How to get attached image alt text in media.php?

First of all, i apologize for my English. I want to get attached image alt text following code in wp-includes/media.php

function img_caption_shortcode( $attr, $content = null ) {
	// New-style shortcode with the caption inside the shortcode with the link and image tags.
	if ( ! isset( $attr['caption'] ) ) {
		if ( preg_match( '#((?:<a [^>]+>\s*)?<img [^>]+>(?:\s*</a>)?)(.*)#is', $content, $matches ) ) {
			$content = $matches[1];
			$attr['caption'] = trim( $matches[2] );
		}
	}

	/**
	 * Filter the default caption shortcode output.
	 *
	 * If the filtered output isn't empty, it will be used instead of generating
	 * the default caption template.
	 *
	 * @since 2.6.0
	 *
	 * @see img_caption_shortcode()
	 *
	 * @param string $output  The caption output. Default empty.
	 * @param array  $attr    Attributes of the caption shortcode.
	 * @param string $content The image element, possibly wrapped in a hyperlink.
	 */
	$output = apply_filters( 'img_caption_shortcode', '', $attr, $content );
	if ( $output != '' )
		return $output;

	$atts = shortcode_atts( array(
		'id'	  => '',
		'align'	  => 'alignnone',
		'width'	  => '',
		'caption' => '',
		'class'   => '',
		'alt'   => '',
	), $attr, 'caption' );

	$atts['width'] = (int) $atts['width'];
	if ( $atts['width'] < 1 || empty( $atts['caption'] ) )
		return $content;

	if ( ! empty( $atts['id'] ) )
		$atts['id'] = 'id="' . esc_attr( $atts['id'] ) . '" ';

	$class = trim( 'wp-caption ' . $atts['align'] . ' ' . $atts['class'] );

	if ( current_theme_supports( 'html5', 'caption' ) ) {
		return '<figure ' . $atts['id'] . 'style="width: ' . (int) $atts['width'] . 'px;" class="' . esc_attr( $class ) . '">'
		. do_shortcode( $content ) . '<figcaption class="wp-caption-text">' . $atts['caption'] . '</figcaption></figure>';
	}

	$caption_width = 10 + $atts['width'];

	/**
	 * Filter the width of an image's caption.
	 *
	 * By default, the caption is 10 pixels greater than the width of the image,
	 * to prevent post content from running up against a floated image.
	 *
	 * @since 3.7.0
	 *
	 * @see img_caption_shortcode()
	 *
	 * @param int    $caption_width Width of the caption in pixels. To remove this inline style,
	 *                              return zero.
	 * @param array  $atts          Attributes of the caption shortcode.
	 * @param string $content       The image element, possibly wrapped in a hyperlink.
	 */

	$caption_width = apply_filters( 'img_caption_shortcode_width', $caption_width, $atts, $content );

	$style = '';
	if ( $caption_width )
		$style = 'style="width: ' . (int) $caption_width . 'px" '; 

	return '<h3>' . $atts['caption'] . '</h3><div ' . $atts['id'] . $style . 'class="' . esc_attr( $class ) . '">'
	. do_shortcode( $content ) . '<p class="wp-caption-text">' . $alt_text . '</p></div>';
}

What can I write instead $alt_text variable for i get attached image alt text?