FERMI OBSERVATIONS OF CASSIOPEIA AND CEPHEUS: DIFFUSE GAMMA-RAY EMISSION IN THE OUTER GALAXY

By A. Abdo, M. Ackermann, M. Ajello, L. Baldini, J. Ballet, G. Barbiellini, D. Bastieri, B. Baughman, K. Bechtol, R. Bellazzini and 144 more. 2009

156 citations recorded by the source corpus.

We present the analysis of the interstellar γ-ray emission measured by the Fermi Large Area Telescope toward a region in the second Galactic quadrant at 100° ⩽ l ⩽ 145° and −15° ⩽ b ⩽ +30°. This region encompasses the prominent Gould Belt clouds of Cassiopeia, Cepheus, and the Polaris flare, as well as atomic and molecular complexes at larger distances, like that associated with NGC 7538 in the Perseus arm. The good kinematic separation in velocity between the local, Perseus, and outer arms, and the presence of massive complexes in each of them, make this region well suited to probe cosmic rays (CRs) and the interstellar medium beyond the solar circle. The γ-ray emissivity spectrum of the gas in the Gould Belt is consistent with expectations based on the locally measured CR spectra. The γ-ray emissivity decreases from the Gould Belt to the Perseus arm, but the measured gradient is flatter than expectations for CR sources peaking in the inner Galaxy as suggested by pulsars. The XCO = N(H2)/WCO conversion factor is found to increase from (0.87 ± 0.05) × 1020 cm−2 (K km s−1)−1 in the Gould Belt to (1.9 ± 0.2) × 1020 cm−2 (K km s−1)−1 in the Perseus arm. We derive masses for the molecular clouds under study. Dark gas, not properly traced by radio and microwave surveys, is detected in the Gould Belt through a correlated excess of dust and γ-ray emission: its mass amounts to ∼50% of the CO-traced mass.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/ac88dc77899f68e8fc20577cae47cb10932614ed

