turtlebot: costmap2d_node won't subscribe to scan topic

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I am working on ros-kinetic and using turtlebot for simluation. Launching turtlebot using the command

 roslaunch turtlebot_gazebo turtlebot_world.launch

publishes the laserscan to topic scan. But I am unable to make costmap2d_node to subscribe to this scan topic.

launch file

<?xml version="1.0"?>
<launch>
  <node pkg="tf" type="static_transform_publisher" name="map_odom_broadcaster"
      args="0 0 0 0 0 0 /map /odom 100"/>
  <node pkg="tf" type="static_transform_publisher" name="map_odom_broadcaster"
      args="0 0 0 0 0 0 /odom /base_link 100"/>
 <param name="/use_sim_time" value="true"/>
         <!-- Run the costmap node -->
          <node pkg="costmap_2d" type="costmap_2d_node" name="costmap_node">
          <rosparam file="./costmap_params.yaml" command="load" ns="costmap" />
         </node>
</launch>

costmap_params.yaml

global_frame: /map
robot_base_frame: base_link
update_frequency: 5.0
publish_frequency: 1.0

#set if you want the voxel map published
publish_voxel_map: true

#set to true if you want to initialize the costmap from a static map
static_map: false

#begin - COMMENT these lines if you set static_map to true
rolling_window: true
width: 6.0
height: 6.0
resolution: 0.025
#end - COMMENT these lines if you set static_map to true

#START VOXEL STUFF
map_type: voxel
origin_z: 0.0
z_resolution: 0.2
z_voxels: 10
unknown_threshold: 10
mark_threshold: 0
#END VOXEL STUFF

transform_tolerance: 0.3
obstacle_range: 2.5
max_obstacle_height: 2.0
raytrace_range: 3.0
#footprint: [[-0.325, -0.325], [-0.325, 0.325], [0.325, 0.325], [0.46, 0.0], [0.325, -0.325]]
robot_radius: 0.46
footprint_padding: 0.01
inflation_radius: 0.55
cost_scaling_factor: 10.0
lethal_cost_threshold: 100
observation_sources: base_scan
base_scan: {data_type: LaserScan, expected_update_rate: 0.4,
observation_persistence: 0.0, topic: scan, marking: true, clearing: true, max_obstacle_height: 0.4, min_obstacle_height: 0.08}

The rqt_graph does not show costmap2d_node subscribing to scan