The evidence for microwave emission from spinning dust grains has been strengthened considerably by its detection in a number of discrete astrophysical objects associated with star formation. These detections, in combination with statistical constraints on its presence on large angular scales in the diffuse ISM, have provided strong observational confirmation of an emission mechanism still referred to as anomalous. This emission has a peaked spectrum with a maximum in the microwave band; the present review discusses the continuum radio emission mechanisms which can contribute to this region of the electromagnetic spectrum, collects published results on the prevalence of anomalous microwave emission in a variety of star formation regions, presents the overall conclusions that may be drawn from the detections so far, and discusses the prospects for future research on the anomalous microwave emission attributed to spinning dust within star forming regions.
The interstellar medium (ISM) of our galaxy and others is volume dominated by a small number of components. These components are differentiated by their temperature, ionization state, and density: the cold neutral medium (CNM) with hydrogen density
In addition to these three phases there also exist quasistatic, long-lived components with pressures far in excess of the ambient ISM, which have a much smaller volume filling factor but represent the bulk of mass in the galaxy. These giant cloud components are virialized and gravitationally bound, and their increased pressure is a result of an ongoing internal struggle to produce pressure gradients which will balance their own self-gravity. Unlike the ambient ISM phases, these clouds are largely molecular with little ionized gas. Their high densities make them opaque to optical or ultraviolet radiation and they have very low temperatures of 10–30 K. These molecular clouds have a partly hierarchical structure, with a self-similar density structure thought to arise naturally from a turbulent medium [
Once massive stars (typically O or B spectral type) have formed within these clouds, the effect of their energetic UV photon flux (
It is these two higher pressure components of the ISM, molecular clouds, and H
In spite of the pervasive nature of dust in star formation regions, there is still debate about many of its properties. Dust is composed of carbonaceous, silicate, and/or metallic grains [
Although star formation regions are generally studied at much shorter wavelengths, the identification of such regions with anomalous microwave emission attributed to spinning dust has led to a surge of observational studies at radio-microwave frequencies. These studies concentrate not only on small angular scales where radio emission is known historically to be associated with protostellar objects, but also on the extended scales of the clouds which contain this activity and indeed the wider complexes of such clouds.
The structure of this review is as follows: in Section
The mechanism and spectrum of the anomalous microwave emission from spinning dust are described elsewhere in this volume, and so I will not repeat that description here. However, at centimetre wavelengths, there are a number of alternative emission mechanisms which also contribute to the overall spectrum of star forming regions. The measured SED is consequently often a combination of multiple types of radiation processes and identifying the contribution from spinning dust alone requires careful separation of these components. Here I give a brief overview of the major alternative mechanisms which may be found to contribute to the overall spectrum. This overview is not an exhaustive description of these mechanisms, for which I refer the reader to the many more detailed investigations referenced in the text, but is intended to provide the reader with a working “toolkit” with which to understand and interpret the following discussions about the observational constraints on anomalous microwave emission from star formation regions.
Anomalous microwave emission is often discussed in the context of the three major radio emission mechanisms: synchrotron, bremsstrahlung, and thermal (i.e., vibrational) dust emission. Although such mechanisms are usually characterized by canonical spectral indices of
There is a variety of mechanisms expected to give rise to radio emission from star formation regions. The most commonly observed of these is that of thermal bremsstrahlung, or
Where the plasma giving rise to the free-free emission is reasonably uniform in density, this leads to a characteristic radio spectrum which has two components delineated by the frequency at which the optical depth equals unity (
The frequency at which an ionized plasma becomes optically thick/thin depends on its size, density, and to a lesser extent its temperature. The first two of these quantities are combined into the emission measure of such a plasma; the turnover frequency where optical depth equals unity is a strong function of the emission measure.
