Galactic latitude. 170°. 172°. 174°. 176°. 178°. 180°. Fig. 1: Upper: Mid: Lower: Cutouts of a 1 à 1 resolution d
Optical Cartography of the Northern Galactic Plane IPHAS i 0-band density map
Hywel Farnhill & IPHAS consortium University of Hertfordshire
Abstract The INT/WFC Photometric Hα Survey of the Northern Galactic Plane (IPHAS, www.iphas.org) observes down to 20th mag. in r 0, i 0 and Hα filters, covering the area 30◦ < ` < 215◦, −5◦ < b < +5◦. DR2 brings IPHAS photometry onto a globally calibrated photometric scale, covering 92% of the total survey footprint[1]. Using artificial source injection and recovery on each of the 14,115 images included in DR2, I have determined corrections to account for incompleteness, and applied these in the generation of stellar number density maps at a range of angular resolutions down to 1 sq. arcmin.. These maps can be used to test the performance of stellar population and extinction models in the Galactic Plane.
30°
32°
34°
` ∼ 30◦
Source density (per sq. arcmin.)
36°
38°
40°
1.0 80 60 40 0.8 20 0 80 0.6 60 40 20 0.4 0 80 60 0.2 40 20 0.00 0.0 -5°
Extinction models Four prescriptions of extinction were applied to the unreddened catalogues returned by the web service of the Besanc¸on model of Galactic population synthesis[2], in cells of size 120 × 120. The Marshall[3] and Sale[4] extinction maps were applied by taking the extinction curve of the sightline closest to the centre of the cell under consideration.
-4°
0.2-3°
-2°
0.4 -1° 0° +1° 0.6 Galactic latitude
+2°
+3°0.8
+4°
+5° 1.0
Fig. 2: IPHAS i 0-band stellar number densities (black) compared to Besanc¸on model predictions extinguished by Marshall (blue), Sale (orange) and diffuse (green) extinction profiles, for a strip through the Galactic Plane at 31◦ ≤ ` ≤ 32◦.
42° 70°
An exponential disc of diffuse extinguishing material was adopted by varying the local normalisation until the best agreement between IPHAS and Besanc¸on bright (r 0 < 15) magnitude distributions. A pseudo-Perseus arm profile was imposed by taking the corrected asymptotic Schlegel[5,6] map value, splitting the extinction between a local component of obscuration, and a Gaussian distribution at the putative distance to the Perseus arm[7].
` ∼ 90◦
Source density (per sq. arcmin.)
Galactic longitude
75°
80°
1.0 40 30 20 10 0.80 40 30 20 0.6 10 0 40 30 0.4 20 10 0 40 0.2 30 20 10 0.00 0.0 -5°
Fig. 5: Four extinction profiles as applied to the sightline (88.9◦, +4.6◦), as described above. -4°
0.2-3°
-2°
0.4 -1° 0° +1° 0.6 Galactic latitude
+2°
+3°0.8
+4°
+5° 1.0
Fig. 3: Comparison as in Fig. 2 only for 88◦ ≤ ` ≤ 89◦, and an additional comparison between IPHAS stellar densities and Besanc¸on predictions extinguished by a pseudo-Perseus arm extinction profile (see right).
The predictions of the Besanc¸on model, obscured by the extinction profiles detailed above, reproduce the observed stellar densities observed at ` ∼ 90◦ & 175◦ - the Sale 3D extinction map[4] in particular is quite successful.
85°
Source density (per sq. arcmin.)
` ∼ 175◦
90° 170°
172°
1.0 10 8 6 0.84 2 0 10 0.68 6 4 0.42 0 10 8 0.26 4 2 0.00 0.0 -5°
Discussion
At ` ∼ 30◦, predictions and observations do not correspond as well. These discrepancies could be due to one or more effects, including mischaracterisation of the star formation history, variations in the reddening law on small scales, and limitations of the extinction curves applied.
-4°
0.2-3°
-2°
0.4 -1° 0° +1° 0.6 Galactic latitude
+2°
+3°0.8
+4°
Fig. 4: Comparison as in Fig. 2 only for 174◦ ≤ ` ≤ 175◦.
+5° 1.0
Including further extinction curves in these comparisons will help to identify which maps are the most successful at reproducing observations. Utilising longer wavelength data to obtain dust column densities could help to determine the optimal reddening law to apply along each sightline.
174°
Density map availability
176°
Completeness-corrected IPHAS density maps in both r 0- and i 0-bands will shortly be made available at a range of resolutions, suitable for a number of applications in addition to testing models as demonstrated above. 178°
180°
-4°
-2°
0°
+2°
+4°
Galactic latitude
Fig. 1: Cutouts of a 10 × 10 resolution density map generated from IPHAS i 0-band data, for objects with i 0 ≤ 18 (G ≤ 18.8). The map has been fully corrected for incompleteness effects via artificial source injection. Regions falling into the 8% area of IPHAS not included in DR2 have been interpolated over. Upper: Colour scale varies from 0-120 sources / sq. arcmin. Mid: Colour scale varies from 0-50 sources / sq. arcmin. Lower: Colour scale varies from 0-30 sources / sq. arcmin.
Cluster searches:
A simple cluster search carried out by fitting a 2-D Gaussian to a rolling 200 × 200 window across the 10 × 10 resolution i 0-band density map found 71 known clusters, and 29 overdensities unassociated with any known clusters.
Target selection:
Lower resolution (up to 1◦ × 1◦) versions of the density maps will be useful for choosing pointings for wide-area multi-object spectrographs.
References [1] [2] [3] [4]
Barentsen et al. (2014), MNRAS, 444, 3230 Robin et al. (2003), A&A, 409, 523 Marshall et al. (2006), A&A, 453, 635 Sale et al. (2014), MNRAS, 443, 2907
[5] Schlegel et al. (1998), ApJ, 500, 525 [6] Schlafly & Finkbeiner (2011), ApJ, 737, 103 ´ (2008), AJ, 13, 1301 [7] Vallee
