ANGULAR MOMENTUM OF DWARF GALAXIES

Kirsty M. Butler1, Danail Obreschkow1, and Se-Heon Oh1,21 International Centre for Radio Astronomy Research (ICRAR), M468, University of Western Australia, WA 6009, Australia

2 Korea Astronomy and Space Science Institute (KASI), Daejeon 305-348, KoreaReceived 2016 November 25; revised 2016 December 8; accepted 2016 December 8; published 2016 December 29

ABSTRACT

We present measurements of baryonic mass Mb and specific angular momentum (sAM) jb in 14 rotating dwarfIrregular (dIrr) galaxies from the LITTLE THINGS sample. These measurements, based on 21 cm kinematic datafrom the Very Large Array and stellar mass maps from the Spitzer Space Telescope, extend previous AMmeasurements by more than two orders of magnitude in Mb. The dwarf galaxies show systematically higher jbvalues than expected from the µj Mb b

2 3 scaling of spiral galaxies, representative of a scale-free galaxy formationscenario. This offset can be explained by decreasing baryon mass fractions =f M MM b dyn (where Mdyn is thedynamical mass) with decreasing Mb (for < M M10b

11 ). We find that the sAM of neutral atomic hydrogen (H I)alone is about 2.5 times higher than that of the stars. The M–j relation of H I is significantly steeper than that of thestars, as a direct consequence of the systematic variation of the H I fraction with Mb.

Key words: galaxies: dwarf – galaxies: fundamental parameters – galaxies: ISM – galaxies: kinematics anddynamics – galaxies: structure

1. INTRODUCTION

It has long been suggested that angular momentum (AM)and mass are the two most fundamental parameters in galaxyformation and evolution (Fall & Efstathiou 1980; Quinn &Zurek 1988; Mo et al. 1998), orchestrating other physicalprocesses within galaxies that produce the properties weobserve. In the Cold Dark Matter (CDM)-dominated scenarioof galaxy formation, AM is acquired through tidal torquesbetween neighboring halos (Peebles 1969). Approximateconservation of AM then dictates the ordered infall of gas intodark matter halos under dissipation of energy (White & Rees1978; Fall & Efstathiou 1980).

Empirically, the pioneering study by Fall (1983) investigatedthe stellar mass M* and sAM j* of 44 spiral (Sb-Sc) galaxiesand 44 elliptical galaxies—a relation that was revisited byRomanowsky & Fall (2012) with 67 spirals and 40 ellipticalsspanning a larger range in morphology (E0-Sc). Totalluminosities were used to derive M* while j* was approxi-mated using kinematic tracers at specific optical radii. Thesample formed two parallel trends of

* *= aj qM with exponents

of a » 2 3 on the M*–j* plane, with elliptical galaxiescontaining three to four times less j* than spirals of equal M*.

High-resolution observations with integral field spectro-graphs (IFS) and/or radio interferometries allow us to obtainspectra for each pixel in spatially resolved images of galaxieson sub-kiloparsec scales in the local universe. AM can then beintegrated pixel-by-pixel. Obreschkow & Glazebrook (2014;hereafter OG14) presented the first precision measurements ofstellar and baryonic AM of 16 spiral (Sab-Scd) galaxies usingneutral hydrogen (H I) kinematic maps in The H I NearbyGalaxy Survey (THINGS; Walter et al. 2008), improving theaccuracy of earlier measurements by an order of magnitude.The a » 2 3 trend found in Romanowsky & Fall (2012) wasreproduced by the sample. Galaxies with equal bulge massfractions (defined as the mass fraction in excess of anexponential disk), however, followed a a » 1 trend. Thisresult refines the connection between sAM and Hubble

morphologies, raising the question of how the M–j relationbehaves at lower masses where bulges are absent.Both theoretically and observationally the low-mass

( < M M10b10 ) end of the M–j plane is not well understood.

At these masses most star-forming galaxies are H I-dominated(Maddox et al. 2015). Stars and molecular gas are subdominantand centrally concentrated, meaning that AM locked up at largeradii is more reliably measured in H I kinematics. This workinvestigates the M–j momentum relation of 14 dwarf irregular(dIrr) galaxies taken from the Local Irregulars That TraceLuminosity Extremes, The H I Nearby Galaxy Survey (LITTLETHINGS: Hunter et al. 2012). In Section 2 we will lay out thesample of dwarf galaxies and provide our measurements oftheir AM and mass. In Section 3 we present a discussion on themean M jb b– plane, the scatter of this relation and the H I, stellarand baryon componentM–j relations. Conclusions are providedin Section 4.