The emission measure of an ionized plasma surrounding a massive young star can be used to establish the relative age of the object with denser, higher emission measure objects corresponding to younger systems. The highest emission measure objects are referred to as hypercompact H
Physical parameters for different types of H
Class | Size | Density | EM |
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(pc) | (cm−3) | (cm−6 pc) | |
Hypercompact | ≤0.03 | ≥106 | ≥1010 |
Ultracompact | ≤0.1 | ≥104 | ≥107 |
Compact | ≤0.5 | ≥5 × 103 | ≥107 |
Classical |
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The integrated flux density spectra, following (
(a) Spectra of different types of H
In practice, the optically thick emission of ultra- and hypercompact H
Frequently the ionized plasma surrounding regions of star formation cannot be well approximated by uniform optical depth but instead has regions of varying density, resulting in a
The canonical geometry for a stellar wind is that of a spherical region of ionized, isothermal (the polytropic case for the same geometry was considered by Chiuderi and Ciamponi [
The same spectral behaviour is also expected from a conical isothermal jet [
Opening angles for outflows from young stellar objects (YSOs) vary with evolution. For example the earliest stage of YSO (Class 0) has highly collimated outflows which become less collimated as they evolve through Class I and, where an outflow remains, leave a wide angle outflow from Class II objects. Typically, however, one may assume that
Outflows which are collimated (
There are two major indirect methods for establishing the expected level of free-free emission from a given region. The first is to use of radio recombination lines (RRLs) to determine the local emission measure and use this to predict the free-free flux density. The integrated line temperature is proportional to both the temperature of the electron plasma and the emission measure,
By assuming an electron temperature, one may determine the emission measure; however generally RRL measurements record both the line temperature and the continuum brightness temperature. By taking the ratio of these two quantities,
The width of RRLs towards H
The second indirect method for predicting free-free emission is to use
Although broadly correct, this treatment is somewhat simplistic in its treatment of the emitting species. Whereas free-free emission will arise from
The spectral behaviour of continuum thermal dust emission is far more coherently expressed than that of free-free emission. Dust spectra are heavily dominated by modified blackbody emission, with an additional subdominant component from the emission lines of individual dust species. The observed continuum flux density from such emission in the Rayleigh-Jeans limit and optically thin limit is characterized as
The opacity index,
For a typical power-law index of
Consequently the contribution from thermal dust at microwave frequencies (
The flux density of dust emission at a single frequency can be used directly to characterize the dust properties of the emitting region. Specifically, the hydrogen column density can be found using
If only free-free emission is involved then the radio spectral index of the flux density with increasing frequency should never be substantially negative. However, this is not the case when free-free is obscured by a dust cloud along the line of sight. The dust and the free-free will have differing optical depths,
Since the optical depth of the dust will be
This kind of absorption can produce highly negative spectral indices in principle. However, in practice the dust column densities required are extremely large compared to the expected value in the ambient ISM; see Section
Of the seven different environments considered as possible sites for producing spinning dust emission by Draine and Lazarian [
There is some confusion in the literature over terminology in this area, and here I explicitly define my nomenclature. Clouds or dark clouds (DC) are regions of high visual extinction on scales of a few parsecs. They contain large quantities of molecular gas, which often harbours
Reflection nebulae (RN) are regions of gas subject to UV flux from nearby stars insufficient to ionize the neutral medium, but strong enough to cause scattering effects which illuminate the dust. Typically they are associated with high mass star formation or particularly luminous premain sequence (PMS; low-mass) stars. In general these low-mass PMS stars are Herbig Ae/Be stars [
Photodissociation regions (PDRs), also sometimes called photon-dominated regions, contain ultraviolet photons from nearby stars, which are not energetic enough to ionize hydrogen and create H
The earliest targeted observations of star formation regions for the purpose of detecting spinning dust emission were made with the Green Bank 43 (140 foot) telescope [
The early tentative detection of spinning dust from the H
As described in Section
Although the emission measure recovered from
Further studies of H
Relative emissivity of
Aside from the difference in observing frequency, a further potential cause of the discrepancy in AME detection between these two samples of H
A significant possibility for misidentification of anomalous microwave emission from H
Unlike low-mass stars, the earliest stages of high-mass star formation where the central object is still undergoing accretion are generally undetectable at radio frequencies. This is because, even though the protostar produces a high enough UV flux to cause significant ionization, the absorption from infalling matter prevents the photons travelling very far from the central source [
Radio recombination lines (RRLs) provide a useful method for establishing the degree of contamination from denser plasma H
More recently the RCW 175 region was examined in detail by Tibbs et al. [
In Figure
Maximum likelihood parameters from the three models described in Section
Model |
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PL+GB |
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PL+SD+GB |
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EM (cm−6 pc) |
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abFF+GB | 5800 |
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SED of RCW 175. (a) Power-law radio emission plus thermal dust greybody. (b) Thermal bremsstrahlung with dust absorption plus thermal dust greybody. (c) Power-law radio emission plus spinning dust (WIM model; DL98) plus thermal dust greybody.