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Citations in the corpus

A general model for the CO–H2 conversion factor in galaxies with applications to the star formation law (2011, background): “…2006; Draine & Li 2007; Pineda, Caselli & Goodman 2008; Leroy et al. 2011; Magdis et al. 2011) and (iii) γ -ray emission arising from the interaction of cosmic rays with H2 (Bloemen et al. 1986; Bertsch et al. 1993; Strong & Mattox 1996; Hunter et al. 1997; Abdo et al. 2010b; Delahaye et al. 2011).”
THE CO-TO-H2 CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES (2012, methodology): “This standard value has been recovered to within a factor of 2 by various techniques: γ-ray measurements (Digel et al. 1996; Abdo et al. 2010); virial masses (Solomon et al. 1987); and dust (Dame et al. 2001; Pineda et al. 22 Sandstrom et al. 2008; Planck Collaboration et al. 2011a).”
FERMI-LAT OBSERVATIONS OF HIGH-ENERGY γ-RAY EMISSION TOWARD THE GALACTIC CENTER (2015, methodology): “Templates tracing γ-ray emission processes were used to determine the γ-ray emissivity of the interstellar gas within several kpc of the Sun (e.g., Abdo et al. 2009a, 2010c; Ackermann et al. 2011).”
THE SPECTRUM OF ISOTROPIC DIFFUSE GAMMA-RAY EMISSION BETWEEN 100 MeV AND 820 GeV (2014, methodology): “This change contributes to a reduced integrated IGRB intensity above 100 MeV of 7.2± 0.6× 10−6 ph cm−2 s−1 sr−1 in comparison to the 1.03± 0.17× 10−5 ph cm−2 s−1 sr−1 reported in Abdo et al. (2010b).”
CO-dark gas and molecular filaments in Milky Way-type galaxies (2014, background): “…the H I 21 cm emission and CO emission show that there is a considerable amount of hydrogen surrounding molecular clouds that is not well traced by either form of emission, and that has therefore been interpreted as being CO-dark molecular gas (Grenier, Casandjian & Terrier 2005; Abdo et al. 2010).”
DEVELOPMENT OF THE MODEL OF GALACTIC INTERSTELLAR EMISSION FOR STANDARD POINT-SOURCE ANALYSIS OF FERMI LARGE AREA TELESCOPE DATA (2016, methodology): “The template method was also applied to LAT observations for studying the interstellar emission in several dedicated regions (Abdo et al. 2010a; Ackermann et al. 2011b, 2012c, 2012a; Ade et al. 2014b, 2014b).”
THE DARK MOLECULAR GAS (2010, methodology): “In general, the X ratios derived from virial mass estimates exceed those based on gamma-ray observations from resolved clouds by factors of 1.5-3.0 (Abdo et al. 2010).”
REGULARITY UNDERLYING COMPLEXITY: A REDSHIFT-INDEPENDENT DESCRIPTION OF THE CONTINUOUS VARIATION OF GALAXY-SCALE MOLECULAR GAS PROPERTIES IN THE MASS-STAR FORMATION RATE PLANE (2013, unknown): “In the past it has been common practice to adopt one of two discrete, “con- sensus” values when estimating molecular gas masses for high-redshift galaxies: αCO =4.4M⊙ (K km/s pc 2)−1 (the conversion factor that is found to apply to GMCs in the Milky Way; e.g. Bolatto et al. 2008, Abdo et al. 2010) f”
Turbulent molecular clouds (2012, background): “Two recent space experiments Fermi/LAT in the γ–ray domain (Abdo et al. 2010, Ackermann et al. 2011) and Planck in the submillimeter domain (Planck collaboration et al. 2011) independently confirm what was already found by the first γ–ray space mission, COS B (Lebrun et al. 1983): there are…”
COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies – I. Relations between H2, H i, stellar content and structural properties (2011, methodology): “Our choice of αCO is roughly the mean of values estimated in the Milky Way and in nearby galaxies (e.g. Strong & Mattox 1996; Dame, Hartmann & Thaddeus 2001; Blitz et al. 2007; Draine et al. 2007; Heyer et al. 2009; Abdo et al. 2010).”
PHIBSS: MOLECULAR GAS CONTENT AND SCALING RELATIONS IN z ∼ 1–3 MASSIVE, MAIN-SEQUENCE STAR-FORMING GALAXIES (2012, unknown): “In that paper we proposed that the lower than expected CO emission is the result of the more extensive UV photodissociation of CO molecules in lower metallicity environments, leading in effect in an increase of the conversion factor α by factors of >3 and >10 between solar and 0.5 and 0.25 times sol”
Sensitivity projections for dark matter searches with the Fermi large area telescope (2016, background): “On the other hand, it is reasonable to assume that variations in the cosmic-ray nuclei spectra are mild on the characteristic scale of interstellar gas complexes due to the moderate energy losses in propagation, so that the γ-ray emission from their mutual interactions can be assumed to be proportio”
The CO-to-H2 Conversion Factor (2013, background): “Another source of uncertainty has become increasingly apparent with Fermi, which resolves the Magellanic Clouds in γ-ray emission (Abdo et al. 2010,b).”
COMBINED CO AND DUST SCALING RELATIONS OF DEPLETION TIME AND MOLECULAR GAS FRACTIONS WITH COSMIC TIME, SPECIFIC STAR-FORMATION RATE, AND STELLAR MASS (2014, unknown): “…αMW = 4.36 ± 0.9 (M /(K km/s pc 2 )), equivalent to XCO=N(H2)/(TRJ=1Δv)= 2x10 20 (cm -2 /(K km/s), Strong & Mattox 1996, Dame, Hartmann & Thaddeus 2001, Grenier, Casandijan & Terrier 2005, Bolatto et al. 2008, Leroy et al. 2011, Abdo et al. 2010, Ostriker, McKee & Leroy 2010, Bolatto et al. 2013).”
ESTIMATING THE STAR FORMATION RATE AT 1 kpc SCALES IN NEARBY GALAXIES (2012, result): “This value is intermediate among recent determinations of the Milky Way αCO (e.g., Strong & Mattox 1996; Dame et al. 2001; Heyer et al. 2009; Abdo et al. 2010) and represents a commonly adopted ”best single value” for work on nearby disk galaxies (e.g., Wong & Blitz 2002; Leroy et al. 2008).”
GLOBAL COSMIC-RAY-RELATED LUMINOSITY AND ENERGY BUDGET OF THE MILKY WAY (2010, methodology): “The spatial distribution of the CR sources is based on pulsars as tracers of supernova remnants (SNRs) used in Strong et al. (2004b), but constant beyond the solar circle as indicated by analysis of theFermi–LAT data for the2nd Galactic quadrant (Abdo et al. 2010a).”