Justin Tolinski (1), Eric Bell (2)

(1) Department of Physics, University of Michigan, 450 Church St., Ann Arbor, MI 48109, USA ; (2) Department of Astronomy, University of Michigan, 1085 S. University, Ann Arbor, MI 48109, USA ;

References (a)  Andromeda, Credit: NASA Galaxy Evolution Explorer; (b) NGC 4565, Credit: Hubble Space Telescope; (c) Messier 101, Credit: Hubble Space Telescope; (b)  Figure A: Credit: Devour B. M. (c)  (1) Chang, Y. Y. et al 2013, ApJ, 773, 149 (d)  (2) Devour B. M., Bell E. F., 2016, MNRAS In Press, [arXiv:1604.00030] (e)  (3) Masters K. L., Nichol R. C., et al. 2010, MNRAS 404, 792 (f)  (4) Bell E. F., De Jong R. S., 2001, ApJ, 550, 212

Dust in z=0.7 Disk Galaxies as Examined by the Dependence of Color and Structure on Inclination

Figure C shows the relationship of a galaxy’s color and its inclination relative to the observer. The eight inset plots show rest-frame U-J as a function of the log of axis ratio (semi-major axis / semi-minor axis). As mentioned in Figure B, each bin consists of the galaxies that are similar based on their apparent 24µm-3.6µm color and apparent 3.6µm magnitude. From analyzing Figure C, one can see a change in the dependence of a galaxy’s color difference on its inclination as the galaxies become fainter and less star forming. Strongest inclination dependence of color difference in the brightest and most star-forming group of galaxies represented by subplot (1). Galaxies in this group have a stronger dust attenuation as the inclination increases; in other words, galaxies have a redder U-J color when viewed edge-on compared to a face-on view likely due to a high dust content. These results correspond to a similar trend seen in studies of dust attenuation for local galaxies in the SDSS at z~0 (2) ; this may be surprising because galaxies at z~0.7 are much more luminous in the IR (therefore expected to be richer in dust) than present day galaxies.

(a) (b) (c)

Figure B shows the relation of color and apparent magnitude of galaxies at z~0.7. 24µm and 3.6µm color [24]-[3.6] as a proxy for star formation rate per unit stellar mass, we can group galaxies that are similar to one another by an inclination-independent means (i.e. dust has little affect on long wavelength emissions (4) ). In general, redder color is attributed to a higher SFR (star formation rate) which implies a higher dust content (2) exists in these galaxies. This is represented by a more negative color in Figure B. Note: The red line (at [24]-[3.6] = 0.65) represents an approximate cutoff for star-forming and non star-forming galaxies--with star formers having a magnitude difference equal to or less than 0.65. (See Table 1 for a general summary)

Dust attenuation accounts for some of the largest systematic errors in our understanding of galaxy properties and evolution. Thus, a pair of star-forming galaxies with nearly identical structures may be observed to have different characteristics if their orientation differs with respect to an observer (e.g. face-on versus edge-on, See Figure A). Current models show dust attenuation is dependent on star dust geometry; however, direct measurement of star-dust geometry is widely unavailable. Thus, it is the goal of this study to provide the first ever direct measurements of dust attenuation in star-forming galaxies at redshift ~0.7. Grouping similar galaxies together based on inclination-independent mid-infrared measurements, we observed the effects of dust on inclination-dependent photometric and morphological quantities—concluding that structures of galaxies with a higher star forming rate and mass have the strongest inclination-dependence. Furthermore, since galaxies in each group have relatively the same intrinsic properties (i.e. star-formation rate and color), we can attribute this dependence to the amount of dust within the galaxies (1).

Dependence of Rest Frame Color on Axis Ratio

Figure C

Inclination-Independent Selection of Similar Galaxies

Further research includes, but is not limited to: •  Measuring the inclination dependence of color and structure using a wider range of absolute magnitudes, and

carefully-constructed absolute magnitudes for sample selection

•  Continue the testing of various common model-dependent dust attenuation measurements (e.g., Kriek et al. 2009, Whitaker et al. 2011) against model-independent measures of attenuation.

•  Critically assess model-dependent galaxy-by-galaxy dust estimates.

•  Developing a model that is completely independent of inclination to measure galaxy dust and account for dust attenuation.

Inclination Dependence of Galaxy Structure Parameters

[3.6µm] 17≤ , ≤ 19.5 17≤ , ≤ 19.5 19.5< , ≤ 21 19.5< , ≤ 21 21< , ≤ 22.5 21< , ≤ 22.5 17≤ , ≤ 22.5 17≤ , ≤ 22.5

[3.6µm]-[24µm] ≤ -0.198 -0.198 ≤ , ≤ 0.65 ≤ -0.311 -0.311 ≤ , ≤ 0.65 -0..213 ≤ -0.213 ≤ , ≤ 0.65 0.65 ≤ , ≤ 1.438 ≥ 1.438

Description Brightest Galaxies Most Star Forming

Optically Blue

Brightest Galaxies Star Forming

Intermediate-luminosity

Star Forming

Intermediate-luminosity Moderate Star-Forming

Faintest Star Forming

Faintest Moderate Star-Forming

Bright to Faint Non Star-Forming

Bright to Faint Non Star-Forming

Optically Red

Parameters for Binning Galaxies

Table 1 gives an overview of how we chose our bins for Figure C and Figure D. Galaxies are deemed similar if they fall within the domains for both its 3.6µm emission and [3.6]-[24] color. By doing this, we are able to study the inclination dependence of galaxies that have similar intrinsic properties (i.e. brightness, star-forming rate, color, etc.).

Table 1

Figure D illustrates the effects of dust and inclination on galaxy structure as discussed in this poster’s introduction and Figure A. Plotting Sérsic indices as a function of axis distribution (for the first time at z~0.7) highlights their dependence inclination. Using our bins, we note that this dependence is strongest for the brightest, most star-forming galaxies—as these generally contain the highest dust content. Current works use Sérsic indices as a predictor of how much dust a galaxy contains (e.g. disk dominated galaxies contain more dust than equally luminous galaxies with a large bulge (3) ). However, by Figure D, dust attenuation may cause a discrepancy between measured and actual Sérsic values for similar galaxies at different inclinations. Thus possibly leading to highly attenuated inclined galaxies being paired with low-inclination partners. In addition, upper panels show that Sérsic indices of very dust poor non-star-forming galaxies (shown by the extreme lack of color dependence on inclination in the top panels of Figure C) are strongly inclination-independent. We speculate that this is not a dust effect, but instead shows Sérsic fits vary systematically with axis ratio (Devour et al., in prep.).

Figure B

Figure D

Further Research

Figure A

Figure A – Examples of face-on galaxies (i - iii) and similar edge-on galaxies (iv - vi) based on similar 3.4µm luminosities, [12]-[3.4] colors, and K-band concentration. As shown above, face-on galaxies observed in the K-band have approximately the same concentrations as their edge-on partners. However, concentration measurements drastically differ between face-on galaxies and their edge-on counterparts when observing their g-band light, which is much more sensitive to dust. Since the g-band is much more subject to dust attenuation than K-band, this demonstrates inclination-dependence of galaxy structures (e.g. Sérsic indices). Note: Figure A galaxies are at z~0. There are no well-resolved rest-frame K-band images available for galaxies at z~0.7.

g-band

K-band

(i) (iii) (ii)

(ii) (i) (iii)

(i) (ii) (iii)

(i) (ii) (iii)

Synopsis

Face-on Edge-on

(1)

Edge-on Face-on

Disk Dominated

Large Bulge

Low SFR

High SFR

Large Mass

Small Mass

Redder

Bluer