2. OBSERVATIONS OF MASS AND AM

2.1. The Sample and Data

To investigate AM at the low-mass end we have taken 14local dIrr galaxies from the LITTLE THINGS sample. Weexclude the four blue compact dwarfs present in LITTLETHINGS as well as galaxies lacking m3.6 m Spitzer data andgalaxies with inclinations less than 40°. Furthermore, DDO155is excluded due to an extremely irregular velocity map alongwith NGC 4163, NGC 1569, and CVnIdwA for which thevelocity profiles are poorly resolved.We make use of high spectral ( -2.6 km s 1) and angular

(~ 6 ) resolved H I kinematic data (two spatial dimensions andone velocity dimension) obtained with the National RadioAstronomy Observatory (NRAO) Very Large Array and3.6 μm mid-infrared Spitzer/IRAC images. Together thesedata sets probe the majority of baryons within our galaxies(Maddox et al. 2015) and allow us to make reliablemeasurements of the sAM ( ºj J M) in the stellar andH I disks. Total baryon mass is calculated by

*= +M M M1.36b H I, where the 1.36 factor accounts for the

The Astrophysical Journal Letters, 834:L4 (6pp), 2017 January 1 doi:10.3847/2041-8213/834/1/L4© 2016. The American Astronomical Society. All rights reserved.

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helium fraction at z=0 (de Blok et al. 2008). We neglectmolecular hydrogen (H2), since CO measurements of nearbydwarf irregulars, whether identified by morphology (Obresch-kow & Rawlings 2009) or stellar mass ( * < M M109 , Boselliet al. 2014), suggest <M M 0.1H2 H I , even if accounting forlow metallicity in the CO–H2 conversion. These low moleculargas fractions can be explained by the relatively low surfacedensities of (non-compact) dwarfs (Obreschkow et al. 2009),backed-up by inefficient H2-formation at low metallicities(Lagos et al. 2011; Xie et al. 2016).

The full sample, along with all measured values, can befound in Table 1.

2.2. Extracting Radial Profiles

As explained in this section, our method to measure the massand AM uses a model of flat, axially symmetric disks.Measurement uncertainties (given in Table 1 and Figure 2)are estimated via a jackknifing method: we only fit the model toa random half of the density and velocity data, and repeat thisfit 1000 times with a different random seed at each iteration.The uncertainties are then estimated from the model deviationsacross all iterations (following Quenouille 1949). Theseuncertainties are accurate if the galaxies satisfy the modelassumptions, but care must be taken in the case of modeldeviations such as irregular structures, warping, and flaring.Since such density and velocity asymmetries will be picked updifferently at each jackknife iteration, they will automaticallyyield larger uncertainties. However, we caution that theseuncertainties might only be lower limits, since most irregula-rities are spatially correlated across many pixels.

Our five-step method optimally combines measurementswith interpolations and extrapolations where data are missing.

Step 1—Data maps: Each H I data cube was run through arobust fitting process whereby the spectrum of each spaxelundergoes a simple parabolic background subtraction and isthen fitted with a Gaussian. The fitted intensity (S), line of sight(LOS) velocity (VLOS), and velocity dispersion (σ) of eachspaxel form 2D maps in the R.A.–decl. plane. A composite

H I and stellar intensity map of DDO133 is shown inFigure 1(a) and the corresponding H I velocity map is inFigure 1(b).Pixels are automatically rejected if S is less than the

background root mean square (rms) value measured for theentire galaxy (Hunter et al. 2012), if > -V 500 km sLOS

1∣ ∣ , if σis less than two velocity bins or larger than the full velocitybandwidth, or if the signal-to-noise ratio of the pixel is less thanthree. The intensity map is then converted from -Jy beam 1 to

