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Register a new userLow CO Luminosities in Dwarf Galaxies
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[Abridged] We present maps of CO 2-1 emission covering the entire star-forming disks of 16 nearby dwarf galaxies observed by the IRAM HERACLES survey. The data have 13 arcsec angular resolution, ~250 pc at our average distance of 4 Mpc, and sample the galaxies by 10-1000 resolution elements. We apply stacking techniques to perform the first sensitive search for CO emission in dwarfs outside the Local Group ranging from single lines-of-sight, stacked over IR-bright regions of embedded star formation, and stacked over the entire galaxy. We detect 5 dwarfs in CO with total luminosities of L_CO = 3-28 1e6 Kkmspc2. The other 11 dwarfs remain undetected in CO even in the stacked data and have L_CO < 0.4-8 1e6 Kkmspc2. We combine our sample of dwarfs with a large literature sample of spirals to study scaling relations of L_CO with M_B and metallicity. We find that dwarfs with metallicities of Z ~ 1/2-1/10 Z_sun have L_CO about 1e2-1e4x smaller than spirals and that their L_CO per unit L_B is 10-100x smaller. A comparison with tracers of star formation (FUV and 24 micron) shows that L_CO per unit SFR is 10-100x smaller in dwarfs. One possible interpretation is that dwarfs form stars much more efficiently, however we argue that the low L_CO/SFR ratio is due to significant changes of the CO-to-H2 conversion factor, alpha_CO, in low metallicity environments. Assuming a constant H2 depletion time of 1.8 Gyr (as found for nearby spirals) implies alpha_CO values for dwarfs with Z ~ 1/2-1/10 Z_sun that are more than 10x higher than those found in solar metallicity spirals. This significant increase of alpha_CO at low metallicity is consistent with previous studies, in particular those which model dust emission to constrain H2 masses. Even though it is difficult to parameterize the metallicity dependence of alpha_CO, our results suggest that CO is increasingly difficult to detect at lower metallicities.
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X-ray-emitting coronae of nearby galaxies are expected to be produced either by accretion from the intergalactic medium and/or by various galactic feedback. We herein present a systematical analysis of the Chandra observations of 53 nearby edge-on disk galaxies over a range of 3 orders of magnitude in SFR. Various coronal properties, such as the luminosity, vertical/horizontal extent, and other inferred parameters, are characterized for all the sample galaxies. For galaxies with high enough counting statistics, we also examine the thermal and chemical states of the coronal gas. Here we concentrate on the coronal luminosity (Lx), estimated in 0.5-2keV and within 5 times the diffuse X-ray vertical scale height. We find Lx strongly correlates with the SFR for the whole sample. But the inclusion of Ia SNe in the total energy input (E_SN) gives an even tighter correlation, which may be characterized with a linear relation, Lx=0.5%E_SN, and with a dispersion of 0.45dex. Moreover, the coronal radiation efficiency (eta=Lx/E_SN) shows little correlation with either the stellar mass or the gravitational mass (M_TF, inferred from the rotation velocity), but is significantly correlated with their ratio (M_TF/M_*), which may be expressed as a linear scaling relation eta=0.35%M_TF/M_* for the entire ranges of galaxy parameters. This joint scaling relation suggests that the coronae are self-regulated by the combination of gravitational confinement and feedback. But SN appears to be the primary heating source, because about half of our galaxies are not massive enough to allow for the accretion to play a major role. The commonly low eta further suggests that the bulk of the SN energy likely flows out into large-scale galactic halos for essentially all the galaxies. Such ubiquitous outflows could have profound implications for understanding the ecosystem, hence the evolution of galaxies.
A linear correlation has been proposed between the CO luminosity ($rm{L}^{prime}_{rm{CO}}$) and full-width at half maximum (FWHM) for high-redshift (z > 1) submillimeter galaxies. However, the controversy concerning the $rm{L}^{prime}_{rm{CO}}$-FWHM correlation seems to have been caused by the use of heterogeneous samples (e.g., different transition lines) and/or data with large measurement uncertainties. In order to avoid the uncertainty caused by using different rotational transitions, in this work we make an extensive effort to select only CO($J = 1-0$) data from the literature. We separate these wide-ranging redshift data into two samples : the low-redshift (z < 1) and high-redshift (z > 1) samples. The samples are corrected for lensing magnification factors if gravitational-lensing effects appeared in the observations. The correlation analysis shows that there exists significant $rm{L}^{prime}_{rm{CO}}$-FWHM correlations for both the low-redshift and high-redshift samples. A comparison of the low- and high-redshift $rm{L}^{prime}_{rm{CO}}$-FWHM correlations does not show strong evolution with redshift. Assuming that there is no evolution, we can use this relation to determine the model independent distances of high-redshift galaxies. We then constrain cosmological models with the calibrated high-redshift CO data and the sample of Type Ia supernovae in the Union 2.1 compilation. In the constraint for wCDM with our samples, the derived values are w_{0} = -1.02 {pm} 0.17, {Omega}_{m0} = 0.30{pm}0.02, and H_{0} = 70.00 {pm}0.60 km,s^{-1},Mpc^{-1}.
The formation and evolution of galaxies is imprinted on their stellar population radial gradients. Two recent articles present conflicting results concerning the mass dependence of the metallicity gradients for early-type dwarf galaxies. On one side, Spolaor et al. show a tight positive correlation between the total metallicity, Z/H and the mass. On the other side, in a distinct sample, we do not find any trend involving Fe/H (Koleva et al.). In order to investigate the origin of the discrepancy, we examine various factors that may affect the determination of the gradients: namely the sky subtraction and the signal-to-noise ratio. We conclude that our detection of gradients are well above the possible analysis biases. Then, we measured the Mg/Fe relative abundance profile and found moderate gradients. The derived Z/H gradients scatter around -0.4 dex/r_e. The two samples contain the same types of objects and the reason of the disagreement is still not understood. Based on observations made with ESO telescopes at La Silla Paranal observatory under program ID076.B-0196.
We performed a series of high-resolution $N$-body simulations to examine whether dark matter candidates in the form of primordial black holes (PBHs) can solve the cusp-core problem in low-mass dwarf galaxies. If some fraction of the dark matter in low-mass dwarf galaxies consists of PBHs and the rest is cold dark matter, dynamical heating of the cold dark matter by the PBHs induces a cusp-to-core transition in the total dark matter profile. The mechanism works for PBHs in the 25-100 M$_{sun}$ mass window, consistent with the LIGO detections, but requires a lower limit on the PBH mass fraction of 1$%$ of the total dwarf galaxy dark matter content. The cusp-to-core transition time-scale is between 1 and 8 Gyr. This time-scale is also a constant multiple of the relaxation time between cold dark matter particles and PBHs, which depends on the mass, the mass fraction and the scale radius of the initial density profile of PBHs. We conclude that dark matter cores occur naturally in halos comprised of cold dark matter and PBHs, without the need to invoke baryonic processes.
We show that cold dark matter particles interacting through a Yukawa potential could naturally explain the recently observed cores in dwarf galaxies without affecting the dynamics of objects with a much larger velocity dispersion, such as clusters of galaxies. The velocity dependence of the associated cross-section as well as the possible exothermic nature of the interaction alleviates earlier concerns about strongly interacting dark matter. Dark matter evaporation in low-mass objects might explain the observed deficit of satellite galaxies in the Milky Way halo and have important implications for the first galaxies and reionization.
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