Distribution of excess microwave emission relative to 100
The first scenario is a simple power-law radio spectrum plus a greybody thermal dust spectrum (PL+GB; Table
A model including a thermal bremsstrahlung emission component suffering from dust absorption at high frequencies (scenario 2; abFF+GB), see Section
A model with both a power-law component and a spinning dust component (scenario 3; FF+SD+GB) is strongly preferred (≫
The emissivity of the excess emission at 31 GHz relative to 100
The Perseus molecular cloud is a well-known star forming cloud nearby in the galaxy. It is associated with three clusters containing premain sequence stars: IC 348, with an estimated age of 2 Myr (and spread of ±1.5 Myr; [
Anomalous emission within Perseus was originally identified using data from the COSMOSOMAS telescope [
Additional
The
An excess of microwave emission was detected towards
The dark cloud L1622 has been extensively studied since it provided the first confirmed detection of an anomalous microwave excess in its SED [
Unlike H
The well-constrained spectrum of the anomalous emission in L1622 was used to investigate the possibility that the excess could be due to rotational emission from fullerenes and fulleranes (hydrogenated fullerenes; [
The first directed sample of a larger number of dark clouds [
The relative emissivity of the two clear detections was found to be in excess of that seen on large scales at high galactic latitude with values of
The five positive detections from this sample of dark clouds were reobserved at higher resolution at radio frequencies [
A further significant result from these high resolution follow-up observations was the first clear morphological correlation of cm-wave radio emission with mid-infrared emission concentrated in
For those dark clouds where a microwave excess is detected the degree of emission relative to the FIR 100
Relative emissivity of microwave emission to 100
A similar correlation of 31 GHz intensity with
Much like dark clouds, reflection nebulae are not expected to emit at radio frequencies as they lack a high degree of ionization. However, significant microwave emission was detected towards the M78 reflection nebula [
The M78 region is part of the dark cloud L1630 within the Orion molecular cloud. It contains a number of reflection nebulae including NGC 2068, NGC 2071, NGC 2064, and NGC 2067. The nebula NGC 2023, which was used to typify the environmental conditions in reflection nebulae by DL98, is also found in the L1630 dark cloud at a distance of approximately
The first, tentative, detection of anomalous microwave emission from a PDR was made with the VSA telescope [
Strong 31 GHz emission from the
In addition to the regions suggested by DL98 as potential sources of spinning dust emission there have been a number of other objects associated with star formation which have been targeted.
An excess of microwave emission towards 18 planetary nebulae was investigated as possibly arising from spinning dust emission by Casassus et al. [
Additional large surveys of 442 planetary nebulae at 30 GHz from the OCRA-p instrument on the Toruń telescope [
The first detection of anomalous microwave emission from an extragalactic star-formation region was made towards the nearby face-on spiral galaxy NGC 6946 [
Observational studies of dark clouds for identifying anomalous microwave emission have largely concentrated on the arcminute scale emission, neglecting the known small-scale radio emission detected from a significant fraction of protostars [
The dense natal dust envelopes which surround YSOs can often conceal their embedded protostars at infrared wavelengths. However, the longer wavelength radiation in the radio band does not suffer to the same extent and is capable of penetrating the dust shell to make such objects detectable at radio frequencies. This radio emission is in addition to that of thermal dust, which is expected to have a spectrum that falls off steeply at long wavelengths, and has been observed to possess a spectrum rising with frequency, indicating that it occurs as a consequence of free-free from partially optically thick ionized plasma, see Section
In the case of low-luminosity protostars, the last of these mechanisms, that of shock ionization associated with the outflow, is often favoured. Although there is considerable uncertainty in measurements of outflow force, most recovered values for observed protostellar jets are considered energetically viable to explain the observed cm-wave radio emission [
A strong correlation between radio and bolometric luminosity has been known for some time in the case of YSOs [
(a) Distribution of radio luminosity,
L675 star-forming dark cloud. Greyscale shows the anomalous emission on large scales as observed with the AMI telescope [
A connection between the presence of active star formation within a dark cloud and the identification of a spinning dust component is more difficult to quantify. The observational uncertainties surrounding the identification of heavily embedded, low luminosity YSOs have led to the proposition that many clouds currently considered to be starless, including a number not previously thought to even be undergoing collapse, do in fact host star formation. This possibility creates complications when investigating the correlation of those clouds hosting star formation with those which are associated with anomalous microwave emission, as highlighted by the case of L675 [
Indeed the inverse correlation of spinning dust emissivity with column density of molecular hydrogen, see Figure
Perseus molecular cloud. Contours show radio emission at 16 GHz from the AMI telescope [
The connection between anomalous microwave emission and polycyclic aromatic hydrocarbon (PAH) molecules was suggested by DL98, who attributed the emission to such very small grains. These PAH molecules represent the extension at small sizes of the standard grain size distribution (e.g., MRL; see Section
Although PAH emission is seen from such a wide variety of galactic objects, the relative strength of this emission to the FIR continuum varies as a function of source type. This is of course true for the dust continuum also; H
