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Register a new userConstraining dark matter halo profiles and galaxy formation models using spiral arm morphology. I. Method outline
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Marc S. Seigar
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We investigate the use of spiral arm pitch angles as a probe of disk galaxy mass profiles. We confirm our previous result that spiral arm pitch angles (P) are well-correlated with the rate of shear (S) in disk galaxy rotation curves, by using a much larger sample (51 galaxies) than used previously (17 galaxies). We use this correlation to argue that imaging data alone can provide a powerful probe of galactic mass distributions out to large lookback times. In contrast to previous work, we show that observed spiral arm pitch angles are similar when measured in the optical (at 0.4 um) and the near-infrared (at 2.1 um) with a mean difference of 2.3+/-2.7 degrees. This is then used to strengthen the known correlation between P and S using B-band images. We then use two example galaxies to demonstrate how an inferred shear rate coupled with a bulge-disk decomposition model and a Tully-Fisher derived velocity normalization can be used to place constraints on a galaxys baryon fraction and dark matter halo profile. We show that ESO 582-G12, a galaxy with a high shear rate (slightly declining rotation curve) at ~10 kpc, favors an adiabatically contracted halo, with high initial NFW concentration (c_vir > 16) and a high fraction of halo baryons in the form of stars (~15-40%). In contrast, IC 2522 has a low shear rate (rising rotation curve) at ~10 kpc and favors non-adiabatically contracted models with low NFW concentrations (c_vir ~ 2-8) and a low stellar baryon fraction <10%.
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We investigate the use of spiral arm pitch angles as a probe of disk galaxy mass profiles. We confirm our previous result that spiral arm pitch angles (P) are well correlated with the rate of shear (S) in disk galaxy rotation curves. We use this correlation to argue that imaging data alone can provide a powerful probe of galactic mass distributions out to large look-back times. We then use a sample of 13 galaxies, with Spitzer 3.6-$mu$m imaging data and observed H$alpha$ rotation curves, to demonstrate how an inferred shear rate coupled with a bulge-disk decomposition model and a Tully-Fisher-derived velocity normalization can be used to place constraints on a galaxys baryon fraction and dark matter halo profile. Finally we show that there appears to be a trend (albeit a weak correlation) between spiral arm pitch angle and halo concentration. We discuss implications for the suggested link between supermassive black hole (SMBH) mass and dark halo concentration, using pitch angle as a proxy for SMBH mass.
In this paper, we investigate the dark matter halo density profile of M33. We find that the HI rotation curve of M33 is best described by a NFW dark matter halo density profile model, with a halo concentration of cvir = 4.0pm1.0 and a virial mass of Mvir = (2.2pm0.1)times10^11 Msun. We go on to use the NFW concentration (cvir)of M33, along with the values derived for other galaxies (as found in the literature), to show that cvir correlates with both spiral arm pitch angle and supermassive black hole mass.
Spiral arms are common features in low-redshift disc galaxies, and are prominent sites of star-formation and dust obscuration. However, spiral structure can take many forms: from galaxies displaying two strong `grand design arms, to those with many `flocculent arms. We investigate how these different arm types are related to a galaxys star-formation and gas properties by making use of visual spiral arm number measurements from Galaxy Zoo 2. We combine UV and mid-IR photometry from GALEX and WISE to measure the rates and relative fractions of obscured and unobscured star formation in a sample of low-redshift SDSS spirals. Total star formation rate has little dependence on spiral arm multiplicity, but two-armed spirals convert their gas to stars more efficiently. We find significant differences in the fraction of obscured star-formation: an additional $sim 10$ per cent of star-formation in two-armed galaxies is identified via mid-IR dust emission, compared to that in many-armed galaxies. The latter are also significantly offset below the IRX-$beta$ relation for low-redshift star-forming galaxies. We present several explanations for these differences versus arm number: variations in the spatial distribution, sizes or clearing timescales of star-forming regions (i.e., molecular clouds), or contrasting recent star-formation histories.
In this paper we study the morphological properties of spiral galaxies, including measurements of spiral arm number and pitch angle. Using Galaxy Zoo 2, a stellar mass-complete sample of 6,222 SDSS spiral galaxies is selected. We use the machine vision algorithm SpArcFiRe to identify spiral arm features and measure their associated geometries. A support vector machine classifier is employed to identify reliable spiral features, with which we are able to estimate pitch angles for half of our sample. We use these machine measurements to calibrate visual estimates of arm tightness, and hence estimate pitch angles for our entire sample. The properties of spiral arms are compared with respect to various galaxy properties. The star formation properties of galaxies vary significantly with arm number, but not pitch angle. We find that galaxies hosting strong bars have spiral arms substantially ($4-6^mathrm{o}$) looser than unbarred galaxies. Accounting for this, spiral arms associated with many-arm structures are looser (by 2$^mathrm{o}$) than those in two-arm galaxies. In contrast to this average trend, galaxies with greater bulge-to-total stellar mass ratios display both fewer and looser spiral arms. This effect is primarily driven by the galaxy disc, such that galaxies with more massive discs contain more spiral arms with tighter pitch angles. This implies that galaxy central mass concentration is not the dominant cause of pitch angle and arm number variations between galaxies, which in turn suggests that not all spiral arms are governed by classical density waves or modal theories.
We investigate galaxy formation in models with dark matter (DM) constituted by sterile neutrinos. Given their large parameter space, defined by the combinations of sterile neutrino mass $m_{ u}$ and mixing parameter $sin^2(2theta)$ with active neutrinos, we focus on models with $m_{ u}=7$ keV, consistent with the tentative 3.5 keV line detected in several X-ray spectra of clusters and galaxies. We consider i) two resonant production models with $sin^2(2theta)=5,10^{-11}$ and $sin^2(2theta)=2,10^{-10}$, to cover the range of mixing parameter consistent with the 3.5 keV line; ii) two scalar-decay models, representative of the two possible cases characterizing such a scenario: a freeze-in and a freeze-out case. We also consider thermal Warm Dark Matter with particle mass $m_X=3$ keV. Using a semi-analytic model, we compare the predictions for the different DM scenarios with a wide set of observables. We find that comparing the predicted evolution of the stellar mass function, the abundance of satellites of Milky Way-like galaxies, and the global star formation history of galaxies with observations does not allow to disentangle the effects of the baryonic physics from those related to the different DM models. On the other hand, the distribution of the stellar-to-halo mass ratios, the abundance of faint galaxies in the UV luminosity function at $zgtrsim 6$, and the specific star formation and age distribution of local, low-mass galaxies constitute potential probes for the considered DM scenarios. We discuss how next observations with upcoming facilities will enable to rule out or to strongly support DM models based on sterile neutrinos.
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