond to the radius where the HI-21 cm warp starts, and we propose
an interpretation in terms of recent star formation, fueled from the
gas accreted from this outer reservoir. A very similar conclusion, in terms of M 33 accreting
gas, has recently
been reached by Magrini et al. (2007), on the basis of O/H, S/H
and [Fe/H] abundances
in the Triangulum Spiral Galaxy M 33.
The HI gas reservoirs in the outer parts of galaxies are observed outside nearly all spiral disks (Sancisi 1983). Their conspicuous warped morphology, even in the absence of any perturbation or companion, implies that this gas is being almost continuously accreted, with a different angular momentum than that of the inner disk (e.g. Binney 1992). Keres et al. (2005) discuss cold gas accretion along filaments in the cosmic web. Accreting systems in gas need not show any signs of accretion in stars, such as the presence of tidal tails, stellar loops or close companions.
The implications of the presence of carbon stars in the outer disk of M 33 immediately beckons the question of a possible ubiquity of such an intermediate age population in the outer domains of other spiral disks out to the Virgo cluster and beyond.
At hand is the potential to
apply our photometric technique of identifying thermally-pulsing
asymptotic giant branch carbon stars to more distant spirals on the basis
of their red
colours in low metallicity domains, an approach
adopted by other teams such as Tsalmantza et al. (2006).
It is a sobering thought that it takes a groundbased
class 8-10 m telescope to secure
individual carbon star spectra in the outer disk of M 33, our second closest
spiral, in realistic amounts of observing time. Hence the urgent need
to identify unresolved sets of carbon stars in distant spiral
galaxies, on the basis of colours alone - without resort to
resolving such stars first with thirty metre class telescopes,
and then imaging them through Sloan or (V, I, 77, 81) filters.
Carbon stars may make an important contribution to the IR luminosity
of high-redshift galaxies. Using the Large Magellanic Cloud as a
guide, carbon stars are produced in large numbers between ages of
about 0.6 and 2 Gyr. Therefore galaxies that undergo a burst of
star formation will have their IR light boosted 0.6 Gyr later, and
this extra light will die away after about 2 Gyr. We know that star
formation in the early universe
(Bouwens et al. 2003) was already proceeding
at redshift of at least .
There may be an epoch starting about 0.6 Gyr after the onset of star formation in the universe (i.e. at
redshifts
)
characterized by large numbers of carbon
stars. The dominant output from the carbon stars will be redshifted
into the mid-infrared where these stars could double the observed
extragalactic flux. Maraston (2005) indeed proposes the use of
carbon stars as an age indicator for high-redshift stellar populations.
Instruments like the Mid-Infrared Instrument on
board the JWST are needed to image such galaxies in their (rest-frame)
2.2 micron band.
Acknowledgements
This paper was completed whilst DLB was a Visiting Professor at the Mount Stromlo and Siding Spring Observatories, Canberra; the hospitality of Ken and Margaret Freeman is very warmly acknowledged. D.L.B. and I.P. are indebted to the Anglo American Chairman's Fund, Mr. C. Sunter, Mrs. M. Keeton and the Board of Trustees. I.P. acknowledges support from the Mexican foundation CONACyT under project 35947-E. This paper is based in part on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the financial support of the W. M. Keck Foundation. This work is also partially based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. R.D.G. and C.E.W. were supported by NASA through an award issued by JPL/Caltech. D.L.B. warmly thanks the Vice-Chancellor of the University of the Witwatersrand for the Vice-Chancellor's Research Award in 2006. D.L.B. also expresses much gratitude to Fani Titi of the TISO Foundation for his stellar support. We thank M. Ashby, P. Wood and S.P. Willner for their input. Finally, we thank the anonymous referee for insightful comments.