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Register a new userThe Opacity of Spiral Galaxy Disks IX; Dust and Gas Surface Densities
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B. W. Holwerda
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Our aim is to explore the relation between gas, atomic and molecular, and dust in spiral galaxies. Gas surface densities are from atomic hydrogen and CO line emission maps. To estimate the dust content, we use the disk opacity as inferred from the number of distant galaxies identified in twelve HST/WFPC2 fields of ten nearby spiral galaxies. The observed number of distant galaxies is calibrated for source confusion and crowding with artificial galaxy counts and here we verify our results with sub-mm surface brightnesses from archival Herschel-SPIRE data. We find that the opacity of the spiral disk does not correlate well with the surface density of atomic (Hi) or molecular hydrogen (H2) alone implying that dust is not only associated with the molecular clouds but also the diffuse atomic disk in these galaxies. Our result is a typical dust-to-gas ratio of 0.04, with some evidence that this ratio declines with galactocentric radius, consistent with recent Herschel results. We discuss the possible causes of this high dust-to-gas ratio; an over-estimate of the dust surface-density, an under-estimate of the molecular hydrogen density from CO maps or a combination of both. We note that while our value of the mean dust-to-gas ratio is high, it is consistent with the metallicity at the measured radii if one assumes the Pilyugin & Thuan calibration of gas metallicity.
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We report the discovery of a well-defined correlation between B-band face-on central optical depth due to dust, tau^f_B, and the stellar mass surface density, mu_{*}, of nearby (z < 0.13) spiral galaxies: log(tau^f_B) = 1.12(+-0.11)log(mu_{*}/M_sol kpc^2)-8.6(+-0.8). This relation was derived from a sample of spiral galaxies taken from the Galaxy and Mass Assembly (GAMA) survey and detected in the FIR/submm in the Herschel-ATLAS survey. Using a quantitative analysis of the NUV attenuation-inclination relation for complete samples of GAMA spirals categorized according to mu_{*} we demonstrate that this correlation can be used to statistically correct for dust attenuation purely on the basis of optical photometry and Sersic-profile morphological fits. Considered together with previously established empirical relationships between stellar mass, metallicity and gas mass, the near linearity and high constant of proportionality of the tau^f_B-mu_{*} relation disfavors a stellar origin for the bulk of refractory grains in spiral galaxies, instead being consistent with the existence of a ubiquitous and very rapid mechanism for the growth of dust in the ISM. We use the tau^f_B-mu_{*} relation in conjunction with the radiation transfer model for spiral galaxies of Popescu & Tuffs (2011) to derive intrinsic scaling relations between specific star formation rate (sSFR), stellar mass, and mu_{*}, in which the attenuation of the UV light used to measure the SFR is corrected on an object-to-object basis. A marked reduction in scatter in these relations is achieved which is demonstrably due to correction of both the inclination-dependent and face-on components of attenuation. Our results are consistent with a picture of spiral galaxies in which most of the submm emission originates from grains residing in translucent structures, exposed to UV in the diffuse interstellar radiation field.
In this paper we explore the relation between dust extinction and stellar light distribution in disks of spiral galaxies. Extinction influences our dynamical and photometric perception of disks, since it can distort our measurement of the contribution of the stellar component. To characterize the total extinction by a foreground disk, Gonzalez et al. (1998) proposed the ``Synthetic Field Method (SFM), which uses the calibrated number of distant galaxies seen through the foreground disk as a direct indication of extinction. The method is described in Gonzalez et al. (1998) and Holwerda et al. (2005a). To obtain good statistics, the method was applied to a set of HST/WFPC2 fields Holwerda et al. (2005b) and radial extinction profiles were derived, based on these counts. In the present paper, we explore the relation of opacity with surface brightness or color from 2MASS images, as well as the relation between the scalelengths for extinction and light in the I band. We find that there is indeed a relation between the opacity (A_I) and the surface brightness, particularly at the higher surface brightnesses. No strong relation between near infrared (H-J, H-K) color and opacity is found. The scalelengths of the extinction are uncertain for individual galaxies but seem to indicate that the dust distribution is much more extended than the stellar light. The results from the distant galaxy counts are also compared to the reddening derived from the Cepheids light-curves Freedman et al. (2001). The extinction values are consistent, provided the selection effect against Cepheids with higher values of A_I is taken into account. The implications from these relations for disk photometry, M/L conversion and galaxy dynamical modeling are briefly discussed.