-M pixel 1 and the m3.6 m stellar images are converted from

-MJy sr 1 to M using a constant mass-to-light ratio of-

M L1 1 (for consistency with OG14). A bilinear interpolationis used to re-grid the stellar images to match the pixel size anddimension of the H I maps.Position angles, system velocities, and inclinations taken

from Oh et al. (2015) are used to compute the deprojected radiir and circular velocity V in each pixel. This involves simpletrigonometry as explained by OG14 in Appendix B of theirpaper. Pixels of position angle within 10◦ to the minor axis aredominated by radial motion due to turbulence and contain littleinformation on the rotation. Pixels in this region are removedfrom the analysis.Step 2—Data profiles:We bin pixels in the H I surface

density SH I, stellar density *S , and velocity maps intoconcentric ellipses of constant deprojected radius. Radialdensity profiles are extracted by taking the mean of SH I ineach ellipse and the median of *S in each stellar ellipse to filterout the effects of foreground stars. The median V in eachellipse is used, as this provides a smoother and more physicalvelocity profile than the mean.Step 3—Model profiles:Assuming the background is fully

subtracted in step 1 the H I density profiles are fitted such thatpS = S - -r r e2 r r

H H ,0 H2 1

I I IH I( ) ( ) . Many of the H I density

profiles exhibit plateaus or dips at small radii where H I gas hasbeen converted to molecular gas or ionized due to stellarfeedback. Therefore, a radius is chosen by eye (green star inFigure 1(c)), and points inside that radius are not included in thefitting process. Next we fit * * *

*pS = S + S- -r r e, 0 2 r r2 1bg( ) ( )

Table 1Measured Values for the 14 dIrr Galaxies in This Letter

Galaxy D MH I *M Mb jH I *j jb Vmax

(Mpc) ( Mlog10 ) ( Mlog10 ) ( Mlog10 ) ( -log kpc km s101) ( -log kpc km s10

1) ( -log kpc km s101) ( -km s 1)

DDO50 3.40 9.25 8.287.70 8.34 7.35

6.78 9.42 8.427.86 2.48 1.42

1.62 1.88 0.850.93 2.45 1.41

1.61 32

DDO52 10.30 8.87 8.428.08 8.02 7.34

7.23 9.05 8.568.25 2.71 2.09

2.42 2.06 1.141.16 2.68 2.08

2.42 66

DDO70 1.30 8.67 7.426.16 7.48 6.07

4.98 8.82 7.566.31 2.17 1.12

1.13 1.55 0.350.08 2.15 1.11

1.12 LDDO87 7.70 8.79 8.48

8.39 7.85 7.587.46 8.95 8.64

8.56 2.66 2.112.12 2.09 1.68

1.57 2.63 2.092.10 53

DDO101 6.40 8.21 7.927.81 8.06 7.69

7.66 8.52 8.198.12 2.29 1.75

1.82 2.00 1.371.38 2.21 1.66

1.73 65

DDO126 4.90 8.37 7.797.73 7.75 7.25

7.11 8.58 7.997.93 2.12 1.35

1.46 1.92 1.311.26 2.10 1.31

1.42 60

DDO133 3.50 8.14 7.757.74 7.75 7.48

7.35 8.38 8.027.99 1.93 1.25

1.26 1.92 1.501.35 1.93 1.31

1.27 47

DDO154 3.70 8.74 8.378.34 7.20 6.87

6.80 8.88 8.518.48 2.55 1.88

1.96 1.60 1.051.00 2.54 1.87

1.95 47

DDO168 4.30 8.64 8.038.00 8.06 7.52

7.41 8.85 8.248.20 2.23 1.40

1.42 2.04 1.251.27 2.20 1.37

1.38 58

DDO210 0.90 6.80 5.965.74 6.18 5.55

5.12 7.00 6.145.95 1.12 0.56

1.00 0.75 0.170.40 1.08 0.51

0.95 17

DDO216 1.10 7.09 6.266.05 7.20 6.47

6.16 7.51 6.686.47 1.31 0.50

0.64 1.17 0.290.42 1.25 0.37

0.51 17

NGC2366 3.40 8.99 7.567.15 8.30 7.06

6.46 9.18 7.747.34 2.44 1.06

1.16 2.12 0.930.63 2.41 1.02

1.12 59

UGC8508 2.60 8.00 7.116.39 6.97 6.07

5.36 8.16 7.236.55 2.67 1.81

2.07 1.59 0.630.85 2.64 1.80

2.05 128

WLM 1.00 8.19 7.176.80 7.51 6.52

6.11 8.39 7.346.99 2.21 1.17

1.32 1.85 0.800.42 2.17 1.14

1.29 39

Note. Distances are taken from Oh et al. (2011) while masses, sAM, and Vmax are measured as in Section 2.2, with the contribution from He only included in thebaryon values. Upper and lower uncertainties on mass and sAM are found by combining distance errors with the 16% and 84% quantiles, respectively, calculated froma 1000 iteration jackknife resampling.

