Using free-free emission measured in the Ka band (26-40 GHz) for 10 star-forming regions in the nearby galaxy NGC 6946, including its starbursting nucleus, we compare a number of star formation rate (SFR) diagnostics that are typically considered to be unaffected by interstellar extinction. These diagnostics include non-thermal radio (i.e., 1.4 GHz), total infrared (IR; 8-1000 μm), and warm dust (i.e., 24 μm) emission, along with hybrid indicators that attempt to account for obscured and unobscured emission from star-forming regions including Hα + 24 μm and UV + IR measurements. The assumption is made that the 33 GHz free-free emission provides the most accurate measure of the current SFR. Among the extranuclear star-forming regions, the 24 μm, Hα + 24 μm, and UV + IR SFR calibrations are in good agreement with the 33 GHz free-free SFRs. However, each of the SFR calibrations relying on some form of dust emission overestimates the nuclear SFR by a factor of ~2 relative to the 33 GHz free-free SFR. This is more likely the result of excess dust heating through an accumulation of non-ionizing stars associated with an extended episode of star formation in the nucleus rather than increased competition for ionizing photons by dust. SFR calibrations using the non-thermal radio continuum yield values which only agree with the 33 GHz free-free SFRs for the nucleus and underestimate the SFRs from the extranuclear star-forming regions by an average factor of ~2 and ~4-5 before and after subtracting local background emission, respectively. This result likely arises from the cosmic-ray (CR) electrons decaying within the starburst region with negligible escape, whereas the transient nature of star formation in the young extranuclear star-forming complexes allows for CR electrons to diffuse significantly further than dust-heating photons, resulting in an underestimate of the true SFR. Finally, we find that the SFRs estimated using the total 33 GHz flux density appear to agree well with those estimated using free-free emission due to the large thermal fractions present at these frequencies even when local diffuse backgrounds are not removed. Thus, rest-frame 33 GHz observations may act as a reliable method to measure the SFRs of galaxies at increasingly high redshift without the need of ancillary radio data to account for the non-thermal emission.

/pdf/calibrating-extinction-free-star-formation-rate-diagnostics-5n1131qv9l.pdf

CALIBRATING EXTINCTION-FREE STAR FORMATION RATE DIAGNOSTICS WITH 33 GHz FREE-FREE EMISSION IN NGC 6946

Abridged: Using free-free emission measured in the Ka-band (26-40GHz) for 10 star-forming regions in the nearby galaxy NGC6946, including its starbursting nucleus, we compare a number of SFR diagnostics that are typically considered to be unaffected by interstellar extinction: i.e., non-thermal radio (i.e., 1.4GHz), total infrared (IR; 8-1000um), and warm dust (i.e., 24um) emission, along with the hybrid (obscured + unobscured) indicators of H\alpha+24um and UV+IR. The 33GHz free-free emission is assumed to provide the most accurate measure of the current SFR. Among the extranuclear star-forming regions, the 24um, H\alpha+24um and UV+IR SFR calibrations are in good agreement with the 33GHz free-free SFRs. However, each of the SFR calibrations relying on some form of dust emission overestimate the nuclear SFR by a factor of ~2. This is more likely the result of excess dust heating through an accumulation of non-ionizing stars associated with an extended episode of star formation in the nucleus rather than increased competition for ionizing photons by dust. SFR calibrations using the non-thermal radio continuum yield values which only agree with the free-free SFRs for the nucleus, and underestimate the SFRs from the extranuclear star-forming regions by a factor of ~2. This result likely arises from the CR electrons decaying within the starburst region with negligible escape compared to the young extranuclear star-forming regions. Finally, we find that the SFRs estimated using the total 33GHz emission agree well with the free-free SFRs due to the large thermal fractions present at these frequencies even when local diffuse backgrounds are not removed. Thus, rest-frame 33GHz observations may act as a reliable method to measure the SFRs of galaxies at increasingly high redshift without the need of ancillary radio data to account for the non-thermal emission.

/pdf/calibrating-extinction-free-star-formation-rate-diagnostics-khx4yzr8r7.pdf

Calibrating Extinction-Free Star Formation Rate Diagnostics with 33GHz Free-Free Emission in NGC6946

We propose a new model for progenitor systems of Type Ia supernovae. The model consists of an accreting white dwarf and a lobe-filling, low-mass red giant. When the mass accretion rate exceeds a certain critical rate, there is no static envelope solution on the white dwarf. For this case, we find a new strong wind solution, which replaces the static envelope solution. Even if the mass-losing star has a deep convective envelope, the strong wind stabilizes the mass transfer until the mass ratio, q, between the mass-losing star and the mass-accreting white dwarf reaches 1.15, i.e., q < 1.15. A part of the transferred matter can be accumulated on the white dwarf at a rate that is limited to cr = 9.0 × 10-7 (MWD/M☉ - 0.50) M☉ yr-1, and the rest is blown off in the wind. The photospheric temperature is kept around T ~ 1 × 105-2 × 105 K during the wind phase. After the wind stops, the temperature quickly increases up to ~1 × 106 K. The white dwarf steadily burns hydrogen and accretes helium, thereby being able to increase its mass up to 1.38 M☉ and explode as a Type Ia supernova. The expected birth rate of this type of supernovae is consistent with the observed rate of Type Ia supernovae. The hot white dwarf may not be observed during the strong wind phase due to self-absorption by the wind itself. The Strong wind stops when the mass transfer rate decreases below cr. Then it can be observed as a supersoft X-ray source.