The quantity of dust in a spiral disk can be estimated using the dusts typical emission or the extinction of a known source. In this paper, we compare two techniques, one based on emission and one on absorption, applied on sections of fourteen disk galaxies. The two measurements reflect, respectively the average and apparent optical depth of a disk section. Hence, they depend differently on the average number and optical depth of ISM structures in the disk. The small scale geometry of the cold ISM is critical for accurate models of the overall energy budget of spiral disks. ISM geometry, relative contributions of different stellar populations and dust emissivity are all free parameters in galaxy Spectral Energy Distribution (SED) models; they are also sometimes degenerate, depending on wavelength coverage. Our aim is to constrain typical ISM geometry. The apparent optical depth measurement comes from the number of distant galaxies seen in HST images through the foreground disk. We measure the IR flux in images from the {it Spitzer} Infrared Nearby Galaxy Survey in the same section of the disk that was covered by HST. A physical model of the dust is fit to the SED to estimate the dust surface density, mean temperature, and brightness in these disk sections. The surface density is subsequently converted into the average optical depth estimate. The two measurements generally agree. The ratios between the measured average and apparent optical depths of the disk sections imply optically thin clouds in these disks. Optically thick disks, are likely to have more than a single cloud along the line-of-sight.
We present a novel method for determining the surface density of protoplanetary disks through consideration of disk dust lines which indicate the observed disk radial scale at different observational wavelengths. This method relies on the assumption that the processes of particle growth and drift control the radial scale of the disk at late stages of disk evolution such that the lifetime of the disk is equal to both the drift timescale and growth timescale of the maximum particle size at a given dust line. We provide an initial proof of concept of our model through an application to the disk TW Hya and are able to estimate the disk dust-to-gas ratio, CO abundance, and accretion rate in addition to the total disk surface density. We find that our derived surface density profile and dust-to-gas ratio are consistent with the lower limits found through measurements of HD gas. The CO ice line also depends on surface density through grain adsorption rates and drift and we find that our theoretical CO ice line estimates have clear observational analogues. We further apply our model to a large parameter space of theoretical disks and find three observational diagnostics that may be used to test its validity. First we predict that the dust lines of disks other than TW Hya will be consistent with the normalized CO surface density profile shape for those disks. Second, surface density profiles that we derive from disk ice lines should match those derived from disk dust lines. Finally, we predict that disk dust and ice lines will scale oppositely, as a function of surface density, across a large sample of disks.
The dust extinction in spiral disks can be estimated from the counts of background field galaxies, provided the deleterious effects of confusion introduced by structure in the image of the foreground spiral disk can be calibrated. Gonzalez et al. (1998) developed a method for this calibration, the ``Synthetic Field Method (SFM), and applied this concept to a HST/WFPC2 image of NGC4536. The SFM estimates the total extinction through the disk without the necessity of assumptions about the distribution of absorbers or the disk light. The poor statistics, however, result in a large error in individual measurements. We report on improvements to and automation of the Synthetic Field Method which render it suitable for application to large archival datasets. To illustrate the strengths and weaknesses of this new method, the results on NGC 1365, a SBb, and NGC 4536, a SABbc, are presented. The extinction estimate for NGC1365 is A_I = 0.6 (+0.6/-0.7) at 0.45 R_25 and for NGC4536 it is A_I = 1.6 (+1.0/-1.3) at 0.75 R_25. The results for NGC4536 are compared with those of Gonzalez et al. (1998). The automation is found to limit the maximum depth to which field galaxies can be found. Taking this into account, our results agree with those of Gonzalez et al. (1998). We conclude that this method can only give an inaccurate measure of extinction for a field covering a small solid angle. An improved measurement of disk extinction can be done by averaging the results over a series of HST fields, thereby improving the statistics. This can be achieved with the automated method, trading some completeness limit for speed. The results from this set of fields are reported in a companion paper Holwerda et al. (2005b).
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