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to the stellar density profile, allowing some vertical offset Sbg tobe subtracted as background light. Due to the low signal-to-noisein the outer stellar disk we visually select a radius (orange star inFigure 1(c)) for which data beyond this radius are not used in thefitting process. The velocity profile is fitted via

= - -V r V e1 r rmax flat( ) ( ), allowing us to extract Vmax . Again,

a radius is chosen by eye (purple star in Figure 1(c)) and pointsbeyond this radius are not used in the fitting process. Manygalaxies exhibit warping in their disks (see Oh et al. 2015) andtherefore non-constant inclinations. This causes the velocityprofile to artificially curve or wiggle as in Figure 1(c), forcing us

to use only the inner velocity curve in our fit. This has little effecton the total sAM measurements, since at this radius j(r) is nearlyconverged (see the red curve in Figure 1(d)). The model profilesare shown in Figure 1(c) as dashed lines.Step 4—Hybrid Profiles:Hybrid maps are formed by

combining raw data with the fitted profiles. Empty H I densitymap pixels lying within the green star’s radius are replacedwith the mean H I density in their ellipses. At larger radii theempty pixels are replaced with the fitted values correspondingto their exact r, allowing a smooth transition from data tomodel. All empty pixels in the velocity map are treated the

Figure 1. (DDO133) (a) Composite raw H I-stellar intensity map, with the blue ellipse marking half-mass radius and the yellow ellipse marking half-AM radius. (b)Raw line of sight velocity map. (c) Radial profiles of H I (green), stellar (orange), and circular velocity (purple) for the raw (dots, scaled in size by fraction of filledpixels), fitted (dashed) and hybrid (solid) profiles. The orange star lies at 10% of the maximum stellar density, while placement of the green star is chosen by eye whereraw data past this radius are used in the H I fit. Similarly, the purple star is chosen by eye where raw data past this radius are not used in the velocity fit. Shaded regionsshow the 16% and 84% quantiles from a 1000 iteration jackknife resampling. (d) Cumulative radial profiles of baryon mass (blue), AM (gold), and sAM (red) for theraw (dots, scaled in size by fraction of filled pixels) and hybrid (solid) data. Shaded regions show the 16% and 84% quantiles from a 1000 iteration jackkniferesampling (Quenouille 1949). The blue and yellow vertical lines indicate the half mass and AM radii, respectively.

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same, replaced by a fitted value calculated for their exact r. Asin step 3 we measure radial profiles, using the mean H I densityand median circular velocity in each concentric ellipse. Thehybrid stellar radial profile contains raw values at radii less thanthe orange star and model values at larger radii. Hybrid profilesare extended out to 15 times the scale radii rH I, predicting massand AM out to larger radii than the extent of observational data(solid lines in Figure 1(c)).

Step 5—Final values: Raw and hybrid total baryon densityprofiles are simply *S = S + S1.36b H I, where the 1.36 factoraccounts for the He fraction ( =M M 0.36He H I ) at z=0.Density radial profiles are converted to mass radial profilesDM r( ) to calculate cumulative mass, AM, and sAM profilesusing

= S D<

M r M , 1ar r

ii

( ) ( )

= S D<

J r M v r , 1br r

i i ii

( ) ( )

=j r J r M r , 1c( ) ( ) ( ) ( )

where the subscript i denotes the ellipse of radius ri in theGalaxy plane. Total integrated values are given by

º ¥M M ( ), º ¥J J ,( ) and º ¥j j ( ). Raw mass and AMcumulative profiles are corrected for the fraction of emptypixels in each ellipse. Hybrid values measured for the gas,stellar, and baryonic M, J, and j can be found in Table 1. TheM rb ( ), J rb ( ), and j rb ( ) profiles are shown in Figure 1(d).