(a) Relationship between the ratio of 6.2
The strength of PAH line emission is also related to the interstellar radiation field (ISRF), which is typically normalized to the measurements of Habing (an alternately used normalization is that of Draine [
In spite of the global trends illustrated in Figure
There are a number of observational issues which affect the identification of spinning dust emission. These stem from the fact that in order to make a (nonstatistical) detection the SED of the observed object must be well constrained from 1–100 GHz. This presents an immediate problem in terms of available data, with objects that are heavily studied at
The majority of radio telescopes operating from 1–15 GHz are calibrated from the scale of Baars et al. ([
Unlike radio telescopes, submillimetre instruments are typically calibrated from planets or bright planetary nebulae. For example the
More recently the Perley-Butler 2010 flux scale which uses the emission of Mars at frequencies
A further issue with compiling spectra over a range of frequencies is that in the case of extended objects two telescopes operating at the same frequency may measure completely different flux densities depending on the range of angular scales to which they are sensitive. For synthesis telescopes (interferometers), this may involve the loss of flux density not only on angular scales larger than the shortest separation of their elements, but also on intermediate scales where there is a gap in the
Currently the bulk of observational data on anomalous microwave emission comes from CMB experiments, which probe large angular scales, and these studies are aimed at detecting the continuum radio emission. Large angular scales are important for identifying regions where anomalous microwave emission is significant as available data indicates that the bulk of the radio signal occurs on these scales. However, information on small angular scales is crucial for probing star formation and circumstellar environments in more detail and understanding the anomalous microwave emission with respect to different ISM conditions requires sufficient resolution to separate distinct environments. Increasingly, galactic surveys undertaken from a non-CMB perspective are expected to contribute to the field of AME science, including those mentioned later in this section, and probe the connections between star formation and spinning dust. Understanding this relationship is important not only in terms of examining the microwave emission itself, but also in terms of probing the VSG and PAH dust populations. VSGs play an important role in the chemical and thermal balance of the ISM; see Section
The existence of VSGs is supported by the physical manifestations of their reprocessing of incident starlight. Such reprocessing by a substantial population of carbonaceous nanoparticles could potentially account for both IR emission features and the strong mid-infrared emission component seen by IRAS. However, the true size distribution of VSGs is poorly known as studies of interstellar extinction are relatively insensitive to its details. The MRN dust size distribution is assumed to significantly underestimate the fraction of ISM carbon content contained in this nanoparticle population and was accordingly modified by DL98 in their spinning dust model to include a significant population of VSGs, assumed to be largely PAH molecules.
Observationally determining the true extent of this VSG/PAH population is nontrivial. The MIR spectral emission features produced by PAHs are only produced when the molecules are subjected to a strong UV flux, which is often absent in the case of heavily embedded pre- and protostellar objects due to local opacity effects. In this respect, spinning dust emission will provide a highly complementary measure of the small grain population to MIR PAH emission in the absence of favorable excitation conditions. At high resolution the anticipated MeerGAL survey at 14 GHz, and additionally the proposed ALMA Band 1 instrument [
In addition to targeted studies with ALMA Band 1, at high angular resolution the new MeerKAT telescope will provide an invaluable resource with its uniquely fast survey speed at 14 GHz. The MeerKAT telescope will have a smaller aperture than the currently available Jansky Very Large Array (JVLA), the consequence of which is that the number of pointings for a 14 GHz survey is comparable to that of an JVLA 5 GHz survey (e.g., CORNISH; [
A balance between the high resolution data that will be available from instruments such as MeerKAT, ALMA, and the JVLA and the low resolution data from CMB experiments such as
Galactic coverage of the MeerGAL [
There is a paucity of surveys currently available in this frequency range due to the comparative expense in terms of observing time of working at higher radio frequencies for fixed aperture telescopes. The cost of a survey with a given sensitivity using a fixed aperture telescope scales as steeply as
Although observational correlations between the degree of detected anomalous microwave emission and other physical characteristics are emerging for a range of physical conditions, see, for example, Sections
Certainly the emerging impression is that those regions of lower density where the ISRF is high provide the most promising environments for detecting anomalous microwave emission. These conditions are linked in the sense that the effective incidence of UV photons on the small grain population is affected by both: a higher ISRF provides a larger incidence rate whereas higher density regions suffer from absorption of UV photons by larger grains. However, a balance is required as the very high ISRF found in some regions may also lead to destruction of the very small grain population.
The prospects for rapid progress in this field are good. Starting from what remains a low level of understanding concerning the necessary triggering mechanisms or local environments suitable for anomalous microwave emission, many questions will be asked and answered with every new dataset that is taken. Whilst information on large scales will be provided by instruments such as the