https://iopscience.iop.org/article/10.1086/310303/pdf

A New Model for Progenitor Systems of Type IA Supernovae

Most elliptical galaxies contain central black holes (BHs), and most also contain significant amounts of hot gas capable of accreting on to the central BH as a result of having cooling times that are short compared to the Hubble time. Why, therefore, do we not see active galactic nuclei (AGNs) at the center of most elliptical galaxies rather than in only (at most) a few percent of them? We here propose the simple idea that feedback from accretion events heats the ambient gas, retarding subsequent infall, in a follow-up to papers by Binney & Tabor and Ciotti & Ostriker. Even small amounts of accretion on a central BH can cause the release of enough energy to reverse the central inflow, when the Compton temperature (TX) of the emitted radiation is higher than the mean galactic gas temperature, the basic assumption of this paper. Well-observed nearby AGNs (3C 273, 3C 279), having TX near 5 × 108 K, amply satisfy this requirement. In this context, we present a new class of one-dimensional hydrodynamical evolutionary sequences for the gas flows in elliptical galaxies with massive central BHs. The model galaxies are constrained to lie on the fundamental plane of elliptical galaxies, and are surrounded by variable amounts of dark matter. Two source terms operate: mass loss from evolving stars, and a secularly declining heating by Type Ia supernovae (SNe Ia). Like the previous models investigated by Ciotti et al., these new models can evolve up to three consecutive evolutionary stages: the wind, outflow, and inflow phases. At this point the presence of the BH dramatically alters the subsequent evolution, because of the energy emitted by the accreting gas flow. The effects of Compton heating and cooling, hydrogen and helium photoionization heating, and bremsstrahlung recycling on the gas flow are investigated by numerical integration of the nonstationary equations of hydrodynamics, in the simplifying assumption of spherical symmetry, and for various values of the accretion efficiency and supernova rates. The resulting evolution is characterized by strong oscillations, in which very fast and energetic bursts from the BH are followed by longer periods during which the X-ray galaxy emission comes from the coronal gas. For a fixed galaxy total mass and structure, the length and the intensity of the bursts depend sensitively on the accretion efficiency and the SN Ia rate. For high efficiency and SN Ia rate values, short and strong bursts are followed by a degassing of the galaxy, with a consequent shut-off of the BH followed by a long period when the mass loss from the stellar population replenishes the galaxy, and after which a new cooling catastrophe another accretion event takes place. In this case, high accretion rates characterize the BH evolution, but the total mass accreted by the BH is very small. For low efficiency and SN Ia rate values, the luminosity evolution is still characterized by strong intermittencies, but the number of global degassing events is considerably reduced, and for very low efficiency values it completely disappears. The remaining instability is then concentrated in the central galactic regions. We also allow for departures from spherical symmetry by examining scenarios in which the central engine is either an advection-dominated accretion flow or a more conventional accretion disk that is optically thick except for a polar region. The general property of highly unstable accretion remains true, with central BHs growing episodically to the mass range 108-109 M☉ (in contrast to ΔMBH 1010-1011 M☉, if feedback is ignored). In all cases the duty cycle (fraction of the time that the system will be seen as an AGN) is quite small and in the range fBH 10-4-2 × 10-3. Thus, for any reasonable value of the efficiency, the presence of a massive BH at the center of a galaxy seems to be incompatible with the presence of a long-lived cooling flow.

https://iopscience.iop.org/article/10.1086/320053/pdf

Cooling Flows and Quasars. II. Detailed Models of Feedback-modulated Accretion Flows

This is the final summary paper of our 15 yr program using the Hubble Space Telescope (HST) to determine the Hubble constant from the Cepheid-calibrated luminosity of Type Ia supernovae. Several recent developments have made it necessary to put the summary on H0 on a broader basis than we originally thought (see the four papers cited in the text). The new Cepheid distances of the subset of 10 galaxies, which were hosts of normal SNe Ia, give weighted mean luminosities in B, V, and I at maximum light of -19.49, -19.46, and -19.22, respectively. These calibrate the adopted SNe Ia Hubble diagram from Paper III to give H0 = 62.3 ± 1.3(random) ± 5.0(systematic) in units of km s-1 Mpc-1. This is a global value because it uses the Hubble diagram between redshift limits of 3000 and 20,000 km s-1, beyond the effects of any local random and streaming motions. Local values of H0 between 4.4 and 30 Mpc from Cepheids, SNe Ia, 21 cm line widths, and the tip of the red giant branch (TRGB) all agree within 5% of our global value. This agreement of H0 on all scales from ~4 to 200 Mpc finds its most obvious explanation in the smoothing effect of vacuum energy on the otherwise lumpy gravitational field due to the nonuniform distribution of the local galaxies. The present value of H0 is consistent with physical methods relying on the time delay of gravitational lenses and the Sunyaev-Zeldovich effect, and it is not in contradiction with the CMB data. Our value of H0 is 14% smaller than the value of H0 found by Freedman et al. because our independent Cepheid distances to the six SNe Ia-calibrating galaxies used in that analysis average 0.35 mag larger than those used earlier.

https://iopscience.iop.org/article/10.1086/508853/pdf

The Hubble Constant: A Summary of the Hubble Space Telescope Program for the Luminosity Calibration of Type Ia Supernovae by Means of Cepheids

In the following the frequencies of SNe in external galaxies (Section 1) and in our Galaxy (Section 2) are discussed. The galactic frequencies are compared with SNRs, pulsars, and white dwarfs. In Section 3 the absolute maximum magnitudes of SNeI and SNeII are discussed, as well as the evidence of SNeI being good standard candles. Speculations on the progenitors of SNe in Section 4 are consistent with the assumption that SNeI come from stars with masses > 8 M ⊙, while SNeI could come from accreting white dwarfs in close binary systems.

G. A. Tammann

Papers

Supernova statistics and related problems