3. RESULTS: M–j PLANE

Figure 2 displays the M jb b– plane with the 14 dIrr galaxiesfrom this work (diamonds) and 16 spiral galaxies from OG14(dots). The relation for dwarfs displays a similar scatter, butsystematically offset to the relation for spirals (Fall &Efstathiou 1980; Romanowsky & Fall 2012; OG14). We shall

now explain these findings in a basic, CDM-dominated galaxyformation framework (White & Rees 1978).

3.1. Mean Baryonic M–j Relation

Spherical CDM halos exhibit the first order relationshiplµj Mh h

3 2 (Mo et al. 1998) between mass Mh, sAM jh, andthe spin parameter λ (Peebles 1969). The mass of baryons Mbthat form the disk makes up a fraction º <f M M 1M b h of thetotal mass. Assuming that the sAM fraction ºf j jj b h is unity,as shown by modern simulations (within 50%; e.g., Stewartet al. 2013) and observations (Fall & Efstathiou 1980), itfollows (OG14),

⎡⎣⎢

⎤⎦⎥l=

--

jf

M

M10 kpc km s1.96

10, 2b

3 1 M2 3 b

10

2 3

( )

where scatter about this relation is accounted for in the halospin parameter λ (Steinmetz & Bartelmann 1995). The grayline in Figure 2 is the M jb b– relation of Equation (2), for aconstant l » 0.03 (typical of CDM halos; Bullock et al. 2001)and »f 0.05M (typical for local Milky Way mass disks). Thegray band represents 80% of the skewed distribution of λ

(Bullock et al. 2001) and agrees well with the THINGSgalaxies. The LITTLE THINGS sample falls systematicallyabove this relation, suggesting λ, fj, or fM must vary with Mb.However, cosmological simulations suggest λ is nearlyindependent of Mb (Knebe & Power 2008) and »f 1j overany halo evolution deprived of major mergers (Stewartet al. 2013).Assuming a universal baryon fraction of 17% (relative to

baryons and CDM), we can rewrite =f 0.17M , where ò is theefficiency of the halo to form a baryonic disk. It is wellestablished that ò (and therefore fM) peaks for galaxies of MilkyWay size (» M1011 ) (Baldry et al. 2008; Behroozi et al. 2013),where the gravitational potential wells are deep enough toretain baryons heated and accelerated by stellar feedback(supernovae, stellar winds etc.). With decreasing baryon massfM decreases to about »f 0.017M at = M M10b

9 . Applyingthis varying f MM b( ), as given by Baldry et al. (2008), theM jb b– relation becomes the red line and shading in Figure 2.While this relation shows a better agreement with our dwarfgalaxies, it still falls slightly below most measurements. This islikely due to the fact that Baldry et al. (2008) only measured fMdown to galaxy masses around = M M10b

9 . Therefore, wealso estimate =f M MM b h directly from the LITTLE THINGSand THINGS data using dynamical masses calculated viaEquation (3) derived by Oh et al. (2011),

-M M V3.29 10 km s , 3dyn5

2001 3· [ ] ( )

whereV200 is assumed to be approximately equal to the velocityat the largest measured radii; for LITTLE THINGS this issimply Vmax . UGC8508 is left out of this analysis, as it is anoutlier in the M–j relation, along with DDO50 and DDO70,which have uncertain Vmax values due to strong disk warping.We apply a log-log fit to f MM b( ) and find

=f M10 0.043M11( ) , decreasing to =f M10 0.003M

9( ) ,lower than that found by Baldry et al. (2008). Our fitted fMvalues predict the region in blue and improves in the

M108 – M109 range.

Figure 2. M jb b– relation of the 14 dIrr galaxies from this work (diamonds) and16 spiral galaxies from OG14 (circles) compared to models for disks formed inspherical CDM halos with differing fM values. A constant fM estimated forMilky Way mass galaxies predicts the light gray region but only agrees withthe THINGS galaxies. A decreasing fM with decreasing Mb better agrees withthe deviated dwarf relation, as this shifts galaxies left on the M–j plane. Thethree white diamonds correspond to the LITTLE THINGS galaxies notincluded in the blue f MM b( ) fit.

