Total Galactic Emission Research Articles

Star-formation-rate estimates from water emission

Context. The star-formation rate (SFR) quantitatively describes the star-formation process in galaxies throughout cosmic history. Current ways to calibrate this rate do not usually employ observational methods accounting for the low-mass end of stellar populations as their signatures are too weak. Aims. Accessing the bulk of protostellar activity within galactic star-forming regions can be achieved by tracing signposts of ongoing star formation. One such signpost is molecular outflows, which are particularly strong at the earliest stages of star formation. These outflows are bright in molecular emission, which is readily observable. We propose to utilize the protostellar outflow emission as a tracer of the SFR. Methods. In this work, we introduce a novel version of the galaxy-in-a-box model, which can be used to relate molecular emission from star formation in galaxies with the SFR. We measured the predicted para-water emission at 988 GHz (which is particularly bright in outflows) and corresponding SFRs for galaxies with LFIR = 108 − 1011 L⊙ in a distance-independent manner, and compared them with expectations from observations. Results. We evaluated the derived results by varying star-forming parameters, namely the star formation efficiency, the free-fall time scaling factor, and the initial mass function. We observe that for the chosen water transition, relying on the current Galactic observations and star formation properties, we are underestimating the total galactic emission, while overestimating the SFRs, particularly for more starburst-like configurations. Conclusions. The current version of the galaxy-in-a-box model only accounts for a limited number of processes and configurations, that is, it focuses on ongoing star formation in massive young clusters in a spiral galaxy. Therefore, the inferred results, which underestimate the emission and overestimate the SFR, are not surprising: known sources of emission are not included in the model. To improve the results, the next version of the model needs to include a more detailed treatment of the entire galactic ecosystem and other processes that would contribute to the emission. Thus, the galaxy-in-a-box model is a promising step toward unveiling the star-forming properties of galaxies across cosmic time.

ARCADE 2 OBSERVATIONS OF GALACTIC RADIO EMISSION

We use absolutely calibrated data from the ARCADE 2 flight in July 2006 to model Galactic emission at frequencies 3, 8, and 10 GHz. The spatial structure in the data is consistent with a superposition of free-free and synchrotron emission. Emission with spatial morphology traced by the Haslam 408 MHz survey has spectral index beta_synch = -2.5 +/- 0.1, with free-free emission contributing 0.10 +/- 0.01 of the total Galactic plane emission in the lowest ARCADE 2 band at 3.15 GHz. We estimate the total Galactic emission toward the polar caps using either a simple plane-parallel model with csc|b| dependence or a model of high-latitude radio emission traced by the COBE/FIRAS map of CII emission. Both methods are consistent with a single power-law over the frequency range 22 MHz to 10 GHz, with total Galactic emission towards the north polar cap T_Gal = 0.498 +/- 0.028 K and spectral index beta = -2.55 +/- 0.03 at reference frequency 1 GHz. The well calibrated ARCADE 2 maps provide a new test for spinning dust emission, based on the integrated intensity of emission from the Galactic plane instead of cross-correlations with the thermal dust spatial morphology. The Galactic plane intensity measured by ARCADE 2 is fainter than predicted by models without spinning dust, and is consistent with spinning dust contributing 0.4 +/- 0.1 of the Galactic plane emission at 22 GHz.

INTEGRAL IBIS Census of the Sky Beyond 100 keV

We report on the first census of INTEGRAL/IBIS detections ($\gtrsim 4\sigma$ significance) above 100 keV based on the Core Program and public Open time observations up to April 2005. There are 49 sources detected in the 100-150 keV band of which 14 are also seen in the 150-300 keV range. The low energy sample is dominated by X-ray binary systems of both low and high mass, but also includes 10 active galaxies. Of the binary systems that are detected above 150 keV, more than 50% are associated with black hole candidates, often reported as microquasars. The present survey results are then used to construct LogN-LogS curves for galactic and extragalactic objects in the 100-150 keV band: above a 1 mCrab sensitivity limit we expect that around 200 galactic sources and almost 350 active galaxies populate the sky above 100 keV. While the contribution of individual point sources to the total Galactic emission has been estimated to be around 70-80% between 100-300 keV, we find that active galaxies detected above 1 mCrab account for only about 3% of the cosmic hard X-ray background in the 100-150 keV band.

Gamma-ray continuum emission from the inner Galactic region as observed with INTEGRAL/SPI

The diffuse continuum emission from the Galactic plane in the energy range 18-1000 keV has been studied using 16 Ms of data from the SPI instrument on INTEGRAL. With such an exposure we can exploit the imaging properties of SPI to achieve a good separation of point sources from the various diffuse components. Using a candidate-source catalogue derived with IBIS on INTEGRAL and a number of sky distribution models we obtained spectra resolved in Galactic longitude. We can identify spectral components of a diffuse continuum of power law shape with index about 1.7, a positron annihilation component with a continuum from positronium and the line at 511 keV, and a second, roughly power-law component from detected point sources. Our analysis confirms the concentration of positron annihilation emission in the inner region (|l|<10), the disk (10<|l|<30) being at least a factor 7 weaker in this emission. The power-law component in contrast drops by only a factor 2, showing a quite different longitude distribution and spatial origin. Detectable sources constitute about 90% of the total Galactic emission between 20 and 60 keV, but have a steeper spectrum than the diffuse emission, their contribution to the total emission dropping rapidly to a small fraction at higher energies. In the SPI energy range the flux is lower than found by OSSE, probably due to the more complete accounting for sources by SPI. The power-law emission is difficult to explain as of interstellar origin, inverse Compton giving at most 10%, and instead a population of unresolved point sources is proposed as a possible origin, AXPs with their spectra hardening above 100 keV being plausible candidates. We present a broadband spectrum of the emission.

Chandra Observations of the X-Ray-luminous Star-forming Galaxy Merger Arp 299

We report results of a Chandra observation of the X-ray luminous star-forming galaxy Arp299 (NGC3690/IC694). We detect 18 discrete X-ray sources with luminosities above ~10^39 erg\s (0.5-8.0 keV band), which contribute ~40% of the total galactic emission in this band. The remaining emission is associated with a diffuse component spatially coincident with regions of widespread star-formation. We detect X-ray emission from both nuclei. One of the discrete sources within the complex nuclear region of NGC 3690 is found to have a very hard spectrum and therefore we associate it with the origin of the AGN-like spectrum that has also been detected at high X-ray energies using Beppo-SAX.