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3.2. Scatter around the Baryonic M–j Relation

OG14 found that the scatter about the spiral M–j relationstrongly correlated with bulge mass fraction β with galaxies ofequal β following a µj M trend. The full sample of dwarfgalaxies in this work lay above the b = 0 trend (no bulge),spinning to fast and sitting in gravitational wells too shallow forbulges to form in situ. As a replacement morphological tracer wecomputed the asymmetry A in the intensity (S) and velocity (V )maps of our sample, using a simple algorithm by Schade et al.(1995). We first cropped the maps out to a radius where the meanH I density dropped below -

M0.5 pc 2 then rotated the maps by180° (S180 and V180) to measure = S - -A S S S2ij ij ijS

180 1( ∣ ∣)( ∣ ∣)and = S + -A V V V2ij ij ijV

180 1( ∣ ∣)( ∣ ∣) , where i and j are the pixelpositions. There was no obvious trend in our data and furthersearches for correlation with other galaxy parameters such as gasfraction, specific star formation rate, or depletion time showed nostatistical significance. This, however, is probably due to thescatter of our data being dominated by uncertainty.

3.3. Component M–j Relations

Figure 3(a) compares the H I (green), stellar (orange), andtotal baryon (purple) M–j relations with their respective onestandard deviation scatters. H I mass MH I increases signifi-cantly faster with H I sAM jH I than that of stellar mass *M withstellar sAM

*j and Mb with jb. Despite the appearance of these

relations,*

> >j j jH bI is always true. When plotted as afunction of Mb (as in Figure 3(b)), the H I and stellar trends liealmost parallel, with H I components on average holding 2.5times more sAM. Stars preferentially form in the centralregions of the galactic disk, where low AM H I has sufficientlycooled and collapsed to form molecular clouds, setting up thestellar disks with systematically lower AM. Further low AMH I is lost through stellar feedback heating up and removingmatter from the disk and new high AM H I accreted at largeradii over time leads to the overall

*> >j j jH bI pattern we

observe. At low masses galaxies have high gas fractions fg(Maddox et al. 2015) and the H I components thereforedominate the M jb b– trend in this regime. At higher masseswhere star formation is more efficient, the stellar componentdominates, resulting in a shallower trend for baryons. Thus, it isthe variation of MH I/ *M with Mb that causes the M jH HI I–relation to be steeper than the * *

M j– relation.

4. CONCLUSION

We have presented measurements of mass and sAM for theH I, stellar, and baryon components of 14 dIrr galaxies from theLITTLE THINGS sample. High-resolution H I kinematic dataand m3.6 m Spitzer maps are combined with kinematic modelsto form hybrid maps, allowing us to accurately integrate the fullmass and AM. These measurements extend on previous AMmeasurements by more than two orders of magnitude in Mb,ranging M10 106 9– . The sample is found to deviate from thespiral relation previously measured by Fall & Efstathiou(1980), Romanowsky & Fall (2012), and OG14. We find thisdeviation to be consistent with CDM theory once we accountfor the decrease in fM with decreasing Mb. This has the effect ofbending the M jb b– relation at the low Mb end. Lastly H I andstellar M–j relations are presented separately, displaying asignificantly steeper trend for H I, explainable by the change infg with Mb. Plotted as a function of Mb, the H I and stellar

relations fall roughly parallel, with 2.5 times more sAM in theH I components.This work demonstrates the enormous importance of 21 cm

radio observations of H Iin measuring the AM of dwarfgalaxies. Already in more massive main sequence galaxies,such observations are crucial because most AM resides at largeradii that are often H I-dominated. In dwarf galaxies thesituation is even more pronounced since these galaxies are H I-dominated at virtually all radii. With future radio telescopessuch as the Square Kilometre Array and its pathfinders comingonline, larger, deeper, and more highly resolved samples willallow a much more complete analysis of the M–j plane across awide mass range.

Figure 3. (a) H I (green), stellar (orange), and baryon component M–j relationsfor the THINGS (circles) and LITTLE THINGS (diamonds) galaxies withshaded regions (colored respectively) outlining the 1σ scatter from eachcomponent fit. (b) H I (green), stellar (orange), and baryon component sAM asa function of Mb for the THINGS (circles) and LITTLE THINGS (diamonds)galaxies with shaded regions (colored respectively) outlining the 1σ scatterfrom each component fit.

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Based on data from the LITTLE THINGS Survey (Hunteret al. 2012), funded in part by the National Science Foundationthrough grants AST-0707563, AST-0707426, AST-0707468,and AST-0707835 to US-based LITTLE THINGS teammembers and with generous support from the National RadioAstronomy Observatory.

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